COMPOSITES & MECHANISM

Presented By

MANISH AGRWAL

07/ME/82

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What Is A Composite?

In general it is a structural material made of two or more different materials.

• Many composite materials are composed of just two phases– one termed the matrix– the other the dispersed or reinforcing phase

• The properties of the composite are a function of– the properties of the constituents– their relative amounts– and geometry of the reinforcing phase

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The physical behavior of composite materials is quite different from that of most common engineering materials that are homogeneous and isotropic. Metals will generally have similar composition regardless of where or in what orientation a sample is taken. On the other hand, the makeup and physical properties of composites will vary with location and orientation of the principal axes. These materials are termed anisotropic, which means they exhibit different properties when tested in different directions. Some composite structures are, however, quasi-orthotropic, in their primary plane.

The bottom illustration in figure is representative of a poor fibre/matrix bond or void within the laminate. The resulting imbalance of stresses between the fiber and matrix can lead to local instability, causing the fiber to shift or buckle

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Isotropic and Anisotropic Materials

Isotropic: Only two independent material constants in the constitutive equation. No normal stress and shear strain coupling, or no shear stress and normal strain

coupling All directions are equivalent

Anisotropic:

Very general term. Simply means that the material is “not isotropic”. May be only slightly anisotropic,or very.

Orthotropic:

3 Orthogonal planes of material symmetry. (Orthogonal means at right angles, like orthographic projection). Intersections of these planes of symmetry are the principal axes of the material.

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Classification Matrix

phase/Reinforcement Phase

Metal Ceramic Polymer

Metal Powder metallurgy parts – combining immiscible metals

Cermets (ceramic-metal composite)

Brake pads

Ceramic Cermets, TiC, TiCN Cemented carbides –

used in tools Fiber-reinforced metals

SiC reinforced Al2O3 Tool materials

Fibreglass

Polymer     Kevlar fibers in an epoxy matrix

Elemental (Carbon, Boron, etc.)

Fibre reinforced metals Auto parts aerospace

  Rubber with carbon (tires)

Boron, Carbon reinforced plastics

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Types of Composite Materials

There are five basic types of composite materials: Fibre, particle, flake, laminar or layered and filled composites.

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Particle Composites Particles usually reinforce a composite equally in all directions (called isotropic).

Plastics, cermets and metals are examples of particles. Particles used to strengthen a matrix do not do so in the same way as fibers. For

one thing, particles are not directional like fibers. Spread at random through out a matrix, particles tend to reinforce in all directions equally.

Particles as the reinforcement

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Flake CompositesFlakes, because of their shape, usually reinforce in 2-D. Two common flake materials are glass and mica. (Also aluminum is used as metal flakes)

A flake composite consists of thin, flat flakes held together by a binder or placed in a matrix. Almost all flake composite matrixes are plastic resins. The most important flake materials are:

1. Aluminum2. Mica3. Glass

Flat flakes as the reinforcement

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Laminar Composites Laminar composites involve two or more layers of the same or different materials.

The layers can be arranged in different directions to give strength where needed. Speedboat hulls are among the very many products of this kind.

In laminar composites outer metal is not called a matrix but a face. The inner metal, even if stronger, is not called a reinforcement. It is called a base.

We can divide laminar composites into three basic types Unreinforced–layer composites

1. All–Metal

2. Metal–Nonmetal

3. Nonmetal Reinforced–layer composites Combined composites

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A lamina (laminae) is any arrangement of unidirectional or woven fibers in a matrix. Usually this arrangement is flat, although it may be curved, as in a shell.

A laminate is a stack of lamina arranged with their main reinforcement in at least two different directions.

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Filled Composites

There are two types of filled composites. In one, filler materials are added to a normal composite result in strengthening the

composite and reducing weight. The second type of filled composite consists of a skeletal 3-D matrix holding a

second material. The most widely used composites of this kind are sandwich structures and honeycombs.

Fillers as the reinforcement

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Combined Composites

It is possible to combine several different materials into a single composite. It is also possible to combine several different composites into a single product. A good example is a modern ski. (combination of wood as natural fiber, and layers as laminar composites)

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The Structure of CompositesConsist of two distinct phases

– matrix and a reinforcing phase

Matrices can be

– metals (Al, Ti)

– ceramics (Al2O3, ZrO2)

– polymers (epoxy, polyester, phenolic)

Reinforcing phases can have different shapes

– fibres, whiskers, particulates

Technologically, fibre composites most important

– glass, carbon, Spectra (PE), Kevlar (aramid)

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Fibre Composites In fibre composites, the fibres reinforce along the line of their length. Reinforcement

may be mainly 1-D, 2-D or 3-D. Figure shows the three basic types of fibre orientation.

