Abstract
Mixed-mode I+II fracture toughness of unidirectional graphite/epoxy laminates, T800H/# 3631. was studied. Mixed-mode propagation of interlaminar cracks was examined by mixed-mode bending tests where crack growth was stabilized by the coordinate conversion control method. Intralaminar fracture toughness was measured by tension tests of off-axis laminates with an inclined crack-like notch. The value of interlaminar fracture toughness increased with crack extension for GI/GIIl1.37 because of development of fiber bridging, while was nearly constant during crack growth for GI/GIIg0.57. There was no big difference between the interlaminar and intralaminar fracture conditions in the range of the mixed-mode ratio of GI/GIIl0.11. For GI/GIIl0.57, the mode I component of mixed-mode fracture toughness, GI, was insensitive to the mode II component, GII. while for GI/GIIl0.57, the GI value decreased to zero as the GI/GII ratio decreased. A new criterion for the mixed-mode fracture condition was proposed, showing a good fit to the experimental results.
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