Flaw Criticality of Graphite/Epoxy Structures
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Published:1979
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The orderly assessment of the serviceability of graphite/epoxy structures requires a knowledge of their flaw criticality. In the Serviceability Program, industry-wide flaw likelihood/criticality surveys have been conducted and the results have been qualitatively assessed. These results were used to set nondestructive evaluation (NDE), analytic study, and experimental verification goals. The progress of this program is reported herein.
Flaws, which can result from the fabrication process, normal service utilization, and battle damage have been categorized into five types, depending on their associated stress field disturbance. Flaw detectability studies have been conducted to determine the optimal NDE process to detect these flaws. Analytic studies have been to conducted to determine the disturbance to the stress field in the vicinity of the flaws and to assess their criticality, that is, their structural integrity.
These results were used to define coupons and elements where the elements represent structures that are in a realistic stress state. Accelerated “worst expected” case environmental fatique testing has been conducted. The preliminary results show, as expected, that advanced composites are relatively damage-tolerant. Since further testing is being conducted, an up-to-date status of the program is given.
Three types of coupons are being tested. The tension coupons contain flaws such as holes, delaminations, ply separations, and scratches. The compression coupons contain delaminations, in untapered and tapered regions, and ply separations. The bolted-joint coupons contain notches at bolt holes, oversized holes, and delaminations. Static tests, both room-temperature dry and 82°C (180°F) temperature wet, are being conducted. Spectrum fatigue tests followed by residual strength tests are also being conducted. Both room-temperature dry and realistic temperature/moisture environments are used. Flaw growth specimens are also being tested. The results for the tension flaw screeing specimens are presented and assessed.
