Riveted Cold Form Steel Stru Articles Test

Standard and/or project Stage TC

ISO 1051:1999 Rivet shank diameters

90.93 TC 2

ISO 14588:2000 Blind rivets -- Terminology and definitions

90.93 TC 2

ISO 14589:2000 Blind rivets -- Mechanical testing

90.93 TC 2

ISO 15973:2000 Closed end blind rivets with break pull mandrel and protruding head -- AlA/St

90.93 TC 2

ISO 15974:2000 Closed end blind rivets with break pull mandrel and countersunk head -- AlA/St

90.93 TC 2

ISO 15975:2002 Closed end blind rivets with break pull mandrel and protruding head -- AI/AIA

90.93 TC 2

ISO 15976:2002 Closed end blind rivets with break pull mandrel and protruding head -- St/St

90.93 TC 2

ISO 15977:2002 Open end blind rivets with break pull mandrel and protruding head -- AIA/St

90.93 TC 2

ISO 15978:2002 Open end blind rivets with break pull mandrel and countersunk head -- AIA/St

90.93 TC 2

ISO 15979:2002 Open end blind rivets with break pull mandrel and protruding head -- St/St

90.93 TC 2

ISO 15980:2002 Open end blind rivets with break pull mandrel and countersunk head -- St/St

90.93 TC 2

ISO 15981:2002 Open end blind rivets with break pull mandrel and protruding head -- AIA/AIA

90.93 TC 2

ISO 15982:2002 Open end blind rivets with break pull mandrel and countersunk head -- AIA/AIA

90.93 TC 2

ISO 15983:2002 Open end blind rivets with break pull mandrel and protruding head -- A2/A2

90.93 TC 2

ISO 15984:2002 90.93 TC 2

Standard and/or project Stage TC

Open end blind rivets with break pull mandrel and countersunk head -- A2/A2

ISO 16582:2002 Open end blind rivets with break pull mandrel and protruding head -- Cu/St or Cu/Br or Cu/SSt

90.93 TC 2

ISO 16583:2002 Open end blind rivets with break pull mandrel and countersunk head -- Cu/St or Cu/Br or Cu/SSt

90.93 TC 2

ISO 16584:2002 Open end blind rivets with break pull mandrel and protruding head -- NiCu/St or NiCu/SSt

90.93 TC 2

ISO 16585:2002 Closed end blind rivets with pull mandrel and protruding head -- A2/SSt

90.93 TC 2

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Mechanical Design Second edition

Peter R. N. Childs BSc (Hons), DPhil, CEng, FIMechE, MIED, ILTM, Mem ASME

University of Sussex, UK

Newnes Mechanical Engineer’s Pocket Book Third edition

Roger L. Timings�

Module10: Special Topics

Learning Unit-1:

M10.1 Mechanical Testing of Composites

The mechanical testing of composite structures to obtain parameters such as strength and stiffiless is a time consuming and often difficult process. It is, however, an essential process, and can be somewhat simplified by the testing of simple structures, such as flat coupons. The data obtained from these tests can then be directly related with varying degrees of simplicity and accuracy to any structural shape. The test methods outlined in this section merely represent a small selection available to the composites scientist. Some, such as the tensile coupon test, are widely recognised as standards, whereas there are dozens of different tests for the measurement of shear properties.

M10.1.1 Tensile Testing

Tensile testing utilizes the classical coupon test geometry as shown below and consists of two regions: a central region called the gauge length, within which failure is expected to occur, and the two end regions which are clamped into a grip mechanism connected to a test machine.

Figure M10.1.1 Typical tensile composite test specimen (all dimensions in mm)

These ends are usually tabbed with a material such as aluminum, to protect the specimen from being crushed by the grips. This test specimen can be used for longitudinal, transverse, cross-

ply and angle- ply testing. It is a good idea to polish the specimen sides to remove surface flaws, especially for transverse tests.

M10.1.2 Compressive Testing

This is much more problematical. The results obtained are essentially dependent on the type of compression fixture used. Also, the gauge length is conical, as if it is too long, the specimen will buckle and flex, resulting in premature failure. If it is too short, then the proximity of the tabs will adversely affect the stress state, resulting in artificially high values. The most widely used compressive test technique is the Celanese fixture, shown below. Cylindrical in design, a small specimen sits within a set of trapezoidal grips, encased in collars and an alignment shell. The gauge length depends on the type of test material and varies between 12.7mm for longitudinal specimens and 6mm for transverse specimens. Again, it is a good idea to tab the specimens.

Figure M10.1.2 Celanese compressive fixture and specimen (all dimensions in mm)

M10.1.3 Intra-laminar Shear Testing

Most of the numerous shear test methods in existence measure intra-laminar shear properties, rather than inter-laminar ones. In theory, however, they should be the same in a perfectly consolidated material. A very popular test is the asymmetric four-point bend test; the essential features of which are shown overleaf. The specimen is 80mm long; 20mm wide and around 3mm thick, depending on the number of plies used in the test laminate. Two notches, 4mm deep, are cut where shown and application of the load will cause shear failure along the notch roots ('axial splits') followed by wholesale intralaminar shearing in the centre section. Strain gauges are

bonded at in the specimen centre, enabling accurate shear modulus measurement (usually

even in poorly consolidated materials)

45

13 12G G

Figure M10.1.3 Schematic representation of the asymmetric four-point bend shear fixture

M10.1.4 Inter-laminar Shear Testing

The most common test for measuring shear delamination is the short beam shear test shown below, where a small specimen (<30mm long) is loaded in three-point bending until a delamination forms in the centre plane at one end of the specimen. �P�.

Figure M10.1.4 The short beam shear test

Such a test is difficult to optimize as failure will often occur by crushing under the central bending nose. In this test, the shear strength, 12 is given as,

12

3( 10.1.1)

4

PM

bh

where �P� is the applied load, �b� is the specimen width and �h� is the specimen thickness.

M10.1.5 (a) Mode-I Fracture Toughness

As a rule, Mode-I delamination in composites is measured using the Double Cantilever Beam

(DCB) test method shown below, rather than the Compact Tension test geometry used for mode-I cracking of most other materials.

Figure M10.1.5 An in situ Double Cantilever Beam (DCB) test

The specimens are long, thin coupons (typically 150mm x 25mm x 3mm), tabbed at one end with aluminium hinges or 'T' tabs and with Teflon insert at the same end in the centre plane of the specimen to represent a delamination of known length. The test is a lengthy affair, as the specimen is loaded and subsequently unloaded many times after a small increment of crack

growth. Referring to the load-displacement plot overleaf, crack growth is detected as a decrease in the trace. Nine load- unload curves are shown, with both the peak load (P) and the

corresponding crack length 1 9......a a recorded. From this, basic beam theory can then be used

to calculate a value of IcG -the mode I fracture toughness -as a function of crack length,

according to equation (M10.1.2), as,

23( 10.1.2)

2P C

G MIc aw

where �C� is the specimen compliance, given as the inverse gradient of the loading portion of each curve, and �w� is the specimen width. This is the basic analytical equation of which numerous modifications exist.

Figure M10.1.6 Typical load-displacement trace for the Double Cantilever Beam (DCB)

test

It is found in most composite materials that IcG will vary with crack length quite significantly.

This phenomenon is known as the 'R curve effect', and is a critical design parameter.

M10.1.5 (b) Mode-II Fracture Toughness

Most mode n testing is conducted using the End Notch Flexure (ENF) test. This test uses identical coupons to the Double Cantilever Beam (DCB) test (minus tabs) and produces shear delamination under three-point bending, as shown Figure M10.1.7,

Figure M10.1.7 An in situ End Notch Flexure (ENF) test

The specimen is placed under a compressive load, continuing until the crack propagates (again, detectable by a drop in the load-displacement trace). If propagation occurs at a load , then

cP IcG

is given by equation (M10.1.3),

2 29( 10.1.3)

2 316

P acG MIIc

E w hf

Unlike the Double Cantilever Beam (DCB) test, only one load cycle is required. As a general guide, Thermosets will exhibit stable mode-I growth and unstable mode-n growth, whereas the opposite is true for thermoplastics.

Learning Unit-2: M10.2

M10.2 Joining Of Composites

M10.2.1 Introduction

As in metal structures, local reinforcement is generally required where any hole (or cut-out) is placed in a structural part. Analogous to metal parts, reinforcement can be bonded or fastened to the structure. Local reinforcement may also be required in the vicinity of joints, either bonded or bolted and in locations where concentrated loads are introduced into the structure.

Joints often occur in transitions between major composite parts and a metal feature or fitting. In aircraft, such a situation is represented by articulated fittings on control surfaces as well as on

wing and tail components, which require the ability to pivot the element during various stages of operation. Tubular elements such as power shafting often use metal end fittings for connection to power sources or for articulation where changes in direction are needed. In addition, assembly of the structure from its constituent parts will involve either bonded or mechanically fastened joints or both.

Joints represent one of the greatest challenges in the design of structures in general and in composite structures in particular. The reason for this is that joints entail interruptions of the geometry of the structure and often, material discontinuities, which almost always produce local highly, stressed areas, except for certain idealized types of adhesive joints such as scarf joints between similar materials.

One of the important factors affecting design of composite materials is the load carrying capability of the composite joints. The two commonly used types of load carrying joints, made of composite laminates, are:

Mechanically fastened joints. Adhesive or bonded joints.

The classification of technological features as indicated in Figure M10.2.1 is conventional in appearance, but the difference lies in the content of joint manufacturing with due regard for the special properties of composite materials.

Figure M10.2.1 The classification of technological features

In principle, adhesive joints are structurally more efficient than mechanically fastened joints because they provide better opportunities for eliminating stress concentrations; Example: advantage can be taken of ductile response of the adhesive to reduce stress peaks. Mechanically

fastened joints tend to use the available material inefficiency. Sizeable regions exist where the material near the fastener is nearly unloaded, which must be compensated for by regions of high stress to achieve a particular required average load. In many cases, however, mechanically fastened joints cannot be avoided because of requirements for disassembly of the joint for replacement of damaged structure or to achieve access to underlying structure. Adhesive joints tend to lack structural redundancy and are highly sensitive to manufacturing deficiencies including poor bonding technique, poor fit of mating parts and sensitivity of the adhesive to temperature and environmental effects such as moisture. Assurance of bond quality has been a continuing problem in adhesive joints. While non-destructive evaluation techniques (ultrasonic and X-ray inspection) may reveal gaps in the bond, there is no present technique, which can guarantee that a bond, which appears to be intact does, in fact, have adequate load transfer capability. Thus mechanically fastened joints tend to be preferred over bonded construction in highly critical and safety related applications such as primary aircraft structural components, especially in large commercial transports, since assurance of the required level of structural integrity is easier to be guaranteed in mechanically fastened assemblies. As a rule, bonded joints prove to be more efficient for lightly loaded/non-flight critical aircraft structures whereas mechanically fastened joints are more efficient for highly loaded structures. Bonded construction tends to be more prevalent in smaller aircraft.

Figure M10.2.2 offers a list of the most common requirement of the joint design. It should be kept in mind that some of these requirements might become design variables in the course of the design process.

Figure M10.2.2 The most common requirement of the joint design

Geometry of the members being joined, for example, could be altered locally to facilitate joint design. Reliability includes an array of requirements, one of which is the implication of joint failure on system performance.

Type of joining to be used requires careful consideration of several parameters with a knowledge of the service that the joint is expected to provide. Load carrying joints usually have an overlap configuration.

Various advantages and disadvantages of bonded and mechanically fastened joints are as follows:

Advantages Disadvantages

Bonded Joints

No stress concentration in adherents.Stiff connection. Excellent fatigue properties. No fretting problems. Sealed against corrosion. Damage tolerant. Small weight penalties. Fewer pieces, lower weight, good load distribution.

Limits to thickness that can be joined with simple joint configuration. Inspection difficulty. Prone to environmental degradation. Requires high level of process control.Sensitive to peel and through- thickness stresses. Residual stress problems when joining dissimilar metals. Disassembly is impossible without component damage. Requires surface preparation.

Table M10.2.1 Advantage and disadvantages of bonded fastened joints

M10.2.2 Mechanically Fastened Joints

The behaviour of composites in bolted joints differs considerably from that of metals. The brittle nature of composites necessitates more detailed analysis to quantify the level of various stress peaks. This is due to the fact that stress concentrations dictate part static strength to a larger extent than in metals. As a result, composite joint design is more sensitive to edge distances and hole spacings than metal joint designs.

Mechanically fastened joints can be divided into two groups, viz. single row and multi-row designs. Typical lightly loaded non-critical joints require a single row of fasteners. The root joint of a wing, or a control surface, is an example of a highly loaded joint where the entire load acting on the aerodynamic surface is distributed into another structure. In such a case, the bolt pattern design consisting of several rows distributes the load for more efficient transfer.

Advantages Disadvantages

Mechanically Fastened Joints

Positive connection. No thickness limitations.

Considerable stress concentration.

Simple process. Simple inspection procedure. Simple joint configuration. Not environmentally sensitive. Provides through-thickness reinforcement and not sensitive to peel stresses.No residual stress problems. No surface preparation of component required.Disassembly possible without component damage. High tolerance to repeated loads.

Relatively compliant connection.Relatively poor fatigue properties.Hole formation may cause damage to composite. Prone to fretting. Prone to corrosion. Large weight penalty.

Table M10.2.2 Advantage and disadvantages of mechanically fastened joints

M10.2.2.1 Design Considerations

The behaviour of mechanically fastened joints is influenced by:

a) Material parameters. b) Configurational parameters. c) Fastener parameters, e.g.

i. Fastener type (screw, bolt, rivet). ii. Fastener size.

iii. Clamping force. iv. Washer size. v. Hole size.

vi. Tolerance.

The primary design considerations for bolted joints include joint strength, fastener type, local reinforcement, joint configuration, holes and pre-load: The process begins with the determination of a configuration for the joint. Single lap joints are normally adequate for thin laminates (up to about 5 mm in thickness). Fastener bending and initial bearing failure are primary areas of concern. Double lap joints are better for cyclic loads and generally stronger.

The use of mechanical fasteners to join composite structures is bound by certain constraints, which do not exist in the design of metallic joints. Care must be taken to select fasteners that are appropriate with the type of composite structures. Special types of fasteners are available for use on composites. These fasteners develop the full bearing capability of the composite without encountering local failure modes and are not susceptible to corrosion.

Fastener selection usually raises issues requiring decisions concerning laminate reinforcement, hole sizes and their location, drilling, fastener installation and inspection. Table M10.2.3 given below identifies various issues and proven design approaches to each issue. The Table indicates that the complexity of designing bolted joints arises from two primary sources, namely, (a)

composite laminates cannot re-distribute high local loads by yielding and plasticity; (b) composites are more easily damaged by drilling and fastener installation than metals.

Issue Approach

Drilling damage. Closely controlled manufacturing operations. Inspection of drilled holes.

High local stresses Larger fastener diameter. Insert (bushing). Increased laminate thickness (locally).

Preload relaxation Larger fastener head. Washers (one or both sides). Limit on installation torque.

Countersunk head Avoid, if possible. Increased laminate thickness (locally).

Damage induced by installation of blind fasteners and drive rivets

Specially designed blind rivets. Verify joint strength with tests.

Table M10.2.3 The various issues and design approaches to each issue

Design of local reinforcement of the laminate to resist local stresses is an important step in the design of bolted joint. If reinforcement is required, a proven approach is to increase laminate thickness by addition of plies placed at ±45° and 90º to the primary load direction. A quasi-

isotropic laminate provides the best bearing strength in any continuous fibre composite. Design of mechanically fastened joints has always been guided by the principle that the material being joined should fail before the fastener, and this is the practice with composites. The major structural limitation in designing mechanically fastened joints is the insufficient through-thickness strength of the laminates. This has given rise to the term pull-through strength.

Another area of concern is the bearing stress, which a fastener applies to the edge of the hole in a composite laminate as its axis rotates due to secondary bending of the joint. This condition can impose a severe limitation on a joint with limited stiffness. Further, composite's inability to support installation stresses of formed fasteners, such as solid rivets or blind fasteners poses another problem. In addition to surface damage, sub-surface damage to the laminate may occur. For this reason, use of these types of fastener is avoided.

M10.2.2.2 Failure Criteria

As in metallic joints, modes of failure in bolted joints of advanced composites are as follows:

a) Tension or tearing failure related to the net area through the fastener hole. The narrower the laminate, the more likely the chances of tensile failure.

b) Shear out failure related to the shear areas emanating from the hole edge parallel to the load and determined by the end distance.

c) Bearing failure based on the projected area of the hole. Determined by the diameter of the hole. Bearing strength is greater than the compressive strength of the composite.

d) Cleavage failure is a mixed mode failure involving tension and bending.

The above failure modes are shown in Figure M10.2.3 and Figure M10.2.4. However, in practice, mixed modes of failure often occur. The allowable stresses in each of these modes are a function of the following:

a) Geometry of the joint including the hole size, plate width and distance of the hole from the edge of the plate.

b) The clamping area and pressure. c) The fibre orientations ply sequence. d) The moisture content and exposure temperature. e) The nature of stressing, e.g. tension or compression, sustained or cyclic and any out-of-

plane loads causing bending.

Figure M10.2.3 The failure modes

Accepted design practice is to select edge distances, plate thicknesses, and fastener diameters so that of all the probable failures would be the net section and the bearing. It is recommended that highly loaded structural joints be designed to fail in a bearing mode to avoid the catastrophic

failures associated with net section failures.

The failure stresses will depend on the degree of anisotropy at the hole and hence on the local fibre orientation. Laminates containing a significant proportion of ±45º fibres have high shear strength and low stress concentrations at the hole. Therefore, they are relatively insensitive to edge distance.

M10.2.2.3 Fastener Selection

Fastener requirements for joining composite structures differ from those joining metallic structures. Fastener selection considerations for joining composites include corrosion

compatibility, fastener material, strength, stiffness, head configuration, importance of clamp-up, lightning protection, etc.


Corrosion Compatibility: Neither fibre glass nor aramid fibre reinforced composites cause corrosion problems when used with most fastener materials. Composites reinforced with carbon fibres are quite cathodic when used with materials such as aluminium or cadmium. Presence of galvanic corrosion between metallic fasteners and non-metallic composite laminates has eliminated several commonly used alloys from consideration. Conventional plating materials are also not being used because of compatibility problems. The choice of fastener materials for composite joints has been limited to those alloys, which do not produce galvanic reactions. The practice followed in aircraft industry is to coat the fasteners with anti-corrosion agent to alleviate galvanic corrosion. Fastener material: The materials currently used in design include alloys of titanium and certain corrosion resistant stainless steels with aluminium being eliminated. The choice is obviously governed by the make up of the composite materials being joined, weight, cost,

and operational environment. Titanium alloy Ti-6Al-4V is the most common alloy used with carbon fibre reinforced composite structures. Bolt bending: Due to increased inter-laminar shear between the composite plies, bending of the bolt occurs more easily. High modulus and high tensile strength fastener material is desired where bending may occur. Susceptibility of bolt bending in composite structure introduces higher reaction loads on the fastener head, which requires more careful consideration of head configuration. Bending should also be considered in multiple component fasteners such as blind fasteners. A threaded core bolt resists bending much better than a smooth bore pull-type blind fastener as shown in Figure M10.2.5.

Figure M10.2.5 The Comparison of bending resistance between the threaded core bolt and

the smooth bore pull-type blind fastener

Head configuration: Composites are sensitive to high bearing loads than are metals. This means fastener heads should be designed with as much bearing surface area as practicable. The larger area improves pull-through and delamination resistance in composites, while reducing over-turning forces from bolt bending. Countersunk or

flush head fasteners are frequently used on exterior surfaces of the aircraft where aerodynamic smoothness is required. Countersunk fasteners for composites include tension head fasteners having the large head depths and shear head fasteners having smaller head depths with head angles ranging from 1000 to 1300 as shown in Figure M10.2.6.

Figure M10.2.6 Countersunk fasteners for composites with different head depths

Countersunk fasteners: tend to bear against the surrounding element more unevenly through the thickness than protruding head fasteners do. Tension head fasteners are generally preferred over shear head fasteners due to greater strength against head pull-through. However, if the joint element is so thin that the countersunk depth is greater than 70% of the element thickness, the tendency towards uneven bearing pressure in tension head fasteners is too great and shear head fasteners are recommended in this case. Caution should be observed in the use of 1300 countersunk head fasteners. Although this type of fastener increases the bearing area of the fastener and permits it to be used in thin laminates, pull-through strength can be adversely affected. Although; close tolerance fit fasteners are desirable for use with composites, interference fit fasteners cannot be used due to potential delamination of plies at the fastener hole. Clamp up: When tolerance fit holes are used, high clamp up appears to be beneficial for joint strength and fatigue life. The clamping forces, however, must be spread out over a sufficient area so that the compressive strength of the resin system is not exceeded and the composite crushed.

M10.2.3 Bonded Joints

As stated previously, adhesive joints are capable of high structural efficiency and constitute a resource for structural weight saving because of the potential for elimination of stress concentrations which cannot be achieved with mechanically fastened joints. However, due to lack of reliable inspection methods and a requirement for close dimensional tolerances in fabrication, aircraft designers have generally avoided bonded construction in primary structure.

In a structural adhesive joint, the load in one component must be transferred through the adhesive layer to another component. The efficiency with which this can be done depends on the joint design, the adhesive characteristics and the adhesive/substrate interface. In order to transfer the load through adhesive, the substrates (or adherend) are overlapped to place the adhesive in shear. Figure M10.2.7 shows some typical joint designs for adhesively bonded joints.

Figure M10.2.7 The typical joint designs for adhesively bonded joints

In general, adhesive joints are characterized by high stress concentrations in the adhesive layer. These originate, in the case of shear stresses, because of unequal axial straining of the adherends, and in case of peel stresses, because of eccentricity in the load path. Considerable ductility is associated with shear response of typical adhesives, which is beneficial in minimizing the effect of shear stress joint strength. The response of typical adhesives to peel stresses tends to be much more brittle than that to shear stresses. Reduction of peel stresses is desirable for achieving good joint performance.

The criteria for selecting an adhesive must be considered in view of the joint design. The joint design must ensure that the adhesive is loaded in shear as far as possible. Tension, cleavage and peel loading as shown in Figure M10.2.8 should be avoided when using adhesives.

Figure M10.2.8 Tension, cleavage and peel loading in adhesives

M10.2.3.1 Design Considerations

The major considerations in the design of a bonded joint can be grouped into five categories. These include joint strength, environmental resistance, joint geometry, selection of the adhesive system and processing. The first step in the process of adhesive joint is to determine a dimensional configuration, which minimizes tensile and peel stresses. Once this is accomplished,

the next task is to select an adhesive system, which best satisfies static strength, fatigue life and environmental requirements. The third step is the development of process specifications for the joint to include details for surface preparation, curing the joint and maintaining pressure during cure, if necessary. The joint strength is typically verified analytically or by structural tests or both. Bonded joint strength vis-a-vis adherends thickness different types of joints is represented in Figure M10.2.9.

From the standpoint of joint reliability, it is vital to avoid adhesive layer to be the weak link in the joint. This means that whenever possible, the joint should be designed to ensure that the adherends fail before the bond layer. This is because failure in the adherends is fibre controlled, while failure in the adhesive is resin dominated and thus subject effects of voids and other defects, thickness variations, environmental effects, processing variations, deficiencies in surface preparation and other factors that are not always adequately controlled. This is a significant challenge since adhesives are inherently much weaker than the composite or metallic elements being joined. However, the objective can be accomplished by recognizing the limitations of the joint geometry being considered and placing appropriate restrictions on the thickness dimensions of the joint for the each geometry. In each type of joint, the adherend thickness may be increased as an approach to achieve higher load capacity. When the adherends are relatively thin, results of stress analysis show that for all types of joints, the stresses in the bond will be small enough to guarantee that the adherends will reach their load capacity before failure can occur in the bond. As the thickness of adherends increases, the bond stresses become relatively larger until a point is reached at which bond failure occurs at a lower load than that for which the adherends fail. This leads to the general principle that for a given joint type, the adherend thicknesses should be restricted to an appropriate range relative to the bond layer thickness. Because of processing considerations and defect sensitivity of the bond material, bond layer thicknesses are generally limited to a range of 0.125 to 0.40 mm.

