An experimental investigation was conducted to characterize the response of three composite material systems to long-term cyclic loading. Thick cross-ply laminates of uniwoven AS/4 carbon fabric were produced using a brittle matrix with and without stitching and a tough-matrix material. Quasi-static tension and compression tests were conducted on specimens with and without circular holes to determine strength, modulus, and failing strain. These tests showed that the measured static mechanical properties were insensitive to the type of matrix material because the laminate response was dominated by the 0° fibers. The stitched specimens had significantly lower static compressive strengths.
A series of fatigue tests (tension-tension, compression-compression, and tension-compression) were conducted to study the influence of an open hole on damage and residual strength. These tests showed that the matrix material and stitching influenced the fatigue behavior of the composite. The tough-matrix material developed less damage than the corresponding brittle-matrix specimens and survived one million cycles of fatigue loading at all stress levels. The stitched composite developed less longitudinal damage than the unstitched materials, but it developed more transverse damage. Both the brittle-matrix and the stitched materials failed prior to one million cycles of tension-compression fatigue loading. The residual strength of the materials was affected by the matrix material and the damage state in the specimen.