Lecture 2 Advanced Composites MEC509J2

8/2/2019 Lecture 2 Advanced Composites MEC509J2

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MEC509J2

Advanced Composite Materials

Composites vs Metals (Aluminium Alloys)

For some time it seemed as if composite materials would replace aluminium as the

material of choice in new aircraft designs. This put pressure on aluminium developers

to improve their products. One result was aluminium-lithium (the first aluminium-

lithium alloy, called 2020, was actually developed in the 1950s for the U.S. Navy).

One of the main efforts of the developers is to save weight and cost compared to

composites because the conventional aluminium manufacturing facilities can be used

on aluminium-lithium.

The early demonstrations of the 25-30% weight savings composites offer over

aluminium constructions plus a substantial reduction in the number of parts required foreach application represents a major attraction of these composites. The obstacles to a

wider use today of composite materials are their high acquisition cost compared with

aluminium, in buying a new generation of production equipment. However, the labour-

intensive construction can be solved by automation of the manufacturing process which

is the key technology in developing composites. The use of tape laying machines, for

example, can cut the time and cost of constructing composite components by a factor of

ten pr more.

The use of composites in the U.S. began in the early 1970s under USAF funding and

the late 1970s NASA instituted a series of programs aimed at developing composite

technology and succeeded in placing primary and secondary structural designs in

commercial services. As a result, aircraft manufacturers became more comfortable with

the materials and more efficient construction techniques were developed; the increased

demand led to lower costs of composite materials.

Metals are isotropic, having structural properties which are the same in all directions.

Composites are anisotropic, a single ply having very high strength and stiffness in the

axial direction but only marginal properties in the crosswise direction. Cross-plying

based on load and function enables composites to meet and surpass the properties of

metals. However, composites can be laid up to quasi-isotropic (having nearly isotropic

properties).

Composites versus Metals

Composites differ from metals as their

Properties are not uniform in all directions

Strength and stiffness can be tailored to meet loads

Possess a greater variety of mechanical properties

Poor through the thickness (i.e. short transverse) strength

Composites are usually laid up in essentially two-dimensional form, while metal

may be used in bars, forgings, castings, etc

Greater sensitivity to environmental heat and moisture

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Greater resistance to fatigue damage

Propagation of damage through delamination rather than through-thickness

cracks

Advantages of Composites over Metals

Light weight

Resistance to corrosion

High resistance to fatigue damage

Reduced machining

Tapered sections and compound contours easily accomplished

Can orientate fibres in direction of strength/stiffness needed

Reduced number of assemblies and reduced fastener count when co-cure or co-

consolidation is used Absorb radar microwaves (stealth capability)

Thermal expansion close to zero reduces thermal problems in outer space

applications

Disadvantages of Composites over Metals

Material is expensive

Lack of established design allowables

Corrosion problems can result from improper coupling with metals, especially

when carbon or graphite is used (sealing is essential) Degradation of structural properties under temperature extremes and wet

conditions

Poor energy absorption an impact damage

May require lightening strike protection

Expensive and complicated inspection methods

Defects can be known to exist but precise location cannot be determined.

Advanced Composite Structures

Therefore the use of composites is based on the demonstration that

Significant weight savings can be achieved

Use of composites can reduce cost, or can be cost effective

Composite structures have been validated by tests as meeting all structural

requirements under aircraft environmental conditions

Cost-weight trade studies should be conducted as part of design activity to

determine appropriate use of composites versus metals

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Structural weight reduction is the key advantage in using composite materials. The

relatively high raw material cost of composites can be offset by carefully evaluating

design and manufacturing processes to minimise the cost of fabrication, inspection and

repair. Obviously the strongest of materials pound by pound, composites draw most of

their strength from their hidden fibres, which come in many types and can be arranged

in various patterns, some in three dimensional shapes by braiding or weaving. Thesecomplex patterns can produce shapes with enormous strength in all directions.

