Strain energy release rate associated with matrix cracking in the theta-layer of unbalanced symmetric (0/theta)_s composite laminate is predicted analytically using a 2-D shear lag stress analysis and the Equivalent Constraint Model (ECM). Its dependence on the crack density and ply orientation angle is examined for glass/epoxy and carbon/epoxy laminates. Suitability of a mixed mode fracture criterion to predict the cracking onset strain is discussed.
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