Stitched uniwoven graphite/epoxy laminates show great promise as a damage-tolerant composite material which is easily fabricated into large net-shaped parts and structures. They also offer the potential of being less costly than other damage-tolerant systems such as toughened matrix composites. Work to date with stitched uniweaves has included evaluations of static properties and damage tolerance as measured by impact resistance, compression after impact, and delamination propagation [1-7]. This paper focuses on experimental characterization of the fatigue response of stitched uniweave composite materials.
The performance of a stitched uniweave material system with an underlying AS4/3501-6 quasi-isotropic lay-up subjected to compression-compression fatigue loading was investigated. In addition, the material's strength and modulus were measured in a limited static compression study. Performance of unnotched specimens having stitch rows at either 0 or 90° to the loading direction was compared. Specimen performances were generally the same regardless of stitch orientation.
Special attention was given to the effects of stitching-related manufacturing defects. The stitches locally compacted the surface layers; microcracks were also evident along the length of the stitches. Damage evaluation techniques included monitoring stiffness and residual compressive strength, X-ray radiography, and edge replication. It was found that the manufacturing defect of inclined stitches had an adverse effect on material performance but only under low-cycle, high-load fatigue conditions. The microcracks did not generally initiate damage or propagate during the compression-compression cycling. The compression failure mode was transverse shear for all specimens, both in static compression and fatigue cycling tests.
Finally, manufacturing advances have improved material performance compared to previously evaluated stitched systems.