Fatigue mixed-mode delamination testing

The separation of adjacent plies in a composite laminate (delamination) has been identified as the dominant failure mechanism in which most advanced composite materials fail. The resistance of a material to delamination is quantified in terms of the delamination fracture toughness, G(c), which is also known to change with the ratio of mode mix. The recently developed mixed mode bending (MMB) testing method is unique in that it allows the measurement of G(c) under any combination of mixed Mode I and Mode II loading. This makes it an ideal tool for mapping the interlaminar fracture behavior of the various material systems. The present work is concerned with the use of the MMB apparatus for fatigue testing, from which G(c) Vs N plots are obtained. These allowed the identification of the threshold value G(threshold). Preliminary static and fatigue results are presented for two aerospace type materials, IM7/8320 graphite/thermoplastic and IM7/5260 graphite/bismaleimide specimens, under various GI/GII ratios.