What is Composite Material?

How Composite Materials Are Manufactured

Manufacturing methods of composite materials vary widely. The key materials that are most commonly described as composites are glass, carbon, and Kevlar® fibers, bonded with resins. In the case of carbon fiber, the strands are pre-impregnated with a heat-activated polyester, vinyl ester, polyurethane, or epoxy resin (prepreg). The flexible sheets of woven fabric (roving) are laid up onto/into a mold and put under pressure. They are then heated to activate the resin which first liquifies, wetting all fibers, then curing to a tough, rigid result.

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The pressure can be applied by tooling that closes and presses the fibers. Vacuum bagging can also apply pressure when a vacuum is applied to &#;pull&#; the shape into place. A third method employs an elastic pressure bladder that uses air pressure to &#;push&#; the shape into place. The end result, after the resin is cured, is a finished part with a shape that follows a mold faithfully, with little or no shrinkage, and a rigid, tough material that exploits the two components&#; best properties. Similar methods are used with glass and Kevlar® reinforcers. The completed structure is then cured at room temperature or at a slightly elevated level and then released from the mold as a finished part or product.

Polymer composites and nanocomposites are very similar in their nature. They require other processes than the more widely recognized composites of carbon fiber and GRP (glass-reinforced plastic). Whether the end result material is a nanocomposite or a macro composite depends only on the scale of the additive&#;macro strands or nanomaterials. In both cases, the manufacturing method is essentially identical. The reinforcer material is considered an additive and is premixed with the polymer when the pellets are first manufactured. Molded parts have evenly distributed reinforcers throughout.

Properties of Composite Materials

Properties of composites are as diverse as the range of materials that fall within this broad classification. Under ideal conditions, the resultant properties of the composite are:

1. High strength/weight ratio.
2. Impact resistance.
3. Chemical/environmental stability.

Chemical Properties of Composite Materials

Composites are often made with a rigid matrix of epoxy resin or a less rigid but still stiff matrix of a thermoplastic polymer. This component generally lends the below primary chemical properties to the resulting material:

1. Can be designed for very high chemical stability. The selection of the matrix material and selecting non-absorbent and non-hygroscopic reinforcement fibers guarantee environmental resilience.
2. Generally highly corrosion/environment resistant.
3. Electrical and thermal conductivity can be engineered to any level from high breakdown voltage insulator up to moderate conductivity, by using appropriate additives and reinforcer materials.
4. Achieving flame resistance or retardancy in composites is also a design property that can be highly controlled.

Physical Properties of Composite Materials

1. The tensile strength of most composites approaches or exceeds that of the reinforcing material. 
2. The compressive strength is similar to or exceeds the compressive strength of the matrix material. This is despite the fact that the overwhelming bulk of most composites is a reinforcer and not a matrix.
3. The bending strength generally greatly exceeds the tensile capacity of the matrix material, as the tensile loads of bending forces are transferred to the reinforcer component and dissipated without fracture.
4. Electrical or thermal conductivity.
5. Abrasion resistance.
6. Reduced weight/density.
7. Magnetic properties.
8. Optical properties.
9. Shock resilience.
10. Fatigue and creep resistance.

Types of Composite Materials

1. Nanocomposites

Nanocomposites are both man-made and naturally occurring. The reinforcer is generally a nanomaterial such as carbon nanotubes or graphene added to a polymer matrix, or silicon nanoparticles added to steel to induce fine crystal growth. In some applications, calcium carbonate or talc can also be effective in making polymers stiffer and stronger.

Typical nanocomposites use the nanomaterial additive to add strength, stiffness, and other properties such as electrical or thermal conductivity to the polymer matrix. Naturally occurring nanocomposite examples are bone and shell. Nanomaterials represent significant health risks in some cases, so manufacture of these materials can be challenging.

2. Metal Matrix Composites (MMCs)

MMCs use a metal matrix like aluminum or magnesium and a high-strength fiber reinforcer in particle or whisker form. Reinforcers are generally carbon fiber or silicon carbide particles. This develops unique properties that go beyond the basic metal component&#;s limits, including increased strength and stiffness, elevated temperature resistance before the onset of weakening, improved wear resistance, and reduced coefficient of thermal expansion. 

MMCs are used in aerospace industries and extreme automotive uses, delivering high strength and low weight. They are also used in electronics, medical devices, and sporting goods. The processing of MMCs is more challenging than most other classes of composites, as high temperatures and the difficulties of uniform reinforcer distribution are challenging.

3. Polymer Matrix Composites (PMCs)

PMCs are the most prevalent and easily understood forms of composite materials. This term encompasses the hand lay-up of carbon fiber and glass fiber fabrics and the manual, injected, or pre-impregnated epoxies and polyester resins that form the matrix. These materials offer various benefits including high stiffness and strength (compared with the part weight), great thermal, chemical, and mechanical resilience, and abrasion resistance. On the other hand, PMC requires highly skilled labor, resulting in higher costs, though these are often not excessive for applications that need a high-strength outcome.

PMCs are widely used in aerospace, automotive, marine, and sporting goods, benefitting from light weight, high strength, and stiffness. Production of PMCs involves assembly methods such as hand lay-up and filament winding, which can be slow. Precise control over the curing process is needed, to achieve ideal material properties.

4. Glass Fiber Reinforced Polymers (GFRPs)

GFRPs are a subset of polymer matrix composites, specific to epoxy and polyester bonded glass fiber materials. The glass fiber can be in chopped strands, lending a degree of anisotropic strength to structures by the mixed orientation of the fibers. The reinforcer can also comprise chopped strand roving (or fabric), making a more orderly process but less well suited to bulk components as fibers are all laid in one plane. Woven roving improves the quality of lay-up and can offer greater strength, at a price.

5. Hybrid Composites

Hybrid composites are those in which two or more different reinforcing fibers are integrated into the final material. This could be a combination of glass and carbon fiber in a lay-up&#;for enhanced impact resistance or cosmetic reasons. It is common to use titanium mesh or strands in the manufacture of racquets for ball sports, to improve tensile and bending performance. These materials can be challenging, as compatibility issues can affect the behavior of the material&#;for example, one fiber may bond better to the matrix than the other. Considerable testing is required to confirm the value or feasibility of the hybrid matrix. They have the same applications as PMCs, but the higher cost restricts their use.