Figure M10.2.9 Bonded joint strength Vs adherends thickness for different types of joints

M10.2.3.2 Failure Criteria

A number of failure modes may occur in bonded composite joints because of their anisotropic nature. In the adherends, failure can be tensile, inter-laminar or transverse. There may be cohesive failure also, which can occur in the adhesive. Various failure modes are shown in Figure M10.2.10 and Figure M10.2.11.



M10.2.3.3 Effect of Joint Geometry

Single and double lap joints with uniformly thick adherends are the least efficient joints. These joints are suitable primarily for thin structures with low running loads i.e. load per unit width. Of these, single lap joints are the least capable because the eccentricity of this type of geometry generates significant bending of the adherends that magnifies the peel stresses. Peel stresses are also present in the case of symmetric double lap and double strap joints and become a limiting factor on joint performance when the adherends are relatively thick.

In case of single lap joint, acceptable efficiencies can be achieved provided the overlap-to-thickness ratio is sufficiently large, of the order of 50:1. As stated above, the efficiency of a single lap joint is limited by the peel stresses. Peel stresses can be reduced to some extent by tapering of adherends (as shown in Figure M10.2.12) but the main method is to use large

overlap-to-thickness ratio. No tapering is needed at ends of the overlap where the adherends butt together because the transverse normal stresses at that location is compressive and small in magnitude.

Figure M10.2.12 Reduction of the peel stresses by overlapping of tapered adherends

For thickness above 1.5 to 2 mm, double lap configuration is used to transfer the strength of the adherends. The optimum overlap-to-thickness ratio in this case is 30: 1. Peel stresses are also present in these types of joints but they are not as severe as the stresses in the case of single lap joints.

Stepped and scarf joints are used for highly loaded adherends, and also where the thickness is more than 6.5 mm. Scarf joints are theoretically the most efficient joints having the potential for complete elimination of stress concentrations. However, practical scarf joints may be less durable because of a tendency towards creep failure associated with a uniform distribution of shear stress along the length of the joint. As a result, scarf joint tends to be used only for repairs of very thin structures. Stepped joints represent a practical solution of bonding thick members. Stepped joints have been extensively used where adherends are subjected to high load intensities. High loads can be transferred if number of short steps of small rise (thickness increment) in each step is used, while maintaining sufficient overall length of the joint. Factors influencing the joining strength are number of steps, length and thickness of each step.

M10.2.3.4 Behaviour of Composite Adherends

Composite adherends are considerably more affected by inter-laminar shear stresses than metals. Therefore, there is a significant need to account for such effects in stress analysis of adhesively bonded composites. Transverse shear deformations of the adherends have an effect analogous to thickening of the bond layer and result in a lowering of both shear and peel stress peaks. In addition, because the resins used for adherend matrices tend to be less ductile than typical adhesives, and are weakened by stress concentrations due to the presence of the fibres, the limiting element in the joint may be the inter-laminar shear and transverse tensile strengths of the adherends rather than the bond strength.

In the case of single lap joints, bending failures of the adherends may occur because of high moments at the ends of the overlap. For metal adherends, bending failures take the form of plastic bending and hinge formation, while for composite adherends the bending failures are brittle in nature. In the case of double lap joints, peel stress build up in thicker adherends can cause the types of inter-laminar failures in the adherends.

The effect of the stacking sequence of the laminates making up the adherends in composite joints is significant. For example, 900 layers placed adjacent to the bond layer theoretically act largely as additional thicknesses of bond material, leading to lower peak stresses, while layers next to the bond layer give stiffer adherend response with higher stress peaks. In practice, 900 layers next to the bond layer tend to weaken the joint due to transverse cracking, as such; advantage cannot be taken of the reduced peak stresses.

In contrast with metal adherends, composite adherends are subject to moisture diffusion effects.Therefore, response of the adhesive to moisture may be more significant issue for composite joints.

M10.2.3.5 Effects of Bond Defects

Defects in adhesive joints, which are of concern, include surface preparation deficiencies,voids and porosity, and thickness variations in the bond layer. Of the various defects, which are of interest, surface preparation deficiencies are probably of greatest concern. These are particularly troublesome because there are no current non-destructive evaluation techniques,which can detect low interfacial strength between the bond and the adherends.

For joints, which are designed to ensure that the adherends are the critical elements, tolerance to the presence of porosity and other types of defect is considerable. Porosity is usually associated with over-thickened areas of the bond, which tend to occur away from the edges of the joint where most of the load transfer takes place. Therefore, it is a relatively benign effect, especially if peel stresses are minimized by adherend tapering. If peel stresses are significant, as in the case of over-thick adherends, porosity may grow catastrophically and lead to non-damage-tolerant joint performance. In the case of bond thickness variations, these usually take place in the form of thinning due to excess resin bleed at the joint edges, leading to overstressing of the adhesive in the vicinity of the edges. Inside tapering of the adherends at the joint edges can be used to compensate for this condition. Bond thicknesses should be limited to ranges of 0.12-0.24 mm to prevent significant porosity from developing. Common practice involves the use of film adhesives containing scrim cloth, some forms of which help to maintain bond thicknesses. It is also common practice to use mat carriers of chopped fibres to prevent a direct path for access by moisture to the interior of the bond.

M10.2.3.6 Surface Pre-Treatment Prior to Bonding

Surface pre-treatment requires removal of contaminants such as oils, mold lubricants or general dirt. Techniques used for surface pre-treatment are:

1. Peel ply method in which one ply of fabric should be installed at the bonding surface and removed just prior to bonding thereby exposing the clean bondable surface. In this technique, a closely woven nylon or polyester cloth is used as the outer layer of the composite during lay-up. This ply is torn or peeled away just before bonding. The basic idea is that the tearing or peeling process fractures the resin-matrix coating and exposes a clean, virgin roughened surface for the bonding process.

2. Abrasion and solvent cleaning to remove abrasion products followed by a solvent wipe. Abrasion increases the surface energy of the surfaces to be bonded and removes any residual contamination. The abrading operation should be conducted with care to avoid exposing or rupturing of the reinforcing fibres from the surface.

A typical cleaning sequence would be to remove the peel ply and then lightly abrade the surface with a dry grit blast. After grit blasting, any remaining residue on the surface may be removed by dry vacuuming or wiping with a clean, dry cheese cloth.

M10.2.3.7 Joint Manufacture

Bonded joints can be made by gluing together pre-cured laminates with a suitable adhesive. Alternatively, bonded joints can be made by forming joints during the manufacturing process in which the joint and the laminate are cured at the same time (co-cured).

M10.2.3.8 Adhesive Selection

The selection of adhesives is based on the strength requirements over the expected service temperature range and the type of equipment available for bonding. Different types of adhesives are available which provide different ranges of adhesive bonding shear and peel strengths at various service temperatures. Adhesives for structural bonding can be categorized into three main physical forms in which they are used - (a) films, (b) pastes and (c) foams. Although films are easier to handle and provide a more uniform bond line thickness than paste adhesives, lack of refrigerated storage equipment sometimes necessitates use of paste adhesives. Foam adhesives are used for stabilizing and splicing pieces of honeycomb core. The criteria for selection of adhesive are as follows:

The adhesive must be compatible with the adherends and able to retain its required strength when exposed to in-service stresses and environment. The joint should be designed to ensure failure in one of the adherends rather than failure within the adhesive bond line. Thermal expansion of dissimilar materials must be considered. Due to large thermal expansion difference between graphite composite and aluminium, adhesively bonded joints between these two materials are likely to fail during cool down from elevated temperature cures as a result of the thermal stresses induced by their differential expansion coefficients. Proper joint design should be used avoiding tension, peel or cleavage loading. If peel forces cannot be avoided, a lower modulus adhesive having high peel strength should be used.Surface preparation should be conducted carefully, avoiding contamination of the bond line with moisture, oil, etc. The adhesive should be stored at the recommended temperature. Use of adhesives that evolve volatiles during cure should be avoided.

The recommended pressure and proper alignment fixtures should be used. The bonding pressure should be great enough to ensure that -the adherends are in intimate contact with each other. Traveller coupons should always be made for testing. The exposed edge of the bond joint should be protected with an appropriate sealing compound.

The major advantages of film adhesives are that they are easier to apply and do not require mixing equipment. They have more uniform viscosity and composition and provide more bond line thickness uniformity in a joint than do paste adhesives. The major disadvantages of film adhesives are that refrigeration is required for storage. In addition, film adhesives are more expensive than pastes and require heat and pressure to achieve satisfactory bonds.

Paste adhesives have a long shelf life and do not require refrigeration. However, they have to be mixed before application, which introduces possible human error of incomplete mixing or improper weighing. Further, paste adhesives have lower strength properties than film adhesives especially for elevated temperature service.

Foam adhesives are used in honeycomb repair to fill gaps in splice areas or between edge members and honeycomb core. In addition, they are used to fill voids and eliminate moisture paths through splice areas.

M10.2.4 Test Verification

In addition to joint coupon testing, which is performed to obtain baseline data, element testing should be performed to verify joint analysis, failure mode, and location. This is particularly important for primary connections and where the load transfer is complex. The purpose of testing is to obtain assurance that the joint behaves in the predicted manner or where analysis is inadequate.

The bolted joint element or sub-component tests are usually performed at ambient conditions to fully characterize load transfer details. Tests at other than ambient conditions are necessary in cases where the low or elevated temperatures with associated moisture contents substantially change the load distributions.

M10.2.5 Typical Joint Designs

A variety of mechanical, adhesive-mechanical and combined joints are shown in the following Table M10.2.3.

Table M10.2.4 A variety of mechanical, adhesive-mechanical and combined joints


M10.3 Environmental Effects on Composites

Composite usage has increased enormously mainly due to the advantages of lightweight, specific strength and stiffness, dimensional stability, tailor-ability of properties such as coefficient of thermal expansion and high thermal conductivity. Environmental effects on these properties may compromise a structure and must be considered during the design process.

This module deals with the major environmental concerns for the composite designer, problems encountered with these environments in the past and some materials or protective systems effectively used.

Different environmental factors along with their effect on composites are briefly discussed in the subsequent paras.

M10.3.1 Biological Attack

Biological attack on composite materials may consist of fungal growth or marine fouling. Fungal growth does not appear to be as damaging as the wet conditions that promote growth. Fungicide has been mixed in with resins to retard this growth. Even though marine organisms will grow on composite surfaces, mechanical properties do not appear to be affected and the fouling can be removed by scraping. Composites with graphite fibres have been used in medical applications for both internal and external purposes. Internal composite structures such as artificial joints or plates for bone fracture support must be bio-compatible or the material may degrade over time. External composite designs (such as artificial limbs or orthotic braces) may experience impact

damage, flexural and torsional loading during use.

M10.3.2 Fatigue

Fatigue, either through mechanical loads or acoustic vibrations, can cause crack growth or local defect formation. Fatigue design depends not only on the load but also on the use temperature range and amount of moisture present. Very cold temperatures (below -50°C) may increase the stiffness of some composite materials thereby increasing the susceptibility to fatigue damage.

M10.3.3 Fluids

M10.3.3.1 (a).Moisture

Moisture is present in the operational environment in which a composite is manufactured and throughout its useful life. Water acts as a plasticiser when absorbed by the matrix, softening the material and reducing some properties of the laminate. Moisture may also migrate along the fibre-matrix interface thereby affecting the adhesion. Moisture in composites reduces matrix dominated properties such as transverse strength, fracture toughness and impact resistance.Lowering of the glass transition temperature may also occur in epoxy and polyimide resins with an increase in absorbed moisture (as shown in Figure M10.3.1). Debonding can occur due to formation of discontinuous bubbles and cracking in the matrix. Mechanical properties can be reduced even further if heat is present or if the composite is under-cured or has a large amount of voids.

Moisture is absorbed into the composite until a saturation point is reached. This has been described as a non-Fickian process, meaning the rate of relaxation in the material due to water absorption is comparable to the diffusion rate of water. As the material properties change, such as decrease in glass transition temperature, the diffusion process changes. The mechanical properties degrade in relation to the amount of moisture absorbed, with no further deterioration after saturation is reached. Strength reductions in polyester laminates have been found to be 10-15% while epoxy resins are less vulnerable.

Figure M10.3.1 The relation between Moisture content and Glass transition temperature

Fibre glass composites with either polyester or epoxy resins have been used extensively in marine structural applications due to their strength to weight characteristics and resistance to the marine environment. Glass reinforcement is preferred over carbon fibres due to carbon's electrical conductivity, which may result in severe dissimilar metals galvanic corrosion with sea water acting as an electrolyte. This is because carbon along with metallic alloys is in the electromotive series of alloys commonly used in aircraft structures. A galvanic cell can thus be formed in the presence of moisture or any other electrolyte between carbon and contacting metal. Carbon, which is the cathodic end of the series and act as a noble metal, is impervious to corrosion itself but will accelerate corrosion in the adjacent less noble metal. Special corrosion control techniques are employed when CFRP components are placed in contact with aluminium components in aircraft assemblies. A fibre glass/epoxy ply is laid up and cocured with the carbon/epoxy plies. A faying surface sealant is applied between the two components. Aluminium parts are anodized, primed and painted prior to assembly (as shown in Figure M10.3.2).

M10.3.3.1 (b) Aircraft fluids

The aircraft fluid environment consists of fuel, hydraulic fluid, lubricants, de-icing compounds and water. Polysulphone has been found to be sensitive to phosphate ester based hydraulic fluids. Some polymer resins such as PEEK may have lower glass transition temperatures after exposure to fluids with a high aromatic content. The fuel-water immersion appeared to be the most damaging, reducing the tensile strength of graphite/epoxy and Kevlar composites by 11 %and 25 % respectively.

Figure M10.3.2 Aluminium parts anodized, primed and painted prior to assembly

M10.3.3.1 (c) Automotive Fluids

The automotive fluid environment consists of gasoline, oil, battery acid, brake fluid, transmission fluid and coolant. Most of the composites in a moist high temperature (150º) environment exhibited micro-cracking. The amount of moisture absorbed, as measured by weight gain, is directly related to the change in mechanical properties. Salt water, antifreeze and gasoline produce most pronounced effects on composites.

M10.3.3.1 (d) Other Fluids

Liquids accidentally spilled on composite surfaces may also affect the mechanical properties. Methylene chloride found in paint strippers may cause severe damage to epoxy resins and a number of other polymers. Solvents, bases and weak acids at room temperature do not appear to affect graphite/epoxies and Kevlar/epoxies.

M10.3.3.2 (a) Weathering

Warm, moist climate may affect the performance of composites. Decrease of 10-20% in tensile strength has been noted for fibre glass/polyester and fibre glass/epoxy where the surface resin has been eroded away due to extended weathering. Erosion due to rain, snow or ice impact may be a problem for some aircraft parts, such as radomes or leading edge parts. Coatings such as polyurethane may be used to make composite parts more resistant to this type of erosion. Effect of weathering on composites depends on the type of material used and whether a protective coating was intact. Studies have indicated that where the paint was intact, the material retained more than 90% of its original strength and 80-90% of modulus. Where the paint had been eroded away, the composite retained only 68% of its original strength.

Composite components are required to qualify the moisture tests, which broadly include condition of structure before and during static and fatigue tests by moisture saturation. Static

tests are carried out following immersion of composite parts in fluids like fuel, hydraulic fluids, cleaning agents, de-icing fluids, etc.

M10.3.3.2 (b) Effect of Contaminants on Weight and Balance

Fluids absorbed by or otherwise introduced into a structure induce weight gains and may cause out-of-balance conditions in flight control surfaces. Contamination detected should always be evacuated, the leakage paths identified, repaired and the structure re-sealed.

M10.3.3.2 (c) Effects of Contaminants on Structural Integrity

Dimensional swelling of the resin matrix generally results from exposure to high humidity at high temperatures, exposure to many aircraft fluids, to chemical paint strippers and to a variety of common solvents. Absorbed moisture lowers the glass transition temperature of a laminate and may be conducive to additional micro cracking within the matrix, which in turn increases the potential for additional moisture absorption. Absorbed chemicals mayor may not affect the structural or mechanical properties of the composite, but generally render the affected part un-repairable.

M10.3.4 Hail

Hail strike to composite structures leads to impact dall1age. For this purpose, composite structure having a skin thickness of 0.8 mm is protected at design stage to withstand a 2-inch hailstone on the ground with a free fall velocity of 33 m/s and energy 35 Joules (Figure M10.3.3). There is no major difference between sandwich and monolithic structures for the same skin thickness as they are equally resistant.

Figure M10.3.3 Impact dall1age due to Hail strike in composite structures

M10.3.5 Foreign Object Damage

This type of damage is caused due to foreign objects striking the surface of composites causing possible localised damage or delamination, etc. It includes ballistic damage, damage from sand, dust, stones and more often from bird strike. The impact resistant of composite materials can be controlled by the choice of reinforcement and matrix. The matrix can be altered by addition of plasticisers, which increase the strain to failure. In addition, radomes and leading edges are designed to protect the structural parts from bird impact damage. Some of these protections are shown in Figure M10.3.4.

Figure M10.3.4 The protections of surface of Composite materials by design of radomes

and leading edges

M10.3.6 Electro Magnetic Effects

Electromagnetic pulse (EMP) effects and electromagnetic interference (EMI) effects are caused by various sources like lightning, precipitation static (p-static) or corona discharge. Its effects can be catastrophic. Non-conductive composites provide little shielding effectiveness, while conductive composites like carbon/epoxy provide varying degree of protection. Fuel tanks, electrical equipment, etc. require isolation from static discharge. Lightning protection schemes can sometimes serve the dual purpose of providing lighting protection and p-static protection.

The objective is to bleed off the static charge prior to any significant build-up that could cause a fire or explosion or precipitate electromagnetic interference with the on-board electrical equipment. The shielding effectiveness of composites can be improved by metal coatings.

M10.3.7 Temperature Effects

Temperature effects on composite materials include cryogenic temperatures, elevated

temperatures and thermal cycling between these extremes. Cryogenic temperatures do not appear to affect the mechanical properties of graphite/epoxies or graphite/polyimides significantly. Elevated temperatures for a prolonged period of time can seriously affect the properties of a composite, with even greater effect if moisture is present. Loss of stiffness with temperature and ageing is indicated in Figure M10.3.5. Susceptibility to matrix softening is not only dependent on the resin but also the lay-up. Temperature effects are not limited to the matrix materials. Extended operation at 350°C (660°F) and 450°C (840°F) can cause oxidation of low modulus PAN-based fibres and high modulus PAN- or Pitch-based fibres, respectively. Oxidation resistance can be improved with higher purity fibres. Thermal cycling conditions are common for a number of applications, including aircraft and spacecraft. Thermal cycling may induce micro-cracking in some composites thereby resulting in reduction of compressive and shear strength.

Figure M10.3.5 The effect of temperature and ageing on stiffness of composite Materials

Protection against temperature effects can be achieved at the design stage itself by:

Selection of resin system with high glass transition temperature.Potential degradation taken into account in the analysis and fatigue test. Protection against moisture exposure.

M10.3.8 Overheat Conditions

Heat generated by lightning strikes has been known to vaporize matrix resins and create large areas of delamination and fibre fracturing on composite rudders, ailerons, wing and stabilizer

tips, nose domes and nacelle cowling. When exposed to hot gases over long periods, polymeric resin binders can become completely destroyed through a process of thermo-oxidation. Preventive methods may consist of application of heat resistant ablative coatings.

M10.3.9 Effect of Ultra Violet Radiation

Ultraviolet radiation is a band of light from 300 to 4000 Å. Ultra-violet radiation may cause degradation through molecular weight change and cross-linking in the resin system. However, this damage is generally limited to darkening of the resin in the surface layer. Coatings, such as thermal control tape, have been used to protect composite materials from degradation.

M10.3.10 Protective Coatings

When an environmentally resistant composite material cannot be utilized, protection of the material through the use of coatings is necessary. A variety of coatings have been developed for protecting composites from various environments. Standard marine paints, pigmented gel coatings and polyurethanes have been used to prevent ultraviolet damage and weathering erosion of marine composites.

M10.3.11 Hygrothermal Stresses and Strains in a Lamina

Composite materials are generally processed at high temperature and then cooled down to room temperatures. For polymeric matrix composites, this temperature difference is in the range of 200 to 300 C, while for ceramic matrix composites, it may be as high as 1000 C. Due to mismatch of the coefficients of thermal expansion of the fibre and matrix, residual stresses result in a lamina when it is cooled down. Also, it induces expansional strains in the lamina. In addition, most polymeric matrix composites can absorb or deabsorb moisture. This moisture change leads to swelling strains and stresses similar to those due to thermal expansion. Laminate where lamina where lamina are placed at different angles have residual stresses in each lamina due to differing hygrothermal expansion of each lamina. The hygrothermal strains are not equal in a lamina in the longitudinal and transverse directions since the elastic constants and the thermal and moisture expansion coefficients of the fiber and matrix are different. In the following sections, stress-strain relationships are developed for unidirectional and angle lamina subjected to hygrothermal loads.

Figure M10.3.6 Maximum normal tensile stresses in the x-direction as a function of angle of lamina using Maximum Stress failure theory.

Figure M10.3.7 Maximum normal tensile stress in the x-direction as a function of angle of lamina using Maximum Strain failure theory.

Figure M10.3.8 Maximum normal tensile stresses in the x-direction as a function of angle of lamina using Tsai-Hill failure theory.

Figure M10.3.9 Maximum normal tensile stresses in the x-direction as a function of angle of lamina using Tsai-Hill failure theory.

M10.3.11.1 Hygrothermal Stress-Strain Relationships for a Unidirectional Lamina

For a unidirectional lamina, the stress-strain relationship with temperature and moisture difference gives,

(M10.3.1)

where the subscripts T and C are used to denote temperature and swelling, respectively. Note that the temperature and moisture changes do not have any shearing strain terms, since no shearing strains are induced in the material axes. The thermal-induced strains are given by,

(M10.3.2)

where 1 and 2 are the longitudinal and transverse coefficients of thermal expansion,

respectively, and is the temperature change. The moisture- induced strains are given by, C

(M10.3.3)

where 1 and 2 are the longitudinal and transverse coefficients of swelling, respectively, and

is the weight of moisture absorption per unit weight of the lamina. T

Equation (M10.3.1) can be inverted to give

(M10.3.4)

M10.3.11.2 Hygrothermal Stress-Strain Relationships for an Angle Lamina

The stress-strain relationship for an angle lamina takes the following form:

(M10.3.5)

Where

(M10.3.6)

and .

(M10.3.7)

The terms x , y , and xy .y are the coefficients of thermal expansion for an angle lamina and

are given in terms of the coefficients of thermal expansion for a unidirectional lamina as

(M10.3.8)

Similarly, x , y and xy are the coefficients of moisture expansion for an angle lamina and are

given in terms of the coefficients of moisture expansion for a unidirectional lamina as

(M10.3.9)

From Equation (M10.3.1), if there are no constraints placed on a lamina, no mechanical strains will be induced in it. This also implies, then, no mechanical stresses are induced. But in a laminate, even if there are no constraints on the laminate, the difference in the thermal/moisture expansion coefficients of the various layers induces different thermal/moisture expansions in each layer. This difference results in residual stresses and will be explained fully in section M10.3.11.4.

M10.3.11.4 HYGROTHERMAL EFFECTS IN A LAMINATE

In Section M10.3.11.3, the hygrothermal strains were calculated for an angle and uni-directional lamina subjected to a temperature change, T , and moisture content change, C . Asmentioned, if the lamina is free to expand, no residual mechanical stresses would develop in the lamina at the macro-mechanical level. How- ever, in a laminate with various plies of different angle or material, each individual lamina is not free to deform. This results in residual stresses in the laminate.

M10.3.11.4.1 Hygrothermal Stresses and Strains

Sources of hygrothermal loads include cooling down from processing temperatures, operating temperatures different from processing temperatures, and humid environment such as an aircraft flying at high altitudes. Each ply in a laminate gets stressed by the deformation differences of adjacent lamina. Only the strains which are in excess of or less than the hygrothermal strains in the unrestricted lamina produce the residual stresses. These strain differences are called mechanical strains and the stresses caused by them are called mechanical stresses.

