Failure of Glass Fiber-Reinforced iPP in Static and Cyclic Loading Characterized by Factorizability of Time, Temperature, and Fiber Orientation

The mechanical properties of fiber-reinforced thermoplastics such as short or long glass fiber-reinforced isotactic polypropylene are anisotropic due to the fiber orientation and time- and temperature-dependent due to the viscoelasticity of the matrix material. Consequently, material characterization involves substantial short- and long-term testing campaigns spanning a broad range of testing conditions, that is, rate, temperature, fiber orientation, and so forth. This study presents a framework based on the combination of previously reported experimental observations captured in phenomenological models for both plasticity and slow-crack growth-driven failure. It is based on the observation that for plasticity-driven failure the influence of applied rate and temperature on the yield stress can be multiplicatively decoupled from the fiber orientation. This implies that capturing the rate and temperature dependence at a single fiber orientation combined with the fiber orientation dependence at a single rate and temperature enables the prediction of plasticity-controlled failure over a broad range of rates, temperatures, and fiber orientations. Additionally, the same factorizability concept is found to be applicable to the slow crack growth-controlled failure regime using the same anisotropic scaling function as obtained for the plasticity-controlled failure regime to predict the fatigue time-to-failure as a function of temperature, fiber orientation, load ratio, and frequency.