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Transcript of MM324
MM 324 JOINING OF MATERIALSMM 324 JOINING OF MATERIALS
SPRING 2011SPRING 2011
Joining is the act or process of putting or bringing things together to make them continuous or to form a unit.
Joining is the process of attaching one component, structural element, detail, or part to create an assembly.
(Joints make complex structures, machines, and devices possible)(Ideal design – No joint)
An assembly is a collection of manufactured parts, brought together by joining to perform one or more than one primary function.
1. Structural 2. Mechanical 3. Electrical
JOININGJOINING
ASSEMBLYASSEMBLY
Assemblies
The primary function is to carry loads - static, dynamic, or both. e.g., buildings, bridges, dams, the chassis of automobiles, or the airframes of aircraft or spacecrafts.
The primary function to create, enable, or permit some desired motion or series of motions through the interaction of properly positioned, aligned, and oriented components e.g., engines, gear trains, actuators
The primary purpose is to create, transmit, process, or store some electromagneticsignal or state to perform some desired function or set of functions e.g., microelectronic packages, printed circuit boards, motors, generators, and power transformers
Structural assembliesStructural assemblies
Mechanical assembliesMechanical assemblies
Electrical assembliesElectrical assemblies
“Joining of Materials and structures”, R W Messler, Elsevier Butterworth–Heinemann, 2004
Electrical assembliesElectrical
assemblies
Mechanical assembliesMechanical assemblies
Structural assembliesStructural assemblies
Why we need to Join materials
• Functionality
• Manufacturability
• Cost
• Aesthetics
Structures Static Dynamic - Size - Complexity - Portability and short term use. - Damage tolerance Special functionality Automobile windows Crack arrestor in structures
“Joining of Materials and structures”, R W Messler, Elsevier Butterworth–Heinemann, 2004
FunctionalityFunctionality
Requirements Structural efficiency: Required structural integrity (e.g., static strength, fatigue strength and/or life, impact strength or toughness, creep strength, etc.) at minimum structural weight.
(small piece assembly Vs machining time, labour) Material utilization (adhesive bonding Vs fasteners) Optimal material selection Functionality (Bull dozer blades)
Manufacturing process (Casting Vs forging)
“Joining of Materials and structures”, R W Messler, Elsevier Butterworth–Heinemann, 2004
ManufacturabilityManufacturability
Joining allows cost to be minimized by
1. Allowing optimal material selection
2. Optimal material utilization
3. Minimum needed weight of material
4. Achieving functionality through large and complex shapes
5. Allowing automated assembly
Cost-effectiveness means more than low cost of manufacture, however. It also means low cost of maintenance, service, repair, and upgrade, all of which are made practical, beyond feasible, by joining.
CostCost
FunctionalityFunctionality
ManufacturabilityManufacturability
CostCost
AestheticsAesthetics
New challenges in Joining
• Joining in extreme conditions (aerospace, marine,
etc)
Offshore drilling platforms demand erection, anchoring, and periodic
repair to occur underwater. Joining in a radiation environment.
• Very big and small structures Smaller machines and microelectro-mechanical systems, Bigger
supertankers and petrochemical refineries demand larger and thicker
section structures be joined and be leak-tight.
• Join material hard to join
Limb re-attachments, intermetallics etc
• Joining of composites and dissimilar material
Methods of joining
Joining can be broadly divided into
• Mechanical
• Chemical
• Physical
• Hybrid
Joining processes
• Mechanical joining
• Adhesive joining
• Soldering
• Brazing
• Welding
Key points in Joining
• Nature of Stress
• Stress level and distribution
• Welding joints under stress
• Mechanical joints under stress
• Adhesives joints under stress
• Soldering and brazing joints under
stress
Joint Stress & Joint Efficiency
Load divided by effective load bearing area of the joint
σjs = P / A (e.g., lbs/in2)
Joint Stress Joint Stress
Joint efficiencyJoint efficiency
It is a measure of the effectiveness of the joint as compared to rest of the structure for carrying the design or service loads.
