Viscoelasticity and memory

When a constant strain is imposed, the stress decreases with time. This is called stress relaxation. The manner of the relaxing stress is a manifestation of the memory loss. 

The relaxation time, A, is expressed as the ratio of viscosity, tj, and elastic (shear) modulus, G, 

When a system has a single relaxation time, this relationship is easy to nnderstand. When there is a distribution of relaxation times, the meaning of rj and G, is not very simple. Therefore, instead of separating the relaxation time into t] and G, the relaxation time itself will be used to describe material behaviour. 

In the rubber processing neither stress nor strain remains constant over time. Therefore, creep or stress relaxation is not an appropriate model. On the other hand a very complicated stress-strain history is difficult to interpret. A simpler model which offers some insight into processability may be found in transient behaviour. One example is a stress-growth under a constant rate of deformation and observation of stress relaxation. To be realistic, the strain must be large and the behaviour must be in nonlinear viscoelasticity. The experiments may be carried out with a rotational rheometer but they are restricted to low rates of deformation, because otherwise rubber slips at the rheometer interface [47, 48]. 

Curve 3 is the stress relaxation from the instantaneous deformation, which retains 100% memory at = 0 and exhibits the memory-loss thereafter. 

---