Lecture 3

Question 1

what reinforcement increases strength at a greater rate, continuous or particulate for each direction?

Answer 1

continuous - gives greater increase in strength for fibre direction but modulus increase in any other direction is lower.
particulate - slower rate of strength increase than continuous fibre.

Question 2

what reinforcement is more anisotropic continuous or discontinuous & aligned?

Answer 2

continuous

Question 3

what are specific strength and specific stiffness?

Answer 3

strength & stiffness divided by specific gravity

Question 4

what exhibit higher strength stiffness and modulus of elasticity out of the particulate and fibre reinforced MMCs

Answer 4

fibre - better modulus of elasticity

particulate - better strength and stiffness

Question 5

why is a smaller fibre material much stronger than the bulk material?

Answer 5

less probability of a flaw present

Question 6

is the diameter of a fibre large or small if the interface surface area is large

Answer 6

interface surface area large — fibre diameter small.

fib diameter & interface SA inversely prop.

Question 7

what are precipitates and what are their effect on metal alloys?
How are they formed?

Answer 7

precipitates - small, uniformly dispersed particles of a second phase.
Improve strength and hardness
formed by heat treatment

Question 8

how does precipitation hardening work to strengthen a material?

Answer 8

dislocation motion is impeded, achieved by incorporating fine oxide particles/non shareable precipitates within the matrix.

Question 9

where is dislocation density highest?

Answer 9

near reinforcements

Question 10

what are dislocations?

Answer 10

where atoms are out of line with the crystal structure

Question 11

which of precipitation hardened or dispersion strengthened systems show better strength characteristics at high/low temps

Answer 11

room temp: precipitation hardened systems have higher strength as it is possible to achieve finer distributions.

high temps: show advantages due to the high thermal stability due to the oxide particles.

Question 12

what are they key element in dispersion strengthened materials?

Answer 12

oxygen

Question 13

how can you improve fracture toughness of a material through changing the fibres and why?

Answer 13

braiding fibres in weaves because of extensive matrix deformation

Question 14

what defects can act as fatigue crack initiation sites?

Answer 14

non-metallic inclusions,

particle clusters

Question 15

how can reinforcing an MMC decrease fatigue and why?

Answer 15

the defects are subjected to lower stress when reinforced as the high stiffness reinforcement will take the load.

Question 16

what does fatigue failure look like for low stresses and high stresses?

Answer 16

splitting of the fibres is the main mode of failure

single crack propagates catastrophically

Question 17

how can second phase particles affect fatigue crack interaction?

Answer 17

they can deflect the crack, causing it to circumvent the 2nd phase materials

Question 18

why do whisker or particle reinforced MMCs produce better creep rate than unreinforced alloys?

Answer 18

There is a better load transfer from the matrix to the reinforcement

Question 19

what does ceramic reinforcement do for the CTE and electrical conductivity?

Answer 19

decreases both CTE and conductivity

Question 20

How can we REDUCE the CTE and conductivity?

Answer 20

adding reinforcement in the matrix,,

altering the distribution of reinforcement,

Question 21

what is something to think about in terms of CTE of matrix and reinforcement?

Answer 21

the difference in CTE