MECHANISMS OF COMPRESSIVE FAILURE IN 3D-COMPOSITES

Angle interlock woven polymer matrix composites have been studied under uniaxial monotonic compression. With considerable variations arising from the geometry of the reinforcement and the degree of constraint in the test, the materials were found to be macroscopically ductile, with compressive strains to failure occasionally exceeding 15%. In contrast, some tests on stitched laminates showed brittle behavior, with essentially no load bearing capacity beyond the strain for peak load (approximately 1%). The mechanisms of failure in the woven composites were determined by a combination of optical microscopy (both in situ and of sectioned specimens), moire interferometry, stereoscopy, and digital image comparison. In all cases, the central failure event was kink band formation in the primary load bearing, axial tows. Various characteristics of the reinforcement geometry were observed to influence kink band formation, including initial misalignment of the load bearing tows and intersections of load bearing tows and through-thickness reinforcing tows. Such geometrical characteristics acted as flaws, tending to lower macroscopic stiffness and strength, but promoting the broad distribution of damage throughout the specimen and averting catastrophic failure. Guidelines for achieving the optimum compromise between strength and damage tolerance may be inferred from these observations.