Prediction of fatigue-crack growth for 7075-T7351 aluminum alloy under various flight-load spectra

A plasticity-induced crack-closure model was used to calculate fatigue-crack-growth lives of middle-crack-tension, M(T), and compact, C(T), specimens made of 7075-T7351 aluminum alloy subjected to nine different simulated aircraft flight-load spectra. Experimental crack-growth results from the literature were compared to crack-growth calculations or predictions made using the FASTRAN life-prediction code, employing material property data extracted from material databases, such as the NASGRO database. In order to account for a loss of crack-front constraint (flat-to-slant crack-growth transition) during crack growth, crack-growth-rate dependent constraint factors were selected to correlate large crack-growth-rate data as a function of the effective stress-intensity-factor range under constant-amplitude loading. FASTRAN calculations or predictions with and without the constraint-loss option, through appropriate specification of constraint factors, resulted in about ±30% difference from the test lives on all nine spectrum loading. Some tests showed as much as 20% scatter between two test lives. Linear-cumulative damage life predictions also did very well on about one-half of the spectrum tests.