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The amount of relaxation which takes place is a function of time, temperature and stress level, thus the actual effect it has on the system is not precisely known, but can be bounded. Stress relaxation describes how polymers relieve stress under constant strain. Because they are viscoelastic, polymers behave in a nonlinear, non-Hookean fashion. [1]
When the back stress is the same magnitude as the applied stress, the material no longer creeps. When the original stress is taken away, the accumulated back stresses will cause the polymer to return to its original form. The material creeps, which gives the prefix visco-, and the material fully recovers, which gives the suffix -elasticity. [2]
During a shearing process, such as extrusion, LLDPE remains more viscous and, therefore, harder to process than an LDPE of equivalent melt index. The lower shear sensitivity of LLDPE allows for a faster stress relaxation of the polymer chains during extrusion, and, therefore, the physical properties are susceptible to changes in blow-up ratios.
Dynamic mechanical analysis (abbreviated DMA) is a technique used to study and characterize materials.It is most useful for studying the viscoelastic behavior of polymers.A sinusoidal stress is applied and the strain in the material is measured, allowing one to determine the complex modulus.
Schematic of the evolution of the instantaneous modulus E(t,T) in a static relaxation test. t is the time and T is the temperature. Consider the relaxation modulus E at two temperatures T and T 0 such that T > T 0. At constant strain, the stress relaxes faster at the higher temperature.
The stress relaxation modulus () is the ratio of the stress remaining at time after a step strain was applied at time =: = (), which is the time-dependent generalization of Hooke's law . For visco-elastic solids, G ( t ) {\displaystyle G\left(t\right)} converges to the equilibrium shear modulus [ 4 ] G {\displaystyle G} :
Figure 4. a) Applied strain in a relaxation test and b) induced stress as functions of time over a short period for a viscoplastic material. As shown in Figure 4, the relaxation test [19] is defined as the stress response due to a constant strain for a period of time. In viscoplastic materials, relaxation tests demonstrate the stress relaxation ...
Dependence of dimensionless stress upon dimensionless time under constant strain. If a Maxwell material is suddenly deformed and held to a strain of , then the stress decays on a characteristic timescale of , known as the relaxation time.
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