Micromechanical behavior of Kevlar-149/S-glass hybrid seven-fiber microcomposites. II: 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 Kevlar 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 microcomposites were stress-rupture tested at 80, 85, and 90% of their Weibull scale parameters for tensile strength. To predict the stress-rupture lifetime of a hybrid composite, a stochastic model was proposed which assumed that the failure of LE fibers in a hybrid follows a continuous-time Markov chain. Both the model and the stress-rupture tests of the hybrid and Kevlar 149 SFT microcomposites indicated that there is a negative hybrid effect for lifetime at high stress ratio while a positive hybrid effect exists at low stress ratio. The higher fraction of the total load for each LE fiber was responsible for the negative hybrid effect 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 predicted using the model is reasonably close to the experimental results.