Stress rupture of unidirectional polymer matrix composites in bending at elevated temperatures

A new method for stress-rupture experiments in bending has been developed and used to characterize unidirectional polymer matrix composites. The method. which makes use of very simple fixtures, led to coherent results. These results have been modeled using the large deflection of buckled bars theory (elastica) and it is possible to predict with good accuracy the strain at each point of the specimen if the end-to-end distance is known. The failure process has been experimentally characterized. The formation and propagation of microbuckles leads to a compressive failure. Based on the elastica and the classical lamination theory, a model for the distribution of the Young's modulus along the length of the specimen has been established. Three different micromechanical models have been applied to analyze the time-to-failure versus strain behavior at two temperatures - one below and one above the glass transition. The first micromechanical model considers the nucleation of the microbuckles as the main cause of failure. In addition, the stiffness and stress distributions at any time before failure are calculated based upon the rotation of the fibers in the damaged region. The second and last models, respectively based upon a Paris Law and energy considerations relate the time-to-failure to the propagation of the main microbuckle. For this last model, a good correlation between experimental and theoretical data has been obtained. Finally the influence of the temperature on these models has been studied.