1-D gives maximum strength in one direction.

2-D gives strength in two directions.

Isotropic gives strength equally in all directions.

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Fibre Composites Usually combinations of ceramic, polymer or glass fibres in a polymer matrix Typically 40-60 % fibre by volume Utilize the very good properties of the fibres Fibre composites have a good combination of stiffness, density and fracture

toughness However they are often expensive They may be difficult to process Often difficult to detect damage (cracks)

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Properties of Fibres

High strength of materials can be achieved due to low probability of flaws in individual fibres

Polymers may be oriented into fibres (Spectra or Kevlar) to utilize the strong C – C bonds of polymer backbone

For carbon fibres, graphite plate structure can be oriented to take advantage of strong bonding

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Properties of Matrix

The matrix binds the fibres together and protects them from external damage It transmits external loads to the fibres

– the matrix itself usually carries only a small fraction of the load It separates the fibres and stops cracks from propagating directly from fibre to fibre It supports the fibres laterally under compression loading It usually has a low density to produce a composite with a low density It is advantageous if the matrix has some ductility

– reinforcing phase often very stiff

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Deformation of Aligned Long Fibre Composites Long fibre composite materials are highly anisotropic Let’s look at two extreme cases

Isostrain

εc = εm =

εf

Isostress

σc = σm =

σf

Loading parallel to the fibers.

Loading perpendicular to the fibers

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Isostrain Analysis Isostrain represents the best possible properties: our composite structure has been

optimised for loading direction (Upper Bound) Assume that under load:

εc = εm = εf

Total load = load carried by fibres + load carried by the matrix

Fc = Ff + Fm

σcAc= σfAf + σmAm

Therefore

σc= σfAf/Ac + σmAm/Ac

Since the length of the fibres = length of matrix = length of composite

σc= σfVf+ σmVm

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We can rearrange to find the modulus of elasticity

Ec = EfVf + EmVm

Let’s now consider the fraction of the load carried by the fibres

Therefore

Thus the load carried by the fibre is

Ff / Fm = σf Af / σmAm = σf Vf / σmVm = ε fEf Vf /

εmEmVm

F f /Fm = Ef Vf/EmVm

Ff/Fc = εf Ef Vf/( εf Ef Vf+ εmEmVm)

Remembering that

Ecε = σc

Ef ε = σf

Emε = σm

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Ff / Fc = Ef/Em/(Ef/Em +Vm/Vf)Notice that the ratio of the loads depends on Ef/Em. Therefore a high modulus fibre is best. However, a high modulus fibre normally has a lower strain to failure, so there is a tradeoff.

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Isostress Analysis

This is the lower bound, the worst case for long fibre aligned composites

Assume that under load:

Strain in the composite is given by

εc = εmVm + εfVf

Therefore

σ/Ec = σ/Em . Vm +σ/Ef . Vf

σc = σm =

σf

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Dividing by σ

1/Ec = Vm/Em + Vf/EfOr

Ec = EmEf/(EmVf +EfVm)

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ExampleData:

– Carbon fibre

– Epoxy matrix

– Vf = 0.5

– Ef = 400 GPa,

– Em= 4 GPa

Isostrain

Isostrain

Isostress

Composite Modulus

Volume fraction of fibre0.25 0.5 0.75

Ef

Em

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For Isostrain (loaded parallel to the fibres)

Ec = Ef V f+ Em Vm

Ec = 400 x 0.5 + 4 x 0.5

Ec= 202 Gpa

For Isostress (loaded perpendicular to the fibres)Ec= EmEf / (EmVf + EfVm)Ec= 400 x 4/(4 x 0.5+400 x 0.5)

Ec= 7.9 GPa

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Why Use Composite Parts? Weight Savings

They weigh 20 percent to 25 percent less than aluminum parts of the same shape. Less weight, less fuel needed.

Thermal Expansion

Composite parts do not expand or contract as much with temperature changes (e.g. aircrafts sits on runways in the tropics and then climbs into the freezing stratosphere).

Corrosion Resistance

Composite parts do not corrode due to moisture in the air, like some metals. Fatigue Resistance

They are less likely to suffer fatigue problems than metals Electric Property:

High dielectric strength

Non-magnetic

Radar transparency