 | It has been suggested that this article besplit into articles titledAutoclave moulding,Resin transfer moulding,Pressure bag moulding andLight resin transfer moulding. (Discuss)(November 2020) |
Acomposite orcomposite material (alsocomposition material) is amaterial which is produced from two or more constituent materials.[1] These constituent materials have notably dissimilar chemical or physical properties and are merged to create a material with properties unlike the individual elements. Within the finished structure, the individual elements remain separate and distinct, distinguishing composites frommixtures andsolid solutions. Composite materials with more than one distinct layer are calledcomposite laminates.
Typical engineered compositematerials are made up of abinding agent forming thematrix and afiller material (particulates orfibres) givingsubstance, e.g.:
Composite materials can be less expensive, lighter, stronger or more durable than common materials. Some are inspired by biological structures found in plants and animals.[3]Robotic materials are composites that include sensing, actuation, computation, and communication components.[4][5]
Composite materials are used forconstruction and technicalstructures such asboat hulls,swimming pool panels,racing car bodies,shower stalls,bathtubs,storage tanks,imitationgranite, andcultured marblesinks and countertops.[6][7] They are also being increasingly used in general automotive applications.[8]
The earliest composite materials were made fromstraw andmud combined to formbricks forbuildingconstruction. Ancientbrick-making was documented byEgyptian tomb paintings.[9]
Wattle and daub might be the oldest composite materials, at over 6000 years old.[10]
Concrete is the most common artificial composite material of all. As of 2009[update], about 7.5 billion cubic metres of concrete are made each year.[17]Concrete typically consists of loose stones (construction aggregate) held with a matrix ofcement. Concrete is an inexpensive material resisting large compressive forces,[18] however, susceptible totensile loading.[19] To give concrete the ability to resist being stretched, steel bars, which can resist high stretching (tensile) forces, are often added to concrete to formreinforced concrete.[20]
Fibre-reinforced polymers includecarbon-fiber-reinforced polymers andglass-reinforced plastic. If classified by matrix then there arethermoplastic composites,short fibre thermoplastics,long fibre thermoplastics orlong-fiber-reinforced thermoplastics. There are numerousthermoset composites, includingpaper composite panels. Many advancedthermoset polymer matrix systems usually incorporatearamidfibre andcarbon fibre in anepoxy resin matrix.[21][22]
Shape-memory polymer composites are high-performance composites, formulated using fibre or fabric reinforcements and shape-memory polymer resin as the matrix. Since a shape-memory polymer resin is used as the matrix, these composites have the ability to be easily manipulated into various configurations when they are heated above their activation temperatures and will exhibit high strength and stiffness at lower temperatures. They can also be reheated and reshaped repeatedly without losing their material properties. These composites are ideal for applications such as lightweight, rigid, deployable structures; rapid manufacturing; and dynamic reinforcement.[23][24]
High strain composites are another type of high-performance composites that are designed to perform in a high deformation setting and are often used in deployable systems where structural flexing is advantageous.[citation needed] Although high strain composites exhibit many similarities to shape-memory polymers, their performance is generally dependent on the fibre layout as opposed to the resin content of the matrix.[25]
Composites can also use metal fibres reinforcing other metals, as inmetal matrix composites (MMC)[26] orceramic matrix composites (CMC),[27] which includesbone (hydroxyapatite reinforced withcollagen fibres),cermet (ceramic and metal), andconcrete. Ceramic matrix composites are built primarily forfracture toughness, not for strength. Another class of composite materials involve woven fabric composite consisting of longitudinal and transverse laced yarns. Woven fabric composites are flexible as they are in form of fabric.
