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A combined experimental and analytical study has been conducted to investigate and predict the damage-onset failure modes of a graphite/epoxy laminate subjected to combined bearing and bypass loading. Tests were conducted in a test machine that allowed the bearing-bypass load ratio to be controlled while a single-fastener coupon was loaded to failure in either tension or compression. Test coupons consisted of 16-ply, quasi-isotropic T300/5208 graphite/epoxy laminates with a centrally located 6.35-mm bolt having a clearance fit. Onset failure modes and loads were determined for each test case. The damage-onset modes were studied in detail by sectioning and micrographing the damaged specimens. In addition, some specimens were loaded to ultimate failure. A two-dimensional finite-element analysis was conducted to determine lamina strains around the bolt hole. Damage onset consisted of matrix cracks, delamination, and fiber failures. Stiffness loss appeared to be caused by fiber failures rather than by matrix cracking and delamination. Fiber failures in the 0-deg plies in the net-section tension and net-compression modes followed the matrix cracking direction in the adjacent 45-deg plies. Fiber failures associated with bearing damage were of two different types: compressively loaded fibers in the 0-deg plies failed by crushing, whereas fibers in the 90-deg plies failed in tension. An unusual offset-compression mode was observed for com-pressive bearing-bypass loading in which the specimen failed across its width, along a line offset from the hole. The computed lamina strains in the fiber direction were used in a combined analytical and experimental approach to predict bearing-bypass diagrams for damage onset from a few simple tests.
laminate, lamina, bolts, bearings, graphite/epoxy, strength, damage, stress analysis, composite materials, joints
Planning Research Corp., Hampton, VA
NASA Langley Research Center, Hampton, VA