Plastic yielding

elastic deformation definition

reversible change in atomic spacing

plastic deformation definition

permanent deformation caused by planes slipping

what is yielding?

the point at which plastic deformation occurs

why does yielding occur gradually in polycrystalline materials?

different grains have different slip planes, so yielding begins at different stresses before macroscopic yielding (luders band)

what is the stress of an elastically-plastic material after yielding?

yield stress

what happens atomically in yielding for BCC yielding?

carbon and nitrogen atoms pin dislocations. when dislocations break free, stress drops and strain decreases, creating luders deformation. normal strain hardening begins after luders band has spread

why is there no sharp yield point in FCC and HCP structures?

dislocations aren’t strongly pinned

what is a flow curve?

a graph of true stress against true plastic strain

process of strain ageing

material is plastically deformed, unloaded, left to age and reloaded

what are the effects of strain ageing?

yield stress increases, upper and lower yield points reappear

what does strain ageing occur after this process?

carbon and nitrogen atoms diffuse back to dislocations and pin them again

what does it mean when N=1 for plastic strain?

linear elastic material

what does it mean when N=3 for plastic strain?

strongly strain hardening material

what does it mean when N>20 for plastic strain?

elastic-perfectly plastic material

what is necking?

local reduction in cross sectional area during tensile testing

how does necking and failure occur?

plastic deformation occurs at the weakest part of the sample, causing the area to contract and the tensile stress to increase. yield stress therefore increases, causing strain hardening, but area contraction isn’t balanced by the increase in strength causing UTS and necking

at what point on a stress-strain curve does necking occur?

UTS

what happens in a tensile test after UTS is reached?

the load decreases, but local stress inside the neck continues increasing until fracture occurs

what is the necking criterion?

necking occurs when strain hardening rate equals the true stress

how do strain and temperature effect FCC and HCP structures?

slip occurs easier, lower yield stress, less sensitive to temperature and strain rate

effect of increasing temperature on BCC structures

more atomic vibrations, so atomic planes come closer together, activating slip systems (making slip yielding and lowering yield stress)

effect of increasing strain rate on BCC structures

less time for thermal activation, hence there’s a lower probability of planes getting close enough for slip (higher yield stress)

why are FCC and HCP more affected by long range obstacles (e.g. precipitates, dislocation networks and second-phase particles)?

these obstacles cannot be overcome by thermal energy, so yield stress isn’t affected much by temperature

what applications are FCC and HCP better suited for?

low temperature vessels and liquid nitrogen systems (less sensitive to temperature)

why are BCC structures more affected by short range obstacles (e.g. solute atoms, pinning of dislocations, lattice friction)?

these obstacles require activation energy to be overcome (e.g. applied stress or thermal energy)