JE = (Joint stress / Stress in the structure) x 100
Two ¼ “ rivets
Actual stress (in tension) in the joint elements for an 1,800-lb force at planes A or C is:
Joint Efficiency
Method Metals Ceramics Glass Polymers Composites
Mech. Joint
75-100 <50 <50 50-100 50-100
Adhesive <20 <20 <20 40-100 20-60
Welding 50-100 30-80 100 100 ≤50
Brazing 40-90 20-70 --- --- ≤50
Soldering 5-20 <20 10-40 --- ≤50
Mortars --- 50-100 --- --- 50-100
“Joining of Materials and structures”, R W Messler, Elsevier Butterworth–Heinemann, 2004
Mechanical Joining
Common examples of mechanical fasteners are nails, bolts (with or without nuts), rivets, pins, and screws. Also paper clips, zippers, buttons, and snaps.Common examples of mechanical fasteners are nails, bolts (with or without nuts), rivets, pins, and screws. Also paper clips, zippers, buttons, and snaps.
Special forms of mechanical fasteners are staples, stitches, and snap-fit fasteners.Special forms of mechanical fasteners are staples, stitches, and snap-fit fasteners.
Joining is achieved by purely mechanical forces i.e. interlocking, interferential forcesJoining is achieved by purely mechanical forces i.e. interlocking, interferential forces
At macroscopic level geometrical shapes interact, interlock and interfere the unwanted movement while on microscopic level peaks and valleys on the interface cause blockage of this movement
At macroscopic level geometrical shapes interact, interlock and interfere the unwanted movement while on microscopic level peaks and valleys on the interface cause blockage of this movement
Types of Mechanical Joining
Joining by supplemental devices (fasteners) called temporary mechanical joining e.g. nut-bolts, screws, key cotters etc.
Joining by integral attachments is called permanent mechanical joining e.g. riveting, socket snaps, cantilever hocks etc
Advantages of mechanical joining
• Dissimilar materials can be join together
• Easy assembly and disassembly
• No change in microstructure or composition
• Easy maintenance and upgrading
• No special joint preparation is needed
• Simple, easy and not costly
• Ability to selectively allow relative motion
between joined parts in some direction(s)
Disadvantages of Mechanical Joining
• Accidental disassembly without precautions
• Stress concentrate on joint points
• Can cause leakage or fluid intrusion if special
requirement have not met
• Crevices in mechanical joints cause corrosion
• Mechanical fasteners add weight into actual
structure
Joint loading
Primary loading is right angle to the fasteners or attachment joints are shear loaded
Primary loading is parallel to the fasteners or attachment joints are tension loaded
Load may be static or dynamic or combination of both.
Shear loaded joints
Friction type shear loaded joints
In FTSL joints, fasteners must create enough forces to hold joint
together and prevent slip e.g. single lap, double strap butt joint
Bearing type shear loaded joints
In BTSL joints, fasteners act as pinning points to prevent
movement of the joint parts e.g. joints from rivets, pins, nails etc.
Shear loaded jointsShear loaded joints
(a)(a) Single lap (b) Double lapSingle lap (b) Double lap
Advantages of single-laps are ease of assembly and cost
Advantages of double laps are elimination of eccentric loading and reduction of shear stresses at each of the multiple shear planes in the fastener in bearing-type joints.
Design consideration for shear loaded joints
• Fastener spacing and edge distance
• Effect of joint area
• Allowable stress level
• Axial and eccentric loading
Spacing between fasteners of ≥ 2.67 times the diameter of the fastener (d), with 3 times preferred,
Edge distances ≥ 1.75 times the diameter of the fastener for sheared edges or
≥1.25 times the diameter of the fastener for rolled edges.
J3.8 & J3.9 developed by AISC
(American Institute of Steel Construction)
Allowable-Stress Design Procedure
For designing shear-loaded fastened jointsFor designing shear-loaded fastened joints
All fasteners are assumed to carry an equal share of the applied loads(perfectly rigid materials)All fasteners are assumed to carry an equal share of the applied loads(perfectly rigid materials)
A joint can reasonably be assumed to be rigid when
(1) Plane sections in the structural member remain plane and do not warp.