Organic matrix/ceramic aggregate composites includeasphalt concrete,polymer concrete,mastic asphalt, mastic roller hybrid,dental composite,syntactic foam, andmother of pearl.[28]Chobham armour is a special type ofcomposite armour used in military applications.[citation needed]
Additionally,thermoplastic composite materials can be formulated with specific metal powders resulting in materials with a density range from 2 g/cm3 to 11 g/cm3 (same density as lead). The most common name for this type of material is "high gravity compound" (HGC), although "lead replacement" is also used. These materials can be used in place of traditional materials such as aluminium, stainless steel, brass, bronze, copper, lead, and even tungsten in weighting, balancing (for example, modifying the centre of gravity of a tennisracquet), vibration damping, and radiation shielding applications. High density composites are an economically viable option when certain materials are deemed hazardous and are banned (such as lead) or when secondary operations costs (such as machining, finishing, or coating) are a factor.[29]
There have been several studies indicating that interleaving stiff and brittle epoxy-basedcarbon-fiber-reinforced polymer laminates with flexible thermoplastic laminates can help to make highly toughened composites that show improved impact resistance.[30] Another interesting aspect of such interleaved composites is that they are able to have shape memory behaviour without needing anyshape-memory polymers orshape-memory alloys e.g. balsa plies interleaved with hot glue,[31] aluminium plies interleaved withacrylic polymers orPVC[32] andcarbon-fiber-reinforced polymer laminates interleaved withpolystyrene.[33]
Asandwich-structured composite is a special class of composite material that is fabricated by attaching two thin but stiff skins to a lightweight but thick core. The core material is normally low strength material, but its higher thickness provides the sandwich composite with highbendingstiffness with overall lowdensity.[34][35]
Wood is a naturally occurring composite comprising cellulose fibres in alignin andhemicellulose matrix.[36]Engineered wood includes a wide variety of different products such as wood fibre board,plywood,oriented strand board,wood plastic composite (recycled wood fibre in polyethylene matrix),Pykrete (sawdust in ice matrix), plastic-impregnated orlaminated paper or textiles,Arborite,Formica (plastic), andMicarta. Other engineered laminate composites, such asMallite, use a central core of end grainbalsa wood, bonded to surface skins of lightalloy or GRP. These generate low-weight, high rigidity materials.[37]
Particulate composites have particle as filler material dispersed in matrix, which may be nonmetal, such as glass, epoxy. Automobile tire is an example of particulate composite.[38]
Advanced diamond-like carbon (DLC) coated polymer composites have been reported[39] where the coating increases the surface hydrophobicity, hardness and wear resistance.
Ferromagnetic composites, including those with a polymer matrix consisting, for example, of nanocrystalline filler of Fe-based powders and polymers matrix. Amorphous and nanocrystalline powders obtained, for example, from metallic glasses can be used. Their use makes it possible to obtain ferromagnetic nanocomposites with controlled magnetic properties.[40]
Fibre-reinforced composite materials have gained popularity (despite their generally high cost) in high-performance products that need to be lightweight, yet strong enough to take harsh loading conditions such asaerospace components (tails,wings,fuselages,propellers), boat andscull hulls,bicycle frames, andracing car bodies. Other uses includefishing rods,storage tanks, swimming pool panels, andbaseball bats. TheBoeing 787 andAirbus A350 structures including the wings and fuselage are composed largely of composites.[41] Composite materials are also becoming more common in the realm oforthopedic surgery,[42] and it is the most common hockey stick material.
Carbon composite is a key material in today's launch vehicles andheat shields for there-entry phase ofspacecraft. It is widely used in solar panel substrates, antenna reflectors and yokes of spacecraft. It is also used in payload adapters, inter-stage structures and heat shields oflaunch vehicles. Furthermore,disk brake systems ofairplanes and racing cars are usingcarbon/carbon material, and thecomposite material withcarbon fibres andsilicon carbide matrix has been introduced inluxury vehicles andsports cars.
In 2006, a fibre-reinforced composite pool panel was introduced for in-ground swimming pools, residential as well as commercial, as a non-corrosive alternative to galvanized steel.
In 2007, an all-composite militaryHumvee was introduced by TPI Composites Inc and Armor Holdings Inc, the first all-compositemilitary vehicle. By using composites the vehicle is lighter, allowing higher payloads.[43] In 2008, carbon fibre andDuPont Kevlar (five times stronger than steel) were combined with enhanced thermoset resins to make military transit cases by ECS Composites creating 30-percent lighter cases with high strength.
Pipes and fittings for various purpose like transportation of potable water, fire-fighting, irrigation, seawater, desalinated water, chemical and industrial waste, and sewage are now manufactured in glass reinforced plastics.
Composite materials used in tensile structures for facade application provides the advantage of being translucent. The woven base cloth combined with the appropriate coating allows better light transmission. This provides a very comfortable level of illumination compared to the full brightness of outside.[44]
The wings of wind turbines, in growing sizes in the order of 50 m length are fabricated in composites since several years.[45]
Two-lower-leg-amputees run on carbon-composite spring-like artificial feet as quick as non-amputee athletes.[46]
High-pressure gas cylinders typically about 7–9 litre volume x 300 bar pressure for firemen are nowadays constructed from carbon composite.Type-4-cylinders include metal only as boss that carries the thread to screw in the valve.