The mechanical strains induced by hygrothermal loads alone,

(M10.3.10)

where the superscript 'M' represents the mechanical strains, 'T' stands for the free expansion thermal strain, and 'C' refers to the free expansion moisture strains. Using stress-strain Equation 2 2 2/ 1/ 22E S , the hygrothermal stresses in a lamina are then given by

(M10.3.11)

where �TC� stands for combined thermal and moisture effects. Hygrothermal stresses induce zero resultant forces and moments in the laminate in the laminate and hence in the n-ply laminate shown in Figure M10.3.10,

Figure M10.3.10 Coordinate locations of plies in a laminate

(M10.3.12)

(M10.3.13)

From Equations (M10.3.11) to (M10.3.13),

(M10.3.14a)

and

(M10.3.14b)

On substituting Equations (M10.3.10) and (M10.3.15), they give

(M10.3.15)

(M10.3.16)

The four arrays on the right-hand side of the above Equations (M10.3.16) and (M10.3.17) are given by,

(M10.3.17)

(M10.3.18)

(M10.3.19)

(M10.3.20)

The loads in Equations (M10.3.18) to (M10.3.25) are called fictitious hygrothermal loads and are known. One can calculate the midplane strains and curvatures by combining Equations (M10.3.17) and (M10.3.18), which is

(M10.3.21)

Using Equation,

0

0

0

x x x

y y

z z

z y

z

One can calculate the global strains in any ply of the laminate. These global strains are the actual strains in the laminate. However, it is the difference between the actual strains and the free expansion strains, which results in mechanical stresses. The mechanical

strains in the ply are given by Equation (10.3.10) as thk

(M10.3.22)

The mechanical stresses in the ply are then calculated by thk

(M10.3.23)

The fictitious hygrothermal loads represent the loads in Equations (M10.3.17) to (M10.3.20) which one can apply mechanically to induce the same stresses and strains as by the hygrothermal load. Hence if both mechanical and hygrothermal loads are applied, one can add the mechanical loads to the fictitious hygrothermal loads to find the ply-by-ply stresses and strains in the

laminate, or one can separately apply the mechanical and hygrothermal loads and then add the resulting stresses and strains from the solution of the two problems.


M10.4 Recycling Of Composite Materials

Recycling is one of the biggest issues facing the composites industry, particularly for large-volume applications. Increasingly stringent environmental regulations are likely to restrict the use of composites in favour of materials that can be recycled cost effectively. This section deals with composite matrices that cannot be commercially depolymerised into monomer.

M10.4.1 Categories of Scrap Composites

Scrap composites can be conveniently divided into three categories:

1. Scrap in the form of offcuts, rejects, sprues etc. arising in the manufacture of composite products. Increasingly, this waste material is used in primary recycling by blending it as filler or reinforcement with virgin plastic of the same chemical origin. This route is subject to the careful control of the levels of contamination in the comminuted composite and to the deterioration in physical properties which may be caused by repeated thermal and mechanical processing. Some thermosets have been successfully recycled since the mid 1980�s without adverse effect on quality.

2. Single grades of contaminated plastic collected from consumers or processors may provide feedstock for primary or secondary recycling, subject to the feasibility of contaminant removal.

3. Mixtures of two or more grades of composite compounds arise as industrial or consumer scraps. This category of scrap poses a substantial problem in composites recycling, because of the problems associated with automatic identification, separation and determination and control of composition. Tertiary recycling is expected to be the more appropriate route for recycling.

M10.4.2 Recycling Methods for: Thermoplastic, Thermoset and Metal Matrix Composites

M10.4.2.1 Thermoplastic Matrix Composites

Thermoplastic composites scrap arising in the first two categories provides for more straightforward recycling than thermoset composites, principally because the thermoplastic can be melted. Fibre attrition and degradation of the matrix polymers lead to reuse applications with less-demanding physical property requirements. A good example of large volume thermoplastic composite recycling is long-glass-fibre-mat reinforced polypropylene (GMT). The offcuts from GMT sheet used for thermoforming of products particularly in the manufacture of automotive parts can be subsequently used after comminution and used over again as raw material for semi-finished sheet. Additionally, the offcuts or reject parts can be ground and used for extrusion or for injection moulding.

M10.4.2.2 Thermoset Matrix Composites

The recycling of thermoset composites presents great difficulties, centered around the irreversibility of cross-linking, the fibre attrition associated with comminution, and a polymer content that may be less than 30% of the total weight. The bulk of the material is often glass-fibre reinforcement or filler, including fire retardants and resin dilutants. In tertiary recycling, the problem is not just one of recycling the polymer.

Greater stability in the supply of scrap is associated with large sources of standardized composite scrap arising from cooperative industry ventures. For example, ERCOM, consortium offour large European composite manufacturers in partnership with a number of leading raw materials suppliers, shred components manufactured from polyester and vinyl-ester-based sheet and bulk modelling compounds to a range of well-defined particle sizes. The resultant fibre and powder fractions can be used in the production of new bulk-moulding compound (SMC/BMC)

components, and can also be used as reinforcing material for thermoplastics and other materials.

This type of initiative is providing for large-scale utilization of composites scrap and the incentive for the development of more comprehensive secondary and tertiary recyclingoperations. Considerable imagination is evident in the research and development work which is in progress to identify profitable routes for the recycling of composite materials. These include recycling machinery development, with attention to comminutive procedures, and new product development that has to overcome the technical, cost and aesthetic advantages of competing virgin materials. Recent research relates to proposed radical and potentially very substantial disposal options for composites scrap. Combustion with heat recovery is proposed as a route for utilizing the energy content of the matrix polymer. The behaviour of a range of composites during combustion, the form of the ash product and the emissions during combustion has been investigated systematically.

Two industrial processes that utilize the energy content and the inert materials arising in the ash have been proposed. In cement manufacture, thermoset composites may be burned in a cement kiln to utilize their energy content and the mineral materials utilized in the cement klinker.Alternatively, polymeric materials filled with calcium carbonate may be of use as a fuel substitute and sulphur oxide removing agents in coal-fired fluidized bed combustion. Large volume applications for selected recycled composites are being developed as shot-blasting media for selective paint removal, and for use as soil conditioners.

Thermoset composite materials represent a large percentage of composites manufacturing, particularly in the automotive sector, and bulk-moulding compound (BMC/SMC's) have been the subject of considerable research and success in recycling.

M10.4.2.3 Metal Matrix Composites

The term metal matrix composite (MMC) encompasses a wide range of materials. Common to all these is a continuous metallic matrix material and the reinforcing phase is usually a ceramic. MMC's are categorized according to the morphology of the reinforcement i.e. continuous

fibers, whiskers, or particles, as shown in Figure M10.4.2.3.

Figure M10.4.2.3 Morphology of the Reinforcement

The high formability of the metallic matrix gives MMC's an advantage over polymeric materials, and they are often available in stock form (e.g. billets, rods and tubes), from which components are shaped and formed during secondary operations. Another advantage is that despite being anisotropic, excellent axial performance can be combined with transverse properties which are more than satisfactory.

M10.4.2.3.1 History

Modem MMC's first appeared in the 1920's with the production of aluminium/alumina dispersion hardened systems. The 1950's saw the development of precipitation hardened materials and in both these materials, the presence of small particles impedes dislocation movement and enhances the toughness of the base metal. Also, small percentages of filler are required (<15%) to obtain this enhancement. Creep is effectively suppressed in these materials as dislocations must climb over the dispersoids by diffusive processes and this result in creep rates decreasing with increasing dispersoids size.

Evolution continued through the 1970's with the introduction of dual phase steels, which are effectively particulate MMC's consisting of up to 20% martensite within a soft ferrite matrix. This form of steel is regarded as the forerunner of modern MMC technology. Fibrous MMC's were developed in the 1960's and were based on tungsten or boron fibres embedded within a copper or aluminium matrix. The morphology of these materials is similar to that of polymer matrix composites however, interest declined as production costs escalated. Nowadays, interest in titanium matrix composite has provided something of a renaissance in this form of material.

The most modem developments in MMC technology have been in the use of whisker reinforcements. The combination of good transverse properties, low cost, high workability and significant increases in performance over unreinforced alloys has made tern currently the most commercially attractive system for many different applications.

M10.4.2.3.2 Fabrication Processes

In modem MMC technology, there are four main routes taken in the manufacture of an MMC component:

1. Primary liquid phase processing: This classification encompasses techniques such as squeeze casting and squeeze infiltration; spray deposition; slurry casting and reactive processing.

2. Primary solid state processing: This includes powder blending and pressing; diffusion bonding of foils and physical vapour deposition.

3. Secondary processing: e.g. extrusion and drawing; rolling; forging; isostatic pressing; superplastic processing and sheet forming.

4. Machining and forming processes: such as electrical, mechanical and fluid-jet cutting, and joining. These processes are rather complex and outside the scope of this course.

M10.4.2.3.3 Applications

Perhaps one of the earliest MMC applications was in the Space Shuttle, in which a structural component in the cargo bay section framework was made from a 60% boron monofilament

reinforced aluminium-based composite. Other major applications are based primarily on the enhanced stiffness and creep characteristics of MMC materials e.g. engine components, drive shafts, bicycle frames and cross-booms on yachts.

Figure M10.4.3.2.3.3 (a) Photograph of a satellite boom/waveguide structure fabricated

from an aluminium-carbon fibre MMC. The fibres are aligned parallel to the axis of the

boom.

Figure M10.4.3.2.3.3 (b) Photograph of a rotor brake disc, made from cast Al-

10wt%Si / 20vol% SiC.

®Duralcan

Figure M10.4.3.2.3.3 (b) Photograph of the Stump-jumper M2 bicycle, the frame is made

from 6061 Al-10wt% alumina ®Duralcan

Proceedings of the 6th Asia-Pacific Structural Engineering and Construction Conference

(APSEC 2006), 5 � 6 September 2006, Kuala Lumpur, Malaysia

A-246

TYPICAL TESTS ON COLD-FORMED STEEL

STRUCTURES

Mahmood Md Tahir, Tan Cher Siang, Shek Poi Ngian Steel Technology Centre, Universiti Teknologi Malaysia, UTM Skudai, 81310 Johor,

Malaysia.E-mail: [email protected]

ABSTRACT: Cold-formed steel has been recently brought into Malaysian construction. It is a steelwork technology that has high potential to be developed in Malaysia, that can offers advantages such as fast erection, lightweight, clean and easier construction. This paper reported a series of research studies carried out in UTM for locally produced cold-formed steel sections and roof truss system. The research work included study on the member capacities for lipped C-section and Hat-section, a full-scale test for roof truss system and the cold-formed steel tek-screw connection capacities. All the studies were based on the requirements of British Standard BS 5950 Part 5 1998. The actual capacities of the proposed sections were ratio from 1.09 - 2.21 compared to the design strength. The full-scale experimental test on roof truss also achieved two times of the estimated design capacities. The tek-screw connection capacities in resistance to shear force and pull-out force ranged 76% - 141% higher than the design requirements. The connection also withstand 10000 times of dynamic load that verified its performance in long-term serviceability. The results of the experimental tests on the proposed cold-formed steel section and roof truss system showed good agreement to the requirements of BS5950 Part 5 1998.

Keywords - Cold-formed steel, member capacities, roof truss, connection.

1. INTRODUCTION

Industrialized Building System (IBS) has been promoted diligently by CIDB, Malaysia since Year 2003. Besides reduced dependency on foreign labour, the simplified construction solutions offer better control of quality, increased productivity and faster completion, less wastage and cleaner environment. Through industrialization of construction, huge amount of work has been shifted to the factory and leaving the construction sites tidier and safer (Sumadi, 2001).

In support of the ongoing process of implementation of IBS in the construction industry, the research and development have been identified to focus in the area of open-building, lightweight materials, joints and sealants, services, and IT and robotics (Nuruddin, 2003). The application of light steel framing design using cold-formed steel is one of the developments of lightweight material.

Light steel framing design is generally based on the use of standard C or Z shaped steel sections (see Figure 1) produced by cold rolling from strip steel. Cold formed sections are generically different from hot rolled steel sections (e.g. Universal Beams and Universal Column), which are used in fabricated steelwork. The steel coil used in cold formed sections is relatively thin, typically 0.5 to 3.2 mm, and is galvanized for corrosion protection (Grubb, 2001).



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Fig 1. Common shapes for cold formed steel

Cold-formed steel sections can be widely used in many sectors of construction, including mezzanine floors, industrial buildings, commercial buildings and hotels and are gaining greater acceptance in the residential sector (see Figure 2). However, the application of light steel framing has not been widely developed in Malaysia, due to the lack of research study on the local practice for such system. This paper gives a series of studies on cold-formed steel section that carried in Universiti Teknologi Malaysia (UTM). It summaries the experimental works on cold-formed steel member capacities, roof truss system and connection capacities. In the end it provides suggestions for future work to enhance the practical use of light steel framing in Malaysia.

Fig 2 Cold formed steel framing (light steel framing) for residential house in UK

(Adopted from Grubb, 2001)



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2. THE BENEFITS OF COLD-FORMED STEEL AND THE LIGHT STEEL

FRAMING

Generally, cold-formed steel sections have several advantages over hot rolled steel sections, timber sections and concrete. The main aspects are listed as follows (Rogan, 1998; Thong, 2003):

1. No insect and fungal infection: The problems such as rotten or discomposed due to insect and fungal infection are eliminated; therefore the material curing and maintenance costs, which are necessary for the timber and concrete construction, could be eliminated as well.

2. Consistency and accuracy of profile: The nature of the manufacturing process: cold rolling - enables the desired profile maintained and repeated for as long as it is required, in a very close tolerance. Moreover, the very little tool wears and the cold rolling process is ideally suited to computerized operation which assists to the maintenance of accuracy.

3. Versatility of profile shape: Almost any desired cross-sectional shape can be produced by cold rolling, such as T-section, Z-section, Sigma-section etc.

4. It could be pre-galvanized or pre-coated: The steel material may be galvanized or coated by plastic materials either to enhance its resistance to corrosion or as an attractive surface finish.

5. Variety of connection and jointing methods: All conventional methods of connecting components, e.g. riveting, bolting, welding, and adhesives are suitable for cold-formed section. The tek-screw is concerned in this study since it is fastest and easiest way which is available in local.

6. Speedy in construction, and suit for site erection: Generally the steel construction has eliminated the curing time which is inevitable in concrete construction; therefore it is far faster than concrete construction. The cold formed steel may more advantageous than the hot rolled steel since it can be cut and erected with very light machine and even only man power.

7. Increase in yield strength due to cold forming: The cold forming process introduces local work hardening in the strip being formed in the vicinity of the formed corners. This local work hardening may results an increment of ultimate yield strength about 25% from its virgin strength.

8. Minimization of material: Since the material used can be very thin in comparison to the lower thickness limits of hot rolled steel sections, it allows the material usage for a given strength or stiffness requirement to be much less than that of the smallest hot rolled sections. The material thickness, or even the cross-sectional geometries, could be controlled to achieve the structural features with minimum material weight.

9. High profitably: In cold rolled process, the manufacturing costs of cold rolled steel section, mainly involve the initial modal of purchasing the rolling machine and the costs of steel strip material later. The machinery cost only expensed once then it could be covered back in the continuous production. The cold formed steel roof truss system, which is mainly interested by local industries, is normally short in



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construction, and involve only some light erection tools. Therefore, the investment required is not high and the return back is sooner than most of the constructional part.

3. EXPERIMENTAL TESTS ON COLD-FORMED STEEL IN UTM

The Steel Technology Centre (STC), Universiti Teknologi Malaysia has carried a series of analytical and experimental research on cold-formed steel structures. The studies include:

Member capacities of cold-formed lipped C-section and Hat-section Full-scale cold-formed roof truss system Tek-screw connection capacities for UNI-Interlocked roof truss system.

The research works is discussed separately in the following sub titles.

3.1 Study on the Member Capacities of Cold-formed Lipped C-section and Hat-section

A research was carried by Steel Technology Centre on cold-formed steel Lipped C-section and Hat-section (Thong, 2003; Tahir, 2005). It aimed to provide a complete design of the proposed Lipped C-section and hat-section (see Figure 3), which is anticipated to be applied as members of conventional school roof truss for JKR.

These sections have been tested for their material yield strength, tension strength, compression strength for both short and slender member and bending capacity. The connection capacity tests included pull-out test and shear test. The capacities estimation and tests are done in accordance to BS5950 Part 5: 1987 (BSI, 1987; Chung, 1993). The experimental layout was shown in Figure 4, and the summary of the results was given in Table 1.

Fig 3 Cold-formed Lipped C-section (Left) and Hat-section (Right)



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(a) Coupon test (b) Tension test (c) Compression test (short member)

(d) Compression test (slender member)

(e) Bending test (f) Connection pull-out test

(g) Connection shear test

Fig 4 Experimental Test Layout for the Member Capacities of the Proposed Cold-formed

Section



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Table 1 Comparison of the experimental test results to the design calculation for member

capacities

Type of Test Type of Specimen Design

Strength

Experimental

Result

Ratio of

Exp/Des*

Coupon test 0.6mm plate1.0mm plate

300 N/mm2

250 N/mm2344 N/mm2

300 N/mm21.151.20

Tension test Lipped C 80×40×38 20.66 kN 26.0 kN 1.26Compression test for short member

Lipped C 80×40×38 33.70 kN 38.0 kN 1.13

Compression test for slender

member

Lipped C 80×40×38 22.84 kN 25.0 kN 1.09

Bending test Hat sectionLipped C 80×40×38

0.26 kNm0.97 kNm

0.38 kNm1.50 kNm

1.461.55

Connection pull-out test

1 screw - Lipped C 2 screws - Lipped C

1 screw - Hat2 screws - Hat

0.73 kN1.46 kN0.53 kN1.06 kN

1.50 kN3.20 kN1.00 kN2.20 kN

2.052.191.892.08

Connection shear test

2 screws - 0.6mm plate4 screws - 0.6mm plate2 screws - 1.0mm plate4 screws - 1.0mm plate

1.90 kN3.80 kN3.40 kN6.80 kN

3.00 kN7.00 kN7.50 kN

12.50 kN

1.581.842.211.84

* Exp/Des = Experimental Result / Design Strength

The test results showed that actual resistance of the cold-formed steel were higher than the design strength, ranged 1.09 - 1.55 for section capacities, and 1.58 - 2.21 for connection capacities. The study concluded that the proposed cold-formed steel section give good agreement compared to the design strength. It is suggested that a full-scale test on the proposed roof truss to be carried out, in order to gain further understanding on the global truss behaviour and to build confidence in utilisation of the system.

3.2 Full-scale Test on the Cold-formed Roof Truss System

Lotez Engineering has proposed the Mega Truss System which is intended to be laid out, fabricated and handled like wood trusses, except that they are built using screw guns rather than plated methods. A full scale testing for the plane truss system was carried out at Laboratory of Structure and Material, UTM, Malaysia under the supervision of Steel Technology Centre (Tahir, 2004). The objectives of the test are:

1. To conform the adequacy of intended use of the proposed system. 2. To define the actual behaviour of the truss system in comparison of theoretical

estimation. 3. To fit the requirements claimed by Public Works Department of Malaysia, for the

application in conventional school buildings.

The proposed truss is spanned over 7.8m over two supports; and cantilevered at a span of 2.3m from each end. The truss height from the support is 2.078m. The cold-formed steel section in used was lipped C-channel. More specification of the truss system and the section properties is given in Table 2 and Table 3.

The roof truss was designed based on BS 5950-1990-Part 5 (BSI, 1998). The design loading were referred to BS 6399 (BSI, 1988). The loadings were to be transferred from the



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purlin to the truss� span at 600mm spacing, as shown in Figure 5. From the design analysis, it was estimated that each point load transferred from the purlin should not less than 8.4kN.

Table 2 Specification of the Roof Truss System

Section C - Lipped Channel (MC 10016),

102mm 51mm 1.6mm thick

Material High Tensile Galvanized Steel

Yield Stress 450 N/mm2

Zinc Coating of Z 275

Connection Class 2 - Self Tapping Screws

Truss Spacing 1.5m

Roof Pitch 25o

Table 3 Section properties of the lipped C-channel and purlin

Section Design strength,

py

(N/mm2)

Effective aea, Aeff

(mm2)

Secondmoment of area, I

(cm4)

Elasticmodulus, Z (cm3)

Moment capacity,Mc (kNm)

Tension capacities,

PT (kN)

Compression capacity, Pc

(kN)

Lipped C

450 297.2 146.35 11.2 4.92 148.05 133.74

Purlin 550 77.63 1.16 0.6 3.22 45.1 42.7



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LC 1

CH 5 CH 6

CH 7

CH 8

CH 13

CH 10

CH 11

CH 15

CH 14

LC 2

CH 12

LC 3

LC 4

Hydraulic jack and load cell set here

Stopper

Support Support

Load distributor

CH9

(a) Load configuration of the experimental test

(b) Experimental Layout

Fig 5 Full-scale test of the roof truss system



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Fig 6 Full-scale test results

The full-scale test on the truss system was carried out successfully and was proven to be valid by computer modelling using MultiFrame 3D. From the study, some conclusions can be made:

1. All the analysis models are valid to represent the actual behaviour of the truss. 2. The loading condition in the test is worse than the designed condition. Therefore if

the truss able to resist the test condition, it should be more than enough to resist the design condition.

3. The truss is able to resist a loading of 16.8kN at each loaded point (or total load of 67.2kN) in the test condition. It is two times higher than the design load. Therefore it can be concluded that the capacity of truss is very safe to be implemented.

4. There are no visible failure observed during the yielding of the truss, therefore the components are expected have a ductile behaviour. The ductile behaviour is favourable in term of safety since it delays the significant collapse of the structure.

From the facts stated above, it is concluded that the truss is having the adequate strength to resist the design loads, and it is safe to be implemented in the intended uses.

3.3 Connection Tests for UNI-Interlocked Roof Truss System

Three sets of connection tests were performed on the product of Tong Yong Sdn. Bhd. - cold formed sections of UNI-Interlocked steel roof truss system at Laboratory of Structural and Material, UTM, on April 2004 (Thong, 2004). The main purpose of the tests was to verify the validity of design method provided by BS 5950 Part 5: Code of practice for design of cold

formed thin gauge sections (BSI, 1998) in estimating the actual behaviour of the connection in the truss system. One of the tests - the fastener dynamic test was aimed to show the durability of the connections under long term dynamic loads. The other two tests were shear test and pull-out test.

All specimens were 0.55mm thick. The dimension of the specimens were shown in Figure 7. They were tested by using DARTEC MACHINE with maximum capacity of 250kN. The loading sequence used was in accordance with European Recommendations (ECCS, 1983).



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The European Recommendations stated that the rate of loading shall not exceed 1kN/min and the rate of straining shall not exceed 1mm/min during the test.

As for fastener dynamic test, the connection of roof truss was inherently subjected to both the dead load and dynamic load especially the vibration of fan. According to Rhodes in Design of Cold Formed Steel Members (1991), it was proven that the strength of shear connections were rarely affected by less 10000 cycles of repeated loading. In this test, 10,000 cycles of cyclic loading (varied between 0 to ±6kN, by 0.5Hz frequency) was imposed onto the connection. The test configuration is shown in figure 8.

(a) Shear Test (b) Fastener Dynamic Test

(C) Pull-out Test

Fig 7 Connection tests on cold-formed steel section with thickness of 0.55mm

Screw Screws

All thickness = 0.55mm

220 mm

30 mm75 mm

300 mm

Screws

20mm

15mm

50mm



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Fig 8 Configuration of fastener dynamic load test and the example of collected data

Table 4 Summary of experimental test results

Type of Test Type of Specimen Experimental

Result (per

Unit Capacity)

Average

Value of

Exp. Result

Design

Strength

Typical model - 2 screws

Typical model - 4 screws

3.43 kN3.00 kN

3.22 kNConnection shear

testOn-site model - 4

screws3.47 kN 3.47 kN

1.44 kN

Connection pull-out test

1 layer 1 screw1 layer 2 screws2 layers 1 screw2 layers 2 screws

1.40 kN1.33 kN1.91 kN1.55 kN

1.55 kN 0.88kN

Fastener dynamic test

-The connections were able to sustain its

strength after 10000 cycles of cyclic loading.

The shear test average result is 3.22kN for typical model and 3.47kN for on-site model. These values are 124% - 141% greater than the design strength. Connection pull-out test average value is 1.55kN, which is 76% greater than their design strength. As for dynamic test, the connections were able to sustain its strength after 10000 cycles of cyclic loading, thus its performance is proven for long-term service.

4. CONCLUSION

A series of experimental tests on cold-formed steel members and structures has been carried out successfully in UTM. The result of the tests can be concluded as follow:

1. For experiment study on the member capacities of cold-formed lipped C-section and Hat-section, all tests result is higher than the design value. The ratio of difference lay between 1.09 and 2.21. The test proven that the proposed sections are suitable for the design of roof truss system.



