The effect of cooling rate and stacking sequence on fatigue behavior was analyzed for notched quasi-isotropic APC-2 laminates. The fatigue behavior of fast (475° F/min) and slow (1° F/min) cooled specimens of the following two layups was studied: Layup A of (-45/0/45/90), and Layup B of (45/90/-45/0),. 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. Parameters such as strain, temperature rise across the notch and number of cycles fatigued were continuously monitored during the fatigue tests. Damage was monitored by the reduction in modulus, penetrant enhanced X-ray radiography, and Scanning Acoustic Microscopy (SAM). Post failure analysis of the specimens was carried out by Scanning Electron Microscopy (SEM). A quasi-3D Finite Element Analysis was performed to compare the differences in the interlaminar stresses arising around the notch in the specimens of the two layups under study.

The ultimate static strengths did not show any appreciable dependence on either cooling rate or stacking sequence. The maximum load in the fatigue cycle was selected as a fraction of the ultimate notched static tensile strength in each case. The fatigue lives showed appreciable difference between the two cooling rates in layup A when tested at the lower load levels. The fatigue behavior was vastly different between the two cooling rates for specimens of layup B. Also, specimens of layup B, both fast and slow cooled, had longer lives than their counterparts from layup A. A model, based on a constant strain-to-failure criterion, was developed for life prediction and the predicted lives are in good agreement with the experimental values.

Fast cooled specimens of layup A showed a gradual degradation in the modulus till failure while slow cooled specimens of the same layup showed a more drastic reduction as they approached failure. No such distinguished behavior was observed in the specimens of layup B. Scanning Electron Micrographs of the fast cooled specimens indicate better fiber/matrix bonding conditions and more matrix plasticity as compared to the slow cooled specimens.

A rotated stacking sequence technique was used for the calculation of the interlaminar stresses around the notch. No single stress seems to control failure but it is likely that failure occurs by the interaction of the different stresses since a three-dimensional stress state exists at the notch. Based on this reasoning, effective stresses were calculated at all those interfaces where one of the interlaminar (normal or shear) stresses has a maximum value. A comparison of the effective stresses calculated showed the layup A to be 1.7 times more prone to delamination than layup B. Damage analysis of the fatigued specimens by X-ray radiography and Scanning Acoustic Microscopy shows the specimens of layup A to be dominated by delaminations as compared to those of layup B. The interfaces predicted to be critical by FEA agreed well with the experimental observations, in general.