Chapter 15 (Plastics as Engineering materials)
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what factors do the results of the stress strain test depend on? (3)
• temperature • strain rate (speed of pulling) • Chemical environment
what are the 3 main types of behaviour for plastics?
• Elastic-Fracture since their brittle • Elastics then Plastic • Completely elastic
what can be said about the elastic modulus of plastics compared to metals
much less than metals
describe the elastic behaviour of plastics
there is an instantaneous deformation and recovery, it is recoverable (low temps)
describe viscous behaviour of plastics
deformation is time dependent and non recoverable (higher temps)
If a polymer is said to be viscoelastic what will the polymer behave as and what type of behaviour will it exhibit?
polymers can behave as rubbery solids and it will have non-linear elastic behaviour
what does chain slippage depend on? (3)
• the rate of strain(speed of pulling) • time • temperature
What happens if the strain is fast on plasitics?
there no time for slippage-» brittle failure occurs
what happens if the strain is slow for plastics?
there is chain slippage and the plastic will react as ductile
what do the graphs of elastic, viscous and visco-elastic behaviour look like respectively?
horizontal line. linear response, positive ln curve
as time increase what happens to stress:
the stress decrease: STRESS RELAXATION
If temperature increases, what happens to the stress
Stress is lower and decreases faster
Describe viscoelastic creeep:
once you apply a constant load: • intially, the plastics will instantaneous strain • over time, the plastic will continue to deform over time. This time dependent deformation is known as creep
What else is the Creep modulus also dependent on
temperature
Match these: Ductile response, Brittle response; Above Tg, below Tg
Above: Ductile Below: Brittle
Describe Crazing
Craze formation prior to cracking: • plastic deformation of spherulites • formation of microvoids and fibrillar bridges
Describe elastic deformation of semi crystalline polymers and state whether its recoverable or not
occurs by bond stretching/bending in the chains. It is recoverable
Describe plasic deformation of semi crystalline polymers and state whether its recoverable or not
• the amorphous regions elongate, and the chains slide past each other • tilting of semi-crystalline regions occurs • blocks orient towards tensile axis • Non recoverable unless heated
Where would continued deformation typically occur and why
in the unnecked region because alignment can occur in the necked region which would lead to strengthening
describe predeformation by drawing:
drawing stretches the polymer prior to use, this aligns the chains to the stretching direction
what are the results of drawing: (3)
• increases the elastic modulus in the stretching direction • increases the tensile strength in the stretching direction • decreases ductility
What does annealing after drawing do? (2)
• decreases alignment 2. reverses the effects of drawing
Which factors affect the strength of polymers and how?
• Average molecular weight of chains: longer chains(heavier)=more entanglement • Degree of crystillinity: affects intermolecular secondary bonding. closer packed=stronger. • Drawing: increases E • Heat treating
what negative effects does an increase in crystillinty have?
increases brittleness