On 5 September 2019,HMD Global unveiled theNokia 6.2 andNokia 7.2 which are claimed to be using polymer composite for the frames.[47]
Composite materials are created from individual materials. These individual materials are known as constituent materials, and there are two main categories of it. One is thematrix (binder) and the otherreinforcement.[48] A portion of each kind is needed at least. The reinforcement receives support from the matrix as the matrix surrounds the reinforcement and maintains its relative positions. The properties of the matrix are improved as the reinforcements impart their exceptional physical and mechanical properties. The mechanical properties become unavailable from the individual constituent materials by synergism. At the same time, the designer of the product or structure receives options to choose an optimum combination from the variety of matrix and strengthening materials.
To shape the engineered composites, it must be formed. The reinforcement is placed onto the mould surface or into themould cavity. Before or after this, the matrix can be introduced to the reinforcement. The matrix undergoes a melding event which sets the part shape necessarily. This melding event can happen in several ways, depending upon the matrix nature, such as solidification from the melted state for a thermoplastic polymer matrix composite or chemicalpolymerization for athermoset polymer matrix.
According to the requirements of end-item design, various methods of moulding can be used. The natures of the chosen matrix and reinforcement are the key factors influencing the methodology. The gross quantity of material to be made is another main factor. To support high capital investments for rapid and automated manufacturing technology, vast quantities can be used. Cheaper capital investments but higher labour and tooling expenses at a correspondingly slower rate assists the small production quantities.
Many commercially produced composites use apolymer matrix material often called a resin solution. There are many different polymers available depending upon the starting raw ingredients. There are several broad categories, each with numerous variations. The most common are known aspolyester,vinyl ester,epoxy,phenolic,polyimide,polyamide,polypropylene,PEEK, and others. The reinforcement materials are often fibres but also commonly ground minerals. The various methods described below have been developed to reduce the resin content of the final product, or the fibre content is increased. As a rule of thumb, lay up results in a product containing 60% resin and 40% fibre, whereas vacuum infusion gives a final product with 40% resin and 60% fibre content. The strength of the product is greatly dependent on this ratio.
Martin Hubbe and Lucian A Lucia considerwood to be a natural composite ofcellulose fibres in amatrix oflignin.[49][50]
Several layup designs of composite also involve a co-curing or post-curing of the prepreg with many other media, such as foam or honeycomb. Generally, this is known as asandwich structure. This is a more general layup for the production of cowlings, doors, radomes or non-structural parts.
Open- and closed-cell-structuredfoams likepolyvinyl chloride,polyurethane,polyethylene, orpolystyrene foams,balsa wood,syntactic foams, andhoneycombs are generally utilized core materials. Open- and closed-cellmetal foam can also be utilized as core materials. Recently, 3D graphene structures ( also called graphene foam) have also been employed as core structures. A recent review by Khurram and Xu et al., have provided the summary of the state-of-the-art techniques for fabrication of the 3D structure of graphene, and the examples of the use of these foam like structures as a core for their respective polymer composites.[51]
Although the two phases are chemically equivalent, semi-crystalline polymers can be described both quantitatively and qualitatively as composite materials. The crystalline portion has a higher elastic modulus and provides reinforcement for the less stiff, amorphous phase. Polymeric materials can range from 0% to 100%[52] crystallinity aka volume fraction depending on molecular structure and thermal history. Different processing techniques can be employed to vary the percent crystallinity in these materials and thus the mechanical properties of these materials as described in the physical properties section. This effect is seen in a variety of places from industrial plastics like polyethylene shopping bags to spiders which can produce silks with different mechanical properties.[53] In many cases these materials act like particle composites with randomly dispersed crystals known as spherulites. However they can also be engineered to be anisotropic and act more like fiber reinforced composites.[54] In the case of spider silk, the properties of the material can even be dependent on the size of the crystals, independent of the volume fraction.[55] Ironically, single component polymeric materials are some of the most easily tunable composite materials known.
Normally, the fabrication of composite includes wetting, mixing or saturating the reinforcement with the matrix. The matrix is then induced to bind together (with heat or a chemical reaction) into a rigid structure. Usually, the operation is done in an open or closed forming mould. However, the order and ways of introducing the constituents alters considerably. Composites fabrication is achieved by a wide variety of methods, includingadvanced fibre placement (automated fibre placement),[56]fibreglass spray lay-up process,[57]filament winding,[58]lanxide process,[59]tailored fibre placement,[60]tufting,[61] andz-pinning.[62]
The reinforcing and matrix materials are merged, compacted, and cured (processed) within a mould to undergo a melding event. The part shape is fundamentally set after the melding event. However, under particular process conditions, it can deform. The melding event for athermoset polymer matrix material is a curing reaction that is caused by the possibility of extra heat or chemical reactivity such as an organic peroxide. The melding event for a thermoplastic polymeric matrix material is a solidification from the melted state. The melding event for a metal matrix material such as titanium foil is a fusing at high pressure and a temperature near the melting point.
