MICROMECHANICAL BEHAVIOR OF KEVLAR-149 S-GLASS HYBRID 7-FIBER MICROCOMPOSITES .2. STOCHASTIC MODELING OF STRESS-RUPTURE OF HYBRID COMPOSITES/

To understand the stress-rupture behavior of hybrid composites, single -fiber type (SFT) and hybrid microcomposites were fabricated with Kevl ar 149 as the low-elongation fiber and S-glass as the high-elongation fiber in a DER 331/DER 732 epoxy mixture (70/30, w/w). The microcompos ites were stress-rupture tested at 80, 85, and 90% of their Weibull sc ale parameters for tensile strength. To predict the stress-rupture lif etime of a hybrid composite, a stochastic model was proposed which ass umed that the failure of LE fibers in a hybrid follows a continuous-ti me Markov chain. Both the model and the stress-rupture tests of the hy brid and Kevlar 149 SFT microcomposites indicated that there is a nega tive hybrid effect for lifetime at high stress ratio while a positive hybrid effect exists at low stress ratio. The higher fraction of the t otal load for each LE fiber was responsible for the negative hybrid ef fect at high stress ratio, while smaller static and dynamic overstress was beneficial to the positive hybrid effect at low stress ratio. The mean lifetime for the hybrid and Kevlar 149 SFT microcomposites predi cted using the model is reasonably close to the experimental results.