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2. For the full-scale test of roof truss system, the truss was able to sustain the total load up to 67.2kN (or point loads of 16.8kN) which is two time higher than the predicted capacity (8.4kN).

3. For the connection tests, the actual strength of the connection in resisting shear force and pull-out force is greater than the design strength. The percentage of difference is 76% - 141%. The connection was able to withstand 10000 cycles load, which also proven its performance for long-term serviceability.

Currently most of the laboratory research and construction practice of cold-formed steel in Malaysia were focus to roof truss system. It is suggested that studies on the member capacities of thicker cold-formed sections (range 1.0mm to 3.2mm) can be carried out in order to widen the application area of steel structures, such as wall stub, floor batten, beam and column. The studies on the partial strength connection for beam-column cold-formed steel structure is also proposed, that it may leads to optimization of the structural robustness and steel weight saving. The cold-formed steel sections can be further applied into sub-structures such as staircases, foundation system, retaining walls and scaffolding.

5. ACKNOWLEDGEMENT

The works reported herein were undertaken in association with Tong Yong Sdn. Bhd and Lotez Engineering Sdn. Bhd. The technical and laboratory works was mainly carried out by Mr Thong Chin Mun (master student of UTM). The technical, financial and material contributions from the above parties are gratefully acknowledged.

6. REFERENCES

British Standard Institution (BSI), 1987, BS5950 Part 5: Code of Practice for Design of Cold

Formed Thin Gauge Sections, British Standard Institution, UK. British Standard Institution (BSI), 1988, BS6399 Part 3: Code of Practice for Imposed Roof

Loads, British Standard Institution, UK. British Standard Institution (BSI), 1998, BS5950 Part 5: Code of Practice for Design of Cold

Formed Thin Gauge Sections, British Standard Institution, UK. Chung, K. F., 1993, Building design using cold formed steel sections: Worked examples, The

Steel Construction Institute, UK. European Convention for Constructional Steelwork (ECCS) , 1983, Publication No. 21

(1983): European recommendations for steel construction: the design and testing of connections in steel sheeting and sections, European Convention for Constructional Steelwork, UK.

Grubb, P.J., Gorgolewski, M.T., and Lawson, R.M., 2001, Building Design using Cold

Formed Steel Sections: Light Steel Framing in Residential Construction, The Steel Construction Institute, UK.

Lawson, R. M., Chung, K.F. and Popo-Ola, S. O., 2002, Building Design using Cold Formed

Steel Sections: Structural Design to BS5950-5: 1998 Section Properties and Load Tables,The Steel Construction Institute, UK.

Rhodes, J., 1991, Design of Cold Formed Steel Members, Elsevier Science Publisher, UK: London.

Rogan, A. L. and Lawson, R. M., 1998, Value and Benefit Assessment of Light Steel Framing

in Housing, The Steel Construction Institute, UK. Nuruddin, M. M., 2003, Research and Development (R&D) for IBS, in Buletin Bulanan

Jurutera Bil 2003 No 6., Institute of Engineers, Malaysia. Sumadi et al. 2001, Promotion Strategies and Future Research & Development Needs on IBS,

National Seminar on IBS, CIDB, 17th Sept 2001.



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Tahir, M.M., Saad, S., Saleh, A.L., Tan, C.K., 2003, Full-scale Testing for Roof Truss System

Using Cold-formed Steel Sections, in 2nd International Conference on Construction in the 21th Century, Korea.

Tahir, M.M., Thong, C.M. and Tan, C.S., 2005, Cold-formed Steel Section Developed for

Roof Truss System, Jurnal Teknologi Vol 42(B), Pernebit UTM, Malaysia, pp11-28. Thong, C.M., 2003, Development of New Cold-formed Steel Sections for Roof Truss System,

Master Thesis, UTM. Thong, C.M., 2004, Test Report of UNI-Interlocked Roof Truss System (Connection),

Technical Report Submitted to UTM. Trebilcock, P. J., 1994, Building Design Using Cold Formed Steel Sections: An Architect�s

Guide, The Steel Construction Institute, UK.

length of 230 mm. Specify a suitable spring for

this application.

11.4 A helical spring is required to exert a force of

2000 N at a length of 200 mm and 1500 N at a

length of 250 mm. Specify a suitable spring for

this application.

11.5 A Belleville spring is required to give a

constant force of 50 � 5 N over a deflection

of �0.2 mm.The spring must fit within a 40 mm

diameter hole.A carbon spring steel with �uts �

1700 MPa has been proposed.

11.6 A Belleville spring is required to give a constant

force of 10 � 1N over a deflection of �0.15mm.

The spring must fit within a 16 mm diameter

hole. A carbon spring steel with �uts �

1700 MPa has been proposed.

11.7 A helical spring is required for a pogo stick to

exert a force of 975 N at a length of 140 mm and

300 N at a length of 220 mm. Specify a suitable

spring for this application if the spring material

is ASTM A232.

Springs

250

Learning objectives achievement

Can you specify the principal parameters for a helical � � � Section 11.2compression spring?

Can you specify the principal parameters for a helical � � � Section 11.3extension spring?

Can you determine the principal dimensions of a Belleville spring � � � Section 11.6washer for a given duty?

d

251

12

FASTENING AND POWER SCREWS

A fastener is a device used to connect or jointwo or more components. Traditional forms offastening include nuts, bolts, screws and rivets. Inaddition, welding and adhesives can be used toform permanent joins between components. Theaim of this chapter is to introduce a wide selec-tion of fastening techniques. Whilst consideringthreaded fasteners the subject of power screws,which are used for converting rotary motion tolinear motion, is also considered here.

LEARNING OBJECTIVES

At the end of this chapter you should be able todetermine the:

• preload and tightening torque for a threadedfastener;

• lifting and lowering torques for a powerscrew;

• safe load for a variety of riveted joints;

• thickness or overlap for simple adhesive lapjoint.

12.1 Introduction topermanent and non-permanent fastening

The joining of components is a frequent necessity

in the design of products.For example, the Boeing

747 has over 2.5 million fasteners.Fastening tech-

niques can also be a major feature in design. Cur-

rent styling for automotive vehicles dictates an

absence of the means of fastening components

together under a cursory inspection.The range of

fastening techniques is extensive including adhe-

sives, welding, brazing, soldering, threaded and

unthreaded fasteners, special purpose fasteners

and friction joints. Some of these techniques are

permanent in nature and some allow the joint to

be dismantled. A variety of basic types of join is

illustrated in Figure 12.1.

Design considerations include:

• whether the joint should be permanent or

non-permanent

• cost

• loads in the fastener or power screw and the

associated components

• life

• tooling

• assembly

• tolerances

• aesthetics

• size

• corrosion

Threaded fasteners are introduced in Section 12.2,

power screws in Section 12.3, rivets in Section

12.4, adhesives in Section 12.5 and welding and

the related subjects of soldering and brazing in

Section 12.6.

12.2 Threaded fasteners

There is a large variety of fasteners available using

a threaded form to produce connection of com-

ponents.The common element of screw fasteners

is a helical thread that causes the screw to advance

into a component or nut when rotated. Screw

threads can be either left-handed or right-handed

Fastening and power screws

252

depending on the direction of rotation desired for

advancing the thread as illustrated in Figure 12.2.

Generally, right-hand threads are normally used

with left-handed threads being reserved for spe-

cialist applications.The detailed aspects of a thread

and the specialist terminology used is illustrated

in Figure 12.3 and defined in Table 12.1.

Specific thread forms, angle of helix, etc., vary

according to specific standards.Common standards

developed include UNS (unified national standard

series threads) and ISO threads. Both of these use

a 60° included angle, but are not interchangeable.

The form for an ISO metric thread for a nut is

illustrated in Figure 12.4. In practice, the root of the

nut and the crest of the mating bolt are rounded.

Both male and female ISO threads are subject to

manufacturing tolerances, which are detailed in BS

3643.A coarse series thread and fine series threads

Butt joint (edge)

Butt joint (tee)

Lap joint

Double lap joint

Cylindrical butt joint

Flange joint

Combination flange

and lap joint

Cylindrical lap joint

Face joint

Cylindrical flange joint

Figure 12.1 A variety of types of join.

Crest

Nut

Root

BoltPitch P

Thread axis

Thread

angle

Root

dia

mete

r

Eff

ective d

iam

ete

r

Majo

r dia

mete

r

Fla

nk

Depth

Pitch_____2

Figure 12.3 Specialist terminology used for describingthreads.

(a) (b)

Figure 12.2 (a) Right-hand thread; (b) Left-hand thread.

Chapter 12

253

are defined in the ISO standard, but fine series

threads tend to be more expensive and may not be

readily available from all stockists.Table 12.2 gives

the standard sizes for a selection of ISO coarse

series hexagon bolts, screws and nuts. ISO metric

threads are designated by the letter M, followed by

the nominal diameter and the pitch required, for

example, M6 � 1.5.

The unified system of screw threads was origin-

ally introduced in the United Kingdom, Canada

and the United States in order to provide a com-

mon standard for use by the three countries.Types

of unified threads in common use include the

unified coarse pitch thread series (UNC) and the

unified fine pitch thread series (UNF). Pertinent

dimensions for selected UNC and UNF threads

are given in Tables 12.3 and 12.4. Unified threads

are specified by notation, in the case of a 1/2 bolt,

in the form ‘1/2 in, 13UNC’ or ‘1/2 in, 20UNF’

depending on whether a coarse or a fine thread is

being used.

The range of threaded fasteners available is

extensive including nuts and bolts,machine screws,

set screws and sheet metal screws. A variety of

machine screws is illustrated in Figure 12.5.

With such an array of types of fastener, the task

of selection of the appropriate type for a given

application can be time-consuming.

Washers can be used either under the bolt head

or the nut or both in order to distribute the clamp-

ing load over a wide area and to provide a bearing

surface for rotation of the nut.The most basic form

of a washer is a simple disc with a hole through

which the bolt or screw passes. There are, how-

ever, many additional types with particular attri-

butes such as lock washers,which have projections

that deform when compressed, producing add-

itional forces on the assembly, decreasing the pos-

sibility that the fastener assembly, will loosen in

service.Various forms of washer are illustrated in

Figure 12.6.

Selection of a particular fastener will depend

upon many different criteria such as:

• strength at the operating temperatures

concerned

• weight

• cost

• corrosion resistance

• magnetic properties

• life expectancy

• assembly considerations.

Threaded fasteners tend to be used such that

they are predominantly loaded in tension. An

example is the bolt shown in Figure 12.7 used to

fasten a flanged joint.As the fastener is tightened

the tension on the bolt increases. It might be

Table 12.1 Thread terminology

Term Description

Pitch The thread pitch is the distance between corresponding points on adjacent threads.Measurements must be made parallel to the thread axis

Outside The outside or major diameter is the diameter diameter over the crests of the thread

measured at right angles to the thread axis

Crest The crest is the most prominent part of thread, either external or internal

Root The root lies at the bottom of the groove between two adjacent threads

Flank The flank of a thread is the straight side of the thread between the root and the crest

Root The root, minor or core diameter is the diameter smallest diameter of the thread measured at

right angles to the thread axis

Effective The effective diameter is the diameter on diameter which the width of the spaces is equal to the

width of the threads. It is measured at right angles to the thread axis

Lead The lead of a thread is the axial movement of the screw in one revolution

Pitch P

60º

1.4433P

Pitchline

Nut

Bolt

P_8

P_4

2_P

6_H H _ 4

_3 8H

H

Figure 12.4 ISO metric thread.


254

envisaged that the strength of a threaded fastener

would be limited by the area of its minor diameter.

Testing,however, shows that the tensile strength is

better defined using an area based on an average of

the minor and pitch diameters.

(12.1)

For UNS threads,

(12.2)

For ISO threads

(12.3)

and

dr � d � 1.226869p (12.4)

The stress in a threaded rod due to a tensile load is

(12.5)

Theoretically one might think that when a nut

engages a thread, all the threads in engagement

would share the load. However, inaccuracies in

thread spacing cause virtually all the load to be

taken by the first pair of threads.

� �tt

F

A

d d pp 0.649519� �

d dN

d dN

p r 0.649519

and 1.299038

� � � �

A d dt p r2

16( �

�� )

Table 12.2 Selected dimensions for a selection of British Standard ISO Metric Precision HexagonBolts. BS 3692:1967

Nominal size Pitch ofand thread thread (coarse

Width across flats Height of headTapping Clearance

diameter pitch series) Max Min Max Min drill drill

M1.6 0.35 3.2 3.08 1.225 0.975 1.25 1.65M2 0.4 4.0 3.88 1.525 1.275 1.60 2.05M2.5 0.45 5.0 4.88 1.825 1.575 2.05 2.60M3 0.5 5.5 5.38 2.125 1.875 2.50 3.10M4 0.7 7.0 6.85 2.925 2.675 3.30 4.10M5 0.8 8.0 7.85 3.650 3.35 4.20 5.10M6 1 10.0 9.78 4.15 3.85 5.00 6.10M8 1.25 13.0 12.73 5.65 5.35 6.80 8.20M10 1.5 17.0 16.73 7.18 6.82 8.50 10.20M12 1.75 19.0 18.67 8.18 7.82 10.20 12.20M14 2 22.0 21.67 9.18 8.82 12.00 14.25M16 2 24.0 23.67 10.18 9.82 14.00 16.25M18 2.5 27.0 26.67 12.215 11.785 15.50 18.25M20 2.5 30.0 29.67 13.215 12.785 17.50 20.25M22 2.5 32.0 31.61 14.215 13.785 19.50 22.25M24 3 36.0 35.38 15.215 14.785 21.00 24.25M27 3 41.0 40.38 17.215 16.785 24.00 27.25M30 3.5 46.0 45.38 19.26 18.74 26.50 30.50M33 3.5 50.0 49.38 21.26 20.74 29.50 33.50M36 4 55.0 54.26 23.26 22.74 32.00 36.50M39 4 60.0 59.26 25.26 24.74 35.00 39.50M42 4.5 65.0 64.26 26.26 25.74 37.50 42.50M45 4.5 70.0 69.26 28.26 27.74 40.50 45.50M48 5 75.0 74.26 30.26 29.74 43.00 48.75M52 5 80.0 79.26 33.31 32.69 47.00 52.75M56 5.5 85.0 84.13 35.31 34.69 50.50 56.75M60 5.5 90.0 89.13 38.31 37.69 54.50 60.75M64 6 95.0 94.13 40.31 39.69 58.00 64.75M68 6 100.0 99.13 43.31 42.96 62.00 68.75

All dimensions in mm.

Bolts are normally tightened by applying

torque to the head or nut, which causes the bolt

to stretch. The stretching results in bolt tension,

known as preload, which is the force that holds a

joint together.Torque is relatively easy to measure

using a torque meter during assembly so this is

the most frequently used indicator of bolt ten-

sion. High preload tension helps to keep bolts

tight, increases the strength of a joint, generates

friction between parts to resist shear and improves

the fatigue resistance of bolted connections.The

recommended preload for reusable connections

can be determined by

Fi � 0.75At�p (12.6)

and for permanent joints by

Fi � 0.9At�p (12.7)

where At is the tensile stress area of the bolt (m2);

�p, proof strength of the bolt (N/m2).

Material properties for steel bolts are given in

SAE standard J1199 and by bolt manufacturers.

If detailed information concerning the proof

strength is unavailable then it can be approxi-

mated by

�p � 0.85�y (12.8)

Once the preload has been determined the

torque required to tighten the bolt can be esti-

mated from

T � KFid (12.9)

where T is wrench torque (N m); K, constant;

Fi, preload (N); d, nominal bolt diameter (m).

The value of K depends on the bolt material

and size. In the absence of data from manufactur-

ers or detailed analysis, values for K are given in

Table 12.5 for a variety of materials and bolt sizes.

Chapter 12

255

Table 12.3 American Standard thread dimensions forUNC screw threads

Nominal Tensile Size major Threads stress area designation diameter (in) per inch (in2)

0 0.06001 0.0730 64 0.002632 0.0860 56 0.003703 0.0990 48 0.04874 0.1120 40 0.006045 0.1250 40 0.007966 0.1380 32 0.009098 0.1640 32 0.0140

10 0.1900 24 0.017512 0.2160 24 0.0242

Fractional sizes1/4 0.2500 20 0.03185/16 0.3125 18 0.05243/8 0.3750 16 0.07757/16 0.4375 14 0.10631/2 0.5000 13 0.14199/16 0.5625 12 0.1825/8 0.6250 11 0.2263/4 0.7500 10 0.3347/8 0.8750 9 0.4621 1.000 8 0.60611⁄8 1.125 7 0.76311⁄4 1.250 7 0.96913⁄8 1.375 6 1.15511⁄2 1.500 6 1.40513⁄4 1.750 5 1.902 2.000 4.5 2.50

Table 12.4 American Standard thread dimensions forUNF screw threads

Nominal Tensile Size major Threads stress area designation diameter (in) per inch (in2)

0 0.0600 80 0.001801 0.0730 72 0.002782 0.0860 64 0.003943 0.0990 56 0.005234 0.1120 48 0.006615 0.1250 44 0.008306 0.1380 40 0.010158 0.1640 36 0.01474

10 0.1900 32 0.020012 0.2160 28 0.0258

Fractional sizes1/4 0.2500 28 0.03645/16 0.3125 24 0.05803/8 0.3750 24 0.08787/16 0.4375 20 0.11871/2 0.5000 20 0.15999/16 0.5625 18 0.2035/8 0.6250 18 0.2563/4 0.7500 16 0.3737/8 0.8750 14 0.5091 1.000 12 0.66311⁄8 1.125 12 0.85611⁄4 1.250 12 1.07313⁄8 1.375 12 1.31511⁄2 1.500 12 1.581


256

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

Figure 12.5 Various machine screw styles. (a) Flatcountersunk head. (b) Slotted truss head. (c) Slotted pan head. (d) Slotted fillister head. (e) Slotted ovalcountersunk. (f) Round head. (g) Hex. (h) Hex washer.(i) Slotted hexagon head.

Figure 12.6 Washers.

Figure 12.7 Flanged joint.

Example 12.1

An M10 bolt has been selected for a re-useable

application.The proof stress of the low carbon

steel bolt material is 310 MPa. Determine the

recommended preload on the bolt and the

torque setting.

Solution

From Table 12.2, the pitch for a coarse series

M10 bolt is 1.5 mm.

For a reusable connection, the recommended

preload is

Fi � 0.75At�p � 13.48 kN

From Table 12.5, K � 0.2.The torque required

to tighten the bolt is given by

T KF d 0.2 13.48 10 0.01

26.96 Nm

i

3� � � � �

�

At2 2

16 8.16) 57.99 mm�

�� ( .9 026

dr 10 1.226869 1.5 8.160 mm� � � �

dp 10 0.649519 1.5 9.026 mm� � � �

The principal applications of threaded fasteners,

such as bolts and nuts is clamping components

together. In such situations the bolt is predomi-

nantly in tension. Both the bolt and the clamped

components will behave as elastic members, pro-

vided material limits are not exceeded. If a load is

applied to a bolted joint that is above the clamp-

ing load, then the behaviour of the joint itself

needs to be considered. As the bolt stretches, the

compressive load on the joint will decrease, alle-

viating some of the load on the bolt. If a very stiff

bolt is used to clamp a flexible member, such as a

soft gasket, most of the additional force, above the

clamping load, is taken by the bolt and the bolt

should be designed to take the clamping force

and any additional force. Such a joint can be clas-

sified as a soft joint. If, however, the bolt is rela-

tively flexible compared to the joint, then nearly

all the externally applied load will initially go

towards decreasing the clamping force until the

components separate.The bolt will then carry all

the external load.This kind of joint is classified as

a hard joint.

Practical joints normally fall between the two

extremes of hard and soft joints. The clamped

components of a typical hard joint have a stiffness

of approximately three times that of the bolt.An

externally applied load will be shared by the bolt

and the clamped components according to the

relative stiffnesses, which can be modelled by

(12.10)

(12.11)

where Fb is final force in the bolt (N); Fi, initial

clamping load (N); kb, stiffness of the bolt (N/m);

kc, stiffness of the clamped components (N/m);

Fe, externally applied load (N); Fc, final force on

the clamped components (N).

F Fk

k kFc i

c

b ce

� �

�

F Fk

k kFb i

b

b ce

� �

�

Chapter 12

257

Table 12.5 Values for the constant K for determiningthe torque required to tighten a bolt

Conditions K

1⁄4 to 1 inch mild steel bolts 0.2Non-plated black finish steel bolts 0.3Zinc plated steel bolts 0.2Lubricated steel bolts 0.18Cadmium plated steel bolts 0.16

Source: Oberg et al., 1996.

Example 12.2

A set of six M8 bolts is used to provide a

clamping force of 20 kN between two compo-

nents in a machine. If the joint is subjected to

an additional load of 18 kN after the initial

preload of 8.5 kN per bolt has been applied,

determine the stress in the bolts.The stiffness

of the clamped components can be assumed to

be three times that of the bolt material. The

proof stress of the low carbon steel bolt mater-

ial is 310 MPa.

Solution

Taking kc � 3kb,

As Fc is greater than zero, the joint remains

tight.The tensile stress area for the M8 bolt can

be determined from

At2 2

16 6.466) 36.61 mm�

�� ( .7 188

dr 8 1.226869 1.25 6.466 mm� � � �

dp 8 0.649519 1.25 7.188 mm� � � �

F Fk

k kF F

k

k kF

F F

c ic

b ce i

b

b be

i e

3

3

3

4 8500

3(18000/6)

4

6250 N

� ��

� ��

� � � �

�

F Fk

k kF F

k

k kF

F F

b ib

b ce i

b

b be

i e

3

1

4 8500

18000/6

4

9250 N

� ��

� ��

� � � �

�

12.3 Power screws

Power screws,which are also known as lead screws,

are used to convert rotary motion into linear move-

ment.With suitably sized threads they are capable of

large mechanical advantage and can lift or move

large loads. Applications include screw jacks and

traverses in production machines. Although suited

to fasteners thread forms, such as the ISO metric

standard screw threads, UNC and UNF series

described in Section 12.2 may not be strong enough

for power screw applications. Instead square,Acme

and buttress thread forms have been developed and

standardized; see Figures 12.8 to 12.10, respectively.

Some of the principal dimensions for standard

Acme threads are given in Table 12.6.

Self-locking in power screws refers to the con-

dition in which a screw cannot be turned by the

application of axial force on the nut.This is very

useful in that a power screw that is self-locking will

hold its position and load unless a torque is applied.

As an example most screw jacks for cars are self-

locking and do not run down when the handle is

let go.The opposite condition to self-locking is a

screw that can be back-driven.This means that an

axial force applied to the nut will cause the screw

to turn.A product application of this is the Yankee

screwdriver, which has a high-lead thread on its

barrel that is attached to the screwdriver bit.The

handle acts as the nut and when pushed the barrel

will turn driving the bit round. Back-driveable

power screws are a useful form of turning linear

motion into rotary motion.


258

The stress in each bolt is given by

This is 82 per cent of the proof stress.The bolts

are therefore safe.

� � ��

��

9250

36.61 10 252.7 MPab

t6

F

A

P _ 2

2_

Pitch PP

Figure 12.8 Square thread.

29↓

Pitch P

D

R

C

Figure 12.9 Acme thread.

7↓ 45↓ H

Pitch PF

R

SH

�S

�

Figure 12.10 Buttress thread.

Table 12.6 Principal dimensions for ACME threads

Major Thread Pitch Minor Tensilediam- Threads pitch diam- diam- stresseter (in) per inch (in) eter (in) eter (in) area (in2)

0.25 16 0.063 0.219 0.188 0.0320.313 14 0.071 0.277 0.241 0.0530.375 12 0.083 0.333 0.292 0.0770.438 12 0.083 0.396 0.354 0.1100.500 10 0.100 0.450 0.400 0.1420.625 8 0.125 0.563 0.500 0.2220.750 6 0.167 0.667 0.583 0.3070.875 6 0.167 0.792 0.708 0.4421.000 5 0.200 0.900 0.800 0.5681.125 5 0.200 1.025 0.925 0.7471.250 5 0.200 1.150 1.050 0.9501.375 4 0.250 1.250 1.125 1.1081.500 4 0.250 1.375 1.250 1.3531.750 4 0.250 1.625 1.500 1.9182.000 4 0.250 1.875 1.750 2.5802.250 3 0.333 2.083 1.917 3.1422.500 3 0.333 2.333 2.167 3.9762.750 3 0.333 2.583 2.417 4.9093.000 2 0.500 2.750 2.500 5.4123.500 2 0.500 3.250 3.000 7.6704.000 2 0.500 3.750 3.500 10.3214.500 2 0.500 4.250 4.000 13.3645.000 2 0.500 4.750 4.500 16.800

A screw thread can be considered to be an

inclined plane wrapped around a cylinder to form

a helix.The forces on a single thread of a power

screw are illustrated for the case of lifting a load

and lowering a load in Figure 12.11.