It is suitable for many moulding methods to refer to one mould piece as a "lower" mould and another mould piece as an "upper" mould. Lower and upper does not refer to the mould's configuration in space, but the different faces of the moulded panel. There is always a lower mould, and sometimes an upper mould in this convention. Part construction commences by applying materials to the lower mould. Lower mould and upper mould are more generalized descriptors than more common and specific terms such as male side, female side, a-side, b-side, tool side, bowl, hat, mandrel, etc. Continuous manufacturing utilizes a different nomenclature.
Usually, the moulded product is referred to as a panel. It can be referred to as casting for certain geometries and material combinations. It can be referred to as a profile for certain continuous processes. Some of the processes areautoclave moulding,[63]vacuum bag moulding,[64]pressure bag moulding,[65]resin transfer moulding,[66] andlight resin transfer moulding.[67]
Other types of fabrication includecasting,[68] centrifugal casting,[69]braiding (onto aformer),continuous casting,[70]filament winding,[71] press moulding,[72]transfer moulding,pultrusion moulding,[73] andslip forming.[74] There are also forming capabilities includingCNC filament winding, vacuum infusion, wet lay-up,compression moulding, andthermoplastic moulding, to name a few. The practice of curing ovens and paint booths is also required for some projects.
The composite parts finishing is also crucial in the final design. Many of these finishes will involve rain-erosion coatings or polyurethane coatings.
The mould and mould inserts are referred to as "tooling". The mould/tooling can be built from different materials. Tooling materials includealuminium,carbon fibre,invar,nickel, reinforcedsilicone rubber and steel. The tooling material selection is normally based on, but not limited to, thecoefficient of thermal expansion, expected number of cycles, end item tolerance, desired or expected surface condition, cure method,glass transition temperature of the material being moulded, moulding method, matrix, cost, and other various considerations.
Usually, the composite's physical properties are notisotropic (independent of the direction of applied force) in nature. But they are typicallyanisotropic (different depending on the direction of the applied force or load). For instance, the composite panel's stiffness will usually depend upon the orientation of the applied forces and/or moments. The composite's strength is bounded by two loading conditions, as shown in the plot to the right.
If both the fibres and matrix are aligned parallel to the loading direction, the deformation of both phases will be the same (assuming there is no delamination at the fibre-matrix interface). This isostrain condition provides the upper bound for composite strength, and is determined by therule of mixtures:
whereEC is the effective compositeYoung's modulus, andVi andEi are the volume fraction and Young's moduli, respectively, of the composite phases.
For example, a composite material made up of α and β phases as shown in the figure to the right under isostrain, the Young's modulus would be as follows:where Vα and Vβ are the respective volume fractions of each phase.This can be derived by considering that in the isostrain case, Assuming that the composite has a uniform cross section, the stress on the composite is a weighted average between the two phases, The stresses in the individual phases are given by Hooke's Law, Combining these equations gives that the overall stress in the composite is Then it can be shown that
The lower bound is dictated by the isostress condition, in which the fibres and matrix are oriented perpendicularly to the loading direction:and now the strains become a weighted averageRewriting Hooke's Law for the individual phases This leads toFrom the definition of Hooke's Lawand, in general,
Following the example above, if one had a composite material made up of α and β phases under isostress conditions as shown in the figure to the right, the composition Young's modulus would be: The isostrain condition implies that under an applied load, both phases experience the same strain but will feel different stress. Comparatively, under isostress conditions both phases will feel the same stress but the strains will differ between each phase. A generalized equation for any loading condition between isostrain and isostress can be written as:[76]
where X is a material property such as modulus or stress, c, m, and r stand for the properties of the composite, matrix, and reinforcement materials respectively, and n is a value between 1 and −1.
The above equation can be further generalized beyond a two phase composite to an m-component system:
Though composite stiffness is maximized when fibres are aligned with the loading direction, so is the possibility of fibre tensile fracture, assuming the tensile strength exceeds that of the matrix. When a fibre has some angle of misorientation θ, several fracture modes are possible. For small values of θ the stress required to initiate fracture is increased by a factor of (cos θ)−2 due to the increased cross-sectional area (A cos θ) of the fibre and reduced force (F/cos θ) experienced by the fibre, leading to a composite tensile strength ofσparallel/cos2 θ whereσparallel is the tensile strength of the composite with fibres aligned parallel with the applied force.
