Composites

how many phases are in a composite microstructure?

2, the matrix and the dispersed phase

are composite microstructures continuous or discontinuous?

discontinuous

what are the two phases in the composite microstructure?

dispersed and the matrix

what is the role of the matrix?

binding together

what is the dispersed phase?

surrounded my the matrix, usually particles

name three composite properties that effects the material properties

• amount of phases

• phase properties

• geometry

• orientation

• distribution

what are particle reinforced composites?

defined by their strengthening mechanism

what is the particulate phase?

harder and stiffer than the matrix which creates a barrier to stop matrix deformation

how does the particulate phase transfer loads?

particles restrain matrix movement resulting in a transfer of load

what is the degree of reinforcement dependant on? (in terms of the composite microstructure)

a high degree of reinforcement is dependant on strong bonding at the matrix particle phase

how should particles be distributed and orientated for effective reinforcement?

for effective reinforcement particles should be in the same direction and evenly distributed throughout the matrix

what is the effect of the material volume fraction?

volume fraction influences behaviour, mechanical properties increase with particulate content

what is the rule of mixtures equation?

shows the upper and lower bounds for the dependence of the elastic modulus on the volume fraction of the phases

what is the equation for the upper bound?

Ec(U) = EmVm+EpVP

where:

• E = elastic modulus

• V = volume fraction

• C = composite

• M = matrix

• U just refers to upper

what is the equation of the lower bound?

Ec(L) = EmVm / VmEp + VpEm

where:

• E = elastic modulus

• V = volume fraction

• C = composite

• M = matrix

• L just refers to upper

what does this equation stand for?

Ec(L) = EmVm / VmEp + VpEm

elastic modulus of the lower bound

where:

• E = elastic modulus

• V = volume fraction

• C = composite

• M = matrix

• L just refers to upper

what are 2 properties of tungsten particles in a copper matrix?

• high heat resistance

• low thermal expansion

what is cermet and 1 one of its properties?

• hard ceramics in a metal matrix

• prevents crack propagation

what is carbon black and what are two properties?

• carbon black is small particles added to rubber

• enhances toughness and abrasion

what is concrete and what is a disadvantage of it?

• concrete is a mix of coarse aggregates, water and cement

• weak and brittle

what is an advantage of reinforced concrete?

better maintains stresses as well as reduced corrosion

what is a dispersed strength composite?

small particles, strengthened at atomic levels

what is are two advantages of a dispersed strength composite?

particles hinder dislocations as well as restricting plastic deformation improving yield and tensile strength

what is a fibre reinforced composite?

within a fibre reinforced composite, the dispersed phase is a fibre that is either continuous or discontinues

what is the difference between continuous and discontinuous fibres?

continuous fibres - aligned

discontinuous fibres - short, aligned and randomly orientated

what do mechanical properties depend on within fibre-reinforced composites?

mechanical properties depend on fibre properties, and the degree in which load is transmitted to the fibres

what happens to the matrix bond when under applied stress?

the fibre matrix bond creases at the fibre ends when under applied stress, yielding a a matrix deformation pattern

what is needed for effective strengthening

critical fibre length

what is the equation for critical length?

where:

• lc = critical length

• σ*f = fibre ultimate strength

• d = fibre diameter

• τc = fibre matrix bond strength
  

what does it mean if fibres are l > lc?

the fibre is continuous

what does it mean if fibres are l < lc?

the fibre is discontinuous

name 2 factors that have a strong influence on fibre strength

• fibre arrangement

• fibre concentration

• distrubution

what is this arrangement?

discontinuous but aligned, short

what is this arrangement?

continuous

what is this arrangement?

discontinuous, randomly aligned

what does tensile stress strain behaviour in longitudinal loading depend on? (give 2)

• stress strain behaviour of the fibre and matrix phases

• phase volume fractions

• direction of stress

are composites generally anisotropic or isotropic?

anisotropic

what happens in stage 1 of a stress strain diagram of a composite material?

• fibres and matrix deform elastically (curve is usually linear)

what happens in stage 2 of a stress strain diagram of a composite material?

