Fatigue damage of notched boron/epoxy laminates under constant-amplitude loading

Fatigue damage in (0, plus or minus 45) and (0, plus or minus 45, 90) boron/epoxy laminates was studied with X-ray radiography and scanning electron microscopy. In addition, limited tests for residual strength and stiffness were performed. Initially, fatigue damage in both (0, plus or minus 45) and (0, plus or minus 45, 90) laminates occurred as intralaminar cracks around the edge of the hole. Then, whenever further damage developed, intralaminar cracks in the plus or minus 45-deg plies began to propagate from the edge of the hole. Finally, in both types of laminates, primarily plus or minus 45-deg fibers broke (prior to two-piece failure) where intralaminar cracks in the plus or minus 45-deg plies had occurred. In the (0, plus or minus 45) laminates, the 45-deg plies developed intralaminar and transthickness cracks along lines parallel to the loading axis and tangent to the hole in the test specimen. This damage, which was most pronounced under compressive loads, had little effect on either strength or stiffness. In contrast, in the (0, plus or minus 45, 90) laminates, the plus or minus 45-deg plies developed intralaminar cracks transverse to the loading axis.