A nonlinear viscoelastic characterization of creep rupture in a reinforced plastic

The long‐term behavior of a glass mat‐reinforced polypropylene has been characterized by accelerated flexural creep tests on samples of the laminate. Considerable data were collected for stress levels ranging from 200 to 7500 psi at six temperatures from room temperature to 140 °C. The material was found to behave in accordance with the linear theory of viscoelasticity except at the longer times where the creep rate increases dramatically and the specimen ruptures. The stress and temperature dependence of this “terminal creep” zone was treated by means of a slightly modified version of Schapery's thermodynamic theory of nonlinear viscoelasticity, which involves the superposition of double logarithmic plots of creep compliance versus time. The stress and temperature dependence of both the free energy function and the entropy production emerge from this treatment. A comparison of nonisothermal creep data with a prediction from the isothermal master curve is marginally acceptable. A statistical synthesis of the data discloses that the incidence of terminal creep is log‐normally distributed. Moreover, the standard deviation associated with the distribution is independent of temperature. It is therefore possible to assess the probability of failure for any constant stress at any constant temperature.