6. Ceramic Matrix Composites (CMCs)

CMCs consist of a ceramic matrix and reinforcing fibers. A ceramic matrix provides extreme temperature and corrosion resistance and excellent wear properties. But ceramics are generally brittle when unreinforced. The addition of silicon carbide, alumina, or carbon fibers can counter the brittleness to make a more serviceable material.

CMCs are used to make gas turbine blades, specialist rocket/aerospace components, and heat exchangers. CMCs are very costly and they remain quite brittle, which limits their use. However, this is a field of intense research, and properties are improving.

7. Natural Fiber Composites (NFCs)

There is an increasing trend toward using natural fibers in composite manufacture, to reduce the environmental impact of materials use. Natural fibers such as jute, flax, cotton, and wood are used in a variety of ways. Automotive interior panels are commonly made from resin-bonded natural fibers which are compression molded to shape and then upholstered in plastics or leather for final surfacing. Wood fibers are added to polymers for FDM/FFF rapid prototyping filaments, to improve strength and produce a wood effect. Skateboard decks make extensive use of natural fiber reinforcement, generally in a polyester resin matrix.

8. Carbon Fiber Reinforced Polymers (CFRPs)

CFRPs are another subset of polymer matrix composites, specific to epoxy and polyester-bonded carbon fibers. For hand lay-up purposes, carbon fiber is generally used as woven roving, with a range of weave patterns used for various types of loading and stress distribution. The fibers are pre-impregnated with thermally activated resins, so the flexible cloth is laid-up and then compressed and baked to liquify and then cure the resin to create a rigid, tough result. Carbon fiber can also be pultruded with a range of polymers, to make continuous lengths of CFRP in complex sections. 

9. Aramid Fiber Reinforced Polymers (AFRPs)

AFRPs are another subset of polymer matrix composites that employ aramid as the reinforcer. Aramid fiber composites are used in the highest-impact applications. The aramid is generally used as woven fabrics that are pre-impregnated with appropriate epoxy and polyester resins, to be processed as per carbon/glass fiber. Another aramid reinforced composite is the paper/aramid honeycomb material used in low-profile flooring panels in aviation&#;layered with aluminum sheets and epoxy bonded, this is a typical high-value hybrid composite.

10. Functionally Graded Composites (FGCs)

FGCs are essentially a subset of any type of composite. These are composite materials in which the constituent parts can be modified in the application or type through the structure to tune performance. A gradual transition in properties is used to avoid stress concentrations at sudden changes. The functional grading can be as simple as adding or altering fiber content at elevated stress points; changes in weave pattern in roving to alter load distribution; or progressive hybridization for impact resilience in regions.

FGCs are used to make lighter and more resilient aircraft and spacecraft components, such as turbine blades and rocket nozzles. Biomedical devices/implants can have varied properties regionalized according to desired tissue interactions. 

Advantages and Disadvantages of Using Composite Materials in 3D Printing

Fiber and metal additives in 3D printing materials offer some potential advantages. These are listed below:

1. Deliver increased strength and stiffness, allowing more functional printed outcomes or reduced weight for the same strength.
2. Can be more durable than matrix material alone. This may enable parts to function at higher temperatures.
3. Some composite additives can add electrical or thermal conductivity or increase breakdown voltage.

Some 3D printing processes use a form of functional grading, by co-printing rigid and elastomeric materials in the same parts, allowing properties to be varied through a build. 

There are also challenges in using composites in some 3D printing processes, as listed below:

1. Additive-loaded materials can bring processing challenges. For example, they can be difficult to shape using established technologies. 
2. Few options are on the market as yet, so availability can be a challenge.
3. Highly functional composite print materials are more costly than their basic alternatives.

Industrial Applications of Composite Materials

Listed below are some industrial applications of composite materials:

1. Motorcycle fairings, kayaks, boat hulls, and aircraft skins.
2. Epoxy-bonded carbon fiber in fishing rods.
3. Plywood for construction.
4. Ferroconcrete for construction.
5. Glass-reinforced plastic for high-strength molding.
6. Aircraft flooring, sandwiching paper honeycomb between two aluminum sheets.
7. Spectacle frames (often mold plastic over a metal structure).

Xometry provides a wide range of manufacturing capabilities, including 3D printing, CNC machining, and injection molding for prototyping and production parts. Get your instant quote today to get your project started.

Copyright and Trademark Notices

1. Kevlar® is a trademark of E. I. DuPont de Nemours and Company

Disclaimer

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Material made from a combination of two or more unlike substances

For the specific carbon and glass fibre based composite materials often referred to loosely as 'composites', see Fibre-reinforced polymer

A black carbon fibre (used as a reinforcement component) compared to a human hairComposites are formed by combining materials together to form an overall structure with properties that differ from that of the individual components

A composite material (also called a composition material or shortened to composite, which is the common name) is a material 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 from mixtures and solid solutions. Composite materials with more than one distinct layer are called composite laminates.

Typical engineered composite materials include:

There are various reasons where new material can be favoured. Typical examples include materials which are less expensive, lighter, stronger or more durable when compared with common materials, as well as composite materials inspired from animals and natural sources with low carbon footprint.[3]

More recently researchers have also begun to actively include sensing, actuation, computation, and communication into composites,[4] which are known as robotic materials.[5]

Composite materials are generally used for buildings, bridges, and structures such as boat hulls, swimming pool panels, racing car bodies, shower stalls, bathtubs, storage tanks, imitation granite, and cultured marble sinks and countertops.[6][7] They are also being increasingly used in general automotive applications.[8]

The most advanced examples perform routinely on spacecraft and aircraft in demanding environments.

