Fatigue Damage and Fracture Mechanism of Notched Woven Laminates

Abstract

Three E-glass/epoxy notched woven laminates (two orthotropic and one quasi-isotropic) were studied under tensile fatigue loading with a stress ratio of 0.1 and a frequency of 20 Hz (sine wave form). At a fixed ratio of maximum fatigue stress to ultimate tensile strength (UTS) (0.5), the apparent secant stiffness changes with the cycle number were measured and the fatigue damage mechanism was investigated. The S-N curve and notched fatigue strength (at 1,000,000 cycles) were determined. All results obtained were compared with those for unnotched specimens. The results show that the normalized apparent secant stiffness change can be divided into three stages. During the first and second stage, damage may be presented in the form of microcracks within the interlacement structure of the woven plies. But they have not been observed and verified with traditional experimental methods such as radiography and microscopy. The last stage corresponds to initiation and evolution of damage observed directly on the specimen surface with a video-camera system. The fatigue life at a same ratio of maximum fatigue stress to UTS for the three laminates is very slightly influenced by the notch. The quasi-isotropic laminate has a lower fatigue strength and an earlier first visible damage than the two orthotropic laminates. However, for the two orthotropic laminates, the fatigue strength and the appearance of the first visible damage are very slighly influenced by the stacking sequences, but, the normalized secant stiffness curve appears to be different from one to another. The notched specimens have the same ratio of the fatigue strength to UTS as the unnotched ones, almost equal to 0.4. The ratio of the notched fatigue strength to the unnotched fatigue strength is amost equal to the static strength reduction factor (SRF) which is equal to the ratio of the static notched strength to the static unnotched strength.