Characterization of interlaminar fracture modes I, II, and I-II of carbon/epoxy composites including in-service related bonding quality conditions

The effects of hot/wet environmental conditions for intervals of one and two years of exposure on fracture modes I, II, and I-II of biaxial carbon/epoxy composites were characterized. Tests were carried out on double cantilever beam, end-notched flexure, four-point end-notched flexure, and mixed mode bending specimens. For the purposes of this study, it was recognized that water absorption was governed by the Fickian mechanism. The effective crack method was used to analyze mode II and mode I-II with a high shear mode participation ratio. Hygrothermal effects degraded initiation toughness in all fracture modes, and extensive fiber bridging along with multiple damage modes within the mid-layers in specimens exposed to heat and humidity altered crack propagation behavior. Mixed mode fracture test results revealed a weak interaction between modes I and II for most parts of G_I/G_II, but the interaction was strongest when the fracture behavior changed from a pure mode to a mixed mode condition.