Lecture 13 Flashcards
describe static tension test measurements
we measure the local deformation of a sample, and get a stress strain curve. This can be done at different temperatures as well.
Clamps hold the specimen and a load is applied.
discuss stress strain curve, what paramaters can we get from the curve
- E (slope of elastic range)
- yield strength (0.2% deformation)
- tensile strength (UTS)
- fracture strength
- elongation at fracture
- we can see the necking too
give examples and explanations for parameters (3) influencing the Young’s modulus
1) temperature : higher T = lower E because bond strength decreases
2) boiling point : higher boiling point = higher E (also related to bond strength)
3) crystal direction : atoms move differently on different crystal planes
explain elstic and plastic deformation in an atomic model
elastic -> we just put load on the bonds without breaking them
plastic -> we create defects, dislocations that can then move
how can plastic deformation occur ? why are polycrystals stronger than single crystals ?
movement of dislocations !
A dislocation moves along a slip plane -> they have preferred crystal directions.
Polycrystals are stronger because grain bounderies are barriers to disloc motion
illuminate the relation between static and dynamic mechanical testing
dynamic testing : we apply cycles of load over time (example alternate compressive and tensile load)
explain how a S-N curve is recorded
you apply cyclic loads at a certain stress and record the nb of cycles before fracture. You do this for many different loads. -> takes a long time !!
what are the 4 strategies for strengthening ?
common goal : reduce the movement of dislocations
1) reduce grain size = more grain bounderies : hall petch, 1/sqrt(diamater)
2) solid solutions : impurities generate lattice strains / stress fields -> sqrt(concentration)
3) precipitation : disloc needs to move around it -> 1/Spacing
4) cold work -> increase density of disloc, but ductility is decreased
what are the 3 annealing stages ?
1) recovery (disloc density decreases)
2) recrystallization (new grains form)
3) grain growth
illustrate how the microstructure influences fatigue strength
the finer the grain size, the more resistant to fatigue = higher stress amplitude is possible.
Annealed material gets weaker but more ductile
what happens at a crack ? (stress related)
Higher stress concentrations at crack tip.
If a crack is parallel to applied load, there is no problem.
classify fractures
1) static :
- ductile : transcrystal (through the grains) or intercrystal (along boundaries)
- brittle (cleavage) : transcrystal or intercrystal
- mixed
2) fatigue fracture :
crack beginning - lines of rest (+striations) - final fracture
explain mechanical advantages of composite materials
the fibers can take up a load of load (depending on their orientation and length).
We can adapt the properties really well.
enumerate different types of hardness measurement
- vickers : pointy square tip -> smaller traces and thin coatings can be tested
- brinell : round tip -> better for more heterogeneous microstructures
- mohs : hardness picks (geology)
enumerate typical sterilization methods (and a few + and -)
1) heat sterilization : autoclave (vapour) or dry heat
2) cold sterilization : gas (ethylene oxyde -> used for heat sensitive substances) or radiation -> well controllable, deep penetration but chemical alteration possible
3) disinfecting with solutions -> careful not to melt polymers, or cause cracks
