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Quasi-isotropic laminates ([0/90/±45]s and [0/±45/90]s) were fabricated from graphite/epoxy and quasi-statically loaded in tension at 76 K until fracture occurred. Fibers in 0-deg plies carry the largest portion of the tensile load; the weaker 90- and 45-deg plies crack at loads much lower than fracture strength. The effect of ply cracks on fracture of load-bearing 0-deg plies was examined to understand how defects affect laminate strength. A generalized plane-strain finite-element model was used to predict stress gradients in the 0-deg ply near the crack tip. Variations in residual stress caused by changes in temperature and absorbed moisture were included in the analysis. The experiments indicated that absorbed moisture significantly alters the fracture strength and fracture surface of a dehydrated [0/90/±45]s laminate tested at 76 K. The 0-deg plies of dehydrated laminates fractured along several 90-deg ply cracks. When moisture saturated a [0/90/±45]s laminate, a single 90-deg ply crack dominated the fracture of the 0-deg ply and the laminate fracture strength decreased 8%. Analysis of residual stresses indicated that a higher residual stress state existed near the 90-deg ply crack when moisture was absorbed.
composite materials, fatigue (materials), fracture mechanics, graphite/epoxy laminates, low temperatures, cracks, residual stress, finite element
Materials research engineer, Fracture and Deformation Division, National Bureau of Standards, Boulder, Colo.