Damage Tolerance of Pressurized Graphite/Epoxy Cylinders Under Uniaxial and Biaxial Loading

摘要

The damage tolerance behavior of internally pressurized, longitudinally slit, graphite/epory tape cylineers was investigated. Specifically, the effects of longitudinal stress, subcritical damage, and structural anisotropy were considered including their limitations on a methodology, developed for quasi-isotropic configurations, which uses coupon fracture data to predict cylinder failure. AS4/3501-6 graphite/expoxy cylinders with (90/0/+-45)_s, (+-45/0)_s, and (+-45/90)_s layups were tested in a test apparatus specially designed and built for this work such that pressurization resulted in only uniaxial (circumferential) loading of the cylinders. All cylinders had a diameter of 305 mm and slit lengths ranged from 12.7 to 50.8 mm. Failure pressure was recorded and fracture paths and failure modes evaluated via post-test reconstruction of the cylinders. These results were compared to results from previous tests conducted in biaxial loading. Structural anisotropic effects were further investigated by testing cylinders with the quasi-isotropic (9/+-45/90)-s layup and comparing these with the results from the other quasiisotropic (90/0/45)_s layup. In all cases, the failure pressures for the uniaxially loaded cylinders fell below those for the biaxially loaded cases and the methodology was not able to predict these failure pressures. These differennces in fracture paths and overall failure pressures. Strain gages placed near the silit tips showed that subcritical damage occurred in all cases. These results, coupled with previous work, show that failure is controlled by local damage mechanisms and the subsequent stress redistribution and damage accumulation scenario. It is thus necessary to assess the local effects, such as subcritical damage, while accounting for the influence of global parameters such as longitudinal load and structural anisotropy. Recommendations are made as to work that should be done to better understand such phenomena and thus lead to the establishment of methodologies to better characterize the failure, and thus damage tolerance, of composite materials and their structures.