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temperature effects on material behaviour, Viscoelasticity, materials show…
temperature effects on material behaviour
component subject load will deform over time
CREEP
temperature dependent
important \[ T_g \] and \[ T_H \]for understanding of creep
materials with low melting points will creep at room temperature under there own weight
others creep at high temperatures and stresses
homologous temperatue
\[ T_H =\frac{T}{T_m}\]
\[ T_H >0.4\]
metal susceptible to creep
rate of creep
stress
temperature
important above \[ 0.4T_m \]
different transition temperatures can apply to other materials
polymers
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depends on the glass transition temperature \[ T_g \]
\[ T_g \]
the temperature at which a amorphous material changes from hard glassy state to softer rubbery state
does not have a sharply defined temperature
partially crystalline
both crystalline and glass transition temperature
regions of crystalline are ordered
polymer chains closely packed and tightly bonded
the \[ T_m higher than T_g \]
\[ T_g\approx\frac{2}{3}T_m \]
low or high temperature affects the properties to a greater extent
Viscoelasticity
viscous
time dependant
permantent
liquids
wont return to original position once stress has been removed
moves as long stress applied
polymers
polymer chains moving past one another in non crystalline region
chains take time to reorganise
atoms in polymer chain
bonded
move in a coordinated way
partially crystalline
cystalline region
elastic
bonds
return original length when load is removed
amorphous regions
some elastic behaviour
force applied the randomy coiled chain
change shape
load removed return to original shape
eleastic recovery is rapid than the load removed
in the viscous component as the chains move past eash other to return to thier original shape
the stress strain between the loading and onloading will not be the same
hysteresis
coefficient of restitution
fraction of energy returned elastically
vary by very large amount depenfing on material, nature of load and external factors
two types of strain
time dependent recoverable eleastic strain
non-recoverable plastic strain
load removed some creep strain recovered
combination of
elastic behaviour
viscous behaviour
elastic response
deformation
instantaneous
recoverable
materials show a time dependent elastic behaviour
response to load
not instantaneous
evolves over time