Characteristics of Composites

The most commonly used advanced composite fibres are carbon and graphite, Kevlar

and boron. Carbon fibres are manufactured by pyrolysis of an organic precursor such

as rayon or PAN (Polyacrylonitrile), or petroleum pitch. Generally as the fibre modulus

increases, the tensile strength decreases. Among these fibres, carbon fibre is the most

versatile of the advanced reinforcements and the most widely used by the aircraft and

aerospace industries. Products are available as collimated, preimpregnated (prepreg)unidirectional tapes or woven cloth. The wide range of products make it possible to

selectively tailor materials and configurations to suit almost any application.

Matrix materials used in advanced composites to interconnect the fibrous

reinforcements are as varied as the reinforcements. Resins or plastic materials, metals,

and even ceramics are used as matrices. Today, epoxy resin is the primary thermoset

composite matrix for airframe and aerospace applications. In all thermoset materials,

the matrix is cured by means of time, temperature and pressure into a dense, low-void

content structure in which the reinforcement is aligned in the direction of anticipated

loads.

An important element in determining the material behaviour is the composition of the

matrix that binds the fibres together. The selected matrix formulation determines the

cure cycle and affects properties such as

Creep,

Compressive and

Shear strengths,

Thermal resistance, moisture sensitivity, and

Ultraviolet sensitivity,

All of which affect the composites long-term stability. Characteristics of selection ofcomposite matrices include:

Epoxy

Most widely used

Best structural characteristics

Maximum use temperature of 200oF (93oC)

Easy to process

Toughened versions now available

The matrix can also be affected by exposure to water. Since it is the matrix and not thefibre (except for Kevlar) that exhibits these hydroscopic properties are seriously

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reduced, especially at high temperatures by exposure to moisture. For airframe

structures, which experience rapid changes of environment, this loss of mechanical

performance due to moisture absorption must be accounted for in design.

Kevlar (Aramid) is the trade name for a synthetic organic fibre. A density of

0.052lb/in3 gives Kevlar a specific tensile strength higher than either boron or most

carbon fibres. When compared to other composite materials such as carbon and born,

Kevlar has poor compressive strength. The inherent characteristic of Kevlar results

from internal buckling of the filaments. However, Kevlar demonstrates a significant

increase in resistance to damage compared to other composite materials. Kevlar fibresare hygroscopic and this fact must be considered in designing with Kevlar.

High performance advanced composites are often used in stiffness-critical applications.

Thus, when developing new materials, the tendency is to maximise longitudinal moduli

while maintaining acceptable levels of strength, impact resistance, strain-to-failure, and

fracture toughness. Tensile properties are fibre-dominated; therefore, the choice of

fibre is dictated by the application.

Compressive properties in unidirectional laminates are both fibre- and matrix

dependent. While compressive moduli are related to the fibre, compressive strength is

dictated by the neat matrix shear modulus. But for homogeneous, isotropic materials,

the neat matrix strength will prevent or minimise intraply cracking in the composite

under impact conditions, and will also insure acceptable transverse properties. Fracture

toughness is important in matrices to minimise the propagation of cracks and defects,

especially at crossly interfaces.

Retention of compressive strength and strain after impact is an important property in

high performance composites. It should be emphasised that, although damage

prevention is important, damage containment is even more crucial. Therefore, to

prevent impact generated cracks from propagating and causing excessive delamination,

adequate interlaminar fracture toughness is required in composites used for the airframestructures.

Guidelines for the synthesis of improved matrices have evolved primarily from

experiential data which highlights weaknesses;

Design criteria which considers the most dangerous threat to performance

degradation

Limitations in process technology

Evaluations of the relationships between neat matrix properties and composite

properties

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These concepts reveal that the most desirable matrix and composite properties (ideal

goals) are shown as follows:

IMPACT

DELAMINATION

COMPRESSION

PROCESSING

Fibre Materials

Types of Fibre Reinforcement

The most common forms of reinforcement for structural composites are FIBRES,

WHISKERS and PARTICULATES. They vary greatly in cost, availability and

properties. The ultimate choice is determined by considerations of property

requirements, processing possibilities and cost effectiveness. Fibres provide the

greatest opportunity to tailor the material properties of the composite such that

materials with whisker or particulate reinforcements are often considered as improved

plastics rather than composite materials.

Fibres have one axis much greater than its others with the smaller axes often being

circular or near circular. Fibres are generally stronger and stiffer along the long axis

due to the process of drawing the fibre.

