Evaluation of Thermal Stresses at the Interface of a Stitch in a Stitched Laminate

The stresses in the vicinity of a single stitch through the thickness of an infinite cylindrical cross-ply laminate subjected to a unit temperature decrease are computed. Interest centers on the potential for microcracking in the vicinity of the through-thickness reinforcement. The infinite cylindrical cross-ply model is a simplification which leads to a tractible axisymmetric formulation, but which retains many of the three-dimensional features of the problem. In the model the stitch is a through-thickness cylinder consisting of fibers and resin, and the laminate consists of plys with fibers oriented radially and plys with fibers oriented circumferentially. Here only a [O/90]2s laminate is examined. Both glass and Kevlar stitches are considered, as is the possibility of resin pockets forming near the stitch. The results indicate that the presence of a resin pocket can alter the stress state in the vicinity of the stitch. In particular, the sign of a peak in the radial stress that occurs at the outside of the laminate changes from compression to tension when resin pockets are considered. The normal stresses in the resin pocket are all positive and their values are larger in the case of Kevlar stitch than in the case of glass stitch. This implies that the Kevlar stitched laminate is more likely to produce microcracking due to high residual stress after curing than the glass stitched laminate.