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The origins of nonlinearity in the Mode II delamination fracture of three organic matrix, carbon fiber composite materials was investigated. This was accomplished by testing specimens with different types of starter cracks and by loading and unloading these specimens several times so that the change in nonlinearity as the delaminations grew could be measured. The load at which crack growth initiated was determined by acoustic emission. Slow crack growth was found to be the principal cause of nonlinearity in the materials tested. The crack velocity obeyed the same power law dependence on GII as has been observed for slow crack growth in visco-elastic polymers. For the first loading from the end of the starter cracks, plastic deformation at the crack tip also contributed to the nonlinearity. Other sources of nonlinearity included an increasing fracture resistance in one of the materials as well as problems associated with producing a clean, sharp starter notch with a straight crack front. The implications of these findings to the use of linear elastic fracture mechanics for predicting delamination behavior of these materials and to the measurement of Mode II interlaminar fracture toughness are also discussed.
delamination, fracture, Mode II, nonlinearity, visco-elastic polymers, plastic, R, -curve, test methods, composite materials, fatigue (materials) fracture
Leader, Defence Research Establishment Pacific, FMO, Victoria, British Columbia