2. It can be defined as the slow & progressive (increasingly
continuing) deformation of a material with time under a
constant stress.
It is both a time & temperature dependent phenemenon.
Creep is probably the most widely studied long-term
property.
It results from the viscoelastic flow of the polymer with
time.
Creep
3. Creep
The method of carrying out creep tests is to subject the specimen
to a constant stress while maintaining the temperature constant
and measuring the extent of deformation.
The resulting data are presented as deformation (strain)-time
curve.
6. Details
When a load is applied at the beginning of a creep test, the instantaneous
elastic deformation (AB) is followed by transient or primary creep (BC) then
the secondary or steady-state creep (CD) and finally by tertiary or
accelerated creep (DE).
Instantaneous deformations Elastic
The primary creep rate has a decreasing rate because of work hardening. It
is similar to delayed elasticity (retarded elasticity) and the deformations are
recoverable.
Secondary creep is essentially viscous in character. The minimum creep rate
(V0) is determined by the slope Δε/Δt.
RT
E
n
eA
t
9. Effect of nano particles in polymers
Many studies shows that the addition of nanoparticles to
neat polymers can significantly improve the creep
resistance of the polymers at various stress and
temperature levels.
During creep deformation, the molecularchains are
stretched and re-oriented.
Creep behaviour is affected by molecularweight,degree of
crystallinity, and the chemical resistance of the polymer.
10. The entangled high aspect ratioCNTs have strong interaction
with the polymer chains and potentially act as physical
barriers for chain re-orientation.
This pseudo physical barrier effectively reduces creep
deformation when the material is stressed.
At low stress level interfacialbonding holds and the CNT forms
a network of crosslinking elements that interconnect the
epoxy chains,this action increases the effective crosslinking
density of the network which reduces mobility and
reorientation of the amorphous epoxy chains.
At higher stress level polymer/CNTinterface are prone to
failure and as a the ability of the nanotubes to limit polymer
chain movement is negligible.
Creep behaviour on epoxy/CNT nanocomposites
12. Creep tests
Creep performance is commonly
represented by creep compliance J(t) and
where ε(t) is creep strain and σ is applied
stress.
For dry atmosphere and NC with different
filler content the creep compliance curves
are given in Fig. 5.
As seen from the figure the inclusion of clay
nanoparticles into epoxyresin led to a
sufficient reduction of creep compliance of
NC specimens dried in atmosphere of 24%
RH. Nevertheless for humid atmospheres
absorbed moisture
significantly affected creep behaviour,
leading to high deformability.
13. Creep of PU/CLAY nanocomposites
Figure 11 shows the creep and
recovery
curves of PU/clay C20A
nanocomposites.
The addition of clay leads to a lower
creep recovery.