(2) A straight line from the center of gravity of the fastener pattern remains straight after a torque is applied.
A joint can reasonably be assumed to be rigid when
(1) Plane sections in the structural member remain plane and do not warp.
(2) A straight line from the center of gravity of the fastener pattern remains straight after a torque is applied.
The assumption of equally shared loading also depends on
1. All fasteners being of the same size and material,
2. All fitting with equal tightness in their fastener holes, and
3. All being equally tightened (at least for threaded fasteners such as bolts).
The assumption of equally shared loading also depends on
1. All fasteners being of the same size and material,
2. All fitting with equal tightness in their fastener holes, and
3. All being equally tightened (at least for threaded fasteners such as bolts).
The various elements of the joint (including structural members and
fasteners) must be sized so that the following conditions are satisfied:
(1) the fasteners will not fail in shear by overload;
(2) the joint plates will not fail in tension by overload;
(3) the fastener holes will not be deformed by bearing loads from the fasteners;
(4) the fasteners will not tear out of the joint plates at edges.
The various elements of the joint (including structural members and
fasteners) must be sized so that the following conditions are satisfied:
(1) the fasteners will not fail in shear by overload;
(2) the joint plates will not fail in tension by overload;
(3) the fastener holes will not be deformed by bearing loads from the fasteners;
(4) the fasteners will not tear out of the joint plates at edges.
Allowable-stress design procedure
Bearing-type shear-loaded jointsBearing-type shear-loaded joints
The allowable-stress design procedure allows the designer to choose the mode by which the structure would ultimately fail.
This procedure allows the designer to choose the ‘‘weakest link’’ in the structure, minimizing serious — especially catastrophic — or costly consequences.
The allowable-stress design procedure allows the designer to choose the mode by which the structure would ultimately fail.
This procedure allows the designer to choose the ‘‘weakest link’’ in the structure, minimizing serious — especially catastrophic — or costly consequences.
Allowable-stress design procedure
ASTM A36 SteelASTM A36 Steel
ASTM A325 steel boltsASTM A325 steel bolts
Thread pitch: 2mm per thread (12 threads per inch)
One shear plate passes through the threaded and one through the unthreaded portion.
Determination of the various stresses produced in the fastener and in the joint plates by a load of 300kN (67,000 lbs. force).Determination of the various stresses produced in the fastener and in the joint plates by a load of 300kN (67,000 lbs. force).
where F is the force in kiloNewtons (or lbs. force), b is the number of shear planes that actually pass through the unthreaded fastener or portion of the shank of a fastener.m is the number of fasteners in the jointAunthreaded is the cross-sectional area of the body of the unthreaded fastener
where d is the bolt’s nominal diameter (in millimeters or inches), n is the number of threads per inch (for the Unified system), and P is the pitch of the threads in millimeters (for the metric system).
The value of shear stress calculated is well within the shear stress of 145MPa (21.0 ksi)allowed for A325 steel bolts, and is therefore acceptable.The value of shear stress calculated is well within the shear stress of 145MPa (21.0 ksi)allowed for A325 steel bolts, and is therefore acceptable.
Determining the shear stress in the fasteners.Determining the shear stress in the fasteners.
“Joining of Materials and structures”, R W Messler, Elsevier Butterworth–Heinemann, 2004
Determining the tensile stress in the joint plates within the joint itselfDetermining the tensile stress in the joint plates within the joint itself
So, the path (or load line) with the smallest area is AB, with an area of 6,325mm2.
This is well within the allowable tensile stress value of 152MPa (22.0 ksi) for A36steel joint material.This is well within the allowable tensile stress value of 152MPa (22.0 ksi) for A36steel joint material.
