Approximte Relations for the Analysis of Single-Step Stress--Relaxation Data in Uniaxial Extension From Experiments Involving a Finite Step Time

In the determination of single-step stress--relaxation behaviour, a finite time is required to reach the desired strain. As a result, an uncertainly is introduced into the observed behaviour at the early times. In the region of linear behaviour, an approximation has previously been derived which can be applied to shear stress--relaxation experiments. Approximate relations are derived which can be applied to uniaxial extension experiments in the region of non-linear behaviour. The derivations are based on the assumption that, under the set of strain histories considered, one can use the Bernstein, Kearsley and Zapas theory (Trans. Soc. Rheol. , 1963, 7, 391) as a one-dimensional description. To demonstrate the validity of the approximate relations, data have been obtained on a linear low-density polyethylene copolymer under conditions of a varied step time and strains well into the region of non-linear behaviour. 8 ref.--AA