History

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The earliest composite materials were made from straw and mud combined to form bricks for building construction. Ancient brick-making was documented by Egyptian tomb paintings.[9]

Wattle and daub is one of the oldest composite materials, at over years old.[10] Concrete is also a composite material, and is used more than any other synthetic material in the world. As of , about 7.5 billion cubic metres of concrete are made each year[11]

Examples

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Composite materials

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Concrete is a mixture of adhesive and aggregate, giving a robust, strong material that is very widely used. Plywood is used widely in construction Composite sandwich structure panel used for testing at NASA

Concrete is the most common artificial composite material of all and typically consists of loose stones (aggregate) held with a matrix of cement. Concrete is an inexpensive material, and will not compress or shatter even under quite a large compressive force.[18] However, concrete cannot survive tensile loading[19] (i.e., if stretched it will quickly break apart). Therefore, to give concrete the ability to resist being stretched, steel bars, which can resist high stretching (tensile) forces, are often added to concrete to form reinforced concrete.[20]

Fibre-reinforced polymers include carbon-fiber-reinforced polymers and glass-reinforced plastic. If classified by matrix then there are thermoplastic composites, short fibre thermoplastics, long fibre thermoplastics or long-fiber-reinforced thermoplastics. There are numerous thermoset composites, including paper composite panels. Many advanced thermoset polymer matrix systems usually incorporate aramid fibre and carbon fibre in an epoxy 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 in metal matrix composites (MMC)[26] or ceramic matrix composites (CMC),[27] which includes bone (hydroxyapatite reinforced with collagen fibres), cermet (ceramic and metal), and concrete. Ceramic matrix composites are built primarily for fracture 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 include asphalt concrete, polymer concrete, mastic asphalt, mastic roller hybrid, dental composite, syntactic foam, and mother of pearl.[28] Chobham armour is a special type of composite 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 tennis racquet), 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-based carbon-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 any shape-memory polymers or shape-memory alloys e.g. balsa plies interleaved with hot glue,[31] aluminium plies interleaved with acrylic polymers or PVC[32] and carbon-fiber-reinforced polymer laminates interleaved with polystyrene.[33]

A sandwich-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 high bending stiffness with overall low density.[34][35]

Wood is a naturally occurring composite comprising cellulose fibres in a lignin and hemicellulose 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 or laminated paper or textiles, Arborite, Formica (plastic), and Micarta. Other engineered laminate composites, such as Mallite, use a central core of end grain balsa wood, bonded to surface skins of light alloy 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]

Products

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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 as aerospace components (tails, wings, fuselages, propellers), boat and scull hulls, bicycle frames, and racing car bodies. Other uses include fishing rods, storage tanks, swimming pool panels, and baseball bats. The Boeing 787 and Airbus A350 structures including the wings and fuselage are composed largely of composites.[41] Composite materials are also becoming more common in the realm of orthopedic surgery,[42] and it is the most common hockey stick material.

Carbon composite is a key material in today's launch vehicles and heat shields for the re-entry phase of spacecraft. 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 of launch vehicles. Furthermore, disk brake systems of airplanes and racing cars are using carbon/carbon material, and the composite material with carbon fibres and silicon carbide matrix has been introduced in luxury vehicles and sports cars.

In , 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 , an all-composite military Humvee was introduced by TPI Composites Inc and Armor Holdings Inc, the first all-composite military vehicle. By using composites the vehicle is lighter, allowing higher payloads.[43] In , carbon fibre and DuPont 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 , HMD Global unveiled the Nokia 6.2 and Nokia 7.2 which are claimed to be using polymer composite for the frames.[47]

Overview

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Carbon fibre composite part.

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 the matrix (binder) and the other reinforcement.[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 the mould 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 chemical polymerization for a thermoset 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 a polymer 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 as polyester, 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 consider wood to be a natural composite of cellulose fibres in a matrix of lignin.[49][50]

Cores in composites

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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 a sandwich structure. This is a more general layup for the production of cowlings, doors, radomes or non-structural parts.

Open- and closed-cell-structured foams like polyvinyl chloride, polyurethane, polyethylene, or polystyrene foams, balsa wood, syntactic foams, and honeycombs are generally utilized core materials. Open- and closed-cell metal 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]

Semi-crystalline polymers

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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.

Methods of fabrication

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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, including advanced fibre placement (automated fibre placement),[56] fibreglass spray lay-up process,[57] filament winding,[58] lanxide process,[59] tailored fibre placement,[60] tufting,[61] and z-pinning.[62]

Overview of mould

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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 a thermoset 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 are autoclave moulding,[63] vacuum bag moulding,[64] pressure bag moulding,[65] resin transfer moulding,[66] and light resin transfer moulding.[67]

Other fabrication methods

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Other types of fabrication include casting,[68] centrifugal casting,[69] braiding (onto a former), continuous casting,[70] filament winding,[71] press moulding,[72] transfer moulding, pultrusion moulding,[73] and slip forming.[74] There are also forming capabilities including CNC filament winding, vacuum infusion, wet lay-up, compression moulding, and thermoplastic moulding, to name a few. The practice of curing ovens and paint booths is also required for some projects.

Finishing methods

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The composite parts finishing is also crucial in the final design. Many of these finishes will involve rain-erosion coatings or polyurethane coatings.

Tooling

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The mould and mould inserts are referred to as "tooling". The mould/tooling can be built from different materials. Tooling materials include aluminium, carbon fibre, invar, nickel, reinforced silicone rubber and steel. The tooling material selection is normally based on, but not limited to, the coefficient 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.

Physical properties

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Plot of the overall strength of a composite material as a function of fiber volume fraction limited by the upper bound (isostrain) and lower bound (isostress) conditions.

Usually, the composite's physical properties are not isotropic (independent of the direction of applied force) in nature. But they are typically anisotropic (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.

Isostrain rule of mixtures

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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 the rule of mixtures:

Figure a) shows the isostress condition where the composite materials are perpendicular to the applied force and b) is the isostrain condition that has the layers parallel to the force.