Intermediate angles of misorientation θ lead to matrix shear failure. Again the cross sectional area is modified but sinceshear stress is now the driving force for failure the area of the matrix parallel to the fibres is of interest, increasing by a factor of 1/sin θ. Similarly, the force parallel to this area again decreases (F/cos θ) leading to a total tensile strength ofτmy /sin θ cos θ whereτmy is the matrix shear strength.
Finally, for large values of θ (near π/2) transverse matrix failure is the most likely to occur, since the fibres no longer carry the majority of the load. Still, the tensile strength will be greater than for the purely perpendicular orientation, since the force perpendicular to the fibres will decrease by a factor of 1/sin θ and the area decreases by a factor of 1/sin θ producing a composite tensile strength ofσperp /sin2θ whereσperp is the tensile strength of the composite with fibres align perpendicular to the applied force.[77]
The majority of commercial composites are formed with random dispersion and orientation of the strengthening fibres, in which case the composite Young's modulus will fall between the isostrain and isostress bounds. However, in applications where the strength-to-weight ratio is engineered to be as high as possible (such as in the aerospace industry), fibre alignment may be tightly controlled.
Panel stiffness is also dependent on the design of the panel. For instance, the fibre reinforcement and matrix used, the method of panel build, thermoset versus thermoplastic, and type of weave.
In contrast to composites, isotropic materials (for example, aluminium or steel), in standard wrought forms, possess the same stiffness typically despite the directional orientation of the applied forces and/or moments. The relationship between forces/moments and strains/curvatures for an isotropic material can be described with the following material properties: Young's Modulus, theshear modulus, and thePoisson's ratio, in relatively simple mathematical relationships. For the anisotropic material, it needs the mathematics of a second-order tensor and up to 21 material property constants. For the special case of orthogonal isotropy, there are three distinct material property constants for each of Young's Modulus, Shear Modulus and Poisson's ratio—a total of 9 constants to express the relationship between forces/moments and strains/curvatures.
Techniques that take benefit of the materials' anisotropic properties involvemortise and tenon joints (in natural composites such as wood) andpi joints in synthetic composites.
In general, particle reinforcement isstrengthening the composites less thanfiber reinforcement. It is used to enhance thestiffness of the composites while increasing thestrength and thetoughness. Because of theirmechanical properties, they are used in applications in whichwear resistance is required. For example, hardness ofcement can be increased by reinforcing gravel particles, drastically. Particle reinforcement a highly advantageous method of tuning mechanical properties of materials since it is very easy implement while being low cost.[78][79][80][81]
Theelastic modulus of particle-reinforced composites can be expressed as,
where E is theelastic modulus, V is thevolume fraction. The subscripts c, p and m are indicating composite, particle and matrix, respectively. is a constant can be found empirically.
Similarly, tensile strength of particle-reinforced composites can be expressed as,
where T.S. is thetensile strength, and is a constant (not equal to) that can be found empirically.
In general, continuousfiber reinforcement is implemented by incorporating afiber as the strong phase into a weak phase, matrix. The reason for the popularity of fiber usage is materials with extraordinary strength can be obtained in their fiber form. Non-metallic fibers are usually showing a very high strength to density ratio compared to metal fibers because of thecovalent nature of theirbonds. The most famous example of this iscarbon fibers that have many applications extending fromsports gear toprotective equipment tospace industries.[82][83]
The stress on the composite can be expressed in terms of thevolume fraction of the fiber and the matrix.
where is the stress, V is thevolume fraction. The subscripts c, f and m are indicating composite, fiber and matrix, respectively.
Although thestress–strain behavior of fiber composites can only be determined by testing, there is an expected trend, three stages of thestress–strain curve. The first stage is the region of the stress–strain curve where both fiber and the matrix areelastically deformed. This linearly elastic region can be expressed in the following form.[82]
where is the stress, is the strain, E is theelastic modulus, and V is thevolume fraction. The subscripts c, f, and m are indicating composite, fiber, and matrix, respectively.
After passing the elastic region for both fiber and the matrix, the second region of the stress–strain curve can be observed. In the second region, the fiber is still elastically deformed while the matrix is plastically deformed since the matrix is the weak phase. The instantaneousmodulus can be determined using the slope of the stress–strain curve in the second region. The relationship betweenstress and strain can be expressed as,
where is the stress, is the strain, E is theelastic modulus, and V is thevolume fraction. The subscripts c, f, and m are indicating composite, fiber, and matrix, respectively. To find the modulus in the second region derivative of this equation can be used since theslope of the curve is equal to the modulus.
