Reading

Question 1

What can a mismatch of thermal expansivity lead to?

Answer 1

Internal residual stresses -> strong effects of mechanical behaviour

Question 2

What is hardness dependent on that stiffness and density is not?

Answer 2

Microstructure

Question 3

Name common reinforcements for PMC, MMC and CMC

Answer 3

PMC - Carbon, glass, aramid
MMC - ceramics
CMC - metallic + ceramic

Question 4

Why is tensile strength data v.approximate espec in ceramic materials?

Answer 4

Ceramics are sensitive to presence of flaws ∴ higher values of strength measured = due to fibres containing few large flaws

Question 5

Why is axial stiffness and strength high in CF

Answer 5

Graphene has a very strong bonding within the plane so alignment in carbon fibres -> stiffness and strength

Question 6

Describe CF anisotropic behaviours in terms of thermal expansivity and conductivity

Answer 6

Higher expansivity in transverse

Higher conductivity in axial direction

Question 7

Why are composites usually bad under transverse loading?

Answer 7

Uniaxial nature means that in transverse loading, the matrix which is usually weak is taking most of the load

Question 8

Describe how processing temperature affects the property of CF

Answer 8

• lower temp —> less dense, less stiff but strong

- higher temp —> more dense, stiff

Question 9

Describe the effects that adding ‘network-modifying’ (lower valence) oxides e.g. Na2O to a quartz will have

Answer 9

1) reduces Tg = easier to deform/draw at low temps
BUT reduces max use temp?

2) stiffness is affected

Question 10

What is a quartz?

Answer 10

Pure silica glass

Question 11

Where are flaws predominantly located in brittle materials?

Answer 11

Surface

Question 12

How can dimensions of brittle fibres affect strength?

Answer 12

Reduction in diameter —> stronger ∵ finer flaws

But practicality difficulties can arise if D is too small

Question 13

Does the toughness of a composite depend on the toughness of the fibre and/or the matrix?

Answer 13

No

Question 14

What are some limitations of polymeric fibres?

Answer 14

• relatively low stiffness

- limited tolerance of heat

Question 15

Describe attractive properties of SiC

Answer 15

• low density
• High stiffness
• good thermal conductivity + stability

Question 16

Describe oxide fibres generally (alumina and silica)

Answer 16

• good resistance to oxidative degradation (problem for other fibres when used @ High temp)
• used for High temp insulation @ 50/50
• alumina fibres w/lower silica content = greater resistance to high temp, higher stiffness + strength

Question 17

What are whiskers?

Answer 17

Elongated single crystals

Question 18

Why do whiskers have v. High strengths?

Answer 18

∵ there are soo fine and have no GB

Question 19

What are some disadvantages to whiskers?

Answer 19

• expensive to produce
• difficult to handle ∵ = readily become airborne = health hazard
• difficult to disperse within a matrix
• despite high tensile strength, toughness likely to be v.low

Question 20

What’s the difference between particles and whiskers?

Answer 20

• cheaper
• easier to process and handle
• can be used as a density reduction

Question 21

What are the main mechanisms of interfacial bonding

Answer 21

1) wetting (van der Waals forces)
2) diffusional processes
3) chemical reactions
4) mechanical keying