The effect of matrix stiffness on the creep-rupture lifetime of carbon fiber/epoxy composites

Seven-fiber, hexagonally-packed microcomposites were made using PAN-based carbon fibers embedded in two epoxy systems, one stiffer than the other. The stiffer system was a bisphenol-A-based epoxy having an initial modulus of 1620 MPa and a strain to failure of 0.09, while the more flexible system, with an initial modulus 538 MPa and a strain to failure 0.32, was a 50/50 (w/w) mixture of the stiffer epoxy and a polyglycol diepoxide. A tetraethylene pentamine hardener was used with both. Creep-rupture tests (N,24) were carried out at 80% and 75% of the value of the Weibull scale parameter (63rd percentile) for microcomposite strength. At these stress levels, the scale parameter for lifetimes was significantly (P<0.05) greater for the stiff epoxy: 74 s vs. 6 s at 80% and 294 s vs. 26 s at 75%. Monte Carlo simulations show that the logarithms of creeprupture lifetime are inversely proportional to the propensity to creep