Mechanical Properties of Materials Flashcards
Before/after melting for crystalline materials
Atomic vibrations increase w/T + volume expansion
At melting for crystalline materials
Crystal formation + high APF - sudden volume decreases
Is there rearrangement of atoms below Tg?
Nah - so thermal expansion coefficient similar to one for crystal
What if T<Tg for polymers?
Polymer is hard + brittle
What if T>Tg for polymers?
Polymer becomes flexible + rubbery
Do polymers most often have crystalline + amorphous zones?
Yes
Do amorphous/glassy materials have a crystalline structure?
No! So dislocations don’t exist + no plastic deformation so totally brittle
Mn
Total wt of polymer/total # of molecules
What number is PDI greater or equal to?
PDI =< Mw/Mn
Isotactic polymer
Side chain always on same side
Syndiotactic polymer
Side chain always on alternate sides
Atactic polymer
Random positioning
Thermoplastic polymers
Flexible plastic that can be moulded when it’s heated
Thermoset polymers
Cross-linked so don’t melt
How do ceramics deform?
Elastically up to point of fracture
Plastic deformation in ceramics
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
Is ductile or brittle fracture preferred?
Ductile
Ductile to brittle transition
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
Why are measured fracture strengths lower than theoretical ones?
B/c of flaws
Any defect will _ the stress applied locally
Increase
What does stress concentration result from?
Reduced cross-section area and shape of defect
When doesn’t tearing take place in balloon?
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
Griffith theory
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
Impact loading
Severe testing case that makes material more brittle + decreases toughness
Why does yield strength decrease with T?
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
Effect of increased loading rate on yield and tensile strength and % elongation?
Increased yield and tensile strength and decreased % elongation b/c an increased rate gives less time for dislocations to move past obstacles
Sigma c
Critical stress value where fast fracture occurs
What happens if first failure mechanism reached is yield stress versus critical stress?
If critical stress reached first then brittle w/no deformation - if yield stress reached first then ductile w/deformation
Why is transition T observed between brittle and ductile behaviours?
Because critical and yield stress have diff behaviours w/temperatures
At increasing T, FCC metals maintain their ____
Ductility
At increasing T, high strength materials maintain their ____
Brittleness
Fatigue
Failure under cyclic stress which varies w/time - fatigue can cause part failure even though max stress < critical stress
Cracks generated by fatigue
These form because cyclic stresses cause dislocations + slipping to move planes at surface
How does slipping occur?
At 45 degree of imposed stress
How can we use fatigue limit (Sfat) to determine if fatigue failure occurs?
S<Sfat if no fatigue failure occurring where S is stress amplitude
How to improve fatigue life?
- Impose compressive surface stress to suppress surface cracks from growing by shot peening or carburizing
- Reduce stress concentrators
What leads to creep?
High T/static mechanical stress
Definition of creep
Sample deformation at constant stress vs. time
Primary creep
Slope (creep rate) decreases w/time (strain hardening effect)
Secondary creep
Steady-state (constant slope - strain hardening-recovery balance)
Tertiary creep
Slope increases w/time (acceleration of rate - microstructure and/or metallurgical changes)
Mechanisms that explain creep
- Stress-induced vacancy diffusion
- Grain boundary diffusion
- Dislocation motion
- Grain boundary sliding
- Dislocation climb
What happens at dislocation climb?
Dislocation moves perpendicular to slip plane so that it can escape from lattice imperfections + continue to slip
