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Proof-testing has shown with a high level of confidence that a unique relationship exists between the static strength and the fatigue life or residual strength in unidirectional graphite/epoxy (T300/5208) composite. However, damage, especially visible damage, caused by proof-testing seems to shorten the life and reduce the residual strength. Failure initiation in fatigue is mainly in the form of longitudinal cracking, starting along the edges at lower stresses. Temperature increase follows the same trend as predicted by the solution of a heat conduction equation with a constant rate of heat generation except immediately before the final failure. Although the reason is not clear, the tensile modulus increases after fatigue. For mathematical simplicity and operational ease, a two-parameter Weibull distribution seems adequate for the analyses of scatter in strength and life.
composite materials, graphite/epoxy composites, fatigue tests, proof-testing, acoustic emission, temperature increase, residual strength, probability, statistical analysis, modulus change
NRC research associateresearch engineer, Air Force Materials Laboratory, Wright-Patterson AFBUniversity of Dayton Research Institute, Dayton, OhioOhio
Research engineeraffiliate professor, University of Dayton Research InstituteWashington University, DaytonSt. Louis, OhioMo