Continuous Fibres (where the fibre reinforcement runs continuously for a significant

portion of the structure) are the most widely exploited type of reinforcement and fibres

of greatly contrasting constitution and properties are widely available. These include

glass, carbon (graphite), silicon carbide (SiC), alumina and organic fibres such as

polyaramid and polyethylene.

Short Fibres are often referred to as discontinuous reinforcement. Their shorter length

may confer processing advantages (e.g. for use in moulding compounds) and is often

either cheaper (due to reduced quality of fibre than used in continuous fibres) or more

readily available.

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damage throughout the material rather than the formation of a single crack. This factor

is very important in the determination of the damage tolerance of composites.

Compressive Strength

The axial compressive strength of fibres is usually not reported as it is a difficult tomeasure. The compressive strength of fibre reinforced composites is controlled

however by the compressive failure modes of the fibres including crushing, shearing

and buckling. The latter is a highly likely mode of failure and occurs by Euler Buckling

of the individual fibres. Whilst the Euler predictions will not provide an accurate

prediction of the compressive strength of a composite (due to the provision of some

lateral stabilisation provided by the matrix) it often provides a good relative assessment

of the ability of a fibre type to resist buckling.

Formability

The size and material properties of a fibre will affect the ease with which they can bedeformed and the radius to which they can be bent before failure both of which affect

the Formability of the fibre. These parameters are especially important if the fibre is to

be used in weaving, sewing, knitting or winding operations. The flexibility of a fibre is

usually expressed as a ratio of the curvature (k) generated by a bending moment (M).

Using the engineers theory of bending the flexibility of a fibre may be determined from:

Where E is the modulus of the fibre and d the fibre diameter

The maximum curvature of the fibre may be determined using:

Where * represents the lower of the tensile and compressive strengths of the fibre.

Carbon (Graphite) Fibres

Carbon fibres have been made inadvertently from nature cellulose fibres such as cotton

or linen for thousands of years. Modern carbon fibres were first introduced in about

1967 as a result of independent development work in the UK and Japan and are much

stiffer than glass with comparable strength. The fibres were first produced from a

polymeric fibre precursor, poly-acrylonitrile (PAN), which was already in tonnage

production for textile applications.

The reduced production costs of high performance carbon fibres (compared to those of

other performance fibres such as boron) brought about a resurgence of interest intoadvanced composites in the civil aerospace industry. At the same time carbon fibres

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were also found to be useful in a variety of recreational equipment. This increase in

applications combined with the energy savings that were possible from making

components from easily formable composites led to the availability of commercial

quantities of the material at reasonable prices which further encouraged uptake of the

technology. In the early 70s the cost of carbon fibre exceeded 200/kg. Modern carbon

fibres posses improved properties and cost around 10-15/kg. As the price continues todrop and the mechanical properties values rises, the number of applications for

continuous filament carbon fibres will undoubtedly grow.

Carbon fibres are long bundles of linked graphite plates, forming a crystal-like structure

known as turbostatic graphite, layered parallel to the fibre axis. This crystal structure

is similar to graphite crystals except that the packing of the layer planes is not as regular

as in the graphite crystals which makes the fibres highly anisotropic, with an elastic

modulus of up to 1000GPa on-axis versus only 35GPa off-axis. Fibres can be made

from several different precursor materials, and the method of production is essentially

the same for each precursor: a polymer fibre undergoes pyrolysis under well controlledheating. The resulting fibre can have a wide range of properties, based on the

orientation, spacing, and size of the graphite chains produced by varying these process

conditions. Carbon fibres are typically 4-8m in diameter.

In early development of carbon fibres it was noticed that the shear strength of the fibre

to matrix bond was quite low. This has led to the development of a number of surface

treatment processes aimed at improving this bond and consequently the inter-laminar

shear strength (ILSS) of the composite. The most common method is known as the

oxidative process, which cleans the carbon surface and then attaches chemical groups

such as hydroxides that can bond with the matrix or size. Organic coatings (sizes) are

also added to the fibres in some cases to further improve the fibre/matrix bonding and

to protect the fibres although their value to the overall performance of the composite is

not as high as when used with glass fibres due to the reduced sensitivity of carbon

fibres to environmental degradation.

