Flexural Fatigue Performance of Glass Fiber/Epoxy Step-Wise Functionally and Non-Functionally Graded Composites of Different Structures

Flexural fatigue performance of newly developed step-wise functionally graded (FG) composites has been investigated experimentally. Two unidirectional glass fiber (UGF)/epoxy FG composites with overall fiber volume fraction of 39.9 +/- 0.3% and thickness of 6.0 +/- 0.1 mm were fabricated using hand lay-up technique. The fabrication proposed technique depends on the variation of the composite structure achieved by three different templates. A non-functionally graded (NFG) composite laminate with the same overall fiber volume fraction and thickness was also fabricated using a fourth template for comparison. Each of the FG and NFG composites is made of six plies. Each ply has a thickness of 1.0 mm. This occurs by controlling the amount of matrix in each ply (constant matrix amount for all plies). Fatigue tests were performed at zero mean stress. A 20% reduction of the initial flexural stiffness was taken as a failure criterion. The S-N curves for the fabricated FG and NFG composites have been constructed as design curves. For FG composites, the effects of the spatial gradient of fiber content and plies stacking sequence on both S-N relationships and the specimens' surface temperature rise were studied. Two-parameter Weibull distribution function was used to statistically analyze the fatigue life results.