MODEL FOR CREEP AND EARLY FAILURE

Tensile creep is described in terms of a generalized phenomenological rheological model that consists of a parallel array of a large number of Maxwell elements modified by irreversible fracture and contact elements. The model, which is equally applicable to small and large deformations (including the failure region), implies non-linear viscoelasticity, memory and irreversibility. It also suggests that the shapes of the creep curves contain information that can indicate early failure in creep. To demonstrate this property of the model, however, an empirical approach had to be adopted and this was done as follows. Published creep curves for five different polymers were presented in the linear form t ε{lunate}(t)= 1 ab+ t a where ε{lunate}(t) is the strain at tie t, and a and b are constants characteristic of the material and the initial stress. The constant a in this context is the asymptotic strain ε{lunate}A and is therefore an indication of the tendency of the strain to stabilize within the given experimental time scale. Consequently, asymptotic moduli EA could also be calculated and plotted versus their corresponding initial stresses σ0. The resulting curves have typical distinct shapes that manifest the existence or absence of dominant strain hardening. In the latter case the curves could be successfully fitted by an equation of the type EA=A-Bq0n where A, B and n are empirical constants. Since tensile failure in terms of the model is reached when EA = 0, the initial stress σF that will eventually lead to early failure can be calculated from qF=( A B) 1 n It is shown that the ranges of published failure data are in agreement with the levels calculated from the above equation with n > 2 and depending on the material. © 1979.