Study of Interlaminar Mechanical Properties for Carbon Fiber Stitched CFRP Laminates

Experimental investigations of CFRP laminates by using carbon fiber (CF) stitching were carried out for evaluating CF stitching effect. The used CF stitch thread is TR 40-1K (612d), in-plane fiber of the CFRP is T700 12K, and resin is TR-A31 for resin transfer molding. Various stitch densities of the CF stitched CFRP laminates for double cantilever beam (DCB) tests were employed here. An insert type of loading fixture which was developed by National Aerospace Laboratory of Japan (NAL) was employed in DCB tests. Pull-out tests of a single carbon stitch thread in the CFRP laminates were also carried out. It is clarified from the DCB test results that the relationship between the volume fraction of CF stitch thread (Vft) and Gl shows a mostly linear function. Moreover, this increase slope of Vft-Gl curve indicates higher one than CFRP stitched by Kevlar-29 1000d. Single CF stitch thread pull-out tests showed that there are two fracture modes of the CF stitch threads in the CFRP laminates, and that a difference in the shape of the load-displacement curve corresponds to these fracture modes. However, there is no big difference in the total energy consumption amount in these different fracture modes. Moreover, CF stitch thread pull-out test results indicate that the energy consumed per unit area of the CF stitch thread was larger than Kevlar-29 1000d stitch thread. This finding agrees with the fact that Interlaminar toughness improvement of CFRP laminate for mode I by CF stitching is more effective than that by Kevlar-29 1000d stitching shown in Vft-Gl relationships.