THE INITIATION AND GROWTH OF DELAMINATIONS INDUCED BY MATRIX MICROCRACKS IN LAMINATED COMPOSITES

A recent variational mechanics analysis of microcracking damage in cross-ply laminates of the form [(S)/90n]s, where (S) is any orthotropic sublaminate much stiffer than [90n], has been extended to account for the presence of delaminations emanating from the tips of microcracks in the [90(2n)]T sublaminate. The new two-dimensional stress analysis is used to calculate the total strain energy, effective modulus, and longitudinal thermal expansion coefficient for a laminate having microcracks and delaminations. These results are used to calculate the energy release rate for the initiation and growth of a delamination induced by a matrix microcrack. At low crack densities, [(S)/90n]s laminates are expected to fail by microcracking and to show little or no delamination. At some critical crack density, which is a function of laminate structure and material properties, the energy release rate for delamination exceeds that for microcracking and delamination is predicted to dominate over microcracking. A quasi-three-dimensional model is used to predict the propagation of arbitrarily shaped delamination fronts. All predictions agree with experimental observations.