In most cases it can be assumed since the second term is much less than the first one.[82]
In reality, thederivative of stress with respect to strain is not always returning the modulus because of thebinding interaction between the fiber and matrix. The strength of the interaction between these two phases can result in changes in themechanical properties of the composite. The compatibility of the fiber and matrix is a measure ofinternal stress.[82]
Thecovalently bonded high strength fibers (e.g.carbon fibers) experience mostlyelastic deformation before the fracture since theplastic deformation can happen due todislocation motion. Whereas,metallic fibers have more space to plastically deform, so their composites exhibit a third stage where both fiber and the matrix are plastically deforming.Metallic fibers havemany applications to work atcryogenic temperatures that is one of the advantages of composites withmetal fibers over nonmetallic. The stress in this region of thestress–strain curve can be expressed as,
where is the stress, is the strain, E is theelastic modulus, and V is thevolume fraction. The subscripts c, f, and m are indicating composite, fiber, and matrix, respectively. and are for fiber and matrix flow stresses respectively. Just after the third region the composite exhibitnecking. The necking strain of composite is happened to be between the necking strain of the fiber and the matrix just like other mechanical properties of the composites. The necking strain of the weak phase is delayed by the strong phase. The amount of the delay depends upon the volume fraction of the strong phase.[82]
Thus, thetensile strength of the composite can be expressed in terms of thevolume fraction.[82]
where T.S. is thetensile strength, is the stress, is the strain, E is theelastic modulus, and V is thevolume fraction. The subscripts c, f, and m are indicating composite, fiber, and matrix, respectively. The composite tensile strength can be expressed as
The critical value ofvolume fraction can be expressed as,
Evidently, the compositetensile strength can be higher than the matrix if is greater than.
Thus, the minimum volume fraction of the fiber can be expressed as,
Although this minimum value is very low in practice, it is very important to know since the reason for the incorporation of continuous fibers is to improve the mechanical properties of the materials/composites, and this value of volume fraction is the threshold of this improvement.[82]
A change in the angle between the applied stress and fiber orientation will affect the mechanical properties of fiber-reinforced composites, especially the tensile strength. This angle,, can be used predict the dominant tensile fracture mechanism.
At small angles,, the dominant fracture mechanism is the same as with load-fiber alignment, tensile fracture. The resolved force acting upon the length of the fibers is reduced by a factor of from rotation.. The resolved area on which the fiber experiences the force is increased by a factor of from rotation.. Taking the effectivetensile strength to be and the alignedtensile strength.[82]
At moderate angles,, the material experiences shear failure. The effective force direction is reduced with respect to the aligned direction.. The resolved area on which the force acts is. The resultingtensile strength depends on theshear strength of the matrix,.[82]
At extreme angles,, the dominant mode of failure is tensile fracture in the matrix in the perpendicular direction. As in theisostress case of layered composite materials, the strength in this direction is lower than in the aligned direction. The effective areas and forces act perpendicular to the aligned direction so they both scale by. The resolved tensile strength is proportional to the transverse strength,.[82]
The critical angles from which the dominant fracture mechanism changes can be calculated as,
where is the critical angle between longitudinal fracture and shear failure, and is the critical angle between shear failure and transverse fracture.[82]
By ignoring length effects, this model is most accurate for continuous fibers and does not effectively capture the strength-orientation relationship for short fiber reinforced composites. Furthermore, most realistic systems do not experience thelocal maxima predicted at the critical angles.[84][85][86][87] TheTsai-Hill criterion provides a more complete description of fiber composite tensile strength as a function of orientation angle by coupling the contributing yield stresses:,, and.[88][82]
Anisotropy in the tensile strength of fiber reinforced composites can be removed by randomly orienting the fiber directions within the material. It sacrifices the ultimate strength in the aligned direction for an overall, isotropically strengthened material.
