Improved variable-amplitude fatigue behavior of a glass-fiber-reinforced hybrid-toughened epoxy composite

A thermosetting epoxy polymer was hybrid-modified by the addition of 9 wt.% of rubber microparticles and 10 wt.% of silica nanoparticles. The GFRP composite laminates employing the unmodified epoxy matrix (GFRP-neat), and the hybrid epoxy matrix (GFRP-hybrid), were produced by a resin-infusion technique. The experimental fatigue lives of both GFRP composites under three different variable-amplitude load sequences, namely (a) a three-step increasing block (IB), (b) a three-step decreasing block (DB), and (c) a random block (RB) load sequence derived from a three-step load block, were determined. The fatigue life of the GFRP-hybrid composite was higher than that of the GFRP-neat composite under all the three load sequence blocks investigated, by about x 2.6 to x 4.0 times. The matrix crack density and the stiffness reduction rate were both lower in the GFRP-hybrid composite compared to the GFRP-neat composite material. The suppressed matrix cracking and reduced delamination growth rates in the hybrid-modified epoxy matrix enhanced the fatigue life of the corresponding GFRP-hybrid composite. Using the constant-amplitude fatigue data generated at various stress ratios, the fatigue lives under these variable-amplitude load sequence blocks were predicted using empirical models. The predicted fatigue lives, although conservative, were in reasonably good agreement with the experimental results.