Determining the bearing stress on the joint plate.Determining the bearing stress on the joint plate.
where F is the applied load, m is the number of fasteners causing bearing (along all loadlines) or the number of fasteners in the pattern (on one side of a joint), d is the fastener’s diameter, and l is the fastener’s total length being acted upon by joint plates (in bearing) or the joint plate ‘‘stack height’’ or ‘‘stack thickness.’’
This falls well within the allowable bearing stress of 335MPa (48.6 ksi) for A36steel plates and is therefore acceptable.This falls well within the allowable bearing stress of 335MPa (48.6 ksi) for A36steel plates and is therefore acceptable.
Determining the fastener tear-out stress.Determining the fastener tear-out stress.
where t is the thickness of the plate, h is the distance from the center of the fastener hole to the edge, m is the number of tear-out shear planes, and F is the applied load.
Since this value is well within the allowable shear strength of 100MPa (14.5 ksi) forA36 steel plates, it would be acceptable for this single-row-of-three arrangement.Since this value is well within the allowable shear strength of 100MPa (14.5 ksi) forA36 steel plates, it would be acceptable for this single-row-of-three arrangement.
Allowable-Stress Design Procedure Applied to a Friction-Type Shear-Loaded JointAllowable-Stress Design Procedure Applied to a Friction-Type Shear-Loaded Joint
Estimation of average preload,
Assuming average pressure is created in each of the bolts in the joints
Evaluation of stresses in the joint caused by the forces applied
These forces can cause shear in the fasteners, tensile overload in the joint plates, elongation of bolt holes under bearing, or fastener tear-out at edges.
Comparison with the force needed to cause slip in the joint under assumed value of preload.
Preloading the bolts high enough to create frictional forces between the joint plates (i.e., at their faying surfaces) such that slip is prevented under the design load of 300kN (67,500 lbs. force), the procedure first involves computing the slip resistance
Allowable-Stress Design Procedure Applied to a Friction-Type Shear-Loaded JointAllowable-Stress Design Procedure Applied to a Friction-Type Shear-Loaded Joint
ASTM A325 steel bolts in ASTM A36 steel joint platesASTM A325 steel bolts in ASTM A36 steel joint plates
Computing slip resistanceComputing slip resistance
The slip resistance of a shear-loaded joint is given by:
where µs is the slip coefficient of the joint, Fp is the average preload in a group or pattern of bolts in kilo Newtons or lbs. force, b is the number of shear planes or faying surfaces, and m is the number of fasteners in the joint
Assuming that the joint surfaces are sandblasted (making µs = 0.47)
If the average preload is assumed to be 77.5kN (17,400 lbs. force) in each of the five bolts in the joint, then the slip resistance is:
Bolt shear determines the ultimate strength of this friction-type jointBolt shear determines the ultimate strength of this friction-type joint
Eccentric Loading of Joints
Typical eccentrically loaded connections. (a) Bracket connection. (b) Beam web splice. (c) Standard beam connections.
If joints are really perfectly rigid, the load on each fastener will not be the same unless the resultant of all externally applied loads passes through the centroid (center of mass or gravity) of the joint’s fastener pattern.
If joints are really perfectly rigid, the load on each fastener will not be the same unless the resultant of all externally applied loads passes through the centroid (center of mass or gravity) of the joint’s fastener pattern.
Axial Shear Versus Eccentric Shear
Eight 1/8 inch diameter A490 bolts in A36 steel arranged and eccentrically loaded with a 40 kip (40,000 lb.) downward force at point P located 8 inches from the centerline of the right-most vertical row of bolts, determine the load on the most highly loaded bolt.