Where K is an empirically determined reinforcement factor; similar to theparticle reinforcement equation. For fibers with randomly distributed orientations in a plane,, and for a random distribution in 3D,.[82]
For real application, most composite isanisotropic material ororthotropic material. The three-dimension stress tensor is required for stress and strain analysis. The stiffness and compliance can be written as follows[89]
and
In order to simplify the 3D stress direction, the plane stress assumption is apply that the out–of–plane stress and out–of–plane strain are insignificant or zero. That is and.[90]
The stiffness matrix and compliance matrix can be reduced to
and
For fiber-reinforced composite, the fiber orientation in material affect anisotropic properties of the structure. From characterizing technique i.e. tensile testing, the material properties were measured based on sample (1-2) coordinate system. The tensors above express stress-strain relationship in (1-2) coordinate system. While the known material properties is in the principal coordinate system (x-y) of material. Transforming the tensor between two coordinate system help identify the material properties of the tested sample. Thetransformation matrix with degree rotation is[90]
for for
The most common types of fibers used in industry areglass fibers,carbon fibers, andkevlar due to their ease of production and availability. Their mechanical properties are very important to know, therefore the table of their mechanical properties is given below to compare them with S97steel.[91][92][93][94] The angle of fiber orientation is very important because of the anisotropy of fiber composites (please see the section "Physical properties" for a more detailed explanation). The mechanical properties of the composites can be tested using standardmechanical testing methods by positioning the samples at various angles (the standard angles are 0°, 45°, and 90°) with respect to the orientation of fibers within the composites. In general, 0° axial alignment makes composites resistant to longitudinal bending and axial tension/compression, 90° hoop alignment is used to obtain resistance to internal/external pressure, and ± 45° is the ideal choice to obtain resistance against pure torsion.[95]
| Symbol | Units | Standard Carbon Fiber Fabric | High Modulus Carbon Fiber Fabric | E-Glass Fibre GlassFabric | Kevlar Fabric | Standard Unidirectional Carbon Fiber Fabric | High Modulus Unidirectional Carbon Fiber Fabric | E-Glass Unidirectional Fiber GlassFabric | Kevlar UnidirectionalFabric | Steel S97 | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Young's Modulus 0° | E1 | GPa | 70 | 85 | 25 | 30 | 135 | 175 | 40 | 75 | 207 |
| Young's Modulus 90° | E2 | GPa | 70 | 85 | 25 | 30 | 10 | 8 | 8 | 6 | 207 |
| In-plane Shear Modulus | G12 | GPa | 5 | 5 | 4 | 5 | 5 | 5 | 4 | 2 | 80 |
| Major Poisson's Ratio | v12 | 0.10 | 0.10 | 0.20 | 0.20 | 0.30 | 0.30 | 0.25 | 0.34 | – | |
| Ult. Tensile Strength 0° | Xt | MPa | 600 | 350 | 440 | 480 | 1500 | 1000 | 1000 | 1300 | 990 |
| Ult. Comp. Strength 0° | Xc | MPa | 570 | 150 | 425 | 190 | 1200 | 850 | 600 | 280 | – |
| Ult. Tensile Strength 90° | Yt | MPa | 600 | 350 | 440 | 480 | 50 | 40 | 30 | 30 | – |
| Ult. Comp. Strength 90° | Yc | MPa | 570 | 150 | 425 | 190 | 250 | 200 | 110 | 140 | – |
| Ult. In-plane Shear Stren. | S | MPa | 90 | 35 | 40 | 50 | 70 | 60 | 40 | 60 | – |
| Ult. Tensile Strain 0° | ext | % | 0.85 | 0.40 | 1.75 | 1.60 | 1.05 | 0.55 | 2.50 | 1.70 | – |
| Ult. Comp. Strain 0° | exc | % | 0.80 | 0.15 | 1.70 | 0.60 | 0.85 | 0.45 | 1.50 | 0.35 | – |
| Ult. Tensile Strain 90° | eyt | % | 0.85 | 0.40 | 1.75 | 1.60 | 0.50 | 0.50 | 0.35 | 0.50 | – |
| Ult. Comp. Strain 90° | eyc | % | 0.80 | 0.15 | 1.70 | 0.60 | 2.50 | 2.50 | 1.35 | 2.30 | – |
| Ult. In-plane shear strain | es | % | 1.80 | 0.70 | 1.00 | 1.00 | 1.40 | 1.20 | 1.00 | 3.00 | – |
| Density | g/cc | 1.60 | 1.60 | 1.90 | 1.40 | 1.60 | 1.60 | 1.90 | 1.40 | – |
| Symbol | Units | Standard Carbon Fiber | High Modulus Carbon Fiber | E-Glass Fiber Glass | Standard Carbon Fibers Fabric | E-Glass Fiber GlassFabric | Steel | Al | |
|---|---|---|---|---|---|---|---|---|---|
| Longitudinal Modulus | E1 | GPa | 17 | 17 | 12.3 | 19.1 | 12.2 | 207 | 72 |
| Transverse Modulus | E2 | GPa | 17 | 17 | 12.3 | 19.1 | 12.2 | 207 | 72 |
| In Plane Shear Modulus | G12 | GPa | 33 | 47 | 11 | 30 | 8 | 80 | 25 |
| Poisson's Ratio | v12 | .77 | .83 | .53 | .74 | .53 | |||
| Tensile Strength | Xt | MPa | 110 | 110 | 90 | 120 | 120 | 990 | 460 |
| Compressive Strength | Xc | MPa | 110 | 110 | 90 | 120 | 120 | 990 | 460 |
| In Plane Shear Strength | S | MPa | 260 | 210 | 100 | 310 | 150 | ||
| Thermal Expansion Co-ef | Alpha1 | Strain/K | 2.15 E-6 | 0.9 E-6 | 12 E-6 | 4.9 E-6 | 10 E-6 | 11 E-6 | 23 E-6 |
| Moisture Co-ef | Beta1 | Strain/K | 3.22 E-4 | 2.49 E-4 | 6.9 E-4 |
Although strength and stiffness ofsteel andaluminum alloys are comparable to fiber composites,specific strength andstiffness of composites (i.e. in relation to their weight) are significantly higher.