• matrix yields and deforms plastically

• fibres continue to stretch

• failure begins once fibres start to fracture

generally, does the matrix or fibres have higher tensile strength?

the fibres have higher tensile strength

why is composite failure not catastrophic?

no all fibres fail at the same time. Once they have failed they are still embedded in the matrix which means they can still support a diminishing load

how does the fibre matrix bond react to longitudinal loading

• the fibre matrix bond is good, meaning that deformation in the matrix and fibre is the same

what does this represent: εc = εm = εf

deformation is the matrix and fibres are the same (iso-strain)

what is the equation for the elastic modulus of a continuous and eloigned fibrous composited in the direction of the alignment.

Ecl = EmVm + EfVf

what is the ratio of the load carried by the fibres to that of the matrix?

Ff / Fm = EfVf / EmVm

what is iso- stress?

the composite and both phases are exposed to same amount of stress

what is the equation for iso-stress?

σc= σm = σf = σ

what is the equation for the modulus of elasticity in a continuous and aligned fibre-reinforced composite in the longitudinal direction?

1 / Ect = Vm / Em + Vf / Ef

what is transverse stress influenced by in fibre reinforced composites? (name 2)

• properties of the matrix of the fibres

• bond strength

• presence of voids

do discontinuous or continuous composites have a higher reinforcement efficiency?

continuous

what is the modulus of elasticity of short composites?

short fibre composites have an elastic modulus of about 90% of continuous fibre composition

why are smaller diameter fibres stronger than the bulk material?

lower probability of a critical surface flaw

name 2 example of fibres

• whiskers

• fibres

• wires

what are whiskers?

thin crystal strands with a high length to diameter ratio

what is an advantage of whiskers?

• high degree of crystalline perfection creating high strength due to less flaws

why are whiskers not used

impractical and expensive

what type of structure can fibres have?

polycrystalline or amorphous

are fibres or wires larger in diameter?

wires

what are wires used for?

reinforcement

what can the matrix phase be made from

either polymer, metal or ceramic

give two functions of the matrix

• binds fibres together

• acts as a medium of stress

• protects fibres from surface damage

• separates fibres stopping propagation

is the matrix brittle of ductile?

ductile

the elastic modulus higher in fibres or the matrix?

fibres

what does the UTS of the composite depend on?

the bond between the matrix and fibres

name 2 advantages of glass fibre

• easily fabricated

• economical

• strong

what is an advantage of carbon fibre reinforced polymers? (give 2)

• high modulus to density ratio

• high specific strength

• retains strength at high temps

name a limitation of glass fibre reinforced polymer composites

• not very stiff

• limited applications

describe the carbon fibre structure

• composed of graphitic and non-crystalline regions

• coated carbon fibres

what are three advantages of aramid fibre reinforced polymer composites

• high strength

• high modulus

• high strength to weight ratio

what is pultrusion used for?

used for continuous lengths and constant cross sectional shapes

describe the process of pultrusion

1. fibres are drawn together as strands

2. strands are impregnated with a thermosetting resin

3. fibres are pulled through a steel die to preform to a desired shape

4. die is heated to initiate the curing of the resin matrix

5. pulling device draws the stock through the dies

6. tubes are created by inserting hollow cores

what is filament winding

process of fibres are positioned in a predetermined pattern to form a hollow shape

what is the process of filament winding

1. fibres are fed through a resin bath

2. fibres are wound onto a mandrel using automated winding equipment

3. curing occurs and mandrel is removed

what is prepreg production

fibre is pre-impregnated with a polymer resin that is only partially cured

what is the pre-preg production process

1. delivered in a tape form

2. directly moulds and fully cures the product without needing to add resin

what is the process of creating a prepreg tape

1. spool wound fibres tows are sandwiched and pressed between sheets of papers using heated rollers

2. a doctor blade spreads resin into a film

3. final product is a thin tape of fibres in partially cured resin

what does final curing require during the pre preg process

heat and pressure

what are structural composites usually composed of?

homogenous and composite materials

what are properties dependant on in structural composites

• material used

• geometrical design

what are sandwich panels

lightweight beams of panels with high stiffness and strength

what are laminar composites?

composed of 2D shapes that have a preferred strength orientation

how do laminar composites create balance?

each board has a different direction of high strength, therefore they are swapped on each layer in order to balance out

what are sandwich boards made up of?

two beams that are separated by a adhesively bonded thicker core

what are the main properties of the core within sandwich boards? (give 2)

• continuous support

• must have shear strength to withstand transverse stresses

• variety of applications