The Mechanism of Creep Depends on Temperature and Stress

As Meyers el al. [3] point out, polymers remain as solid material even when these parts of their chains are rearranging in order to accompany the stress, and as this occurs, it creates a back stress in the material. When the back stress is of 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. 

Viscoelastic materials can follow at least three different behaviors, i.e. linear vis-coleasticity, nonlinear viscoelasticity and anelastic behaviour. 

Linear viscosity is that when the function is splitted in both creep response and load. All linear viscoelastic models can be represented by a classical Volterra equation connecting stress and strain [1-9]  

Linear Viscoelasticity is usually applicable only for small deformations [2,3,5, 23-25], 

The analysis viscoelasticity performed by David Roylance [25] is a nice outline about the mechanical response of polymer materials. This author consider that viscoelastic response is often used as a probe in polymer science, since it is sensitive to the material s chemistry and microstructure [25], While not all polymers are viscoelastic to any practical extent, even fewer are linearly viscoelastic [24,25], this theory provide a usable engineering approximation for many applications in polymer and composites engineering. Even in instances requiring more elaborate treatments, the linear viscoelastic theory is a useful starting point. 

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