Fibrous Reinforcement

Fibres used as reinforcement for composite components are available in a number of

different configurations. Each fibre configuration has a range of factors that influence

how they will perform in a composite. These factors are identified as;

Orientation Length

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Shape

Composition

It has been identified that the fibre orientation as the most important contributing factor

to composite performance. The orientation of the fibres in a reinforcement can bebroken down into three distinct categories, one dimensional or uni-directional

reinforcement, planar reinforcement or 2 dimensional and 3-dimensional reinforcement.

The one-dimensional geometry will have the maximum strength and modulus with the

direction of the fibres. The planar geometry will have different strengths in each

direction according to the direction of the fibres, e.g. 50% of the fibres at 0 o and 50% at

90o will yield a value for strength of approximately half of the one dimensional value in

each of the directions. The three-dimensional arrangement can be regarded as being

isotropic but with greatly reduced reinforcing values in each of the principal directions.

Traditional Fabric Lay-ups

A fabric lay-up is a structure comprised of layers of fibre plies as the reinforcement

within the composite structure. The fibre plies usually take the form of one of the four

main fibre orientation categories: Unidirectional, Woven, Multiaxial, and

Other/random. The figure shows a simple 2 layer laminate of unidirectional fibres, the

fibres are laid at 90o to each other in the X-Y plane.

Laminated composites are widely used to manufacture high strength structural

components. The configuration of the laminate effects strength in the in-plane

directions according to the lay-up sequence of the structure. Figure shows two different

lay-up sequences of unidirectional plies. The unidirectional lay-up will have very high

strength in the direction of the fibre but much lower strength transverse to the fibres.

The quasi-isotropic lay-up will have strength equal in each in-plane direction, however,

the actual strength value in each direction will be much lower than the strength of the

unidirectional lay-up in the fibre direction.

Control of the lay-up sequence in the manufacture of composite components fromlaminates is an important consideration for final component mechanical performance.

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The hand laying of laminates is a time consuming and hence expensive process. Thick

composites require a large number of plies and debulking routines before conversion to

a composite. One advantage offered by new manufacturing methods for reinforcements

is the ability to preform the reinforcement.

Textile reinforcement

Textile manufacture has taken place for thousands of years. With the coming of the

industrial revolution the advanced production of textiles has changed dramatically i.

Hand looms and mechanically controlled manufacturing facilities have been replaced

by high-speed computer controlled manufacturing centres capable of producing large

quantities of textile in short periods of time. The use of CAD/CAM machinery to design

and manufacture textiles has resulted in low cost, high speed, and high volume textileproduction.

Textile reinforcement manufacture for advanced composites has been completed in a

number of different ways including weaving, braiding, knitting and variations of each.

Each of these processes imparts different properties to the preforms and therefore the

choice of process is dictated by the required properties of the product. The goal of

these methods has been to enhance not only the mechanical properties of the composite

components manufactured from the preforms, but also to ease the processing of the

final composite and hence facilitate reduced manufacturing costs. Textile design

methods allow the tailoring of yarn placement and direction and hence strength. This iseasily demonstrated by comparing the transverse strength of a plain weave composite

against a unidirectional composite. The use of the plain weave structure with 50% fibre

in the 0 degree direction and 50% in the 90 degree direction gives the composite

transverse strength. Unidirectional fibre composites while extremely strong in the

direction of fibre alignment have greatly reduced strength transverse to the fibres, the

strength in the transverse direction coming mostly from the matrix.

Textile Manufacturing

Weaving

Weaving is the most widely used textile manufacturing technique and accounted for

over 70% of all two dimensional fabrics produced in 1989ii. Weaving of glass, carbon

and aramid is achieved on a conventional loom with little or no modification. The use

of conventional machinery allows cost to be kept to a minimum during the manufacture

of 2D reinforcements

It is stated that the fabric architecture can be altered to meet the specific mechanical

properties required by the composite application. This is achieved by changing the

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weave pattern, fibre type, tow type, tow count etc. The number of possible

architectures is unlimited.