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E C = &#; i = 1 V i E i {\displaystyle E_{C}=\sum _{i=1}V_{i}E_{i}}

where EC is the effective composite Young's modulus, and Vi and Ei 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: E C = V α E α + V β E β {\displaystyle E_{C}=V_{\alpha }E_{\alpha }+V_{\beta }E_{\beta }} where Vα and Vβ are the respective volume fractions of each phase. This can be derived by considering that in the isostrain case, &#; C = &#; α = &#; β = &#; {\displaystyle \epsilon _{C}=\epsilon _{\alpha }=\epsilon _{\beta }=\epsilon } Assuming that the composite has a uniform cross section, the stress on the composite is a weighted average between the two phases, σ C = σ α V α + σ β V β {\displaystyle \sigma _{C}=\sigma _{\alpha }V_{\alpha }+\sigma _{\beta }V_{\beta }} The stresses in the individual phases are given by Hooke's Law, σ β = E β &#; {\displaystyle \sigma _{\beta }=E_{\beta }\epsilon } σ α = E α &#; {\displaystyle \sigma _{\alpha }=E_{\alpha }\epsilon } Combining these equations gives that the overall stress in the composite is σ C = E α V α &#; + E β V β &#; = ( E α V α + E β V β ) &#; {\displaystyle \sigma _{C}=E_{\alpha }V_{\alpha }\epsilon +E_{\beta }V_{\beta }\epsilon =(E_{\alpha }V_{\alpha }+E_{\beta }V_{\beta })\epsilon } Then it can be shown that E C = ( E α V α + E β V β ) {\displaystyle E_{C}=(E_{\alpha }V_{\alpha }+E_{\beta }V_{\beta })}

Isostress rule of mixtures

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The lower bound is dictated by the isostress condition, in which the fibres and matrix are oriented perpendicularly to the loading direction: σ C = σ α = σ β = σ {\displaystyle \sigma _{C}=\sigma _{\alpha }=\sigma _{\beta }=\sigma } and now the strains become a weighted average &#; C = &#; α V α + &#; β V β {\displaystyle \epsilon _{C}=\epsilon _{\alpha }V_{\alpha }+\epsilon _{\beta }V_{\beta }} Rewriting Hooke's Law for the individual phases &#; β = σ E β {\displaystyle \epsilon _{\beta }={\frac {\sigma }{E_{\beta }}}} &#; α = σ E α {\displaystyle \epsilon _{\alpha }={\frac {\sigma }{E_{\alpha }}}} This leads to &#; c = V β σ &#; β + V α σ &#; α = ( V α &#; α + V β &#; β ) σ {\displaystyle \epsilon _{c}=V_{\beta }{\frac {\sigma }{\epsilon _{\beta }}}+V_{\alpha }{\frac {\sigma }{\epsilon _{\alpha }}}=({\frac {V_{\alpha }}{\epsilon _{\alpha }}}+{\frac {V_{\beta }}{\epsilon _{\beta }}})\sigma } From the definition of Hooke's Law 1 E C = V α E α + V β E β {\displaystyle {\frac {1}{E_{C}}}={\frac {V_{\alpha }}{E_{\alpha }}}+{\frac {V_{\beta }}{E_{\beta }}}} and, in general,

1 E C = &#; i = 1 V i E i {\displaystyle {\frac {1}{E_{C}}}=\sum _{i=1}{\frac {V_{i}}{E_{i}}}}

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: E C = ( E α E β ) / ( V α E β + V β E α ) {\displaystyle E_{C}=(E_{\alpha }E_{\beta })/(V_{\alpha }E_{\beta }+V_{\beta }E_{\alpha })} 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]

( X c ) n = V m ( X m ) n + V r ( X r ) n {\displaystyle (X_{c})^{n}=V_{m}(X_{m})^{n}+V_{r}(X_{r})^{n}}

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:

( X c ) n = &#; i = 1 m V i ( X i ) n {\displaystyle (X_{c})^{n}=\sum _{i=1}^{m}V_{i}(X_{i})^{n}}

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 of σparallel /cos2 θ where σparallel 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 since shear 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 of τmy /sin θ cos θ where τmy 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 of σperp /sin2θ where σperp is the tensile strength of the composite with fibres align perpendicular to the applied force.[77]

The graph depicts the three fracture modes a composite material may experience depending on the angle of misorientation relative to aligning fibres parallel to the applied stress.

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, the shear modulus, and the Poisson'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 involve mortise and tenon joints (in natural composites such as wood) and pi joints in synthetic composites.

Mechanical properties of composites

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Particle reinforcement

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In general, particle reinforcement is strengthening the composites less than fiber reinforcement. It is used to enhance the stiffness of the composites while increasing the strength and the toughness. Because of their mechanical properties, they are used in applications in which wear resistance is required. For example, hardness of cement 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]

The elastic modulus of particle-reinforced composites can be expressed as,

E c = V m E m + K c V p E p {\displaystyle E_{c}=V_{m}E_{m}+K_{c}V_{p}E_{p}}

where E is the elastic modulus, V is the volume fraction. The subscripts c, p and m are indicating composite, particle and matrix, respectively. K c {\displaystyle K_{c}} is a constant can be found empirically.

For more copper composite materialinformation, please contact us. We will provide professional answers.

Similarly, tensile strength of particle-reinforced composites can be expressed as,

( T . S . ) c = V m ( T . S . ) m + K s V p ( T . S . ) p {\displaystyle (T.S.)_{c}=V_{m}(T.S.)_{m}+K_{s}V_{p}(T.S.)_{p}}

where T.S. is the tensile strength, and K s {\displaystyle K_{s}} is a constant (not equal to K c {\displaystyle K_{c}} ) that can be found empirically.

Continuous fiber reinforcement

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In general, continuous fiber reinforcement is implemented by incorporating a fiber 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 the covalent nature of their bonds. The most famous example of this is carbon fibers that have many applications extending from sports gear to protective equipment to space industries.[82][83]

The stress on the composite can be expressed in terms of the volume fraction of the fiber and the matrix.

σ c = V f σ f + V m σ m {\displaystyle \sigma _{c}=V_{f}\sigma _{f}+V_{m}\sigma _{m}}

where σ {\displaystyle \sigma } is the stress, V is the volume fraction. The subscripts c, f and m are indicating composite, fiber and matrix, respectively.

