Local delamination in laminates with angle ply matrix cracks. Part 2: Delamination fracture analysis and fatigue characterization

Constant amplitude tension-tension fatigue tests were conducted on AS4/3501-6 graphite/epoxy (02/ theta sub 2/ -(theta sub 2))sub s laminates, where theta was 15, 20, 25, or 30 degrees. Fatigue tests were conducted at a frequency of 5 Hz and an R-ratio of 0.1. Dye penetrant enhanced x-radiography was used to document the onset of matrix cracking in the central -(theta) degree plies, and the subsequent onset of local delaminations in the theta/ -(theta) interface at the intersection of the matrix cracks and the free edge, as a function of the number of fatigue cycles. Two strain energy release rate solutions for local delamination from matrix cracks were derived: one for a local delamination growing from an angle ply matrix crack with a uniform delamination growing from an angle ply matrix crack with a triangular shaped delamination area that extended only partially into the laminate width from the free edge. Plots of G(max) vs. N were generated to assess the accuracy of these G solutions. The influence of residual thermal and moisture stresses on G were also quantified. However, a detailed analysis of the G components and a mixed-mode fatigue failure criterion for this material may be needed to predict the fatigue behavior of these laminates