Fatigue performance impact of hybridization on Kevlar/flax and glass/flax hybrid composites

This research investigates the effects of hybridization with flax in two distinct laminates: Kevlar/flax (Kevlar/flax/epoxy, KFE) and glass/flax (glass/flax/epoxy, GFE), each featuring two flax fiber orientations (0° and ±45°) under stress-controlled conditions. Employing the wet hand lay-up method, the laminates, comprising 16 plies in a sandwich structure, undergo tension–tension stress-controlled loading at 5 Hz with R = 0.1 until failure or completion of 106 cycles. A comprehensive analysis, encompassing fatigue life, damage modulus, residual strain, fatigue modulus, and hysteresis energy, is conducted to discern the synergies and influences of Kevlar and glass with flax fibers. Results indicate that KFE hybrid specimens exhibit exceptional fatigue strength, surpassing other laminates in load endurance by a significant margin (1.22x–2.46x) over the same cycle count. Conversely, GFE hybrids, despite demonstrating initially higher strength, experience a rapid decline in endurance, particularly evident in the 0° GFE hybrids, which exhibit a 0.43x reduction. Moreover, both KFE hybrids demonstrate a more gradual rate of decline compared to their corresponding GFE hybrids (0.82x and 0.63x) and KFE UD (unidirectional) samples show less sensitivity than FE UD (0.87x). These findings suggest that Kevlar forms a highly effective hybrid with flax, whereas glass, despite common comparisons with flax, does not yield a favorable hybrid for structural applications.