Although the stress&#;strain behavior of fiber composites can only be determined by testing, there is an expected trend, three stages of the stress&#;strain curve. The first stage is the region of the stress&#;strain curve where both fiber and the matrix are elastically deformed. This linearly elastic region can be expressed in the following form.[82]

σ c &#; E c &#; c = &#; c ( V f E f + V m E m ) {\displaystyle \sigma _{c}-E_{c}\epsilon _{c}=\epsilon _{c}(V_{f}E_{f}+V_{m}E_{m})}

where σ {\displaystyle \sigma } is the stress, &#; {\displaystyle \epsilon } is the strain, E is the elastic modulus, and V is the volume 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 instantaneous modulus can be determined using the slope of the stress&#;strain curve in the second region. The relationship between stress and strain can be expressed as,

σ c = V f E f &#; c + V m σ m ( &#; c ) {\displaystyle \sigma _{c}=V_{f}E_{f}\epsilon _{c}+V_{m}\sigma _{m}(\epsilon _{c})}

where σ {\displaystyle \sigma } is the stress, &#; {\displaystyle \epsilon } is the strain, E is the elastic modulus, and V is the volume 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 the slope of the curve is equal to the modulus.

E c &#; = d σ c d &#; c = V f E f + V m ( d σ c d &#; c ) {\displaystyle E_{c}'={\frac {d\sigma _{c}}{d\epsilon _{c}}}=V_{f}E_{f}+V_{m}\left({\frac {d\sigma _{c}}{d\epsilon _{c}}}\right)}

In most cases it can be assumed E c &#; = V f E f {\displaystyle E_{c}'=V_{f}E_{f}} since the second term is much less than the first one.[82]

In reality, the derivative of stress with respect to strain is not always returning the modulus because of the binding interaction between the fiber and matrix. The strength of the interaction between these two phases can result in changes in the mechanical properties of the composite. The compatibility of the fiber and matrix is a measure of internal stress.[82]

The covalently bonded high strength fibers (e.g. carbon fibers) experience mostly elastic deformation before the fracture since the plastic deformation can happen due to dislocation 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 have many applications to work at cryogenic temperatures that is one of the advantages of composites with metal fibers over nonmetallic. The stress in this region of the stress&#;strain curve can be expressed as,

σ c ( &#; c ) = V f σ f &#; c + V m σ m ( &#; c ) {\displaystyle \sigma _{c}(\epsilon _{c})=V_{f}\sigma _{f}\epsilon _{c}+V_{m}\sigma _{m}(\epsilon _{c})}

where σ {\displaystyle \sigma } is the stress, &#; {\displaystyle \epsilon } is the strain, E is the elastic modulus, and V is the volume fraction. The subscripts c, f, and m are indicating composite, fiber, and matrix, respectively. σ f ( &#; c ) {\displaystyle \sigma _{f}(\epsilon _{c})} and σ m ( &#; c ) {\displaystyle \sigma _{m}(\epsilon _{c})} are for fiber and matrix flow stresses respectively. Just after the third region the composite exhibit necking. 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, the tensile strength of the composite can be expressed in terms of the volume fraction.[82]

( T . S . ) c = V f ( T . S . ) f + V m σ m ( &#; m ) {\displaystyle (T.S.)_{c}=V_{f}(T.S.)_{f}+V_{m}\sigma _{m}(\epsilon _{m})}

where T.S. is the tensile strength, σ {\displaystyle \sigma } is the stress, &#; {\displaystyle \epsilon } is the strain, E is the elastic modulus, and V is the volume fraction. The subscripts c, f, and m are indicating composite, fiber, and matrix, respectively. The composite tensile strength can be expressed as

( T . S . ) c = V m ( T . S . ) m {\displaystyle (T.S.)_{c}=V_{m}(T.S.)_{m}}

V f {\displaystyle V_{f}}

V c {\displaystyle V_{c}}

( T . S . ) c = V f ( T . S . ) f + V m ( σ m ) {\displaystyle (T.S.)_{c}=V_{f}(T.S.)_{f}+V_{m}(\sigma _{m})}

V f {\displaystyle V_{f}}

V c {\displaystyle V_{c}}

The critical value of volume fraction can be expressed as,

V c = [ ( T . S . ) m &#; σ m ( &#; f ) ] [ ( T . S . ) f + ( T . S . ) m &#; σ m ( &#; f ) ] {\displaystyle V_{c}={\frac {[(T.S.)_{m}-\sigma _{m}(\epsilon _{f})]}{[(T.S.)_{f}+(T.S.)_{m}-\sigma _{m}(\epsilon _{f})]}}}

Evidently, the composite tensile strength can be higher than the matrix if ( T . S . ) c {\displaystyle (T.S.)_{c}} is greater than ( T . S . ) m {\displaystyle (T.S.)_{m}} .

Thus, the minimum volume fraction of the fiber can be expressed as,

V c = [ ( T . S . ) m &#; σ m ( &#; f ) ] [ ( T . S . ) f &#; σ m ( &#; f ) ] {\displaystyle V_{c}={\frac {[(T.S.)_{m}-\sigma _{m}(\epsilon _{f})]}{[(T.S.)_{f}-\sigma _{m}(\epsilon _{f})]}}}

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]

The effect of fiber orientation

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Aligned fibers

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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, θ {\displaystyle \theta } , can be used predict the dominant tensile fracture mechanism.