The shear stress per bolt is:
40 kip / 8 bolts = 5 kip (5, 000 lbs) per bolt,
The torque or moment M is:
40Kpsi[8 + 5.5/2] = 430Kip-in
For the four bolts close to the centroid:r2
i = 22 + (5.5/2)2 = 11.56 in2
The sum of r21s for these four bolts is 46:25 in2
For the four bolts far from the centroid:r2
2 = 62 + (5.5/2)2 = 43.56 in2
The sum of r22s for these four bolts is 174.25 in2
The vertical and horizontal components of the secondary shear force for the most highly loaded bolt are,
The vector sum of all primary and secondary shear components,
The value is acceptable since the shear allowable for A490 is 24.1 kpsi
Tension loaded joints
If applied or internally generated stress are parallel to the joint axes, the joint is under tension loading
Preload (Before service) should be high enough to reduce the effect of the applied loadings
If applied or internally generated stress are parallel to the joint axes, the joint is under tension loading
Preload (Before service) should be high enough to reduce the effect of the applied loadings
Tension loaded joints
Threaded fasteners like nut-bolts and screws can create considerable clamping force thus making the joint suitable for tension loading.Threaded fasteners like nut-bolts and screws can create considerable clamping force thus making the joint suitable for tension loading.
This preload must be high to compress the joint structural elements, thereby improving the resistance of the joint to externally applied tensile loads.This preload must be high to compress the joint structural elements, thereby improving the resistance of the joint to externally applied tensile loads.
The preload in the fastener is commonly called the ‘‘working load’’The preload in the fastener is commonly called the ‘‘working load’’
The preload and the externally applied tensile load should not exceed the yield strength of the fastener.The preload and the externally applied tensile load should not exceed the yield strength of the fastener.
Bolted joints joined elements are stiffer than the fastener (compressive load in the elements is high and tensile load in the fastener is low)Bolted joints joined elements are stiffer than the fastener (compressive load in the elements is high and tensile load in the fastener is low)
Low tensile working load desirable to reduce fatigue in the dynamic applications whether in static or dynamic structures.Low tensile working load desirable to reduce fatigue in the dynamic applications whether in static or dynamic structures.
Tension loaded joints
Stiffness characteristics (kS) of a bolt/screw with +ive slope
Stiffness characteristics (kP) of clamped joint with –ive slope
When an external tensile load, designated as FAX is imposed on the preloaded joint, the bolts elongate and joint element compression decreases.
The resulting deflections are fSA and fPA with fSA > fPA
If the clamped joint elements are stiffer than the bolts are, only a small fraction, designed FSA, of the external load FAX is added to the load in the bolts FS. The balance of the external load, FPA, is resisted by the clamped joint elements. So, as long as FSA can be kept small, then fatigue loading on the bolts will also be low.
1. The maximum clamping load must be greater than the required nominal preload to allow for embedment as well as to offset expected scatter in tensioning i.e., FM(Max) > FV.
2. The magnitude of the above maximum clamping force must be no greater than 70% of the 0.2% offset yield point of the fastener material; i.e., FM(Max) < 0.7F0.2
3. The bolt working load must be no greater than 10% of the 0.2% offset yield load; i.e., FSA < 0.1F0.2
4. The bolt working load must also not exceed the endurance limit load for the bolt if fatigue is a consideration; i.e., FSA < FE.
For strong and reliable working joints, the joint preload and the working load must meet the following requirements
Preload in tension loaded fastened joints
FM(min) = Minimum acceptable clamping load (lbs. force or kN)
FM(max) = Maximum clamping load (lbs. force or kN)
FS = Total bolt load (lbs. force or kN)
FX = Load allowance for embedment (lbs. force or kN)
FSP = Clamping load limit (lbs. force or kN)
F0.2 = Load at 0.2% offset yield point (lbs. force or kN)
FE = Bolt endurance load (lbs. force or kN)
FSA = Bolt working load (lbs. force or kN)
FPA = Clamped joint element working load (lbs. force or kN)
FAN = Effective external load (lbs. force or kN)
FNR = Residual clamping load (lbs. force or kN)
φ = Theoretical resiliency ratiof = Residual load factorks = Bolt stiffness (lb./in. or kN/mm)
kp = Clamped joint element stiffness (lb./in. or kN/mm)
1. Estimate the external (as well as any internally generated) loads seen by the bolted joint, including static (tension, shear, bending, torsion), dynamic (fatigue, impact, seismic), and inertial loads.
