Fatigue behavior of un-notched and open-hole quasi-isotropic pseudo-ductile thin-ply carbon/glass hybrid laminates

This paper investigates the fatigue behavior of un-notched and open-hole Quasi-Isotropic (QI) thinply interlayer hybrids made of thin-ply T300 carbon/epoxy plies sandwiched between standard thickness S-glass/epoxy plies. Samples were initially quasi-statically loaded to failure to determine the critical stress level (pseudo-yield stress (σpy)) at which significant damage starts, i.e. multiple fragmentation of the carbon plies. Quasi-static results showed successful pseudo-ductility and notch-insensitivity before the final failure. The hybrid configurations were loaded at 4 Hz in tension-tension fatigue at different percentages of the σpy below that required to cause significant damage. It was observed that, for the un-notched samples cycled at 80% of the σpy, there is a small stiffness reduction from the early cycles caused by matrix cracking, and there is little stiffness reduction up to 100,000 cycles. By increasing the stress level to 90% of the σpy, there is a gradual stiffness reduction due to the appearance of the fragmentation and delamination which leads to the failure of the un-notched samples after 10,000 cycles. A similar behavior was observed for the open-hole laminates, where initially matrix cracking and subsequently some stiffness reduction were observed for the low stress level (50% of the σpy), with no more stiffness reduction up to 100,000 cycles. However a gradual delamination growth was observed for the high stress level (70% of the σpy), which leads to the final failure after 30,000 cycles. Overall good fatigue performance of pseudo-ductile hybrid composites, with gradual fatigue damage growth was observed, that shows their applicability for industrial applications.