At small angles, θ &#; 0 &#; {\displaystyle \theta \approx 0^{\circ }} , 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 cos &#; θ {\displaystyle \cos \theta } from rotation. F res = F cos &#; θ {\displaystyle F_{\mbox{res}}=F\cos \theta } . The resolved area on which the fiber experiences the force is increased by a factor of cos &#; θ {\displaystyle \cos \theta } from rotation. A res = A 0 / cos &#; θ {\displaystyle A_{\mbox{res}}=A_{0}/\cos \theta } . Taking the effective tensile strength to be ( T.S. ) c = F res / A res {\displaystyle ({\mbox{T.S.}})_{\mbox{c}}=F_{\mbox{res}}/A_{\mbox{res}}} and the aligned tensile strength σ &#; &#; = F / A {\displaystyle \sigma _{\parallel }^{*}=F/A} .[82]

( T.S. ) c ( longitudinal fracture ) = σ &#; &#; cos 2 &#; θ {\displaystyle ({\mbox{T.S.}})_{\mbox{c}}\;({\mbox{longitudinal fracture}})={\frac {\sigma _{\parallel }^{*}}{\cos ^{2}\theta }}}

At moderate angles, θ &#; 45 &#; {\displaystyle \theta \approx 45^{\circ }} , the material experiences shear failure. The effective force direction is reduced with respect to the aligned direction. F res = F cos &#; θ {\displaystyle F_{\mbox{res}}=F\cos \theta } . The resolved area on which the force acts is A res = A m / sin &#; θ {\displaystyle A_{\mbox{res}}=A_{m}/\sin \theta } . The resulting tensile strength depends on the shear strength of the matrix, τ m {\displaystyle \tau _{m}} .[82]

( T.S. ) c ( shear failure ) = τ m sin &#; θ cos &#; θ {\displaystyle ({\mbox{T.S.}})_{\mbox{c}}\;({\mbox{shear failure}})={\frac {\tau _{m}}{\sin {\theta }\cos {\theta }}}}

At extreme angles, θ &#; 90 &#; {\displaystyle \theta \approx 90^{\circ }} , the dominant mode of failure is tensile fracture in the matrix in the perpendicular direction. As in the isostress 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 sin &#; θ {\displaystyle \sin \theta } . The resolved tensile strength is proportional to the transverse strength, σ &#; &#; {\displaystyle \sigma _{\perp }^{*}} .[82]

( T.S. ) c ( transverse fracture ) = σ &#; &#; sin 2 &#; θ {\displaystyle ({\mbox{T.S.}})_{\mbox{c}}\;({\mbox{transverse fracture}})={\frac {\sigma _{\perp }^{*}}{\sin ^{2}\theta }}}

The critical angles from which the dominant fracture mechanism changes can be calculated as,

θ c 1 = tan &#; 1 &#; ( τ m σ &#; &#; ) {\displaystyle \theta _{c_{1}}=\tan ^{-1}\left({\frac {\tau _{m}}{\sigma _{\parallel }^{*}}}\right)}

θ c 2 = tan &#; 1 &#; ( σ &#; &#; τ m ) {\displaystyle \theta _{c_{2}}=\tan ^{-1}\left({\frac {\sigma _{\perp }^{*}}{\tau _{m}}}\right)}

where θ c 1 {\displaystyle \theta _{c_{1}}} is the critical angle between longitudinal fracture and shear failure, and θ c 2 {\displaystyle \theta _{c_{2}}} 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 the local maxima predicted at the critical angles.[84][85][86][87] The Tsai-Hill criterion provides a more complete description of fiber composite tensile strength as a function of orientation angle by coupling the contributing yield stresses: σ &#; &#; {\displaystyle \sigma _{\parallel }^{*}} , σ &#; &#; {\displaystyle \sigma _{\perp }^{*}} , and τ m {\displaystyle \tau _{m}} .[88][82]

( T.S. ) c ( Tsai-Hill ) = [ cos 4 &#; θ ( σ &#; &#; ) 2 + cos 2 &#; θ sin 2 &#; θ ( 1 ( τ m ) 2 &#; 1 ( σ &#; &#; ) 2 ) + sin 4 &#; θ ( σ &#; &#; ) 2 ] &#; 1 / 2 {\displaystyle ({\mbox{T.S.}})_{\mbox{c}}\;({\mbox{Tsai-Hill}})={\bigg [}{\frac {\cos ^{4}\theta }{({\sigma _{\parallel }^{*}})^{2}}}+\cos ^{2}\theta \sin ^{2}\theta \left({\frac {1}{({\tau _{m}})^{2}}}-{\frac {1}{({\sigma _{\parallel }^{*}})^{2}}}\right)+{\frac {\sin ^{4}\theta }{({\sigma _{\perp }^{*}})^{2}}}{\bigg ]}^{-1/2}}

Randomly oriented fibers

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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.

E c = K V f E f + V m E m {\displaystyle E_{c}=KV_{f}E_{f}+V_{m}E_{m}}

Where K is an empirically determined reinforcement factor; similar to the particle reinforcement equation. For fibers with randomly distributed orientations in a plane, K &#; 0.38 {\displaystyle K\approx 0.38} , and for a random distribution in 3D, K &#; 0.20 {\displaystyle K\approx 0.20} .[82]

Stiffness and Compliance Elasticity

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For real application, most composite is anisotropic material or orthotropic material. The three-dimension stress tensor is required for stress and strain analysis. The stiffness and compliance can be written as follows[89]

