Analysis of the effect of stitching on the fatigue strength of single-lap composite joints

The influence of selective stitching on the fracture behaviour of co-cured single-lap joints under fatigue loading was investigated by means of an experimental and numerical analysis in order to identify the mechanics of through-thickness reinforcement and to quantify its effect on the fracture response of delaminated joints. It was observed that while stitching does not improve the static strength of joints, it significantly extends both the crack initiation and crack propagation phases. The extension of the delamination growth stage, in particular, was attributed to the bridging action of stitches on delaminated adherends, which – as indicated by finite element calculations – suppresses the mode I component of strain energy release rate and thus reduces crack propagation rates. By exploiting the experimental observations and the results of FE analysis, the fatigue crack propagation and the final failure of stitched joints were successfully predicted by an energy-based approach adopting a Paris power-law.