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
explain what to look out for when sterilizing different types of materials
1) if material can withstand high temperatures and vapour
2) gas : some materials may absorb the gas
3) radiation : can chemically or mechanically alter some materials
4) solutions can cause discoloration, degradation / melting, or corrosion
