Uniaxial creep or stress-relaxation response

We recognize at the onset that the structural relaxations promoted in the polymer by an applied stress are of a local distortional nature, producing shear strain responding to the applied shear stress in a thermally assisted manner. Some 

Mueh of the formal development of this section follows that of McCrum et al. (1967), to which the reader is also directed. 

Consider the application of a constant shear stress ao to a viscoelastie solid at t = 0 in simple shear (or alternatively in tension as pure shear). The time-dependent response is ideally an instantaneous elastic flexure followed by time-dependent creep as depicted schematically in Fig. 5.1(a), which at a monotonically decreasing rate asymptotieally approaches a constant shear strain proportional to the applied shear stress. Removal of the shear stress at any time t results in an instantaneous elastic recovery followed by a reverse creep response that asymptotically returns the solid to its initial state, as also depicted in Fig. 5.1(a). 

Consider next a complementary response of the solid to the application of a constant shear strain /q at t = 0. Then, the idealized unit stress response as a function of time is depicted in Fig. 5.2 as an instantaneous resistive shear stress governed by elasticity, that in time relaxes from an initial unrelaxed level to a final asymptotic level with a complementary stress relaxation response given by 

4)= a t)lyQ is defined as the relaxation modulus, changing from an initial high unrelaxed level /ry eventual low, fully relaxed level asymptotically 

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