Effect of span/thickness ratio on fatigue properties of thick quasi-isotropic CFRP laminates subjected to three-point bending loading

Abstract

In this study, three-point bending fatigue tests were conducted to investigate damage growth behavior of thick quasi-isotropic carbon fiber reinforced plastic (CFRP) laminates. The span/thickness ratio L/h was varied for both static tests and fatigue tests to determine failure mechanisms. In the static tests, the failure mode changed from delamination to compressive buckling as span/thickness ratio increases. The apparent interlaminar shear strength was higher for the specimens with lower span/thickness ratios, and the shear failure was likely to be caused by oblique crack in 90° plies. In the fatigue tests, the specimens with span/thickness ratio L/h=7 caused delamination regardless of stress level. For the specimens with span/thickness ratio L/h =15, three types of failure modes were obtained depending on stress level; delamination near the neutral plane, buckling at compressive area and matrix crack at tensile area. During the fatigue tests, stiffness of specimens with different span/thickness ratios which fail in shear was compared using digital image correlation (DIC) method. The specimens with span/thickness ratio 4 suddenly decreased in stiffness without increasing normal strain over cycles. Both specimens with span/thickness ratio 4 and 11 finally failed by delamination but they showed different failure process in how normal or shear strain developed.