| Carbon Fiber Composite (aerospace grade) | Carbon Fiber Composite (commercial grade) | Fiberglass Composite | Aluminum 6061 T-6 | Steel, Mild | |
| Cost $/LB | $20 – $250+ | $5 – $20 | $1.50 – $3.00 | $3 | $0.30 |
| Strength (psi) | 90,000 – 200,000 | 50,000 – 90,000 | 20,000 – 35,000 | 35,000 | 60,000 |
| Stiffness (psi) | 10 x 106– 50 x 106 | 8 x 106 – 10 x 106 | 1 x 106 – 1.5 x 106 | 10 x 106 | 30 x 106 |
| Density (lb/in3) | 0.050 | 0.050 | 0.055 | 0.10 | 0.30 |
| Specific Strength | 1.8 x 106 – 4 x 106 | 1 x 106 – 1.8 x 106 | 363,640–636,360 | 350,000 | 200,000 |
| Specific Stiffness | 200 x 106 – 1,000 x 106 | 160 x 106 – 200 x 106 | 18 x 106 – 27 x 106 | 100 x 106 | 100 x 106 |
Shock, impact of varying speed, or repeated cyclic stresses can provoke the laminate to separate at the interface between two layers, a condition known asdelamination.[98][99] Individual fibres can separate from the matrix, for example,fibre pull-out.
Composites can fail on themacroscopic ormicroscopic scale. Compression failures can happen at both the macro scale or at each individual reinforcing fibre in compression buckling. Tension failures can be net section failures of the part or degradation of the composite at a microscopic scale where one or more of the layers in the composite fail in tension of the matrix or failure of the bond between the matrix and fibres.
Some composites are brittle and possess little reserve strength beyond the initial onset of failure while others may have large deformations and have reserve energy absorbing capacity past the onset of damage. The distinctions in fibres and matrices that are available and themixtures that can be made with blends leave a very broad range of properties that can be designed into a composite structure. The most famous failure of a brittle ceramic matrix composite occurred when the carbon-carbon composite tile on the leading edge of the wing of theSpace Shuttle Columbia fractured when impacted during take-off. It directed to the catastrophic break-up of the vehicle when it re-entered the Earth's atmosphere on 1 February 2003.
Composites have relatively poor bearing strength compared to metals.
Composites are tested before and after construction to assist in predicting and preventing failures. Pre-construction testing may adopt finite element analysis (FEA) for ply-by-ply analysis of curved surfaces and predicting wrinkling, crimping and dimpling of composites.[100][101][102][103] Materials may be tested during manufacturing and after construction by various non-destructive methods including ultrasonic, thermography, shearography and X-ray radiography,[104] and laser bond inspection for NDT of relative bond strength integrity in a localized area.
A particulate composite is characterized as being composed of particles suspended in a matrix. Particles can have virtually any shape, size or configuration. Examples of well-known particulate composites are concrete and particle board. There are two subclasses of particulates: flake and filled/skeletal
The term 'pultrusion' combines the word 'pull' and 'extrusion.' It is a continuous manufacturing process to produce products with constant cross sections such as profiles and sheets. Fig. 5.25 is a schematic illustration of general pultrusion setup. As shown in the figure, continuous fiber reinforcements are saturated (wet out) with desired resin matrix either in a resin bath or in resin injection chamber. The coated fibers then pass through heating and forming dies where curing of the resin and forming of the shape occur. After the die the composite is allowed to postcure while being pulled to the saw which cuts it into stock length. Different resin–fiber combinations are used to achieve the final desired properties
