The effect of delaminations induced by transverse cracks and splits on stiffness properties of composite laminates

In an effort to evaluate stiffness degradation due to delaminations growing at the 0 degrees/90 degrees interface from the tips of transverse cracks in the 90 degrees plies and splits in the 0 degrees plies of cross-ply [0(m)/90(n)](s) laminates, a new theoretical approach was developed. It employs the Equivalent Constraint Model of the damaged lamina [Fan J, Zhang J. In-situ damage evolution and micro/macro transition for laminated composites. Composites Science and Technology 1993;47:107-118], which allows one to avoid cumbersome consideration of the repeated laminate element defined by the intersecting pairs of transverse cracks and splits. It also uses an improved 2D shear-lag analysis [Zhang J, Soutis C, Fan J. Strain energy release rate associated with local delamination in cracked composite laminates. Composites 1994;25(9):851-862] to determine the stress fields in the explicitly damaged lamina and the In situ Damage Effective Functions to describe its reduced stiffness properties. Reduced stiffness properties of the damaged lamina are found to depend explicitly upon the crack density and relative delamination area associated with that lamina and implicitly upon two damage parameters associated with the neighbouring lamina. Theoretical predictions reveal that transverse crack tip delaminations cause significant reduction in the shear modulus and Poisson's ratio of cross-ply and symmetric balanced [+/-theta(m)/90(n)](s) laminates. Dependence of the laminate reduced elastic properties on the orientation angle of the constraining ply is examined. Contribution of each damage mode (transverse cracking, transverse crack tip delaminations, splitting and split tip delaminations) into stiffness loss is established. (C) 2000 Elsevier Science Ltd. All rights reserved.