[ σ 1 σ 2 σ 3 σ 4 σ 5 σ 6 ] = [ C 11 C 12 C 13 C 14 C 15 C 16 C 12 C 22 C 23 C 24 C 25 C 26 C 13 C 23 C 33 C 34 C 35 C 36 C 14 C 24 C 34 C 44 C 45 C 46 C 15 C 25 C 35 C 45 C 55 C 56 C 16 C 26 C 36 C 46 C 56 C 66 ] [ ε 1 ε 2 ε 3 ε 4 ε 5 ε 6 ] {\displaystyle {\begin{bmatrix}\sigma _{1}\\\sigma _{2}\\\sigma _{3}\\\sigma _{4}\\\sigma _{5}\\\sigma _{6}\end{bmatrix}}={\begin{bmatrix}C_{11}&C_{12}&C_{13}&C_{14}&C_{15}&C_{16}\\C_{12}&C_{22}&C_{23}&C_{24}&C_{25}&C_{26}\\C_{13}&C_{23}&C_{33}&C_{34}&C_{35}&C_{36}\\C_{14}&C_{24}&C_{34}&C_{44}&C_{45}&C_{46}\\C_{15}&C_{25}&C_{35}&C_{45}&C_{55}&C_{56}\\C_{16}&C_{26}&C_{36}&C_{46}&C_{56}&C_{66}\end{bmatrix}}{\begin{bmatrix}\varepsilon _{1}\\\varepsilon _{2}\\\varepsilon _{3}\\\varepsilon _{4}\\\varepsilon _{5}\\\varepsilon _{6}\end{bmatrix}}} and [ ε 1 ε 2 ε 3 ε 4 ε 5 ε 6 ] = [ S 11 S 12 S 13 S 14 S 15 S 16 S 12 S 22 S 23 S 24 S 25 S 26 S 13 S 23 S 33 S 34 S 35 S 36 S 14 S 24 S 34 S 44 S 45 S 46 S 15 S 25 S 35 S 45 S 55 S 56 S 16 S 26 S 36 S 46 S 56 S 66 ] [ σ 1 σ 2 σ 3 σ 4 σ 5 σ 6 ] {\displaystyle {\begin{bmatrix}\varepsilon _{1}\\\varepsilon _{2}\\\varepsilon _{3}\\\varepsilon _{4}\\\varepsilon _{5}\\\varepsilon _{6}\end{bmatrix}}={\begin{bmatrix}S_{11}&S_{12}&S_{13}&S_{14}&S_{15}&S_{16}\\S_{12}&S_{22}&S_{23}&S_{24}&S_{25}&S_{26}\\S_{13}&S_{23}&S_{33}&S_{34}&S_{35}&S_{36}\\S_{14}&S_{24}&S_{34}&S_{44}&S_{45}&S_{46}\\S_{15}&S_{25}&S_{35}&S_{45}&S_{55}&S_{56}\\S_{16}&S_{26}&S_{36}&S_{46}&S_{56}&S_{66}\end{bmatrix}}{\begin{bmatrix}\sigma _{1}\\\sigma _{2}\\\sigma _{3}\\\sigma _{4}\\\sigma _{5}\\\sigma _{6}\end{bmatrix}}}

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 σ 3 = σ 4 = σ 5 = 0 {\displaystyle \sigma _{3}=\sigma _{4}=\sigma _{5}=0} and ε 4 = ε 5 = 0 {\displaystyle \varepsilon _{4}=\varepsilon _{5}=0} .[90]

[ ε 1 ε 2 ε 3 ε 4 ε 5 ε 6 ] = [ 1 E 1 &#; ν 21 E 2 &#; ν 31 E 3 0 0 0 &#; ν 12 E 1 1 E 2 &#; ν 32 E 3 0 0 0 &#; ν 13 E 1 &#; ν 23 E 2 1 E 3 0 0 0 0 0 0 1 G 23 0 0 0 0 0 0 1 G 31 0 0 0 0 0 0 1 G 12 ] [ σ 1 σ 2 σ 3 σ 4 σ 5 σ 6 ] {\displaystyle {\begin{bmatrix}\varepsilon _{1}\\\varepsilon _{2}\\\varepsilon _{3}\\\varepsilon _{4}\\\varepsilon _{5}\\\varepsilon _{6}\end{bmatrix}}={\begin{bmatrix}{\tfrac {1}{E_{\rm {1}}}}&-{\tfrac {\nu _{\rm {21}}}{E_{\rm {2}}}}&-{\tfrac {\nu _{\rm {31}}}{E_{\rm {3}}}}&0&0&0\\-{\tfrac {\nu _{\rm {12}}}{E_{\rm {1}}}}&{\tfrac {1}{E_{\rm {2}}}}&-{\tfrac {\nu _{\rm {32}}}{E_{\rm {3}}}}&0&0&0\\-{\tfrac {\nu _{\rm {13}}}{E_{\rm {1}}}}&-{\tfrac {\nu _{\rm {23}}}{E_{\rm {2}}}}&{\tfrac {1}{E_{\rm {3}}}}&0&0&0\\0&0&0&{\tfrac {1}{G_{\rm {23}}}}&0&0\\0&0&0&0&{\tfrac {1}{G_{\rm {31}}}}&0\\0&0&0&0&0&{\tfrac {1}{G_{\rm {12}}}}\\\end{bmatrix}}{\begin{bmatrix}\sigma _{1}\\\sigma _{2}\\\sigma _{3}\\\sigma _{4}\\\sigma _{5}\\\sigma _{6}\end{bmatrix}}}

The stiffness matrix and compliance matrix can be reduced to

[ σ 1 σ 2 σ 6 ] = [ E 1 1 &#; ν 12 ν 21 E 2 ν 12 1 &#; ν 12 ν 21 0 E 2 ν 12 1 &#; ν 12 ν 21 E 2 1 &#; ν 12 ν 21 0 0 0 G 12 ] [ ε 1 ε 2 ε 6 ] {\displaystyle {\begin{bmatrix}\sigma _{1}\\\sigma _{2}\\\sigma _{6}\end{bmatrix}}={\begin{bmatrix}{\tfrac {E_{\rm {1}}}{1-{\nu _{\rm {12}}}{\nu _{\rm {21}}}}}&{\tfrac {E_{\rm {2}}{\nu _{\rm {12}}}}{1-{\nu _{\rm {12}}}{\nu _{\rm {21}}}}}&0\\{\tfrac {E_{\rm {2}}{\nu _{\rm {12}}}}{1-{\nu _{\rm {12}}}{\nu _{\rm {21}}}}}&{\tfrac {E_{\rm {2}}}{1-{\nu _{\rm {12}}}{\nu _{\rm {21}}}}}&0\\0&0&G_{\rm {12}}\\\end{bmatrix}}{\begin{bmatrix}\varepsilon _{1}\\\varepsilon _{2}\\\varepsilon _{6}\end{bmatrix}}} and [ ε 1 ε 2 ε 6 ] = [ 1 E 1 &#; ν 21 E 2 0 &#; ν 12 E 1 1 E 2 0 0 0 1 G 12 ] [ σ 1 σ 2 σ 6 ] {\displaystyle {\begin{bmatrix}\varepsilon _{1}\\\varepsilon _{2}\\\varepsilon _{6}\end{bmatrix}}={\begin{bmatrix}{\tfrac {1}{E_{\rm {1}}}}&-{\tfrac {\nu _{\rm {21}}}{E_{\rm {2}}}}&0\\-{\tfrac {\nu _{\rm {12}}}{E_{\rm {1}}}}&{\tfrac {1}{E_{\rm {2}}}}&0\\0&0&{\tfrac {1}{G_{\rm {12}}}}\\\end{bmatrix}}{\begin{bmatrix}\sigma _{1}\\\sigma _{2}\\\sigma _{6}\end{bmatrix}}}

Two different coordinate systems of material. The structure has a (1-2) coordinate system. The material has a (x-y) principal coordinate system.

