Analysis of damage in composite laminates under bending

The focus of this research was damage formation in composite laminates subjected to bending. Matrix cracking and internal delamination are common damage modes before final failure for a composite laminate under thermo-mechanical loading. Two configurations of cross-ply laminates, namely [0_m/90 _n]_s and [90_m/0_n]_s, were considered for the study. Approximate analytical solutions for the stress states in the two laminates subjected to constant bending moment, with matrix cracks in the 90° layers, were derived using a variational approach. The evolution of matrix cracking under monotonically increasing load was studied for a number of composite materials. The analytical predictions showed an initial stage of rapid matrix crack multiplication followed by a slowing down in the crack multiplication. In the case of [0_m/90_n] _s laminate, 0° ply failure in tension or compression was found to be likely even at the initial stages of matrix cracking for laminates with thin 90° layer. The 0° ply failure is delayed for laminates with thicker 90° layer.; The analytical model for the stress state in the [90_m/0 _n]_s laminate was extended to include delamination from the matrix crack-tip along the 0/90 interface. The bending moment required to initiate crack-tip delamination was computed as a function of the crack density. By comparing this result with matrix crack evolution, the relative dominance of the two modes of damage could be determined. The critical crack density beyond which delamination dominates matrix cracking is obtained from the analysis. The critical crack density is interpreted as the stage beyond which growth of delamination rather than matrix cracking is likely to occur. Parametric studies conducted by varying the laminate configuration showed that the critical crack density for delamination onset strongly depends on the thickness of the 90° layer and the distance of the 90° layer from the laminate mid-plane.; Quasi-static growth of delamination under monotonic loading was studied for the laminate with matrix cracking of critical density. Comparison of the load for delamination growth with the load for possible formation of further matrix cracking was done to determine the possibility of matrix cracking after delamination. Predictions for different laminate lay-ups of a graphite/epoxy system showed the possibility of matrix cracking after delamination grew to a critical length. This critical delamination length was found to be close to the crack spacing in most of the cases considered, i.e. further matrix cracking is likely to occur only after a large portion of the laminate between two consecutive cracks delaminates. Possible failure of 0° layer before further matrix cracking was predicted for laminates with thin 90° layer.