The inclination of the plane is called the lead

angle .

(12.12)

In the case of lifting a load, summing the forces

gives

(12.13)

(12.14)

So

(12.15)

So

(12.16)

where is the coefficient of friction between the

screw and the nut.The coefficient of friction can

typically be taken as 0.15.

Solving Eqs 12.14 and 12.16 to give an expres-

sion for F gives

(12.17)

The screw torque required to lift the load is

given by

(12.18)

It is convenient to rewrite this equation in terms

of the lead L, so substituting with Eq. 12.12 and

rearranging gives:

(12.19)

The thrust collar also contributes a friction torque

that must be accounted for.The torque required

to turn the thrust collar is given by

(12.20)

where dc is diameter of the thrust collar; c, coef-

ficient of friction in the thrust bearing.

The total torque to lift the load for a square

thread is

(12.21)

Similar analysis can be performed for lowering

a load, in which case:

(12.22)

For Acme threads the equivalent torque relation-

ship for lifting a load is:

(12.23)

and for lowering:

(12.24)

The work done on a power screw is the product

of the torque and angular displacement. For one

revolution of a screw,

Win � 2�T (12.25)

TPd d L

d LP

dd

p p

pc

c cos )

2( cos )

2�

� � �

� � � �

(

TPd d L

d LP

du

p p

pc

c cos )

2( cos )

2�

� � �

� � � �

(

TPd d L

d LP

dd

p p

pc

c )

2( )

2�

� �

� � �

(

TPd d L

d LP

du

p p

pc

c )

2( )

2�

� �

� � �

(

T Pd

c cc

2�

TPd d L

d Ls

p p

p

)

2( )�

� �

� �

(

T Fd Pd

s

p p

2

cos sin )

2(cos sin )� �

�

�

(

FP

( cos sin )

cos sin�

�

�

NP

cos sin

� �

F N f P

N N P

y 0 cos sin

cos sin

→ � � � �

� � �

F N ( cos sin )� �

F F f N

F N N

x 0 sin

cos sin

� � � �

� � �

→ cos

tan p

��

L

d

Chapter 12

259

y

x F

P

NL � lead

f

!

! dp

!

F

! dp

fN

P

L

(a) (b)

Figure 12.11 Force analysis at the interface of a leadscrew and nut (a) lifting a load; (b) lowering a load.

The work delivered for one revolution is

Wout � PL (12.26)

The efficiency of the system defined as work

out/work in is

(12.27)

Substituting for the torque for an Acme thread

using Eq. 12.23, neglecting collar friction, gives

(12.28)

Simplifying using Eq. 12.12 gives

(12.29)

For a square thread, � � 0, so cos � � 1, giving

(12.30)

Standard Acme screws have lead angles between

about 2° and 5°.Assuming a coefficient of friction

of 0.15 gives the efficiency between 18 and 36 per

cent.This shows the disadvantage of power screws.

Higher efficiencies can, however, be attained by

reducing the friction and one of the ways of doing

this has been found to be the use of ball screws

although these potentially add to the cost of

production.

A screw will self-lock if

(12.31)

or

� tan cos � (12.32)

For a square thread, where cos � � 1,

(12.33)

or

� tan (12.34)

It should be noted that Eqs 12.31 to 12.34 are for

a statically loaded screw.Dynamic loading, such as

vibration, can reduce the effective friction and

cause a screw to back-drive.

In the design of a power screw, consideration

should also be given to buckling of the screw and

choice of material for the screw and nut.

��

p

L

d

��

� cosp

L

d

��

�

1 tan

1 cot

��

� �

cos tan

cos cot

��

� � � �

( cos )

( cos )

p

p p

L d L

d d L

��

2

PL

T


260

Example 12.3

A self-locking power screw is required for a

screw jack.An initial proposal is to use a single

start 1.25–5 Acme power screw.The axial load

is 4000 N and collar mean diameter is 1.75 in.

Determine the lifting and lowering torques,

the efficiency of the power screw and whether

the design proposal is self-locking.

Solution

Single start thread, so the lead, L, is equal to the

pitch, P.

N � 5 teeth per inch

Assume sliding friction, � 0.15.

The torque to lift the load is given by

Eq. 12.23.

T 4000 0.02921

0.02921 0.00508 cos 14.5

0.02921 cos 14.5 0.15 0.00508

0.15 40000.04445

2

58.420.01376 4.9182 10

7.62 10 13.34

12.39 13.34 25.73 N m

3

4

��

��

� � �

� �

��

� ��

� � �

�

�

2

0 15

0 08884

.

.

!

!!

!

!!

dc 1.75 44.45 mm� � �

dp 1.15 29.21 mm� � �

L 0.2 5.08 mm� � �

PN

1

1

5 0.2� � � �

12.4 Rivets

Rivets are non-threaded fasteners that are usually

manufactured from steel or aluminium.They con-

sist of a preformed head and shank, which is

inserted into the material to be joined and the

second head that enables the rivet to function as a

fastener is formed on the free end by a variety of

means known as setting.A conventional rivet before

and after setting is illustrated in Figure 12.12.

Rivets are widely used to join components in

aircraft, boilers, ships and boxes and other enclos-

ures. Rivets tend to be much cheaper to install

than bolts and the process can be readily auto-

mated with single riveting machines capable of

installing thousands of rivets an hour.

Rivets can be made from any ductile material,

such as carbon steel, aluminium and brass.A var-

iety of coatings is available to improve corrosion

resistance. Care needs to be taken in the selection

of material and coating in order to avoid the pos-

sibility of corrosion by galvanic action. In general,

a given size rivet will be not as strong as the

equivalent threaded fastener.

The two main types of rivet are tubular and

blind and each type are available in a multitude of

varieties.The advantage of blind rivets is that they

require access to only one side of the joint.A fur-

ther type of rivet with potentially many overall

advantages, from the production perspective, is self-

piercing rivets that do not require predrilled holes.

The rivet is driven into the target materials with

high force, piercing the top sheets and spreading

outwards into the bottom sheet of material under

the influence of an upsetting die to form the joint.

Factors in the design and specification of rivets

include the size, type and material for the rivet,

the type of join, and the spacing between rivets.

There are two main types of riveted joint: lap-

joints and butt-joints (Figure 12.13). In lap joints

the components to be joined overlap each other,

while for butt joints an additional piece of material

is used to bridge the two components to be joined

which are butted up against each other. Rivets

can fail by shearing through one cross-section

known as single shear, shearing through two

cross-sections known as double shear, and crush-

ing. Riveted plates can fail by shearing, tearing

and crushing.

For many applications, the correct use of rivets

is safety critical and their use is governed by con-

struction codes. For information and data con-

cerning joints for pressure vessels reference to the

appropriate standards should be made, such as the

ASME boiler code.

Chapter 12

261

The torque to lower the load is given by

Eq. 12.24.

The design will be self-locking if

� 0.15, so the design is self-locking.

��

� ��

�

cos 0.00508

0.02921cos 14.5

0.05359

p

L

d

��

��

��

��

��

2

4000 0.00508

2 12.39 0.261

4000 0.00508

2 25.73 0.126

screw

both

PL

T

T 58.420.01376 4.9182 10

0.08884 7.62 10 13.34

5.765 13.34 19.11 Nm

3

��

� ��

� � �

�

�4

Before setting

After setting

Figure 12.12 Conventional rivet before and after setting.

Riveted joints can be designed using a simple

procedure (Oberg et al., 1996) assuming that:

• the load is carried equally by the rivets;

• no combined stresses act on a rivet to cause

failure;

• the shearing stress in a rivet is uniform across the

cross-section;

• the load that would cause failure in single

shear would have to be double to cause failure

in double shear;

• the bearing stress of the rivet and plate is dis-

tributed equally over the projected area of the

rivet;

• the tensile stress is uniform in the section of

metal between the rivets.

The allowable stress for a rivet is generally

defined in the relevant standard. For example, the

ASME boiler code lists an ultimate tensile stress

for rivets of 379 MPa, ultimate shearing stress of

303 MPa and an ultimate compressive or bearing

stress of 655 MPa. Design stresses are usually 20

per cent of these values, i.e. for tensile, shear and

bearing stresses the design limits are 75, 60 and

131 MPa, respectively.

For a single lap joint, the safe tensile load based

on shear is given by

L � nAr�d (12.35)


on compressive or bearing stress is given by

L � nAb�c (12.36)

Ab � td (12.37)


on tensile stress is given by

L � Ap�t (12.38)

where L is load (N); Ar, cross-sectional area of

rivet (m2); Ab, projected bearing area of rivet

(m2); Ap, cross-sectional area of plate between

rivet holes (m2); t, thickness of plate (m); d, diam-

eter of rivet (m); �d, allowable shear stress (N/m2);

�c, allowable bearing or compressive stress

(N/m2); �t, allowable tensile stress (N/m2).

The efficiency of a riveted joint is given by

(12.39)

A selection of rivets specified in BS 4620:1970

and ANSI B18.1.2-1972 is included in Tables 12.7

to 12.10 as examples. Rivets, however, are avail-

able as stock items from specialist manufacturers

and suppliers in a much wider variety than the

small selection presented in Tables 12.7 to 12.10.

For any given application the relevant standard

should be referenced and a range of manufactur-

ers’ products considered.

� least safe load

ultimate tensile strength of

unperforated section


262

t

p

t

p

p

P

t

tc

tc

Single riveted lap-joint

Double riveted lap-joint

Double riveted butt-joint

Figure 12.13 Some types of riveted joints.

Chapter 12

263

Table 12.7 British Standard hot-forged rivets for general engineering purposes

Head dimensions

60° csk and raised csk head Snap head Universal head

Nom shank Tol on Nom Height of Nom Nom Nom Nom diam d diam d diam D raise W diam D depth K diam D depth K Rad R Rad r

(14) 21 2.8 22 9 28 5.6 42 8.416 �0.43 24 3.2 25 10 32 6.4 48 9.6

(18) 27 3.6 28 11.5 36 7.2 54 1120 30 4.0 32 13 40 8.0 60 12

(22) �0.52 33 4.4 36 14 44 8.8 66 1324 36 4.8 40 16 48 9.6 72 14

(27) 40 5.4 43 17 54 10.8 81 1630 45 6.0 48 19 60 12.0 90 18

(33) �0.62 50 6.6 53 21 66 13.2 99 2036 55 7.2 58 23 72 14.4 108 22

(39) 59 7.8 62 25 78 15.6 117 23

Extracted from BS 4620:1970. Note see the standard for full ranges. All dimensions are in millimetres. Sizes shown inparentheses are non-preferred.

Hot-forged rivets

60� csk and raised csk head

d

K

D

60↓�2.5

L

W

Universal headSnap head

D

RL

d

D

r

d

K

L

K

Example 12.4

Determine the safe tensile, shear and bearing

loads and the efficiency for a 300mm section of

single-riveted lap joint made from 1/4�� plates

using six 16-mm diameter rivets. Assume that

the drilled holes are 1.5mm larger in diameter

than the rivets.The values for the design limits

for tensile, shear and bearing stress can be taken

as 75, 60 and 131MPa, respectively.

Solution

The safe tensile load, L, based on shear of the

rivets is given by

The safe tensile load based on bearing or com-

pressive stress is given by

L 6 0.016 6.35 10 131 10

79.86 kN

3 6� � � � � �

�

�

L nA 60.016

4 75 10 90.48 kNr d

62

� � � � � � �


264

Table 12.8 British Standard cold-forged rivets for general engineering purposes

Head dimensions

90° cskhead Snap head Universal head Flat head

Nom shank Tol on Nom Nom Nom Nom Nom Nom Nomdiam d diam d diam D diam D depth K diam D depth K Rad R Rad r diam D depth K

1 �0.07 2 1.8 0.6 2 0.4 3.0 0.6 2 0.251.2 2.4 2.1 0.7 2.4 0.5 3.6 0.7 2.4 0.31.6 3.2 2.8 1.0 3.2 0.6 4.8 1.0 3.2 0.42 4 3.5 1.2 4 0.8 6.0 1.2 4 0.52.5 5 4.4 1.5 5 1.0 7.5 1.5 5 0.63 6 5.3 1.8 6 1.2 9.0 1.8 6 0.8

(3.5) �0.09 7 6.1 2.1 7 1.4 10.5 2.1 7 0.94 8 7 2.4 8 1.6 12 2.4 8 1.05 10 8.8 3.0 10 2.0 15 3.0 10 1.36 12 10.5 3.6 12 2.4 18 3.6 12 1.5

(7) �0.11 14 12.3 4.2 14 2.8 21 4.2 14 1.88 16 14 4.8 16 3.2 24 4.8 16 210 20 18 6.0 20 4.0 30 6 20 2.5

12 �0.14 24 21 7.2 24 4.8 36 7.2 – –(14) – 25 8.4 28 5.6 42 8.4 – –16 – 28 9.6 32 6.4 48 9.6 – –

Extracted from BS 4620:1970. Note see the standard for full ranges.All dimensions are in millimetres. Sizes shown in parentheses are non-preferred.

Cold-forged rivets

K

L

d

D

90��2.5�

d

L

KD

d

L

K

D

L

K

d

D

R

r

Snap head Universal head Flat head90� csk head

The safe load based on tensile load is given by

L � Ap�t.

The area of the plate between the rivet

holes, Ap is given by

Ap3

3

3 2

0.00635 (0.3 6(16 10

1.5 10 ))

1.238 10 m

� � �

� �

� �

�

�

�

The safe tensile load would be the least of the

three values determined, i.e. L � 79.86 kN.

The efficiency is 56 per cent.

��

� � ��

79.86 10

0.3 75 10 0.56

3

60 00635.

L A t 1.238 10 131 10

162.2 kN

p3 6� � � � � �

�

�

Chapter 12

265

Table 12.9 Selected American National Standard large button, high button, cone and pan head rivets

Head diam A (in) Height H (in) Head diam A (in) Height H (in)

Mfd Driven Mfd Driven Mfd Driven Mfd DrivenNom body diam Da note 1 note 2 note 1 note 2 note 1 note 2 note 1 note 2

Button head High button head (acorn)1/2 0.875 0.922 0.375 0.344 0.781 0.875 0.500 0.3755/8 1.094 1.141 0.469 0.438 0.969 1.062 0.594 0.4533/4 1.312 1.375 0.562 0.516 1.156 1.250 0.688 0.5317/8 1.531 1.594 0.656 0.609 1.344 1.438 0.781 0.6091 1.750 1.828 0.750 0.688 1.531 1.625 0.875 0.68811⁄8 1.969 2.062 0.844 0.781 1.719 1.812 0.969 0.76611⁄4 2.188 2.281 0.938 0.859 1.906 2.000 1.062 0.84413⁄8 2.406 2.516 1.031 0.953 2.094 2.188 1.156 0.93811⁄2 2.625 2.734 1.125 1.031 2.281 2.375 1.250 1.00015⁄8 2.844 2.969 1.219 1.125 2.469 2.562 1.344 1.09413⁄4 3.062 3.203 1.312 1.203 2.656 2.750 1.438 1.172

Cone head Pan head1/2 0.875 0.922 0.438 0.406 0.800 0.844 0.350 0.3285/8 1.094 1.141 0.547 0.516 1.000 1.047 0.438 0.4063/4 1.312 1.375 0.656 0.625 1.200 1.266 0.525 0.4847/8 1.531 1.594 0.766 0.719 1.400 1.469 0.612 0.5781 1.750 1.828 0.875 0.828 1.600 1.687 0.700 0.65611⁄8 1.969 2.063 0.984 0.938 1.800 1.891 0.788 0.73411⁄4 2.188 2.281 1.094 1.031 2.000 2.094 0.875 0.81213⁄8 2.406 2.516 1.203 1.141 2.200 2.312 0.962 0.90611⁄2 2.625 2.734 1.312 1.250 2.400 2.516 1.050 0.98415⁄8 2.844 2.969 1.422 1.344 2.600 2.734 1.138 1.06213⁄4 3.062 3.203 1.531 1.453 2.800 2.938 1.225 1.141

Extracted from ANSI B18.1.2–1972, R1989. Note see the standard for full ranges.

All dimensions are given in inches.a Tolerance for diameter of body is plus and minus from nominal and for 1⁄2-in. size equals �0.020, �0.022; for sizes 5⁄8 to 1-in, incl,

equals �0.030, �0.025; for sizes 11⁄8 and 11⁄4-in equals �0.035, �0.027; for sizes 13⁄8 and 11⁄2-in equals �0.040, �0.030; for sizes 15⁄8 and 13⁄4-in

equals �0.040, �0.037.

Note 1: Basic dimensions of head as manufactured.

Note 2: Dimensions of manufactured head after driving and also of driven head.

Note 3: Slight flat permissible within the specified head-height tolerance.

The following formulae give the basic dimensions for manufactured shapes: Button head, A � 1.750D; H � 0.750D; G � 0.885D. High button head, A �

1.500D � 0.031; H � 0.750D � 0.125; F � 0.750D � 0.281; G � 0.750D � 0.281. Cone head, A � 1.750D; B � 0.938D; H � 0.875D. Pan head,

A � 1.600D; B � 1.000D; H � 0.700D. Length L is measured parallel to the rivet axis, from the extreme end to the bearing surface plane for flat bear-

ing surface head type rivets, or to the intersection of the head top surface with the head diameter for countersunk head type rivets.

A

D

GH

LL

H

D

G

A L

H

D

A

0.094

F

0.500

B

D L

B

A

H

Button head High button Cone head Pan head

see note 3


266

Table 12.10 Selected American National Standard large flat and oval countersunk rivets

Body diameter D (in) Head diam A (in) Head depth H (in)Oval crown Oval crown

Nominala Max. Min. Max.b Min.c Ref. heighta C (in) radiusa G (in)

1/2 0.500 0.520 0.478 0.936 0.872 0.260 0.095 1.1255/8 0.625 0.655 0.600 1.194 1.112 0.339 0.119 1.4063/4 0.750 0.780 0.725 1.421 1.322 0.400 0.142 1.6887/8 0.875 0.905 0.850 1.647 1.532 0.460 0.166 1.9691 1. 000 1.030 0.975 1.873 1.745 0.520 0.190 2.25011⁄8 1.125 1.160 1.098 2.114 1.973 0.589 0.214 2.53111⁄4 1.250 1.285 1.223 2.340 2.199 0.650 0.238 2.81213⁄8 1.375 1.415 1.345 2.567 2.426 0.710 0.261 3.09411⁄2 1.500 1.540 1.470 2.793 2.652 0.771 0.285 3.37515⁄8 1.625 1.665 1.588 3.019 2.878 0.831 0.309 3.65613⁄4 1.750 1.790 1.713 3.262 3.121 0.901 0.332 3.938

Extracted from ANSI B18.1.2–1972, R1989. Note see the standard for full ranges.All dimensions are given in inches.a Basic dimension as manufactured. For tolerances see table footnote on Table 12.9.b Sharp edged head.c Rounded or flat edged irregularly shaped head (heads are not machined or trimmed).

Flat and oval countersunk head

D

L

Q

H

A

C

G

H

L

D

A

Q

Flat csk head Oval csk head

Example 12.5

Determine the maximum safe tensile load that

can be supported by a 1 m section of double

riveted butt joint with 15 mm thick main

plates and two 8 mm thick cover plates.There

are six rivets in each of the outer rows and

seven rivets in each of the inner rows. The

rivets are all 20 mm in diameter. Assume that

the drilled holes are 1.5 mm larger in diameter

than the rivets.The values for the design limits

for tensile, shear and bearing stress can be

taken as 75, 60 and 131 MPa, respectively.

Solution

In analysing a double riveted joint, it is only

necessary to analyse one side due to symmetry.

The safe tensile load based on double shear-

ing of the rivets is equal to the number of rivets

288 Mechanical Engineer’s Pocket Book

4.2 Riveted joints

4.2.1 Typical rivet heads and shanks

2D 2D

Round orsnap head

Le

ng

th

Pan head Mushroom head

60°

D

D

fD

32

2D2D 2D

Le

ng

th

90°

90° countersunkhead

DfD

4

Flat headConoidal head

Plain orsolid shank

Semi-tubularshank

Tubularshankor eyelet

Split orbifurcatedshank

Driveshankfor softmaterials

4.2.2 Typical riveted lap joints

Single row lap joint

Double row (chain) lap joint

Double row (zigzag) lap joint

3D 3D

3D

La

p 6

D

Pitch

Pitch

1.5

D

3D1.5D 3D 3D

3D1.5D

1.5

D

La

p 6

D

3D 3D

Pitch

3D

Pitch

D 5 rivet shank diameter3D1.5D

1.5

D

La

p 3

D

3D 3D

Pitch

Fastenings 289

4.2.3 Typical riveted butt joints

Single strap chain riveted butt joint (single row)

Note: This joint may also be double row riveted, chain or zigzag. The strap width 5 12Dwhen double riveted (pitch between rows 5 3D).

Double strap chain riveted butt joint (double row)

Note: This joint may also be double row zigzag riveted (see Section 4.2.2) or it may be single riveted as above.

1.5D 3D 3D 3D

1.5

D3

D1

.5D

1.5

D3D

Pitch

Pitch

Pitch

Str

ap

or

cove

r p

late

wid

th =

12

D

Pitch

Strap or

cover plate

1.5

D1

.5D

Str

ip w

idth

5 6

D

1.5

D

1.5D 3D D 5 Rivet shank diameter3D3D


Fastenings 291

4.2.4 Proportions for hole diameter and rivet length

1.0625D

D

1.5D


Dimensions in mm

Nominal Tolerance Nominal Tolerance Nominal Tolerance Toleranceshank on head on head on head on

diametera diameter diameter diameter depth depth lengthd d D D K K L

1 1.8 0.6 10.21.2 2.1 60.2 0.7 20.01.6 60.07 2.8 1.02.0

3.5 60.24 1.2 10.242.5

4.4 1.5 20.03.0

5.3 1.8 10.520.0

(3.5) 6.1 60.29 2.1 10.294 60.09 7.0 2.4 20.05 8.8 3.06

10.5 3.610.35

60.35 20.0(7) 12.3 4.2 10.88 60.11 14.0 4.8 20.0

1018.0 6.0

12 21.0 60.42 7.2 10.42 11.0(14) 60.14 25.0 8.4 20.0 20.016 28.0 9.6

aRivet sizes shown in brackets are non-preferred.For further information see BS 4620: 1970.

4.2.5 Cold forged snap head rivets

d

L

K

D

With d 5 16 mm or smaller

D 5 1.75d

K 5 0.6d

L 5 length

Fastenings 293

Dimensions in mm

Nominal Tolerance Nominal Tolerance Nominal Tolerance Toleranceshank on head on head on head on length

diametera diameter diameter diameter depth depth L

d d D D K K

(14) 22 9 11.016 60.43 25 61.25 10 11.00 20.0

18 28 11.5 20.0

20 32 13 11.5 11.6(22) 60.52 36 11.8 14 20.0 20.024 40 16

(27) 43 17 12.030 48 62.5 19 20.0(33) 60.62 53 21 13.0

20.0

36 58 63.0 23 12.539 62 25 20.0

aRivet sizes shown in brackets are non-preferred.For further information see BS 4620: 1970.

