Materals- Composites

Question 1

What is wrong with monolithic (single) materials?

Answer 1

Strong ones are relatively dense. Increasing yield strength or stiffness generally results in a decrease in toughness.

Question 2

What are composites?

Answer 2

Combinations of several dissimilar materials aiming to get the best properties of each and none of the disadvantages.

Question 3

Describe pearlitic steels as composites

Answer 3

Microstructure consists of alternating layers of α-ferrite and cementite. Ferrite phase is soft and ductile. Cementite is hard and very brittle. Combined characteristics are reasonably high ductility and strength.

Question 4

What is bone made of?

Answer 4

Composite of strong yet soft protein collagen and hard, brittle material apatite.

Question 5

What are the phases in a composite?

Answer 5

Matrix phase (main phase). Reinforcement (dispersed) phase (the addition)

Question 6

Examples of types of matrix and reinforcements in composites

Answer 6

Often matrix is a polymer.

Reinforcements can be particles, short fibres, long fibres

Question 7

What influences the properties of a composite?

Answer 7

The properties of the constituent phases. Their relative amounts. Geometry of reinforced phase. The shape of the bonding between the phases.

Question 8

Describe large particles-reinforced composites

Answer 8

Large meaning order of microns and above. Particulate phase harder and stiffer than matrix. The particles restrain movement of matrix phase in the vicinity of each particle. Matrix transfers some of applied stress to the particles which bear a fraction of the load. Degree of reinforcement depends on strong bonding at the matrix-particle interface. Example is concrete.

Question 9

What are the rule of mixture equations for large-particle composites?

Answer 9

Upper limit:
Ec(u)=EmVm+EpVp
Lower limit:
Ec(l)=(EmEp)/(VmEp+VpEm)
Where u and l mean upper and lower
E means elastic modulus
c means composite
V means volume fraction
m and p mean matrix and particle

Question 10

Graph of upper and lower bound for elastic modulus against particle concentration

Answer 10

Upper bound is positive linear

Lower bound joins upper bound at either end but hangs a bit below (always positive gradient).

Question 11

Describe dispersion-strengthened particle-reinforced composites

Answer 11

Particles much smaller (10 to 100nm diameter). Strengthening mechanism similar to precipitation hardening. Matrix bears major portion of applied load. Dispersed particles hinder or impede motion of dislocations, restricting plastic deformation.

Question 12

What are cermets?

Answer 12

Examples of ceramic-metal composites. Composed of extremely hard particles of refractory carbide ceramic such as WC or TiC embedded in a matrix of a metal such as cobalt or nickel. WC and TiC are brittle.

Question 13

Examples of properties of phases in reinforced concrete composite

Answer 13

Concrete: moderate density, strong in compression, weak in tension, can be made on site.
Steel: high density, strong in compression and tension, must be processed in factory.

Question 14

How does prestressed concrete work?

Answer 14

Cracking in concrete would normally take place on tensile side. Steel bars are in concrete. They are stretched elastically while concrete is setting. Steel pulls back when released which compresses the concrete. This reduces the tensile force when concrete is bent.

Question 15

Describe fibre-reinforced composites

Answer 15

Dispersed phase is in the form of a fibre. Designed for high strength and/or stiffness with low weight. Mechanical properties depend on properties of fibre and the degree to which an applied load is transmitted to the fibres by the matrix phase which depends on interfacial bond between the fibre and matrix phase.

Question 16

Formula for critical fibre length

Answer 16

lc=σfxd/2τc
σf is ultimate tensile strength of fibre
d is fibre diameter
τc is fibre-matrix bond strength (or shear yield strength of matrix phase, whichever is smaller)

Question 17

What happens if a fibre is of the critical fibre length?

Answer 17

When an applied load is equal to the fibre’s UTS, the centre of fibre takes this force.

Question 18

What happens if a fibre is greater than the critical fibre length?

Answer 18

When load equal to its UTS is applied, there is a region in the middle that takes this load.

Question 19

What happens if a fibre is shorter than the critical fibre length?

Answer 19

When a load equal to the UTS of the fibre is applied, the fibre never takes this load, it is just applied to the matrix so the strength of the composite is just the strength of the matrix.

Question 20

What are continuous fibres?

Answer 20

Fibres that are much longer than the critical fibre length and so significantly improve the strength of the composite.

Question 21

What are discontinuous fibres?

Answer 21

Fibres that are much shorter than the critical fibre length and so don’t improve the strength of the composite.

Question 22

What influence the properties of fibre-reinforced composites?

Answer 22

The arrangement and orientation of the fibres relative to one another. Fibre concentration. Fibre distribution.

Question 23

When are better overall fibre-reinforced composite properties achieved?

Answer 23

When the fibre distribution is uniform.

Question 24

Roles of matrix phase in fibre-reinforced composites

Answer 24

Often metal or polymer because some ductility is desirable.
Binds fibres together and acts as the medium by which an externally applied stress is transmitted and distributed to the fibres.
Protect the individual fibres from surface damage as a result of mechanical abrasion or chemical reactions with the environment.
Serves as Barrie to crack propagation.

Question 25

Describe glass fibre-reinforced polymer (GFRP) composites and their advantages and disadvantages

Answer 25

Glass is easily drawn into high-strength fibres and is readily available and economically fabricated into a composite. As a fibre, glass is strong and when embedded in a plastic matrix produces a composite having very high specific strength. Chemically inert in variety of corrosive environments when coupled with various plastics. However not stiff.

Question 26

Describe carbon fibre-reinforced polymer (CFRP) composites and their advantages

Answer 26

Carbon fibres have highest specific modulus and specific strength and retain these high values at elevated temperatures. At room temperature carbon fibres are not affected by moisture or a wide variety of solvents, acids and bases. They exhibit a diversity of physical and mechanical characteristics. Inexpensive fibre and composite manufacturing processes.

Question 27

How do structural composites work?

Answer 27

They are composed of 2D sheets or panels that have a preferred high-strength direction. The layers are stacked and cemented together such that the orientation of the high-strength direction varies with each successive layer.

Question 28

Describe wood as a natural composite

Answer 28

It has strong fibres (cellulose) is a softer matrix of lignin. It is a polymer-polymer composite. On the large scale it contains porosity.

Question 29

Describe how would is arranged?

Answer 29

Long thin cellulose fibres aligned in one direction held together by lignin

Question 30

Describe toughness and how strength varies in wood

Answer 30

Strength is affected by density, drying and defects present. Strength across grain may be 1/20th of strength along the grain.
Wood doesn’t crack easily so is tough. The pulling apart of fibrous structure absorbs energy.

Question 31

Describe particle board wood

Answer 31

It’s wood separated into small fragments (may be waste from wood machining. Fragments combined with resin under pressure to make a board. Depending on fragment size can make chipboard or strandboard (flakes orientated in chip plane.

Question 32

Describe fibreboard wood

Answer 32

Wood is broken down to its fibres by grinding. Fibres then mixed with resin and pressed at temperature to bond. Medium density fibreboard (MDF) is most common variety but cannot be used for structural applications.

Question 33

How is plywood made?

Answer 33

Veneers produced by rotary cutting of small trees (soaking or steaming may be needed). Layers are glued perpendicular to adjacent ones to overcome tendency to split.