Effect of Stitching on the Static and Fatigue Performance of Co-cured Composite Single-lap Joints

The paper investigates the effect of Kevlar stitching on the tensile static and fatigue properties of co-cured single-lap joints with cross-ply graphite- epoxy adherends. The variables considered in this study were the overlap length, the stitch spacing, and the off-edge distance. Experimental results indicate that while stitching does not improve the static strength, stitched joints exhibit much better fatigue performance than unstitched joints. Ultrasonic analyses and continuous monitoring of stiffness change during fatigue life allowed us to follow delamination progression and to clearly identify the nucleation and the propagation stages, in particular, stitching was observed to increase the durations of both the crack initiation and crack propagation phases, with stitches remaining intact until complete failure. Moreover, in the range of values examined, the fatigue behavior of stitched joints is improved by reducing the stitch spacing, while it appears to be insensitive to the variation of the off-edge distance of stitches.