Damage-Caused Residual Curvatures in Symmetric Cross-Ply Laminates

Thermo-mechanical response of [90_n/0_m]_s carbon/epoxy and glass/epoxy cross-ply laminates in 4-point bending is analyzed experimentally and analytically. Intralaminar cracks in surface 90°-plies and local delaminations introduced in one of the 90°-plies at large deflections reduce the laminate bending stiffness and make the laminate asymmetric due to differences in the damage state in the layers. The latter leads to residual thermal curvature that increases with intralaminar crack density and with growing local delaminations. In the present study optical microscopy was used for crack density quantification. It was also found experimentally that small local delaminations develop in the initial stage of damage evolution and under increasing load they grow rapidly from the existing and newly created crack tips. The effect of damage on residual curvature and the bending stiffness was analyzed using an analytical method, where the concept of the effective stiffness of damaged ply is used in the classical laminate theory. Analytical results were validated with a 3-D FEM simulation of the damaged laminate in a 4-point bending test. In the literature a phenomenon that the microdamage in laminate layers causes redistribution of in-plane thermal stresses is often overlooked. The present paper shows that the used analytical approach gives an accurate description of experimental results regarding two independent sets of data: the residual curvature; and the laminate bending stiffness with evolving micro-damage. The present study also renders a better insight in the mechanics of the phenomena and allows estimation of the extent of local delaminations that is difficult to measure in tests.