Fatigue life of individual composite specimens based on intrinsic fatigue behavior

A method is presented to predict fatigue life which explicitly accounts for specimen specific damage histories and the statistical distribution of quasi-static tensile strengths. First a ‘critical element’ is identified as that portion of the composite which controls failure, and the stress on that element is monitored. Then a cumulative damage model based on the remaining strength of the critical element, is used to extract an ‘intrinsic’ S-N curve for the critical element fatigue response from the fatigue data and the stress history. Using unidirectional specimens, such an intrinsic S-N curve was derived, and subsequent fatigue predictions were vastly improved over the traditional S-N curve. Fatigue tests on (90°/0°) s crossply materialf were then run and lives predicted based on the intrinsic S-N curve with no adjustable parameters. The agreement between experiment and prediction validates the method. This method may have application in real-time durability evaluation of individual composite components using real-time stiffness to continuously evaluate remaining life during service.