2. Compute the stiffness or spring rate of fasteners, ks, using an empirical relationship such as,
kS = AAABE / LSAB + LBAS
where the various parameters relate the areas of the threaded and unthreaded portions of the bolt, the threaded and unthreaded lengths of the bolt, and the bolt material’s modulus of elasticity.
3. Determine the joint’s stiffness, kP, experimentally by applying an external tension load to the fastener and measuring the tension load in the fastener with a strain gauge or ultrasonically. Then, when the stiffness of the bolt is known (from these tests), a technique known as joint-diagram, which relates to the load-deflection diagrams for the bolt and joint materials, is used to estimate the joint stiffness. Alternatively, plots of joint resilience can be employed and parameters needed for calculating preload can be determined graphically.
The procedure for determining a target preload rely on a combination of analytical, empirical, graphical, and experimental means.
4. Select a target preload, which is generally the greatest load (known as the ‘‘allowable upper limit’’ or FM(max)) that the bolt can withstand without yielding, accounting for torque (e.g., from nuts) and other loads (e.g., shear, bending), but considering the ‘‘acceptable lower limit’’ or FM(min) to prevent failure by leaking, vibrating loose, or shortening needed fatigue life.
5. Determine whether or not the tension normally developed in any bolt will exceed the maximum allowable tensile strength for the particular bolt material. Assembly tool (e.g., torque wrench) errors and operator problems (e.g., skill level, worker fatigue, bolt accessibility) must be taken into account by adding to the preliminary target preload.
6. Consider actual lower limits on clamping force brought about due to relaxation effects (e.g., plastic flow in bolt threads or in new parts), often called ‘‘embedment relaxation’’ and/or elastic interactions between bolts (e.g., the effect of tightening sequence). These effects must be subtracted from the preliminary target preload.
Preload in tension loaded fastened joints
Application of torque on the bolt or the screw is a common way to control the preload.
The torque preload interaction is imprecise due to the following factors:
1. The finish of threads in the screw or nut/bolt
2. The fit between the male and female threads
3. The finish of joint members at the faying surface
4. The size of holes and their perpendicularity with the joint
5. The hardness of all parts
6. The speed of tightening
7. The age, temperature, quantity, condition, and type of any lubricant used
The correct torque is usually selected using ‘‘short-form torque equation,’’
T = kd FPT
k is an experimentally determined constant, which defines the relationship between the applied torque and the achieved preload in a given situation, d is the nominal diameter of the bolt or machine screw (in mm or in.), and FPT is the target preload (in kilo-newtons or lbs. force).
Typical Scatter in Preload for Various Threaded Fastener Tightening Methods
Achieving a Desired (Target) Preload in Bolts
Preferred methods for achieving precise values of preload do so by - inducing the preload by literally and directly stretching the bolt, - directly measuring the stretch induced by some method of torsional loading or tightening,- directly measuring the clamping force with special bolts or washers or bolt-washer assemblies. Some other methods still rely on developing preload indirectly by torquing, without making any direct measurements.
Turn-of-the-nut control technique:Tightening of the nut preload (60 to 80% of σy of the bolt) air-powered impact
wrench (with a torque limited clutch).
The nut is marked and then turned another half turn to cause yielding in the bolt.
This technique relies on a predetermined correlation of bolt tension and applied torque.
(Scatter in the values of bolt tension that reduces at very high levels of torque)
Microprocessor-controlled torque-turn tools:
These devices measure both the applied torque and the angle through which the nut turns to monitor and control fastener preload.
These also rely on predetermined correlations between induced tension and applied torque but reduce scatter by more precisely controlling the applied torque.
Hydraulic tensioners or bolt headers
Stretch a large bolt from the threads to a desired preload based on actual extension
After achieving the desired tension the nut is snugged down.
Ultrasonic devices
Calculation of the bolt tension by measuring the change in length of the tensioned bolt through ultrasonic method or by measuring the velocity changes in the ultrasonic waves caused by the residual stresses.
