Damage development in bolt bearing of composite laminates

The buildup of damage in bolt-loaded specimens has been investigated. Destructive evaluation by staining, dissection, and microscopy was used to evaluate the complete three-dimensional geometry of the damage zone at several load levels and loading conditions for glass and graphite fiber-reinforced epoxy laminates with the stacking sequence [ [O/ + 457 45] 5/0] s. The assumed stress hybrid finite element method was used to determine the elastic stress distributions, including simulations of no friction and no sliding. Three sets of tests are included: a fastened bolt joint in tension, a fastened bolt joint in compression, and a fastened bolt hole in tension. Complete damage characteristics and stress distributions are even ply by ply through the thickness as a function of load. The local strength around the hole boundary is examined pointwise by the Tsai-Hill criterion; the maximum stress criterion is then used to determine the failure mode. First-ply failure criteria are applied and compared with experiments. The three sets of tests indicate that the damage zones of the net-tension and bearing-compression regions are independent at both the initiation and failure stages. Implications for gross failure criteria are discussed, and the need for more sophisticated methods is demonstrated. Attempts have been made to correlate actual damage zones with Whitney-Nuismer distance parameters, but no physical interpretation of these parameters appears justified.

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