The Application of Kinetic Fracture Mechanics to Life Prediction for Polymeric Materials

A recently developed theory of kinetic crack growth is applied to the problem of the time-dependent failure of polymeric materials subject to a timewise constant state of stress. The theory is generalized to include a rather complete treatment of statistical variability. When restricted to the power law range of behavior, it is found that an assumption of a Weibull distribution of static strengths implies rigorously that the corresponding lifetime distributions are also Weibully distributed. In fact, an extremely simple interrelation is found whereby the shape parameter for lifetime is equal to the product of the shape parameter for static strength times the magnitude of the log stress versus log lifetime slope of the scale parameter for the lifetime distribution. These theoretical predictions are compared with the experimental results available from the testing of a particular polymeric fiber composite material, Kevlar 49/epoxy. The theoretical prediction for the lifetime distribution compares favorably with the observed data.