Analysis of a delayed fracture criterion for lifetime prediction of viscoelastic polymer materials

In this work a multi-axial yield/failure model for viscoelastic/plastic materials is applied, which was developed by Naghdi and Murch (in J. Appl. Mech. 30:321-328, 1963) and later extended and refined by Crochet (in J. Appl. Mech. 33:327-334, 1966), to predict long-term creep rupture of polymers. The criterion defines a function, which depends on time, the viscoelastic properties and applied stress, to establish an empirical law with creep yield (fracture). In this work a linear relationship is proposed, defined as a time-dependent failure criterion, which can be applied for extrapolation purposes. A comparative analysis using energy-based failure criteria is performed. It is proved, for the polymers considered in this study, that the proposed time-dependent failure criterion holds for long times. Experimental data are used to illustrate the applicability of this time-dependent failure criterion.