4.2.6 Hot forged snap head rivets

D

L

K

d

With d 5 14 mm or larger D 5 1.6d

K 5 0.65d

L 5 length

294

Mech

anica

l Engin

eer’s P

ock

et B

ook

Dimensions in mm

Nominal Nominal length* Lshank

3 4 5 6 8 10 12 14 16 (18) 20 (22) 25 (28) 30 (32) 35 (38) 40 45 50 55diametera d

1.0 3 3 3 3 3 3 3 3 3 3

1.2 3 3 3 3 3 3 3 3 3 3

1.6 3 3 3 3 3 3 3 3 3 3 3 3

2.0 3 3 3 3 3 3 3 3 3 3 3 3

2.5 3 3 3 3 3 3 3 3 3 3 3 3

3.0 3 3 3 3 3 3 3 3 3 3 3 3

(3.5)4.0 3 3 3 3 3 3 3 3 3

5.0 3 3 3 3 3 3 3 3 3 3 3 3 3 3

6.0 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3

aSizes and lengths shown in brackets are non-preferred and should be avoided if possible. The inclusion of dimensional data is not intended to imply that all the productsdescribed are stock production sizes. The purchaser should consult the manufacturer concerning lists of stock production sizes.For the full range of head types and sizes up to and including 39 mm diameter by 160 mm shank length see BS 4620: 1970.

4.2.7 Tentative range of nominal lengths associated with shank diameters

Fastenings 295

4.2.8 POP® rivets

POP® or blind riveting is a technique which enables a mechanical fastening to be made whenaccess is limited to only one side of the parts to be assembled, although reliability, predictabil-ity, reduction of assembly costs and simplicity in operation, mean that blind rivets are alsowidely used where access is available to both sides of an assembly. POP® and other brands ofblind riveting systems have two elements, the blind rivet and the chosen setting tool.

The blind rivet is a two-part mechanical fastener. It comprises of a headed tubular body mounted on a mandrel which has self-contained features that create (when pulledinto the body during setting) both an upset of the blind end and an expansion of thetubular body, thus joining together the component parts of the assembly. The setting toolbasically comprises an anvil which supports the head of the rivet and jaws which grip and pull the mandrel to cause it to set the rivet before the mandrel breaks at a pre-determined load.

Many different styles of blind rivet are available but the most widely used is the Open EndRivet Body type defined in BS EN ISO 14588: 2000 as: ‘A blind rivet having a body hollowthroughout its length and able to use an standard mandrel’. The principle of blind rivet-ing using open style rivets is shown in the following figure.

With the mandrel held in the setting tool, the rivet body is inserted into the pre-punchedor pre-drilled component. Operation of the setting tool pulls the mandrel head into therivet body causing it to expand on the blind side of the assembly, whilst drawing the com-ponents together and eliminating any gaps between them as it does so. At a predeter-mined point, when the blind side head is fully formed, continued operation of the settingtool causes the mandrel to break, the spent portion of the mandrel is pulled clear and theinstallation of the rivet is complete.

The Closed End Rivet Body type is defined in BS EN ISO 14588: 2000 as: ‘A blind rivet bodywhich is closed and remains closed after setting’. This type is also commonly known as‘sealed’. The closed end rivet prevents ingress of vapour or moisture through the bore ofthe installed rivet and also ensures mandrel head retention, particularly important in elec-trical equipment, for example.

POP® blind rivets are available in a variety of materials, body styles and head forms to pro-vide fastening options for a broad spectrum of assembly and environmental requirementsfrom brittle and fragile materials such as acrylic plastics through to stainless steel. A sum-mary is shown in Section 4.2.9. This figure and the following tables are taken from thepublications of Emhart Teknologies (Tucker Fasteners Ltd.) from whom further informa-tion can be obtained. This company’s address is listed in Appendix 3.

4.2.9 POP® range guide

Head styleThe low-profile domed head is suitable for most applications but, where soft or brittlematerials are fastened to a rigid backing, the large flat head variety should be considered.The 120° countersunk head style should be used wherever a flush surface is required.

Mandrel typesPOP® open type rivets are normally supplied with ‘Break stem’ mandrels (code BS)designed to retain the mandrel head when the rivet is set. ‘Break head’ (code BH) man-drels, designed to eject the mandrel head from the rivet body, can be supplied for mostopen rivets and are particularly useful in the pre-assembly of electrical circuit boards.

Standard open type rivet

Closed end type rivet

Peel type rivet

‘MGR’

‘LSR’ aluminium

Wide range of rivets suitable for use where applications donot have high load bearing requirements.

For situations where fastening has to be watertight,pressure tight, or where mandrel retention is a requirement.

Suitable for joining plastics, rubber, wood, GRP or laminates.

Wide grip range. Ideal for use with inconsistent holes.

Rivet with load spreading characteristics for use in soft,friable or brittle materials.


Fastenings 297

Finishl Rivet body standard finishes: Steel and nickel-copper rivet bodies are normally supplied

zinc plated.l Paint and other finishes: Rivets with differing surface finishes and paint colours can be

provided on request. Aluminium alloy rivets are available anodized and dyed, matt orgloss for aesthetic and environmental reasons.

4.2.10 Good fastening practice

Blind riveting is a highly reliable and proven method of fixing material together perman-ently. To achieve a superior fastening, the following principles should be considered.

Workpiece materialsWhen materials of different thickness or strengths are being joined, the stronger material –if possible – should be on the blind side. For example, if plastic and metal are to be joined,the plastic sheet should be beneath the rivet head and the metal component should beon the blind side.

Hole size and preparationAchieving a good joint depends on good hole preparation, preferably punched and, ifnecessary, de-burred to the sizes recommended in the POP® blind rivet data tables.

Rivet diameterAs a guide for load-bearing joints, the rivet diameter should be at least equal to the thick-ness of the thickest sheet and not more than three times the thickness of the sheet imme-diately under the rivet head. Refer to data tables for rivet strength characteristics.

Edge distanceRivet holes should be drilled or punched at least two diameters away from an edge – butno more than 24 diameters from that edge.

Rivet pitchAs a guide to the distance between the rivets in load-carrying joint situations, this dis-tance should never exceed three rivet diameters. In butt construction it is advisable toinclude a reinforcing cover strip, fastening it to the underlying sheet by staggered rivets.

Rivet materialChoosing rivets of the correct material normally depends on the strength needed in theriveted joint. When this leads to rivets of material different to the sheets being joined it isimportant to be aware that electrolytic action may cause corrosion. (See Section 4.2.12.)

Setting and safetyThe type of setting tool is usually selected to suit the production environment. The toolmust be cleared of spent materials before setting the next rivet and, in the case of poweroperated tools, must not be operated without the mandrel deflector or mandrel collec-tion system being in position. Safety glasses or goggles should always be worn.

4.2.11 Selection of POP® (or blind) rivets

Joint strengthFirst assess the tensile strength and the shear-load strength required by the joint, both ofwhich can be achieved by the correct number and spacing of fastenings, and by choosinga rivet with a body of the correct material and diameter. The strength columns on the datapages enable a rivet of the correct strength to be chosen.


Joint thicknessThe next stage is to work out the combined thickness of the materials to be joined,remembering to allow for any air gaps or intermediate layers such as sealants. Then iden-tify the selected rivet in the size with the necessary grip by consulting the data page. It isimportant to do this because a rivet with the incorrect grip range cannot satisfactorily gripthe back of the workpiece or assembly.

Corrosion acceleration (nature of materials)Finally, follow the general rule that the rivet chosen should have the same physical andmechanical properties as the workpiece. A marked difference in properties may cause jointfailure through metal fatigue or galvanic corrosion. Corrosion is accelerated by certaincombinations of materials and environments. Generally, avoid contact between dissimilarmetals. The significance of the letters A, B, C and D in the following chart is as follows:

A The corrosion of the metal considered is not accelerated by the contact metal.B The corrosion of the metal being considered may be slightly accelerated by the contact

metal.C The corrosion of the metal considered may be markedly accelerated by the contact

metal.D When moisture is present, this combination of metal considered and contact metals is

inadvisable, even under mild conditions, without adequate protective measures.

Where two symbols are given (for instance B or C) the acceleration is likely to change withchanges in the environmental conditions or the condition of the metal.

Rivet material Contact metal

Nickel Stainless Copper Steel Aluminium ZincCopper Steel andAlloy Alloys

Nickel Copper Alloy – A A A A AStainless Steel A – A A A ACopper B or C B or C – A A ASteel C C C – B AAluminium and Alloys C B or C D B or C – AZinc C C C C C –

4.2.12 Design guidelines

Soft materials to hardA large flange rivet can be used with the flange on the side of the soft material.Alternatively, POP® LSR type rivets spreads the clamping loads over a wide area so as toavoid damage to soft materials.

Plastics and brittle materialsFor fragile plastics and brittle materials, POP® riveting offers a variety of application solu-tions. Soft-set/All Aluminium rivets offer low setting loads, whereas both the ‘Peel’ and

Large flange

Hard

Soft

POP® LSR

Fastenings 299

‘LSR’ ranges afford enhanced support to the materials being joined. For stronger plastics,standard POP® rivets – open or closed end products – may be used.

Channel section materialAn extended nosepiece can be used to reach to the bottom of a narrow channel section (A)(see figure below). A longer mandrel rivet should be used and the maximum nosepiecediameter should be equal to that of the rivet flange. Alternatively, the rivet can be set fromthe other side (B) or the channel widened to accept a standard rivet and setting tool (C).

Thick/thin sheetWhen materials of different thickness are to be fastened, it is best to locate the thickerplate at the fastened side (A) (see figure below). When the hole diameter in the thinnerplate is large, a large flange rivet should be used (B). When the thinner plate is located atthe fastened side, either use a backing washer (C) or ensure that the diameter of the holein the thicker plate is smaller than the one in the thinner plate (D).

Blind holes and slotsThe setting of a POP® or blind rivet against the side of a blind hole, or into and against amilled slot, intersecting hole or internal cavity, is possible because of the expansion of therivet body on installation.

A B C D

A B C

Extendednosepiece

POP® LSRLarge flange Peel rivetSoft set (PAD)

Aluminium (AD/ABS)


Pivot fasteningUse of a special nosepiece will provide a small gap between the rivet flange and the assem-bly, so providing for pivot action.

Hole diametersWhilst standard hole diameters are the rivet body diameter plus 0.1 mm, component holesizes may not always be this accurate, for example in pre-punched components. In caseswhen the hole on the fastened (blind) side is larger, POP® MGR rivets should be selectedbecause of its superior hole filling characteristics. POP® LSR and POP® Peel rivets are alsopossible alternative solutions in these circumstances, especially when working with friableor fragile materials. Alternatively, when then larger hole is on the flange side, a largeflange rivet should be chosen.

When, however, good hole filling and retained mandrels are used to give high shear andvibration resistance POP® ‘F’ series rivets should be specified. (See Section 4.2.13.)

POP® 'F' series rivet

MGR rivet Large flange

Pivot nosepiece

Gap

Part

A B C

Fastenings 301

Elevated temperature performanceElevated temperature strengths will vary from the figures quoted in the data tables. Thefollowing curves are offered for guidance.

99.5% PURE ALUMINIUM

Elevated temperature



100

90

80

70

60

50

100

90

80

70

60

50

50 100 150 °C

50 100 150 200 °C

50 100 150 200 °C

%

stated

strength

value

%

stated

strength

value

100

90

80

70

60

50

%

stated

strength

value


50 100 150 200 °C

100

90

80

70

60

50

%

stated

strength

value


100 200 300 400 °C

100

90

80

70

60

50

%

stated

strength

value


100 200 300 400 °C

100

90

80

70

60

50

%

stated

strength

value


50 100 300 400 °C

100

90

80

70

60

50

%

stated

strength

value


50 100 150 200 °C

100

90

80

70

60

50

%

stated

strength

value

ALUMINIUM 2.5% MAGNESIUM ALLOY

ALUMINIUM 3.5% MAGNESIUM ALLOY

ALUMINIUM 5% MAGNESIUM ALLOY STAINLESS STEEL

NICKEL COPPER ALLOY

MILD STEEL

COPPER

302

Mech

anica

l Engin

eer’s P

ock

et B

ook

4.2.13 POP® ‘F’ series

‘F’ Series: Domed headCarbon steel (AISI 1006) mandrel: carbon steel

7.60–8.253.80–3.98

0.90–1.40

L

9.20–9.854.70–4.88

1.15–1.65

L

Rivet code

Tensilestrength

(N)

Shearstrength

(N)

Mandreldiameter

(mm)

Holediameter

(mm)

Nominalrivet

diameter

(mm)

Griprange

(mm)(mm)

‘L’Length

98–10.6 1.5–3.5

12.2–13.0 3.5–6.0

1.5–3.5

3.5–6.0

11.3–12.1

14.2–15.0

16.3–17.1 6.0–8.5

4.0

4.8

4.1–4.2

4.9–5.0

2800 3000

3000

3600

4600

5700

2800

4250

4000

3550

2.64

3.20

FSD 4010 BS

FSD 4012 BS

FSD 4812 BS

FSD 4815 BS

FSD 4817 BS

Faste

nin

gs

303

‘F’ Series: Domed headAluminium 3.5% Magnesium alloy (5052) mandrel: carbon steel

Note: Shear and tensile strengths are typical values. Joint strengths will be dependent upon the following application criteria:1. Hole size, 2. Materials to be fastened, 3. Application material thicknessesIt is recommended users conduct their own test(s) to determine suitability for their application(s).

9.20–9.854.70–4.88

1.15–1.65

L

Rivet code

Tensilestrength

(N)

Shearstrength

(N)

Mandreldiameter

(mm)

Holediameter

(mm)

Nominalrivet

diameter

(mm)

Griprange

(mm)(mm)

‘L’Length

1.5–3.5

3.5–6.0

10.3–11.1

13.2–14.0

15.6–16.4 6.0–8.5

4.8 4.9–5.0

1850

2100

3100

2050

2200

2200

3.20

FSD 4811 BS

FSD 4814 BS

FSD 4816 BS

304

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The following symbols are used in the tables in Sections 4.2.14–4.2.16.

This is the recommended This is the recommended The shear and tensile figures This is the nominal diameter maximum thickness of the diameter of the drilled/ quoted are indicative of the of the mandrel and is shown to materials to be riveted together punched hole which for performance of the rivet under assist in the selection of the correctassuming a hole diameter reliable rivet setting should standard test conditions. Actual ‘POP®’ rivet tool nosepiece – indicated as nominal in the be burr free. performance will depend on essential for correct rivet setting specification tables. The rivet will in most material types and thickness. For performance.The thickness should include circumstances set satisfactorily safety, testing in the application is any gap between materials in holes up to 0.1 mm greater recommended for critical prior to setting. than the nominal quoted. assemblies.

Nominal diameter hole Tensile/shear

performance

Nominal mandrel diameterMaximum riveting

thickness

Faste

nin

gs

305

4.2.14 Open type aluminium 3.5% magnesium alloy

Material composition: aluminium 3.5% magnesium alloy; mandrel: carbon steel

TAPD 31 BS

TAPD 33 BS

TAPD 36 BS

SNAD 3050 BS

SNAD 3065 BS

SNAD 3080 BS

SNAD 3100 BS

SNAD 3120 BS

Rivetcode

carbonsteel

mandrel

Nominal

(mm)

Rivet dimensions and limits

(mm)

Nominal

(mm)

Max.riveting

thickness

(mm)

2.4

3.0

‘L’Nominal

rivetbody

length

(mm)

Nominalrivetdia.

(mm) (N) (N)

3.5

7.5

6.5

2.5

3.1

3.5

5.0

5.0 2.0

4.8

0.8

2.4

8.0

9.0

6.0

5.0

10.0

12.0

3.08–2.90

1.10 max.

6.3–5.7

L

2.48–2.30 5.00–4.00

0.90 max.

L

550

1000 800

400 1.42

1.83

continued

306

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6.65–6.053.28–3.10

1.10 max.

L

Rivetcode

carbonsteel

mandrel

(N) (N)

Nominal

(mm)

Nominal

(mm)


(mm)

Nominalrivetdia.

(mm)

Max.riveting

thickness

(mm)

‘L’Nominal

rivetbody

length

(mm)

15.0 12.0

4.5 1.6

3.26.0

8.0

9.7

11.5 7.9

13.5 9.5

15.0

17.0

3.2 3.3 1200 800 1.83

12.7

11.1

4.8

6.4

SNAD 3150 BS

TAPD 42 BS

TAPD 44 BS

TAPD 46 BS

TAPD 48 BS

TAPD 410 BS

TAPD 412 BS

TAPD 414 BS

TAPD 416 BS

Section 4.2.14 (continued)

Faste

nin

gs

307

10.00–9.003.28–3.10

1.50 max.

L

14.3

16.7

20.7

3.2

7.9

9.5

3.2

4.0

3.3 1200 800 1.83

3.2

4.8

6.4

7.9

18.5

20.0

24.0

6.2

8.0

9.7

11.5

13.5

7.0

8.5

10.5

12.2

17.0 12.7

6.4

4.8

TAPD 418 BS

TAPD 421 BS

TAPD 425 BS

TAPD 44 BSLF9.5

TAPD 46 BSLF9.5

TAPD 48 BSLF9.5

TAPD 410 BSLF9.5

TAPD 412 BSLF9.5

TAPD 416 BSLF9.5

TAPD 54 BS

TAPD 56 BS

TAPD 58 BS

TAPD 510 BS

continued

308

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Rivetcode

carbonsteel

mandrel

Rivet dimensions and limits (mm)

(N) (N)

Nominal

(mm)

Nominal

(mm)

Nominalrivetdia.

(mm)

Max.riveting

thickness

(mm)

‘L’Nominal

rivetbody

length

(mm)

14.0 9.5

15.7 11.1

12.717.5

18.5

20.2

22.5 17.4

24.7 19.8

8.5

10.5

4.0 4.1 1910 1330 2.29

6.4

4.8

13.5

15.9

TAPD 514 BS

TAPD 512 BS

TAPD 516 BS

TAPD 517 BS

TAPD 520 BS

TAPD 522 BS

TAPD 525 BS

TAPD 56 BSLF12

TAPD 58 BSLF12

8.22–7.624.08–3.90

1.35 max.

L


Faste

nin

gs

309

12.2 7.9

9.514.0

15.7 11.1

17.5 12.7

19.4

7.5

9.2 4.8

11.0 6.4

4.0

4.8 4.9

4.1 1910 1330 2.29

2800 2020 2.6414.7 9.5

16.5 11.1

13.519.0

25.5 19.8

12.7 7.9

3.2

14.3 TAPD 518 BSLF12

TAPD 516 BSLF12

TAPD 514 BSLF12

TAPD 512 BSLF12

TAPD 510 BSLF12

TAPD 64 BS

TAPD 66 BS

TAPD 68 BS

TAPD 610 BS

TAPD 612 BS

TAPD 617 BS

TAPD 614 BS

TAPD 625 BS

TAPD 633 BS32.0 26.2

4.08–3.90

1.60 max.

L

12.150–11.50

4.88–4.70

1.60 max.

L

9.80–9.20

continued

310

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Rivetcode

carbonsteel

mandrel

(N) (N)

Nominal

(mm)


(mm)

Nominal

(mm)

Nominalrivetdia.

(mm)

Max.riveting

thickness

(mm)

‘L’Nominal

rivetbody

length

(mm)

38.2 32.8

44.5 39.1

9.20

11.0

12.7 7.9

14.5 9.5

16.5

4.8

4.8

4.9

4.9

2710 1950 2.64

2800 2020 2.64

11.1

4.8

6.4

TAPD 6175 BSa

TAPD 6150 BSa

TAPD 66 BSLF14

TAPD 68 BSLF14

TAPD 610 BSLF14

TAPD 612 BSLF14

TAPD 614 BSLF14

4.88–4.70

2.00 max.

L

14.30–13.70

8.97–8.374.88–4.90

1.20 max.

L


Faste

nin

gs

311

19.0

25.5 19.8

3.56.5

8.0 4.5

10.0 6.0

12.0

14.0

16.0 11.5

18.0 13.5

5.0

6.0

5.1 2600 2200 2.64

22.0 17.5

25.0 20.5

3.58.0

20.0 15.5

9.5

7.5

13.5

SNAD 5120 BS

SNAD 5100 BS

SNAD 5080 BS

SNAD 5065 BS

TAPD 625 BSLF14

SNAD 5140 BS

SNAD 5160 BS

SNAD 5180 BS

SNAD 5200 BS

SNAD 5220 BS

SNAD 6080 BS

SNAD 5250 BS

TAPD 617 BSLF14

5.08–4.85

1.60 max.

L

9.30–8.70

continued

312

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Rivetcode

carbonsteel

mandrel

(N) (N)

Nominal

(mm)


(mm)

Nominal

(mm)

Nominalriverdia.

(mm)

Max.riveting

thickness

(mm)(mm)

‘L’Nominal

riverbody

length

12.3–11.76.08–5.85

2.10 max.

L

13.00–12.406.48–6.25

2.10 max.

L

3800 3000 3.20

4525 3200 3.66

10.0

12.0

14.0

16.0

18.0

20.0

12.7

12.7

19.8

19.5

26.2

11.5

13.5

15.5

6.4

9.5

7.5

5.5

6.0

6.4

6.1

6.5

SNAD 6100 BS

SNAD 6120 BS

SNAD 6140 BS

SNAD 6160 BS

SNAD 6180 BS

SNAD 6200 BS

TAPD 88 BS

TAPD 816 BS

TAPD 824 BS


Faste

nin

gs

313

4.2.15 Open type carbon steel

Material composition: carbon steel; mandrel: carbon steel

Rivetcode

carbonsteel

mandrel

(N) (N)

Nominal

(mm)

Nominal

(mm)


(mm)

Nominalrivetdia.

(mm)

Max.riveting

thickness

(mm)

‘L’Nominal

rivetbody

length

(mm)

5.84–5.242.88–2.90

1.32 max.

L

6.3–5.73.08–2.90

1.1 max.

L

930 715 1.83

1400 1100 1.83

5.3

5.0

6.5

8.0

7.0

9.0

10.0

12.0

2.0

3.5

5.0

2.92.8

3.0

2.9

3.1

SNSD 3050 BS

SNSD 3080 BS

TSPD 33 BS

SNSD 3100 BS

SNSD 3120 BS

SNSD 3065 BS

continued

314

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Nominalrivetdia.

Rivetcode

carbonsteel

mandrel

(N) (N)

Nominal

(mm)


(mm)

Nominal

(mm)

Max.riveting

thickness

(mm)

‘L’Nominal

rivetbody

length.

(mm)(mm)

6.65–6.053.28–3.10

1.10 max.

L

1550 1150 1.93

15.0

4.5

6.0

8.0

9.5

11.5

13.5

1.65.0

4.8

6.4

7.9

9.5

3.2

1.6

12.0

3.2 3.3

SNAD 3150 BS*

TSPD 412 BS

TSPD 48 BS

TSPD 42 BS

TSPD 44 BS

TSPD 46 BS

TSPD 410 BS

TSPD 52 BS

3.27.0 TSPD 54 BS


Faste

nin

gs

315

8.12–7.724.08–3.85

1.35 max.

L

9.82–9.224.88–4.65

1.60 max.

L

2500 1730 2.29

3500 2620 2.9

8.5 4.8

6.410.5

12.2 7.9

14.0 9.5

6.5 2.4

9.0 4.8

11.0 6.4

12.7 7.9

14.5 9.5

16.5 11.1

18.3 12.7

19.0 13.5

7.5 3.2

4.0

4.8

4.1

4.9

TSPD 56 BS

TSPD 58 BS

TSPD 510 BS

TSPD 512 BS

TSPD 63 BS

TSPD 64 BS

TSPD 66 BS

TSPD 68 BS

TSPD 610 BS

TSPD 612 BS

TSPD 614 BS

TSPD 616 BS

TSPD 617 BS

15.9 11.1

12.717.6

TSPD 514 BS

TSPD 516 BS

continued

316

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Rivetcode

carbonsteel

mandrel

(N) (N)

Nominal

(mm)


(mm)

Nominal

(mm)

Nominalrivetdia.

(mm)

Max.riveting

thickness

(mm)

‘L’Nominal

rivetbody

length

(mm)

9.30–8.705.08–4.85

1.6 max.

L

3790 2880 2.95.0 5.1

6.5 2.5 SNSD 5065 BS

8.0 4.0 SNSD 5080 BS

10.0 6.0 SNSD 5100 BS

12.0 8.0 SNSD 5120 BS

14.0 10.0 SNSD 5140 BS

16.0 12.0 SNSD 5160 BS

18.0 14.0 SNSD 5180 BS

4.010.0 SNSD 6100 BS


Faste

nin

gs

317

12.3–11.76.08–5.85

2.1 max.

L

11.15–10.556.51–6.40

1.55 max.

