Flexural performance and damage mechanisms of stitched composites under different stitch patterns and densities by acoustic emission

The flexural performance of standard composites has been extensively studied; however, the response of stitched composites with different stitch patterns and densities has received little attention and is not well understood. This study systematically explored the flexural performance of stitched composites with different stitch patterns and densities. The stitches weakened the flexural strength while improving the residual strength of the composites, particularly under a high stitch density. In addition, the stitch threads in the longitudinal stitch pattern preferably resisted the deformation of the bottom side of the specimen, showing superior flexural strength compared with the diagonal and transversal stitch patterns. However, the stitch threads in the diagonal and transverse stitch patterns impeded crack propagation and enhanced the residual strength of the composites. In addition, acoustic emission was used to confirm that the acoustic emission signals and the first moment of rapid increase in cumulative acoustic emission energy were detected earlier under a high stitch density, suggesting that the increase in stitch defects promotes damage initiation and severe damage. The durations with high values and a continuous increase in the cumulative acoustic emission energy after brittle failure indicated that the stitched composites experienced non-catastrophic failure.