EFFECT OF COOLING RATE AND STACKING-SEQUENCE ON THE FATIGUE BEHAVIOR OF NOTCHED QUASI-ISOTROPIC AS4 PEEK LAMINATES/

The effect of cooling rate and stacking sequence on fatigue behavior w as analyzed for notched quasi-isotropic AS4/PEEK laminates. The fatigu e behavior of fast (475 degrees F/min) and slow (1 degrees F/min) cool ed 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 subje cted to a load-controlled, tension-tension fatigue loading with a stre ss ratio R = 0.1 at a frequency of 5 Hz. Damage was monitored by the r eduction in dynamic stiffness and penetrant enhanced X-radiography. Po st-failure analysis was carried out by scanning electron microscopy (S EM). Influence of cooling rate on the fatigue properties was different in the two stacking sequences investigated. The fatigue behavior chan ged drastically with the variation in the stacking sequence. A model, based on a constant strain-to-failure, was developed for life predicti on and the predicted lives are in good agreement with the experimental values. To help understand the fatigue behavior of the two stacking s equences studied, the interlaminar stress distributions were calculate d 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 de lamination as compared to those of Lay-up B and this is consistent wit h experimental observations.