Lecture 13

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.

• E (slope of elastic range) • yield strength (0.2% deformation) • tensile strength (UTS) • fracture strength • elongation at fracture • we can see the necking too

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

elastic -> we just put load on the bonds without breaking them plastic -> we create defects, dislocations that can then move

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

dynamic testing : we apply cycles of load over time (example alternate compressive and tensile load)

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 !!

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

1) recovery (disloc density decreases) 2) recrystallization (new grains form) 3) grain growth

the finer the grain size, the more resistant to fatigue = higher stress amplitude is possible. Annealed material gets weaker but more ductile

Higher stress concentrations at crack tip. If a crack is parallel to applied load, there is no problem.

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

the fibers can take up a load of load (depending on their orientation and length). We can adapt the properties really well.

• vickers : pointy square tip -> smaller traces and thin coatings can be tested • brinell : round tip -> better for more heterogeneous microstructures • mohs : hardness picks (geology)

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

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