Fatigue strength of knitted fabric reinforced laminates

Abstract

Results from both theoretical experimental investigations on laminated epoxy composites reinforced with plain weft knitted carbon fiber fabrics of different arrangements subjected to fatigue loads are presented in this paper. The stacking sequences considered are [0/0/0/0], [0/45/-45/0], and [90/90/90/90], where 0 refers to the fabric wale direction and 90 to the course direction. Tensile fatigue with stress control was applied to the laminates as well as to the monolithic matrix material, with a stress ratio of R=0.1 and a cycling frequency of 5Hz. The theoretical prediction of the laminate fatigue strength was made using a general methodology, which is a combination of the classical lamination theory and the bridging micromechanics model. The stresses sustained by each lamina layer in the laminate were determined according to the classical lamination theory, whereas the internal fatigue stresses generated in the fiber and matrix materials of the lamina were obtained by virtue of the bridging model. The internal stresses were detected against the constituent fatigue strengths at the same loading conditions. Thus, the laminate fatigue strength was predictable using only the constituent fatigue data and the laminate geometric parameters, which were measured/determined independently. Reasonably good agreement has been found between the predicted and experimental S-N curves of all the three laminates.