L

5500 4200 3.65

6900 5000 3.86

6.012.0

6.0

6.4 6.5

6.1

SNSD 6120 BS

8.014.0 SNSD 6140 BS

10.016.0 SNSD 6160 BS

12.018.0 SNSD 6180 BS

3.89.5 TSPD 8095 BS

7.613.0 TSPD 8130 BS

12.718.5 TSPD 8185 BS

aBody 25% magnesium alloy.

318

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4.2.16 Closed end type aluminium 5% magnesium alloy

Material composition: aluminium 5% magnesium alloy; mandrel: carbon steel or stainless steel

Rivetcode

carbonsteel

mandrel

Rivetcode

stainlesssteel

mandrel

(N) (N)

Nominal

(mm)


(mm)

Nominal

(mm)

Nominalrivetdia.

(mm)

Max.riveting

thickness

(mm)

‘L’Nominal

rivetbody

length

(mm)

6.30–5.703.28–3.10

1.10 max.

L

8.22–7.624.08–3.90

1.50 max.

L

1400 1110 1.63

2220 1640 2.18

6.0

7.5

9.0

11.0

12.0

8.0

9.5

6.411.0

6.4

7.9

3.2

4.8

4.8

3.2

1.6

3.2

4.0

3.3

4.1

AD 42 SB

AD 44 SB

AD 46 SB

AD 48 SB

AD 410 SB

AD 54 SB

AD 56 SB

AD 58 SB

AD 42 SS

AD 44 SS

AD 46 SS

AD 48 SS

AD 410 SS

AD 54 SS

AD 56 SS

AD 58 SS

Faste

nin

gs

319

9.85–9.204.88–4.70

1.75 max.

L

3110 2260 2.64

4800 4000 3.66

7.912.5

4.8

6.4

4.9

6.5

AD 510 SB

3.28.30 AD 64 SB

4.810.0 AD 66 SB

6.411.5 AD 68 SB

7.913.0 AD 610 SB

9.514.5 AD 612 SB

12.718.0 AD 616 SB

15.922.0 AD 620 SB

AD 510 SS

AD 64 SS

AD 66 SS

AD 68 SS

AD 610 SS

AD 612 SS

AD 616 SS

AD 620 SS

6.413.0 AD 84 H

9.516.0 AD 86 H

–

–

13.34–12.066.48–6.32

2.51 max.

L


4.2.17 Blind rivet nuts

Blind rivet nuts are especially designed to offer a means of providing a stronger threadedjoint in sheet and other materials and, like blind rivets, only require access to one side ofthe workpiece for installation. They are not only a form of captive nut but, unlike someother types, they also allow components to be riveted together as well providing a screwthread anchorage. The principle of their use is shown below.

The POP® blind rivet nut is screwed onto the threaded mandrel of the setting tool and isthen inserted into the drilled or punched hole.

The tool is operated, retracting the mandrel. The unthreaded part of the nut expands on the blind side of the workpiece to form a collar and applies a powerful clenching force that rivets the components firmly together.

With the POP® nut firmly in position the tool mandrel is simply unscrewed from the nut.The threaded portion is now ready to act as a secure anchorage point.

Blind rivet nuts are generally available in thread sizes M3 to M12 in numerous combin-ations of head style, body form and material. Three head styles are available, flat head,90° countersunk and, for thin gauge materials, a small flange which provides a near flushappearance without the need for countersinking.

Standard bodies are round with open or closed ends. The closed end prevents the ingressof moisture or vapour through the bore of the nut. Where a higher torque resistance isrequired, body forms may be fully or partially hexagonal (set in a hexagonal hole), orsplined (set in a round hole).

Fastenings 321

4.2.18 POP® Nut Threaded Inserts: application

POP® Nut Threaded Inserts provide a simple and effective way to join materials with thebenefit of an internal thread, in a variety of applications.

POP® Nut Threaded Inserts are the perfect solution for providing high-quality, load-bearing threads in various materials where alternative methods cannot maintain torque andpull out loads. POP® Nut Threaded Inserts are suitable for single sheets down to 0.5 mm.

POP® Nut Threaded Inserts enable components, which are assembled later in the productioncycle, to be adjusted.


POP® Nut Threaded Inserts are ideally suited to applications where access is only availablefrom one side of the workpiece.

4.2.19 POP® Nut Threaded Inserts: installation

POP® Nut Threaded Inserts are easily installed from one side of the workpiece withoutdamaging surrounding surfaces of previously finished or delicate components and aresuitable for use with all materials in today’s manufacturing environment:

l Available in a variety of materialsl Wide range of stylesl Complete range of Hand and Power setting toolsl Bulk and small pack available.

Install/screw the POP® Nut Threaded Insert to the tool’s threaded mandrel.

Operating the tool then retracts the mandrel. The unthreaded part of the nut then com-presses to form a collar on the blind side of the workpiece, applying a powerful clenchingforce that firmly joins the components together.

Fastenings 323

With the POP® Nut Threaded Insert firmly in position, the tool mandrel is simplydemounted/unscrewed from the insert.

The threaded part of the insert then acts as a secure anchorage point for subsequentassembly work.

324

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4.2.20 POP® Nut: Steel

Flat head open end (with knurls)

Thread Description Length Grip Hole Barrel Flange Flange Bulk box Small pack d (mm) (mm) dia. (mm) dia. (mm) dia. (mm) thickness (mm) quantity quantity

L e D B S

M4PSFON430 10.0 0.3–3.0

6.0 5.9 9.0 1.010 000 500

PSFON440 11.5 3.1–4.0 10 000 500

M5PSFON530 12.0 0.3–3.0

7.0 6.9 10.0 1.08000 500

PSFON540 15.0 3.1–4.0 5000 500

M6PSFON630 14.5 0.5–3.0

9.0 8.9 12.0 1.54000 500

PSFON645 16.0 3.1–4.5 4000 500

M8PSFON830 16.0 0.5–3.0

11.0 10.9 15.0 1.52000 250

PSFON855 18.5 3.1–5.5 2000 250

M10PSFON1030 17.0 0.5–3.0

12.0 11.9 16.0 2.01500 200

PSFON1060 22.0 3.0–6.0 1500 200

M12 PSFON1240 23.0 1.0–4.0 16.0 15.9 22.0 2.0 1500 200

L

B d D

S

e

Faste

nin

gs

325


L e D B S

M5 PSFCN530 18.0 0.3–3.0 7.0 6.9 10.0 1.0 5000 500

M6 PSFCN630 20.5 0.5–3.0 9.0 8.9 12.0 1.5 2000 500

M8 PSFCN830 25.0 0.5–3.0 11.0 10.9 15.0 1.5 1500 250

L

B

S

d D

e

Flat head closed end (with knurls)

326

Mech

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Flat head open end hexagonal


L e M1 M B S

M4 PSFOH430 11.5 0.5–3.0 6.1 6.0 9.3 1.0 10 000 500

M5 PSFOH530 13.5 0.5–3.0 7.1 7.0 10.3 1.0 8 000 500

M6 PSFOH630 15.5 0.5–3.0 9.1 9.0 12.3 1.5 4 000 500

M8 PSFOH830 17.5 0.5–3.0 11.1 11.0 14.3 1.5 2 000 250

M10 PSFOH1040 22.0 1.0–4.0 13.1 13.0 16.3 2.0 1 500 200

L

B

S

d

M

e

M1

Faste

nin

gs

327

Countersunk head open end (with knurls)


L e D B S

M4 PSKON435 11.5 2.0–3.5 6.0 5.9 9.0 1.5 10 000 500

M5 PSKON540 13.5 2.0–4.0 7.0 6.9 10.0 1.5 8 000 500

M6 PSKON645 16.0 2.0–4.5 9.0 8.9 12.0 1.5 4 000 500

M8 PSKON845 19.0 2.0–4.5 11.0 10.9 14.0 1.5 2 000 250

M10 PSKON1045 21.0 2.0–4.5 12.0 11.9 14.7 1.5 1 500 200

L

B

S

d D90°

e

For further information see the wide range of publications concerning POP® riveting issued by Emhart Teknologies (Tucker Fasteners Ltd) – See Appendix 3.

ECCS TC7 TWG 7.10 Connections in Cold-formed Steel Structures

The Testing of Connections with Mechanical

Fasteners in Steel Sheeting and Sections

2nd

Edition, 2009

ECCS Publication - The Testing of Connections with Mechanical Fasteners in Steel Sheeting and Sections

FREE Download Publications www.eccspublications.eu

The Testing of Connections with Mechanical Fasteners in Steel Sheeting and Sections

2

The Testing of Connections with Mechanical Fasteners in Steel Sheeting and Sections Nº124, 2

nd edition, 2009

Published by:

ECCS ! European Convention for Constructional Steelwork

[email protected]

www.steelconstruct.com

All rights reserved. No parts of this publication may be reproduced, stored in a retrieval sys-

tem, or transmitted in any form or by any means, electronic, mechanical, photocopying, re-

cording or otherwise, without the prior permission of the copyright owner.

ECCS assumes no liability regarding the use for any application of the material and informa-

tion contained in this publication.

Copyright © 2009 ECCS ! European Convention for Constructional Steelwork

ISBN: 92-9147-000-91

Printed in Multicomp, Lda - Mem Martins, Portugal



Preface

3

PREFACE

This document intends to provide guidance on the testing of mechanical fasteners, used

to form structural connections in cold-formed steel sheeting and sections. It updates an earlier

document European Recommendation for Steel Construction: The Design and Testing of

connections in Steel Sheeting and Sections; Publication no. 21 of the European Convention

for Constructional Steelwork, 1983. There are major differences between the current docu-

ment and the previous publication. Since that document was published, Eurocodes have been

introduced which provide more detailed guidance for the structural design of connections.

Whereas the earlier document provided guidance on the design of connections, this has been

omitted in the present document as it is now covered by the Eurocodes.

This document has been prepared by the ECCS working group TWG 7.10 and approved

by the Technical Committee TC7. The final draft was circulated to TC7 in May 2007.

The members of working group TWG 7.10 who contributed to the document are:

A. Belica Luxembourg

A. Arnedo Pena Spain

H. Hofmeyer Netherlands

R. Hettmann Germany

J. Kesti Finland

L. Sokol France

K. Kathage Germany

Z. Nagy Romania

R. Podleschny Germany

H. Saal Germany

W. Siokola Austria

T. Toma Netherlands

T. Vrany Czech Republic

R. Pedreschi (chairman) United Kingdom

Facing the first edition, in the present second edition some additions and corrections were

made by an ad-hoc working group consisting of

K. Kathage Germany

Th. Misiek Germany

H. Saal Germany

The final draft was circulated to TC7 in September 2009.




4



Contents

5

CONTENTS

1. INTRODUCTION ........................................................................................................ 7

1.1 Aims of the document ................................................................................................ 7

1.2 Scope .......................................................................................................................... 7

1.2.1 Types of connections .......................................................................................... 7

1.2.2 Types of material ................................................................................................ 8

2. DEFINITIONS ............................................................................................................. 9

2.1 Failure modes of connections ..................................................................................... 9

2.1.1 Failure modes of connections for fasteners loaded in shear .............................. 9

2.1.1.1 Bearing failure of connection .................................................................... 10

2.1.1.2 Net section tension failure ......................................................................... 11

2.1.1.3 Shear of fastener ........................................................................................ 11

2.1.1.4 End failure of connection .......................................................................... 12

2.1.1.5 Tilting and pull-out failure of connection ................................................. 12

2.1.2 Failure modes for fasteners loaded in tension .................................................. 12

2.1.2.1 Pull-out failure .......................................................................................... 13

2.1.2.2 Fastener failure .......................................................................................... 13

2.1.2.3 Pull-over and pull-through ........................................................................ 13

2.2 Characteristic resistance of connection under static load ........................................ 14

2.3 Derivation of an empirical formula for characteristic resistance ............................. 15

2.4 Characteristic resistance of connections under repeated load determined by test. .. 16

2.5 Alternative method of estimating the design resistance under repeated load .......... 18

2.6 Definition of failure resistance and requirements for minimum deformation ......... 18

2.6.1 Definition of failure resistance ......................................................................... 18

2.6.2 Deformation requirements in tension tests ....................................................... 19

2.6.2.1 Requirements to ensure sufficient deformation capacity .......................... 19

2.6.2.2 Requirements to ensure a sound structure in serviceability limit state ..... 19

2.6.3 Deformation requirements in shear tests .......................................................... 20

2.6.3.1 Requirements to ensure sufficient deformation capacity .......................... 20

2.6.3.2 Requirements to ensure a sound structure in service state ........................ 20

2.7 Design resistance of connections under static load .................................................. 21

2.8 Design resistance of connections under combined loading ..................................... 22

2.9 Shear flexibility ........................................................................................................ 22

3. TEST PROCEDURES ............................................................................................... 25

3.1 General requirements ............................................................................................... 25

3.1.1 Properties of steel samples ............................................................................... 25

3.1.2 Fastener properties ........................................................................................... 25

3.1.3 Testing equipment ............................................................................................ 25

3.1.4 Manufacture of the test samples ....................................................................... 26

3.1.5 Number of tests ................................................................................................ 26

3.2 Shear tests ................................................................................................................. 27




6

3.2.1 Standard shear test ............................................................................................ 27

3.2.1.1 Single fastener test .................................................................................... 27

3.2.1.2 Two fastener test ....................................................................................... 28

3.2.1.3 Measurement of deformation .................................................................... 28

3.2.2 Pull out by shear/fastener failure test ............................................................... 29

3.2.3 Test procedure for shear ................................................................................... 29

3.3 Tension test .............................................................................................................. 30

3.3.1 Standard tension test arrangements .................................................................. 30

3.3.2 Pull-over/pull-trough resistance ....................................................................... 31

3.3.3 Test method of determining pull-over/pull-through resistance for special

apllications ........................................................................................................ 32

3.3.4 Pull-over/pull-trough resistance for special applications obtained from standard

tests ................................................................................................................... 33

3.3.5 Pull-out resistance ............................................................................................ 34

3.3.6 Test procedure for tension ................................................................................ 36

3.4 Test report and recording of test results ................................................................... 36

BIBLIOGRAPHY ........................................................................................................ 39



Introduction

7

1. INTRODUCTION

This document replaces the earlier publication by the European Convention of Construc-

tional Steelwork, European Recommendations for the Testing of Connections in Profiled

Steel Sheeting and Sections, Publication no. 21, first published in 1977 and revised in 1983.

Since this publication the use of profiled steel and cold-formed steel has grown considerably,

particularly in the use of cold-formed steel sections in structural framing. The current docu-

ment has been restructured to conform with guidance in current Eurocodes, which have also

been introduced since 1983. The document has been prepared by the Technical Working

Group 7.10, part of the ECCS Technical Committee TC7, Cold-formed steel structures.

1.1 Aims of the document

This document intends to provide information and guidance on the testing of connections

using mechanical fasteners in cold-formed steel structures and steel cladding to determine the

design resistance and flexibility. Guidance is provided on the test methods, the interpretation

of the results and the preparation of test reports. A list of current references is provided in the

appendices.

1.2 Scope

Important definitions:

- Fastener: the connecting element in fastening;

- Fastening: the interaction of a fastener with the surrounding material;

- Connection: a group of one or more fastenings.

1.2.1 Types of connections

The recommendations of this document cover conventional mechanical connections us-

ing fasteners, such as self tapping and self drilling screws, bolts, rivets and powder-actuated

fasteners (powder-actuated fasteners are also referred to as cartridge fired pins).

For further guidance on fastener specification, refer to ECCS publication no. 42 Me-

chanical Fasteners (currently under review) for use in steel sheeting and sections.

Fasteners specifically excluded from the recommendations of this document are:

- fasteners for use in composite sandwich cladding panels, these are considered in sepa-

rate documents (ECCS no. 115: European Recommendations for Sandwich Panels part

I ! Design, 2001 and ECCS no. 127: Preliminary European Recommendations for

Testing and Design of Fasteners for Sandwich Panels, 2009);

- non-standard forms of mechanical fastener such as self-piercing rivets, mechanical

clinching and rosettes. These will be the subject of subsequent further guidance docu-

ments;

- fasteners in composite structures between steel and other materials;




8

- high strength friction grip bolts.

- fastenings made by gluing or welding.

1.2.2 Types of material

The recommendations in this document refer to steel sheets and sections commonly used

in profiled metal cladding and cold-formed steel structural sections and hot rolled steel sec-

tions used as the substrate for mechanical fasteners. The sheet materials may be unprotected

or protected by a suitable coating such as hot dipped galvanising or organic coatings.



Definitions

9

2. DEFINITIONS

This section provides definitions of failure modes, test parameters, results and outputs.

2.1 Failure modes of connections

The tests described in this document consider connections subjected to two different

force actions, shear and tension. These force actions relate to the predominant force in the fas-

tener itself. The failure mode of the connection may actually be by tension of the connected

steel whilst the fastener itself is subjected to shear. The failure mode of a particular connec-

tion of either at least two sheets or at least one sheet and the substrate by a mechanical fas-

tener is influenced by a number of factors:

- type of fastener;

- the method of installation of the fastener;

- the tensile strength and thickness of the steel being connected;

- the lay-up of the sheets/sections in relation to the fastener;

- shape of the profiled sheet in pull-over tests (see 2.1.2.3).

The range of tests described in this document considers both the force action on the fas-

tener itself and the failure mode of the connection. When carrying out tests on connections it

is important and necessary to record the failure mode. This section describes typical failure

modes for connections. The figures in this section are illustrated using screws; however the

failure modes are applicable to all types of fasteners considered by this document.

2.1.1 Failure modes of connections for fasteners loaded in shear

If the fastener is loaded in shear, failure of a connection may occur in either the fastener

itself, in the steel sheets being connected or in the attachment of the fastener to the steel

sheets. The characteristics of the various failure modes are summarised in table 2.1.




10

Table 2.1: Summary of failure characteristics for connections tested in shear

Failure

mode

Elongation

of hole

Piling of

material

Necking/

tearing

Pull-out of

fastener

Location of

failure

bearing

failure

either one or

both sheets

net section

failure

either one or

both sheets

shear of

fastener fastener

end

failure

either one or

both sheets

tilting and

pull-out

between sheet

and fastener

2.1.1.1 Bearing failure of connection

Fig. 2.1: Bearing failure

Typical characteristics:

- elongation of holes;

- piling of material in front of fastener;

- out of plane curling of sheet;

- no necking of sheet;

- possible diagonal cracks originating from material adjacent to fastener;

- failure occurs in either one or both sheets.



Definitions

11

2.1.1.2 Net section tension failure

Fig. 2.2: Net section tension failure


- necking across width of sheet;

- necking across thickness;


2.1.1.3 Shear of fastener

Fig. 2.3: Shear of fastener


- failure by fracture or necking of fastener.




12

2.1.1.4 End failure of connection

Fig. 2.4: End failure of connection


- elongation of holes;

- longitudinal tear in the direction of applied force;

- piling of material;


2.1.1.5 Tilting and pull-out failure of connection

Fig. 2.5: Tilting and pull-out failure


- curling of sheets;

- failure occurs by pull-out of fastener from lower sheet.

2.1.2 Failure modes for fasteners loaded in tension

If the fastener is loaded in tension, failure of a connection may occur either in the fastener

itself, the steel sheet/section or the attachment of the fastener to the steel sheet/section.

Curling pull-out



Definitions

13

2.1.2.1 Pull-out failure

Fig. 2.6: Pull-out failure


- thread stripping in screws or bolts and/or in the substrate;

- distortion of head of fastener;

- distortion of steel substrate;

- failure occurs by dislocation of fastener from substrate.

2.1.2.2 Fastener failure

Fig. 2.7: Fastener failure


- failure occurs by fracture of fastener or necking of fastener shank.

2.1.2.3 Pull-over and pull-through

This type of failure may occur in the connection between sheet and substrate (see 3.3.2).




14

Fig. 2.8: Pull-over and pull-through failure


- tearing and distortion of sheet around head of fastener - pull-over;

- fastener remains connected to the substrate;

- failure occurs in material adjacent to the head of the fastener;

- some steel may be left under the fastener;

- profile may disconnect completely from fastener ! pull-through.

2.2 Characteristic resistance of connection under static load

The characteristic resistance Rk of a fastening follows from a statistical evaluation of test

results:

skRR mk

where:

Rm = mean value of the adjusted test results Radj of the failure resistance defined in

clause 2.6.1 obtained from a minimum of five tests:

Pull through separation

from fastener

Original form of sheet

Distortion and tearing

Substrate

Some steel may be left under the fastener



Definitions

15

k = the coefficient which depends on the number of test observations, the type of dis-

tribution, the fractile part and confidence level chosen. The appropriate value of k

will be found in EN 1990 (Annex D)

s = standard deviation

Radj = is the adjusted test result

Radj,i = Robs,i / R

in which R is the resistance adjustment coefficient given by:

cor

corobs

u

obsuR

t

t

f

f ..

where:

fu.obs = the actual measured ultimate resistance of the material

fu = nominal ultimate resistance

tobs.cor = the actual measured material core thickness

tcor = core thickness of the material

= 1, if fu.obs > fu and = 0, if fu.obs fu

and

Robs = is the observed test result

The observed values may be used without application of (2.2) if failure occurs in the fas-

tener. The values for the material where failure occurred should be used with (2.3).

The actual measured basic ultimate resistance fu.obs should not deviate by more than 25%

from the nominal fu, basic ultimate resistance i.e. fu.obs > 0,75 fu.

The actual measured thickness tobs should not exceed the nominal material thickness tnom

by more than 12%. The core thickness, tcor, is the bare metal thickness, i.e. the actual thick-

ness minus any protective coatings such as galvanising, (According to EN 1993-1-3 clause

3.2.3 with the usual coating of 275 g/m2, the core thickness can be taken as tnom ! 0,04).

2.3 Derivation of an empirical formula for characteristic resistance

For tests for which either the thickness t or tensile strength fu of the sheet where failure

occurs is varied systematically, an empirical relationship may be derived for the resistance of

the fastener, provided:

- the same mode of sheeting failure occurs, and

- at least three distinct values of fu·t are considered, and

- at least five sets of test observations are available for each value of fu·t

The empirical formula for characteristic resistance Rk, is given by:

2

1cc

uk tfR




16

Coefficients c1 and c2 may be estimated from regression analysis of the test data to give

the straight line of best fit passing through the origin for a plot of ultimate load per fastener

against the function fu·tc2.

The empirical formula so derived may be used for the prediction of fastening resistance

for sheeting of thickness within the range of five sets of tests.

Similar extrapolation limits shall be applied when predicting fastening resistances of dif-

ferent tensile strengths of sheeting of the same thickness. The empirical formula may be ap-

plied to both shear and tension test results.

For tests where the failure occurs in the sheeting for which either the thickness t or the

resistance fu of the sheeting where the failure occurs is varied systematically, the characteris-

tic resistance Rk at intermediate values may be obtained by linear interpolation provided

- the same mode of sheeting failure occurs, and

- for pull-over/pull-trough tests with varied thickness t of the sheet to be fixed, the

range for interpolation is limited to

tmax = 0,125 mm for t 0,75 mm

tmax = 0,25 mm for 0,75 mm < t 2,00 mm

- for pull out tests with varied thickness t1 of the substrate, the range for interpolation

is limited to

t1,max = 0,25 mm for t1 0,75 mm

t1,max = 0,5 mm for 0,75 mm < t1 2,00 mm

t1,max = 2,0 mm for t1 2,00 mm if tests for t1 < 2,00 mm are performed, too, and

t1,max = 1,0 mm for t1 2,00 mm if no tests for t1 < 2,00 mm are performed

- for shear tests with varied thickness, the above mentioned ranges of interpolation

for t and t1 apply.

- for tests with varied tensile strengths, the range of interpolation is limited to

fu,max = 50 N/mm² and

fu,max / fu,max = 25 %

Extrapolation to higher thicknesses is not allowed.

2.4 Characteristic resistance of connections under repeated load determined by

testing

The characteristic resistance of fasteners subjected to repeated loads can be determined

by the test methods described in this section.

If the Palmgren-Miner-Rule is not applicable, then the actual load spectrum has to be

considered in the tests.

The behaviour of a fastening under repeated load shall be shown in a S-N curve (Wöhler

curve). The characteristic S-N curve should be determined as follows:

- carry out the repeated loading tests for the defined load spectrum. In constant amplitude

tests this range is defined by minimum and maximum load. If the load direction is not

alternating a minimum load of 10% of the maximum load is recommended.

- tests should be carried out for at least three different levels of maximum load.

