Abstract
Published studies have reported outstanding improvements in Mode I fracture toughness and compression after impact (CAI) strength of fibrous composites due to through-the-thickness (z-fiber) reinforcement. One of the methods of through-the-thickness reinforcement successfully experimented by the authors is stitching of textile composites. Our results have shown improvements of about 30 times higher Mode I fracture toughness as characterized by critical strain energy release rate (GIc) and up to 400% increase in the CAI strength over the unstitched textile composites. In this paper we illustrate the damage growth and its underlying mechanisms in the case of stitched composites as observed in our studies. The role and interplay of starter crack, first line of stitches, needle and bobbin yarns, type of stitch lock, stitch density and other processing parameters are discussed. Double Cantilever Beam (DCB) tests were conducted on 24 ply graphite/epoxy (AS4/3501-6) uniweave textile laminates stitched with different yarns and densities to measure Mode I fracture toughness. 48 ply AS4/3501-6 uniweave textile laminates with different simulated impact damage were tested in compression to measure CAI strength. In addition, the effect of stitching on thinner 16 ply AS4/3501-6 plain weave textile laminates was studied where the gains in CAI strength, though noticeable, are much less than for the thicker laminates.