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. The transformation matrix with θ {\displaystyle \theta } degree rotation is [90]

T ( θ ) &#; = [ cos 2 &#; θ sin 2 &#; θ cos &#; θ sin &#; θ s i n 2 θ cos 2 &#; θ &#; cos &#; θ sin &#; θ &#; 2 cos &#; θ sin &#; θ 2 cos &#; θ sin &#; θ cos 2 &#; θ &#; sin 2 &#; θ ] {\displaystyle T(\theta )_{\epsilon }={\begin{bmatrix}\cos ^{2}\theta &\sin ^{2}\theta &\cos \theta \sin \theta \\sin^{2}\theta &\cos ^{2}\theta &-\cos \theta \sin \theta \\-2\cos \theta \sin \theta &2\cos \theta \sin \theta &\cos ^{2}\theta -\sin ^{2}\theta \end{bmatrix}}} for [ &#; ´ ] = T ( θ ) &#; [ &#; ] {\displaystyle {\begin{bmatrix}{\acute {\epsilon }}\end{bmatrix}}=T(\theta )_{\epsilon }{\begin{bmatrix}\epsilon \end{bmatrix}}} T ( θ ) σ = [ cos 2 &#; θ sin 2 &#; θ 2 cos &#; θ sin &#; θ s i n 2 θ cos 2 &#; θ &#; 2 cos &#; θ sin &#; θ &#; cos &#; θ sin &#; θ cos &#; θ sin &#; θ cos 2 &#; θ &#; sin 2 &#; θ ] {\displaystyle T(\theta )_{\sigma }={\begin{bmatrix}\cos ^{2}\theta &\sin ^{2}\theta &2\cos \theta \sin \theta \\sin^{2}\theta &\cos ^{2}\theta &-2\cos \theta \sin \theta \\-\cos \theta \sin \theta &\cos \theta \sin \theta &\cos ^{2}\theta -\sin ^{2}\theta \end{bmatrix}}} for [ σ ´ ] = T ( θ ) σ [ σ ] {\displaystyle {\begin{bmatrix}{\acute {\sigma }}\end{bmatrix}}=T(\theta )_{\sigma }{\begin{bmatrix}\sigma \end{bmatrix}}}

Types of fibers and their mechanical properties

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The most common types of fibers used in industry are glass fibers, carbon fibers, and kevlar 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 S97 steel.[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 standard mechanical 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]

Mechanical properties of fiber composite materials

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Fibres @ 0° (UD), 0/90° (fabric) to loading axis, Dry, Room Temperature, Vf = 60% (UD), 50% (fabric) Fibre / Epoxy Resin (cured at 120 °C)

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Symbol Units Standard

Carbon Fiber

Fabric

High Modulus

Carbon Fiber

Fabric

E-Glass

Fibre Glass Fabric

Kevlar

Fabric

Standard

Unidirectional

Carbon Fiber

Fabric

High Modulus

Unidirectional

Carbon Fiber

Fabric

E-Glass

Unidirectional

Fiber Glass Fabric

Kevlar

Unidirectional Fabric

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 600 350 440 480 990 Ult. Comp. Strength 0° Xc MPa 570 150 425 190 850 600 280 &#; 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 &#;

Fibres @ ±45 Deg. to loading axis, Dry, Room Temperature, Vf = 60% (UD), 50% (fabric)

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Symbol Units Standard

Carbon Fiber

High Modulus

Carbon Fiber

E-Glass

Fiber Glass

Standard

Carbon Fibers

Fabric

E-Glass

Fiber Glass Fabric

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

Mechanical properties of aerospace grade & commercial grade carbon fiber composites, fiberglass composite, and aluminum alloy and steel

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This table is demonstrating one of the most important features and advantage of fiber composites over metal, that is specific strength and specific stiffness. Although the steel and the aluminum alloy have comparable strength and stiffness with fiber composites, the specific strength and stiffness of composites are around higher than steel and the aluminum alloy.

Comparison of Cost, Specific Strength, and Specific Stiffness

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Carbon Fiber Composite (aerospace grade) Carbon Fiber Composite (commercial grade) Fiberglass Composite Aluminum 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

10 6 {\displaystyle 10^{6}}

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

Failure

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Shock, impact of varying speed, or repeated cyclic stresses can provoke the laminate to separate at the interface between two layers, a condition known as delamination.[98][99] Individual fibres can separate from the matrix, for example, fibre pull-out.

Composites can fail on the macroscopic or microscopic 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 the mixtures 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 the Space 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 .

Composites have relatively poor bearing strength compared to metals.

Testing

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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.

See also

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References

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Further reading

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• Robert M. Jones (). Mechanics of Composite Materials (2nd ed.). Taylor & Francis. ISBN .

• Cederbaum G, Elishakoff I, Aboudi J, Librescu L (). Random Vibration and Reliability of Composite Structures. Technomic. Bibcode:tech.book.....C.

• Librescu L., Song O. (). Thin-Walled Composite Beams: Theory and Application. Springer.

• Polymers and Polymeric Composites: A Reference Series. Springer. . Archived from the original on -03-21 .

• Autar K. Kaw (). Mechanics of Composite Materials (2nd ed.). CRC. ISBN 978-0---1.

• Handbook of Polymer Composites for Engineers By Leonard Hollaway Published Woodhead Publishing
• Madbouly, Samy, Chaoqun Zhang, and Michael R. Kessler. Bio-Based Plant Oil Polymers and Composites. William Andrew, .
• Matthews, F.L.; Rawlings, R.D. (). Composite Materials: Engineering and Science. Boca Raton: CRC Press. ISBN 978-0---1.

• Andreas Haka (). Engineered Stability. The History of Composite Materials in the 19th and 20th Centuries. Cham: Springer. ISBN 978-3-658--5.

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