Other Techniques
Calibrated washers have diametrically opposed bumps on each face that deform under the appropriate bolt preload (or squeezing force)
Calibrated bolts have a small protrusion at their shank end that breaks off when the proper preload value is reached or a spring-like, fluted (or warped) integral collar under the bolt head that deforms elastically to become flat against the joint element when the proper preload is attained.
Proper lubrication is required for proper fastening of the structures in most of these techniques.
Loss of Preload in Service
At least five mechanisms have been identified by which the initial preload in a tightened fastener can be lost in service.
Embedment relaxation
Initially the high points and members in contact High stress produces localized yielding Yielding continues until enough area is in contact to produce stabilization.
Loss of 5% – 25% preload with 5% - 10% is common.
Embedment relaxation can be partially overcome by installing, loosening and then reinstalling the bolt / screw.
Gasket creep
Gaskets are used to seal a fastened joint against fluid leakage or infiltration. To work effectively, gaskets are intentionally made of compliant materials (e.g., cork, felt, rubber, plastics, or soft metals, such as lead or copper), they invariably relax over time.
This can be compensated by retightening of the screws from time to time
Elastic interactions
During the tightening of bolts / screws in a pattern one by one, loss of preload occurs in the initially tightened bolts / screws due to elastic effects.
Loss of 40% - 100% can occur.
Tightening fasteners in a sequence that attempts to balance joint compression throughout tightening
Retightening during a second (or subsequent) pass
Vibration loosening
First the fastener loose preload over time and once the preload has fallen so far that it is no longer able to prevent transverse slip between male and female threads or bolt head or nut and joint surfaces, then the loosening action accelerates rapidly and can result in the complete loss of the nut or the bolt or machine screw or both.
For vibration that is parallel to the axis of the fastener, 20 to 40% loss of initial preload is typical, but for vibration that is transverse to the axis of the fastener, complete loss can readily occur.
Vibrational loosening can be prevented by:
- properly designing the joint,- choosing a fastener that can be preloaded high enough to prevent transverse slip,- choosing a fastener design or fastening system that inherently resists vibration- a device that prevents vibrational loosening.
Special thread forms, so called ‘‘locking’’ adhesives on the threads, use of spring-nuts or lock-washers, and wiring of nuts to bolt shanks can all help.
Stress relaxation
Extremely high service temperatures or nuclear radiation environment. Stress relaxation occurs due to creep.
The only way to avoid it is the use of creep resistant materials
Rotational motion
Rotational motion of the assembly containing fasteners can cause them to loosen.
Left-handed threads are used in certain applications in a bid to avoid loosening
FATIGUE LOADING OF FASTENED JOINTS
Fatigue loading is present in all dynamic structures and many static structures
Loads associated with the various stages of combustion in a four-cycle or a two-cycle internal combustion engine.
Motion induced vibrations or shaking associated with improperly balanced rotating, mechanisms.
Various g-loadings during maneuvers in aircraft or motor vehicles, or from takeoffs and landings in aircraft, and bumps negotiated by vehicles.
Loading rates can be high frequency or low frequency and high stress or low stress, fully periodic or random, sinusoidal, square, or seemingly random in waveform.
Adequate preloading is the prime factor necessary to meet or exceed anticipated cycling loading on the joint.
(a) Careful designing (b) Experimental testing of the joint
Lower level of allowable stresses for fatigue loaded joints as compared to static loading.
FATIGUE LOADING OF FASTENED JOINTS
Indication of fatigue failure
1. If cyclic (especially tensile) loads were present during service, whether planned
or accidental.
2. If there was no warning of the onset of the eventual failure by localized necking, plastic stretch marks, or obvious wear.
3. If the rough appearance of the fracture surface(s) where overload occurred contains a smooth (almost polished) appearance where fatigue was causing slow crack propagation, possibly with tell-tale striations or crack arrest marks.
