Mechanical Properties of Materials

Question 1

Before/after melting for crystalline materials

Answer 1

Atomic vibrations increase w/T + volume expansion

Question 1

At melting for crystalline materials

Answer 1

Crystal formation + high APF - sudden volume decreases

Question 2

Is there rearrangement of atoms below Tg?

Answer 2

Nah - so thermal expansion coefficient similar to one for crystal

Question 3

What if T<Tg for polymers?

Answer 3

Polymer is hard + brittle

Question 4

What if T>Tg for polymers?

Answer 4

Polymer becomes flexible + rubbery

Question 5

Do polymers most often have crystalline + amorphous zones?

Answer 5

Yes

Question 6

Do amorphous/glassy materials have a crystalline structure?

Answer 6

No! So dislocations don’t exist + no plastic deformation so totally brittle

Question 7

Mn

Answer 7

Total wt of polymer/total # of molecules

Question 8

What number is PDI greater or equal to?

Answer 8

PDI =< Mw/Mn

Question 9

Isotactic polymer

Answer 9

Side chain always on same side

Question 10

Syndiotactic polymer

Answer 10

Side chain always on alternate sides

Question 11

Atactic polymer

Answer 11

Random positioning

Question 12

Thermoplastic polymers

Answer 12

Flexible plastic that can be moulded when it’s heated

Question 13

Thermoset polymers

Answer 13

Cross-linked so don’t melt

Question 14

How do ceramics deform?

Answer 14

Elastically up to point of fracture

Question 15

Plastic deformation in ceramics

Answer 15

Undergo this through dislocations + plane slipping but this is less favoured by ionic bonding b/c of necessity to maintain electrical neutrality - this doesn’t occur around room temp

Question 16

Is ductile or brittle fracture preferred?

Answer 16

Ductile

Question 17

Ductile to brittle transition

Answer 17

At low strain rate/high T/low stress concentrations all materials are ductile - if strain rate increases/T decreases/stress concentration increases then becomes brittle

Question 18

Why are measured fracture strengths lower than theoretical ones?

Answer 18

B/c of flaws

Question 19

Any defect will _ the stress applied locally

Answer 19

Increase

Question 20

What does stress concentration result from?

Answer 20

Reduced cross-section area and shape of defect

Question 21

When doesn’t tearing take place in balloon?

Answer 21

If the work done by the gas pressure inside the balloon + the release of elastic energy from the membrane as the crack grows < energy of tearing

Question 22

Griffith theory

Answer 22

Release of elastic energy from membrane as crack grows (since before crack material is stretched and after crack stress is released) < energy of tearing needed to create new surface

Question 23

Impact loading

Answer 23

Severe testing case that makes material more brittle + decreases toughness

Question 24

Why does yield strength decrease with T?

Answer 24

As T increases the increased atomic mobility helps in movement of dislocations + removes some of defects stopping dislocations –> material goes to plastic domain more easily

Question 25

Effect of increased loading rate on yield and tensile strength and % elongation?

Answer 25

Increased yield and tensile strength and decreased % elongation b/c an increased rate gives less time for dislocations to move past obstacles

Question 26

Sigma c

Answer 26

Critical stress value where fast fracture occurs

Question 27

What happens if first failure mechanism reached is yield stress versus critical stress?

Answer 27

If critical stress reached first then brittle w/no deformation - if yield stress reached first then ductile w/deformation

Question 28

Why is transition T observed between brittle and ductile behaviours?

Answer 28

Because critical and yield stress have diff behaviours w/temperatures

Question 29

At increasing T, FCC metals maintain their ____

Answer 29

Ductility

Question 30

At increasing T, high strength materials maintain their ____

Answer 30

Brittleness

Question 31

Fatigue

Answer 31

Failure under cyclic stress which varies w/time - fatigue can cause part failure even though max stress < critical stress

Question 32

Cracks generated by fatigue

Answer 32

These form because cyclic stresses cause dislocations + slipping to move planes at surface

Question 33

How does slipping occur?

Answer 33

At 45 degree of imposed stress

Question 34

How can we use fatigue limit (Sfat) to determine if fatigue failure occurs?

Answer 34

S<Sfat if no fatigue failure occurring where S is stress amplitude

Question 35

How to improve fatigue life?

Answer 35

1. Impose compressive surface stress to suppress surface cracks from growing by shot peening or carburizing
2. Reduce stress concentrators

Question 36

What leads to creep?

Answer 36

High T/static mechanical stress

Question 37

Definition of creep

Answer 37

Sample deformation at constant stress vs. time

Question 38

Primary creep

Answer 38

Slope (creep rate) decreases w/time (strain hardening effect)

Question 39

Secondary creep

Answer 39

Steady-state (constant slope - strain hardening-recovery balance)

Question 40

Tertiary creep

Answer 40

Slope increases w/time (acceleration of rate - microstructure and/or metallurgical changes)

Question 41

Mechanisms that explain creep

Answer 41

• Stress-induced vacancy diffusion
• Grain boundary diffusion
• Dislocation motion
• Grain boundary sliding
• Dislocation climb

Question 42

What happens at dislocation climb?

Answer 42

Dislocation moves perpendicular to slip plane so that it can escape from lattice imperfections + continue to slip