Effect of Cooling Rate and Stacking Sequence on the Fatigue Behavior of Notched Quasi-Isotropic AS4/PEEK Laminates

Abstract

The effect of cooling rate and stacking sequence on fatigue behavior was analyzed for notched quasi-isotropic AS4/PEEK laminates. The fatigue behavior of fast (475°F/min) and slow (1°F/min) cooled specimens of the following two lay-ups was studied: Layup A of (−45/0/45/90)s and Layup B of (45/90/−45/0)s. All specimens were subjected to a load-controlled, tension-tension fatigue loading with a stress ratio R = 0.1 at a frequency of 5 Hz. Damage was monitored by the reduction in dynamic stiffness and penetrant enhanced X-radiography. Post-failure analysis was carried out by scanning electron microscopy (SEM). Influence of cooling rate on the fatigue properties was different in the two stacking sequences investigated. The fatigue behavior changed drastically with the variation in the stacking sequence. A model, based on a constant strain-to-failure, was developed for life prediction and the predicted lives are in good agreement with the experimental values. To help understand the fatigue behavior of the two stacking sequences studied, the interlaminar stress distributions were calculated with a quasi-3D finite element analysis (FEA) for the regions around the hole. Specimens of Lay-up A were predicted to be more prone to delamination as compared to those of Lay-up B and this is consistent with experimental observations.