ON THE PHENOMENON OF CURVED MICROCRACKS IN [(S) 90(N)](S) LAMINATES -THEIR SHAPES, INITIATION ANGLES AND LOCATIONS/

A variational analysis of the stress state in microcracked cross-ply l aminates has been used to investigate the phenomenon of curved microcr acking in [(S)/90n]s laminates. Previous investigators proposed that t he initiation and orientation of curved microcracks are controlled by local maxima and stress trajectories of the principal stresses. We hav e implemented a principal stress model using a variational mechanics s tress analysis and we were able to make predictions about curved micro cracks. The predictions agree well with experimental observations and therefore support the assertion that the variational analysis gives an accurate stress state that is useful for modeling the microcracking p roperties of cross-ply laminates. An important prediction about curved microcracks is that they are a late stage of microcracking damage. Th ey occur only when the crack density of straight microcracks exceeds s ome critical value-the critical crack density for curved microcracking . The predicted critical crack density for curved microcracking agrees well with experimental observations.