4. If failure appears to have initiated at points of high stress concentration at surfaces from things such as
(a) sharp radii on joint elements or fasteners (e.g., under heads) (b) machining marks or gouges on the fastener or joint elements (c) at thread run-out areas on bolts or where bolt threads just engage a nut or internally threaded joint element (d) in areas of wear or fretting in friction-type joints (e) where a joint splice plate or ‘‘doubler’’ or section change ends.
FATIGUE LOADING OF FASTENED JOINTS
Reducing the Tendency for Fatigue Failure
Fatigue in mechanical joints can be reduced by the combination of:
a) Proper design (b) Proper analysis
c) Proper manufacturing (d) Proper use
The following points should be considered
1. Material selection: Materials with higher σy and with larger difference between σy
and σUTS. Materials low notch sensitivity. Decarburization during processing and
presence untempered martensite can increase the susceptibility to fatigue.
2. Careful treatment of joints and parts: Nut faces and the under-surfaces of bolt heads must be perpendicular to the fastener axis, and the fastener hole must be perpendicular (or normal) to the joint element (2% error can reduce life 25%). Threads should be well lubricated. Increasing the slip resistance increases the joint’s fatigue life (friction type).
3. Prevention of crack initiation: Machining marks, handling scratches, and other surface blemishes should be removed by polishing (or other finishing processes) in areas of potential fatigue or high loading (e.g., edges, section transitions, holes, etc.).
Shot peening, burnishing, and roll-forming or planishing to introduce compressive stresses at the surface.
Reducing the Tendency for Fatigue Failure
4. Reduction in load excursions: Keep the ratio of the minimum to the maximum
load as near to unity as possible. Avoid high tensile loads, and in particular keep
preload high. High stiffness joint / stiffness bolt ratio.
5. Other Methods:
a) There are at least three threads above and three threads below or within a
nut face or internally threaded part’s surface;
b) Avoiding having the thread run-out coincide with a shear plane in the joint;
using large head-to body fillets;
c) Using a large thread root radius;
d) Using rolled-in rather than cut threads;
e) Using collars between the head and the joint plates and between the nut and
the joint plates to increase the length-to-diameter ratio of the bolts;
f) Turning down the diameter of the bolt body just below the head to reduce the
bolt’s stiffness relative to the joint’s stiffness;
g) Using long or thick nuts;
h) Using spherical washers to help a bolt adjust to bending loads;
i) Using so-called ‘‘tension nuts’’ to reduce the level of stress in the threads.
Corrosion can lead to the complete failure of a fastener or a joint
Corrosion and Environmental Degradation
Metals tend to react chemically with electropositive elements, which include O2, Cl2, Br2 , S and some oxides of S, and carbonates (oxidation)
Avoid combination of metal/alloys which are widely apart in the galvanic series. Avoid contact with electrolytes. Rain, dew, snow, or high humidity. (galvanic corrosion).
Avoid metals near some electronegative nonmetals like graphite.
In stress corrosion cracking, corrosion generally starts at a point of high stress concentration, a crack nucleates, and propagation occurs under continued exposure to the corrosive agent and the stress state, particularly if that stress state is tensile in nature.
Protective coatings or finishes should be employed. These can include primers, paints, inhibitors, conversion coatings, oils, greases, or platings.
Two surfaces in contact experience slight, periodic relative rubbing motion (Fretting corrosion)
Structural joints are seldom loaded in pure tension or compression or shear. Actual service frequently reveals that other types of loads, most notably bending loads, are operating.
Bending Loads
Bending loads increase stress concentration effects at notches, such as thread roots,thread run-outs, and fastener head-to-shank radii.
Bending can drastically change the way in which
the joint acts and the fastener must behave.
(1) Bending introducing tension into fasteners
intended for what were supposed to be shear-
loaded joints operating in bearing;
(2) Bending introducing tension out of the plane
of a shear-loaded joint intended to operate with
fastener-induced friction; and
(3) Bending causing joint opening between
fasteners used with seals or gaskets, creating
leaks.