Tensile fatigue properties of 3D composites with through-thickness reinforcement

The tensile fatigue properties of specific types of 3D woven, stitched and z-pinned composites with through-thickness reinforcement are compared in this paper. Tensile tests under monotonic and cyclic loading were performed on the 3D composite materials to determine the influence of the z-reinforcement type - woven z-binder, stitch or z-pin - on the tensile modulus, strength and fatigue life. The in-plane Young's modulus of the composites was not affected by the type or volume content of the z-reinforcement. The tensile strength of the 3D woven and stitched composites was also not affected by the z-reinforcement, however the strength of the z-pinned composite dropped steadily with increasing volume content of z-reinforcement. The fatigue life of the 3D composites was reduced by the z-reinforcement, regardless of whether they were woven z-binders, stitches or z-pins. The fatigue lives of the 3D composites decreased with increasing volume content of z-reinforcement. The tensile fatigue properties are degraded by the z-reinforcement causing damage to the microstructure of the 3D composites. The fatigue damage mechanisms caused by the different types of z-reinforcement are described. The results indicate that through-the-thickness reinforcement is detrimental to the tensile fatigue life, although the study was restricted to specific types of materials and further research into a wider variety of 3D woven, stitched and z-pinned composites is required for a general assessment of their fatigue performance. (C) 2008 Elsevier Ltd. All rights reserved.