Compression properties and damage mechanisms of stitched carbon/epoxy composites

The effect of stitching on compression properties and damage mechanisms of carbon/epoxy composites is experimentally investigated. The waviness angle of load-bearing tows is also measured in both in-plane and out-of-plane directions, and a correlation is made between compression strength and the waviness angle. It is found that stitching generally reduces the compression strength of carbon/epoxy composites of up to 16%. The average compression strength has a stronger correlation with the in-plane waviness angle rather than the out-of-plane one. A comparative damage assessment, which studies the initiation, progression and final failure in stitched and unstitched composites under axial compression, reveals that the early cracking in resin-rich region is responsible for the lower compression strength experienced by the stitched composites. The interaction between resin cracking and fiber splitting near the flank of wavy fibers accelerates the development of fiber kinking, and causes an early failure in stitched composites.