Adaptations of linear theory - differential models

Recall that the first term on the right is equal to the stress after a long time, or equivalently the stress produced by loading at an infinitely slow rate. The equation can be generalized to 

Kitagawa et al, in their work on polypropylene, and Kitagawa and Takagi, working on polyethylene, used torsion testing. Their equivalent of Equation (10.6) in terms of shear stress and shear strain takes the form 

Stress relaxation tests were used to evaluate K. According to Equation (10.10) 

Similar work with polyethylene [9] revealed a comparable level of accuracy. The differential approach has been applied also to high-density polyethylene by Zhang and Moore [11,12]. 

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