To clarify the basic characteristics of off-axis ply cracking, unidirectional and angle-ply laminates were tested. Laminate configurations were [θ]_8 and [+- θ]_s, where 9 = 15, 30, 45, 60 and 75. For unidirectional composite, pre-cracks were introduced at the center of either side of the free edge using razor blade. Static tensile tests were conducted at room temperature. Microscopic damage progress was observed using video microscopy and soft X-ray radiography. For unidirectional laminates, mode Ⅰ (G_Ⅰ) and Ⅱ (G_(Ⅱ)) energy release rate associated with crack progress were calculate using FE method. Mode Ⅰ and Ⅱ energy release rate became larger as the crack progress. G_(Ⅱ) ratio to G_Ⅰ became larger as the ply angle θ became smaller. That is, mixed mode fracture criterion were necessary to predict the crack progress in small angle ply. For angle-ply laminates, damage mechanics analysis was used to predict the modulus reduction caused by matrix cracking. Analysis underestimated modulus reduction. Nonlinear analysis which include plastic deformation is necessary for accurate prediction.
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