The use of single layer woven glass, carbon and aramid tows is well established within

the composite structures industry for both aerospace and non-aerospace applications.Traditionally composite components manufactured from single or multiple layer woven

reinforcement are manufactured using the lay-up technique. Fibre-reinforced laminates

have been used for many years in the boat building industry, aerospace and automotive

industries. The use of laminates has been limited by a number of factors, including

manufacturing costs and some inferior in plane mechanical properties, when compared

to some of the more mainstream materials such as aluminium alloys.

In the lay-up process, layers of reinforcement in the form of dry reinforcement or resin

impregnated plies (prepreg), are stacked on top of one another to form a laminate and

then processed by various methods depending on the application of the finalcomponent. Hand lay- up of fibre laminates is a long and arduous process requiring a

high amount of human contact time and does not readily lend itself to the cost

conscious and lean production schedules of today. In the case of prepreg laminates, a

high-added cost due to the specialised handling and storage requirements is incurred

during their use. With rising interest in the use of composite materials for applications

not related to the high tech aerospace and high-performance car markets, there is a

need for the development and understanding of other methods of preform manufacture

and composite processing that will be more attractive to these sectors.

3D Textile reinforcements

3D textile reinforcements have proportions of yarn in each of the X Y and Z directions.

The Z direction is also known as through the thickness because the yarn will pass

through the fabric from top to bottom or in the case of 3D woven fabric it may pass

layer to layer. This quantity of yarn in the Z or through the thickness direction is the

indicator of textile type. 3D textiles will have a much higher amount of yarn in the Zdirection than the flat 2D textiles.

3D textile reinforcements also known as technical textiles offer the advantages such as

shaped preforming, thereby reducing the preparation time for composite manufacture.

Technical textiles can also provide improvements in mechanical properties of

composite components. Areas highlighted in the literature where these improvements

can occur are interlaminar shear strength, damage tolerance and impact resistance.

Methods of manufacturing these technical textiles for use as composite reinforcements

include 3D weaving, 3D braiding, 3D knitting, and stitching. Each of these methodshas specific advantages and disadvantages.

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3D Woven Textiles

Designers and engineers have taken out numerous patents on dedicated weaving

machines that will place yarn in specific directions throughout the preform. Orthogonal,

layer interlock and multi-axis woven textiles represent the three main categories of

woven preform with distance fabrics and woven sandwich panels.

In general orthogonal and angle/layer interlock 3D woven technical textiles can be

easily manufactured with modification of standard weaving machinery used to produce

cloth for textile applications. This simple manufacturing route provides a capability for

cost reduction in composite component production. Multi axis weaving describes a

process that incorporates the insertion of +- 45 o yarns into the fabric architecture. The

process of manufacturing of these multi-axis preforms at present requires very

specialised machinery and is undertaken by very few organisations.

Results from mechanical testing state that 3-D woven composite components have

benefits over existing materials used within the aerospace industry. A major advantage

of technical textile usage is that techniques involved in the manufacture of these

advanced textile structures allow the production of near net shaped preforms, thereby

reducing a portion of the cost of component manufacture.

The advantages of 3D woven composites over their 2D counterparts have been shownin publications from several authors. There are however a number of distinct

disadvantages that have to be taken into consideration when deciding to employ a

technical textile as reinforcement in a composite component.

However the use of a composite based on a 3D woven preform will only be of benefit if

the overall performance and cost aspects are balanced against each other

In the Engineering Composites Research Centre 3D weaving approach, an orthogonal

textile is produced using a standard weaving loom. The warp (or 0o

direction) tows areused to provide through-the-thickness (TTT) binders that consolidate the preform The

TTT tows are arranged in different patterns and levels of the reinforcement according to

the net shape and mechanical properties required for the final composite component.

3D Woven reinforcement design

The use of high-powered CAD systems coupled with advanced manufacturing

techniques have made it possible to design and manufacture 3D textile reinforcements

under computer control. Computer control of the weaving design and manufacture

offers the advantages of visualisation of the weave structure and lift plan prior to

manufacture as well as providing basic information for down stream applications suchas finite element analysis. Complex weave architectures in the case of 3D weaving can

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be lain out on the computer and visualised before production takes place.

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Lecture 2 Advanced Composites MEC509J2

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