- for each load level at least five tests should be carried out.

- Specimen failure may be defined by either a resistance or deformation criterion.



Definitions

17

Resistance criterion:

- inability to attain the maximum load shall constitute failure for the test.

Deformation criterion:

- shear test: deformation failure will be considered to occur when a nominated

maximum slip is exceeded;

- tension test: serviceability failure may, when relevant, be taken as the observation

of cracking around the fastener.

It may be assumed that a force which is sustained 107 times will not cause failure at

larger number of force repetitions (endurance limit).

- failure occurs when the test sample can no longer sustain the maximum load or exceeds

the maximum deformation. The number of cycles of load at which this occurs should be

recorded.

- the characteristic number of cycles to failure for a particular level of maximum load fol-

lows from:

log (Nc) = (log N)m - k·s

where:

Nc = the characteristic number of cycles to failure at a certain force level

N = the number of cycles to failure of a test.

(log N)m = log N divided by the number of tests

s = the standard deviation of log N

k = a statistical factor which depends on the number of test observations, the type

of distribution, the fractile part and confidence level chosen (EN 1990, Annex

D).

The characteristic S-N curve can be determined by connecting the log (Nc)-points by

straight lines.

Comments:

If there is a potential of unscrewing due to repeated loading special tests may be necessary to

demonstrate that unscrewing does not occur. The test procedures in this document do not cover this In

such cases manufacturer!s recommendations or alternative connections may be useful.

In principle the same type of specimens can be used as for the static test (see section 3).

For the maximum load of the highest load level a value of (2/3) Rk is recommended

where Rk is the characteristic static resistance of the fastening.

In the above Rk is the characteristic static resistance of the fastening. The minimum load

should be close to zero, not more than 10% of the maximum load.

Loading frequency:

- for tension test maximal 5,0. Hz;

- for shear test a maximum of 5,0 Hz is recommended; higher frequencies may be used

provided resonance is avoided and that there is no influence on the material properties

by the effect of hysteresis load.




18

max

min

time

load

Fig. 2.9: Limits for repeated loads

The load ranges are appropriate for most applications. To determine the endurance limit

of the fastener it may be necessary to reduce the lowest maximum load level.

2.5 Alternative method of estimating the design resistance under repeated load

Comments:

Alternative methods for the determination of the design resistance of fastenings to take account of

repeated loads are available:

In many situations repeated loads are most likely due to temperature changes and provided the

ductility requirements of 2.8.2 and 2.8.3 are met then it may not be necessary to carry out repeated

load tests.

Design resistance of connection subjected to repeated wind load will be given by

23

2

M

kd

RR

where:

Rd = design resistance of a fastening loaded by wind which takes into account the influence of

repeated load

Rk = characteristic static resistance according to Chapter 2.1

M2 = partial factor for resistance

2.6 Definition of failure resistance and requirements for minimum deformation

2.6.1 Definition of failure resistance

The ultimate resistance of the fastening shall be taken as the maximum load recorded dur-

ing the test. With double fastener connections the ultimate load per fastener shall be taken as

one-half the ultimate load of the connection. In shear tests, it is recommended to define a fail-



Definitions

19

ure load as peak load in a deformation of 3 mm. The maximum load within 3 mm deformation

is illustrated in Fig. 2.10.

Fig. 2.10: Failure limits for tests

Comments:

In certain situations, where, under extreme loading condition , for example, earthquake loads the

deformation requirement of 3 mm may not be applied and the peak load may be taken as the ultimate

load, provided there is sufficient post yield deformation.

2.6.2 Deformation requirements in tension tests

2.6.2.1 Requirements to ensure sufficient deformation capacity

The deformation at ultimate resistance of the fastening has to be sufficient to ensure re-

distribution and equalisation of forces in connections and to avoid consideration of secondary

forces.

2.6.2.2 Requirements to ensure a sound structure in serviceability limit state

An acceptable serviceability state will be achieved when the recovery of deflection after

loading up to service load (approximate characteristic resistance multiplied by 0,6) and re-

moval of the loading has not led to excessive deformation.

Comments:

To avoid the need to consider secondary forces the deformation at ultimate resistance of the fas-

tening should be greater than 3 mm.

This limit will generally be reached when all of the following conditions are fulfilled for profiled

sheeting:

- single sheet thickness 1,5 mm

- yield resistance of the sheet material 240 N/mm2

- difference between the width of the sheet flange through which is fastened and the diameter

of head of the fastener or washer 14 mm.

- the characteristic resistance of the fastener or anchorage of the fastener is sufficient (no

pull-out and fastener-failure).

3mm 3mm

Rmax Rmax




20

The deformation requirements have also to be satisfied for the most critical combination

of thickness and resistance within the tolerance limits of the sheet material and fastener. For

this reason the characteristic resistance of a connection with insufficient deformation capacity

at failure should be greater than the characteristic resistance of the fastening.

When:

Rk1 is the characteristic resistance belonging to a failure mode with insufficient deforma-

tion capacity.

Rk2 is the characteristic resistance belonging to a failure mode with sufficient deformation

capacity.

Then it is desirable that:

Rk1 1,0·Rk2

The factor 1,0 is sufficient because at a tension force with a value about Rk2 the fastening

still possesses sufficient deformation capacity. Therefore variations in yield stress of the ap-

plied sheet materials and the scatter in test results are not so significant. When the occasion

arises the corrected values (according to clause 2.2) for Rk1 and Rk2 have to be taken. A de-

formation less than 3 mm may be sufficient, provided secondary forces are considered.

2.6.3 Deformation requirements in shear tests

To prevent brittle failure (minimum required deformation) and to ensure a sound struc-

ture under serviceability conditions (allowable deformation) sufficient deformation is required

prior to failure. Sufficient ductility of the connection is especially important in structures af-

fected by thermal stresses (e.g. claddings).

2.6.3.1 Requirements to ensure sufficient deformation capacity

The deformation at failure of the fastening has to be sufficient to ensure redistribution

and equalisation of forces in connections and to avoid consideration of secondary forces.

A fastening to the substructure needs a larger deformation capacity than a seam fastening

when secondary forces and forces caused by temperature variation are not considered.

2.6.3.2 Requirements to ensure a sound structure in service state

An acceptable serviceability state will be achieved if the recovery of deflection after

loading up to service load (approximate characteristic resistance multiplied by 0,6) and re-

moval of the loading has not led to excessive permanent deformation. At the level of 0,6 of

the characteristic load the sample should still exhibit linear elastic behaviour.

Comments:

In cladding applications, deformation less that 0,5 mm at failure of the connection might be con-

sidered as a fastening with insufficient deformation capacity. To avoid the need to consider secondary

forces in seam fastenings in profiled sheeting a deformation at failure of 0,5mm is sufficient. To avoid



Definitions

21

consideration of secondary forces and forces caused by temperature variation for profiled sheeting to

substructure fastenings the following should be adopted: The deformation at failure of the connection

is preferably larger than 3 mm without excessive hole deformation of the substructure. (The most criti-

cal fastening for this requirement is that with four sheet layers-combined longitudinal and transverse

lap joint. In clause 3.2 a test specimen is shown to check the requirement). Furthermore when ultimate

load is reached before a deformation of 3 mm then the remaining resistance Rr should be greater than

the design resistance Rd (The design resistance is defined in clause 2.4).

Fig. 2.11: Deformation limits at failure

These requirements have also to be satisfied for the most critical combination of thick-

ness and resistance within the tolerance limits of the sheet material and fastener. For this rea-

son the characteristic resistance for a failure mode with insufficient deformation capacity

should be greater than the characteristic resistance for a failure mode with sufficient deforma-

tion.

When:

Rk1 is the characteristic resistance belonging to a failure mode with insufficient deforma-

tion capacity.

Rk2 is the characteristic resistance belonging to a failure mode with sufficient deformation

capacity.

Then it is desirable that:

Rk1 > 1,3 Rk2

When occasion arises the corrected values (according to clause 2.4) for Rk1 and Rk2 have

to be taken. The intention of the factor 1,3 is to take into account the variation in yield stress

of the applied sheet materials in reality and the scatter in test results for Rk1 and Rk2.

A deformation capacity less than 3 mm can also be sufficient, but then secondary forces

and forces caused by temperature variation should be considered.

2.7 Design resistance of connections under static load

The design resistance of fastenings under static load is defined as the characteristic static

resistance divided by an appropriate partial safety factor:

applied load

Rr = remaining resistance

Rd = design resistance Rd

Rr

3mm deformation




22

2M

kd

RR

where:

Rd = design resistance of a fastening

Rk = characteristic resistance of a fastening under static load (clause 2.2)

M2 = partial factor for resistance

Comments:

The partial factor for resistance takes into account:

- statistical base of Rk and accepted risks;

- uncertainty in material properties and geometric tolerances.

According to Annex D, EN 1990, partial factor of resistance, M2 should be taken from the appro-

priate Eurocode provided there is sufficient similarity between the tests and the usual field of applica-

tion of the partial factor as used in calculations. According to EN 1993-1-3, partial factor of resis-

tance may be chosen M2 = 1,25 for fastenings.

2.8 Design resistance of connections under combined loading

If in practice a fastening can be loaded by shear and tension at the same time, then the

behaviour of the fastening under the combined load has to be considered.

Comments:

Design formulae for combined loadings are given in EN 1993-1-3.

2.9 Shear flexibility

The shear flexibility of a fastening, ch, shall be determined from:

ch

1

Rd / 1

ah

n

where:

ah = the slip of a fastening ( corrected with the elongation of the test specimen over the

measuring length) at a load equivalent to Rd / 1

Rd = design resistance of a fastening

1 = an appropriate factor

n = number of test specimens

Comments:

The shear flexibility shall be determined according to clause 3.2 for the connection of sheet to

sheet using either the two fasteners or one fastener test.



Definitions

23

For fastening sheets to the substructure with the one-fastener test the appropriate value of 1 is

1,5 the corresponding partial load factor for wind action.

The shear flexibility is based on the averaged measured deformation obtained from shear tests.

The level of applied shear force in the connection at which the deformation is taken is the maximum

force at working load level i.e. the design resistance of the fastener divided by the partial factor of

safety for the particular load in question. In the two fastener shear test the applied load should be di-

vided by two to determine the force at working load in a single fastener.




24



Test procedures

25

3. TEST PROCEDURES

3.1 General requirements

3.1.1 Properties of steel samples

Connection resistance depends on the resistance and the thickness of the steel sheetings

or sections that are connected by the fastener, but is limited by the load-bearing resistance of

the fastener.

For each batch of steel that is used, tests shall be carried out to determine the following

material properties, fy (fp0.2), fu and A.

Where:

fy, fp0,2 = yield stress, 0,2% proof stress

fu = ultimate tensile strength

A = percentage elongation at fracture

For each steel type at least three tensile tests shall be carried out in accordance with the

technical delivery conditions.

The sheet thickness of the base material exclusive of galvanising and coating shall be

measured to an accuracy of 0,05 mm.

3.1.2 Fastener properties

Nominal fastener and washer dimensions shall be verified to an accuracy of 0,01 mm.

Tests should also be carried out in accordance with the European Recommendations for

Steel Construction Publication no. 42, (currently under review) in order to determine the me-

chanical properties of the fastener.

Comments:

For washers, these dimensions refer to the metal and to the sealing material.

For screws and blind rivets, tension tests and shear tests have to be carried out in order to de-

termine the tension resistance and the shear resistance of the fastener.

For screws, drill-drive, thread forming and torque tests have to be carried out.

3.1.3 Testing equipment

The testing apparatus shall be such that the rate of loading can be controlled, and constant

loads maintained.

The testing apparatus should be calibrated. Applied loads should be measured to an accu-

racy of at least 1%.

Support systems shall be such that the original direction of loading shall be maintained

throughout the test.




26

3.1.4 Manufacture of the test samples

Fasteners shall be fixed within 1 mm of the positions specified on the test pieces.

All test samples shall be made in a consistent manner with the fasteners applied in accor-

dance with the producer"s recommendation or the procedure to be adopted on site. This ap-

plies for:

- diameter of pre-drilled holes;

- depth setting control or alternatively tightening torque, depending on the specifica-

tions of the manufacturer for threaded fasteners;

- type of tool and powder load for powder actuated fasteners;

- strength (steel grade) and thickness of substrate.

Comments:

Care must be taken to ensure that laboratory test samples and inspection procedures are not in-

compatible with site practice. Usually, the fasteners are mounted by control of the setting depth.

With the specimens for the testing of riveted connections under shear the mandrel has to

be removed by drilling or pressing out.

Comments:

The mandrel is not a part of the load-bearing system of the connection. The mandrel may get lost

during the lifetime of the connection (for e.g. may pop out or corrode). Also the location of the break-

ing point of the mandrel may change depending on tools and production lots. If the mandrel is not re-

moved the test results may not be used for the design because of the aforementioned reasons and inde-

pendent of this may not be used for connections with larger clamping size.

3.1.5 Number of tests

The characteristic fastening resistances are determined using a statistical evaluation of

test results.

The minimum number of tests from which the characteristic resistance is calculated is in-

fluenced by the variability of the results, and may be determined as follows:

a) If the test series includes only one nominal thickness or includes several thicknesses

according to clause 2.3, a minimum of ten tests shall be carried out. If any one of

these tests results in an ultimate load that differs from the mean ultimate load by

more than 10 % of the mean at least five further tests shall be carried out. Test results

shall be evaluated in accordance with section 2.

b) If the test series includes several nominal thicknesses where the difference between

the actual thickness is at least 0,1 mm, or several nominal tensile strengths where the

difference between the actual tensile strengths is at least 30 N/mm2, at least five tests

shall be carried out for each thickness x strength value. The total number of tests in

the series shall be at least nine. Test results shall be evaluated in accordance with

section 2.

Test results may only be disregarded if a reason for the deviation is explained and docu-

mented. Such a non-representative result obtained from any one test, may be replaced by



Test procedures

27

those from two or more equivalent tests. Any test so rejected shall be reported, and the reason

for the rejection clearly stated.

Comments:

The minimum number of five tests allows the variability of the fastening to be established. The

characteristic fastening resistance will generally increase if the number of tests increases, as the frac-

tile factor k (k according to EN 1990) for the test series will be improved. To obtain better results ten

tests per test series is recommended.

3.2 Shear tests

3.2.1 Standard shear test

Two test arrangements are recommended, described in Fig. 3.1 and Fig. 3.2, using either

one or two fasteners per test sample.

Comments:

Mean ultimate load values per fastener for single fastener connections are normally little differ-

ent from those obtained for double fastener connections. Differing characteristic resistances may

however be obtained following statistical evaluation of test results.

3.2.1.1 Single fastener test

250

mm

25

0m

m

e1

e1

L0

Extensometer50 mm

Fig. 3.1: Shear test specimen with one fastener

If the thickness of either of the steel pieces forming the connection exceeds 2 mm then

packing pieces or shims are recommended to be used in order to apply an axial load to the test

specimen.




28

3.2.1.2 Two fastener test

In cases where curling or distortion of the connection is expected (for example for con-

nections of two thin sheets such as side lap connections) the test with two fasteners (Fig. 3.2)

may provide results with a lower variation as the influence of the curling and the distortion of

the connection is reduced.

25

0m

m

25

0m

m

e1

e1

L0

Extensometer

s1

50 mm

Fig. 3.2: Shear test specimen with two fasteners

Table 3.1: Specimen dimensions

Fastener

diameter

d [mm]

Specimen [mm]

w L0 e1 p1

6,5 60 150 30 60

> 6,5 10ád 20ád+30 5ád 10ád

Tolerance ± 2 ± 6 ± 1 ± 1

3.2.1.3 Measurement of deformation

Deformation should be measured using an appropriate extensometer.

A single extensometer may be used provided it can be ensured that it is connected on the

central axis of the connection. The extensometer shall be a single unit fixed across the width

of the tests specimens. Alternatively two separate extensometers, one fixed to each side

(width or thickness) of the specimen shall be used. In this case deformation shall be taken as

the average of the recordings of the two extensometers.

The required accuracy of measurement is given in clause 3.2.3.



Test procedures

29

Comments:

Shear flexibility (i.e. the shear deformation per unit shear load) obtained from double fastener

tests will be greater than shear flexibility values obtained from single fastener tests. If it is necessary

to calculate the shear flexibility, following clause 2.10, for a single fastener from a double fastener

test then the flexibility should be based on Rd is taken as 50% of the design load obtained for the dou-

ble fastener connection and the deformation as recorded during the test.

Deformations obtained by measurement of the relative travel of the grips of the testing machine

shall not be used for the determination of the deformation of the connection unless:

a) there is no slip of the specimen in the grips of the testing device, and

b) deformation readings are corrected to account for the elastic extension of the specimen be-

yond the specified extensometer length.

3.2.2 Pull out by shear/fastener failure test

For connections with more than one steel sheet pull-out by shear as well as fastener fail-

ure can govern. For the resistance and deformation requirements see section 2.

25

0m

m

250

mm

e1

e1

L0

Extensometer

2 x t1

1 x t2

50 mm

Fig. 3.3: Test specimen for pull out by shear/fastener failure

Comments:

In the case of fastening more than one layer, the deformations decrease. For powder actuated

fasteners and screws with short penetration depth also the pull-out resistance may decrease.

3.2.3 Test procedure for shear

The rate of loading shall not exceed 1 kN/min and the rate of deformation shall not ex-

ceed 1 mm/min.




30

During the test either the rate of deformation or the rate of load application must be con-

trolled to ensure that both of the above limits are not exceeded.

Deformation shall be measured to an accuracy of ± 0,2 mm for fastenings loaded in shear.

The test should be continued beyond 3 mm of relative displacement. The inclination of

the fastener should be measured at the unloaded specimens. The elongation of the holes

should be determined with the disassembled specimen as the difference of the measured di-

ameter in force direction and that normal to force direction.

Comments:

The specified loading rate is necessary to ensure adequate time within the range of slip.

For the design of diaphragms and deformations requirements it is essential to have an accurate

knowledge of the shear flexibility of the fastening (i.e. the shear deformation per unit shear load).

An inclination of the fastener of more than 10 degrees can be regarded as a failure of the sub-

structure. If a hole elongation of more than 0,1 mm is measured in a sheet, failure can be regarded as

occurred in this sheet.

3.3 Tension test

3.3.1 Standard tension test arrangements

The test fixture for the standard tension test is shown in Fig. 3.4 in principal. The dimen-

sions of the sheeting are the same for all tension tests (see Fig. 3.5, 3.7 and 3.10). Different

failure modes are obtained by using different material thicknesses.

Fig. 3.4: Set-up and fixture for the standard tension test



Test procedures

31

3.3.2 Pull-over/pull-trough resistance

The tests to determine the pull-over/pull-trough resistance should be carried out with

specimens according to Fig. 3.5.

Fig. 3.5: Pull-over/pull-trough test specimen

The flat sheeting, from which the specimen is fabricated, shall be that actually used in

practice for the profiled sheeting. The support material shall be sufficiently thick, to resist pull

out failure of the fastener.

For cartridge fired pins and blind rivets the presence of the support material is a neces-

sary requirement for their proper installation. Therefore for these fasteners the support mate-

rial must be included in the pull-over/pull-trough test according to Fig. 3.5. Deformation of

the support may require to chose an appropriate geometry of the support, see Table 3.2.

For self-drilling and self-tapping screws an alternative pull-over/pull-trough test is shown

in Fig. 3.6, for these fasteners, the support material is not essential for this type of failure.

Fig. 3.6: Pull-over/pull-trough test without support material

Comments:

The formed sheeting of the specimen with its fixed dimensions together with the rigid clamping of

its webs serves only as a model of real profiled sheeting. This model will give satisfactory results for

many steel sheeting profiles.

60




32

3.3.3 Test method of determining pull-over/pull-through resistance for special apllications

The pull-over/pull-trough resistance for the special applications of Fig. 3.7 can be deter-

mined by tests according to Fig. 3.8 and Fig. 3.9. As a simple and conservative alternative the

results from the standard tests according to clause 3.3.2 may be reduced by application of the

coefficients given in clause 3.3.4.

Fig. 3.7: Special applications

The test according to Fig. 3.8 takes into account that the pull-over/pull-trough resistance

can be reduced considerably if the fastener is inclined, owing to asymmetrical deformation of

the support member, such as in case of thin (t 3 mm) C-, L- or Z-shaped members (condi-

tion 4 of Fig. 3.7). The standardised dimensions of the support member shown in Fig. 3.8 will

give satisfactory results for most C-, L- or Z-shaped members used in practise.

The alternative tension test according to Fig. 3.9 is appropriate for the applications speci-

fied in conditions 1, 2 and 3 of Fig. 3.7. For this test the dimensions of the sheeting and the

support member as well as the adjustment of the fasteners should comply with the real condi-

tions on site. The alternative tension test can also be used in cases where detailed information

about the sheet deformation is required.



Test procedures

33

Fig. 3.8: Pull-over/pull-trough test specimen with flexible support member

Fig. 3.9: Alternative pull-over/pull-trough test

Comments:

The test span Lz depends on the geometry of the profile. A value of Lz = 6·b is recommended, where b

is the width of the connected flange of the profile. If failure of the profile occurs, the test span should

be decreased to Lz 3 Mu/Fz where Mu is the ultimate moment capacity for a single corrugation of the

test sheet and Fz is the estimated ultimate fastening load.

3.3.4 Pull-over/pull-trough resistance for special applications obtained from standard tests

The pull-over/pull-trough resistance determined by tests according to clause 3.3.2 is not

appropriate for the connection conditions specified in Fig. 3.10. The pull-over/pull-trough re-

sistance for the applications specified in Fig. 3.10 can be estimated by multiplying the pull-

60 = =




34

over/pull-trough resistance, determined by tests according to clause 3.3.2 with the coefficients

given in Fig 3.10.

Fig. 3.10: Special applications - Coefficients for the pull-over/pull-trough resistance

3.3.5 Pull-out resistance

Generally the tests to determine the pull-out resistance of the fastener from the substrate

should be carried out with specimens according to Fig. 3.11.



Test procedures

35

Fig. 3.11: Pull out test specimen

The support shall be chosen according to the table 3.2.

Table 3.2: Types of supports

Thickness of support

material t1 6 mm t1 < 6 mm

type of support to be

used in practice all types hot rolled sections

cold formed sections,

hollow sections,

sheeting (when testing

sheeting to sheeting

connections)

standardised support

to be used in test

hot rolled flat steel hot rolled angle cold formed channel

30

For pull out tests with self-drilling and self-tapping screws an alternative test is shown in

Fig. 3.12. In this test the support shall also be chosen according to the table above and the

penetration depth of the fastener into the support material should comply with the actual con-

ditions of application in practice.

40

t1 40

70

t1

60

t1

60




36

Fig. 3.12: Pull out test without sheeting specimen

Comments:

The depth of penetration of the fastener into the substrate has a major influence on the resistance

of the connection. The penetration depth itself depends on the total thickness of the sheeting connected

to the support. This thickness is a maximum (4·t) for the sheeting fastening where longitudinal and

transverse lap joints coincide. The influence of this should be taken into account by using specimens in

accordance with Fig. 3.11 and 3.12. It should be noted that asymmetrical deformation of thin sup-

ported members! e.g. C-, L- or Z-shaped members may increase the pull out resistance, especially in

the case of self-tapping screws. Such increase shall be avoided.

3.3.6 Test procedure for tension

The rate of loading shall not exceed 1 kN/min and the rate of deformation shall not ex-

ceed 5 mm/min.

Deformation shall be measured to an accuracy of ± 0,5 mm for fastenings loaded in ten-

sion.

3.4 Test report and recording of test results

The test report shall include the following data as applicable:

- A reference to this recommendation

- Type of test and test specimen;

- Details of fastener and washers, dimensions, material, identification data, including

drawings;

- Details of fastener application, including pre-drilled hole diameter, tightening torque,

powder load, setting tools;

- Details of sheeting, including dimensions of elements, identification data, mechanical

properties and thickness according to clause 3.1.1;



Test procedures

37

- For tensile tests: tabulation of ultimate load values and failure mode for each speci-

men;

- For tensile tests: tabulation of maximum load values, load at 3 mm deformation, de-

formation at maximum load as well as hole elongation and inclination of the fastener

as characteristics of the failure mode for each specimen;

- Load-deformation curves for each specimen;

- Note of calibration of the used test equipment or calibration certificates;

- Other information as appropriate.




38



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39

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