Effect of stress ratio on fatigue-crack growth in 7075-T6 aluminum-alloy sheet

Abstract Axial-load fatigue-crack-propagation tests were conducted on 12 in. (305 mm) wide sheet specimens made of 7075-T6 aluminum alloy. These tests were made at stress ratios R (ratio of the minimum stress to the maximum stress) ranging from −1.0 to 0.8 and at maximum stress levels ranging from 5 to 50 ksi (34–340 MN / m 2 ) to study the effects of stress ratio on fatigue-crack growth. The fatigue-crack-propagation data were analyzed using Paris' stress-intensity analysis. The data from tests at negative R values fell into a relatively narrow scatterband along with the results from the R = 0 tests on a plot of rate against stressintensity range. Apparently, the compression portion of the loading cycle did not significantly affect crack growth in these tests. The data from tests at different positive stress ratios fell into discrete bands on these plots. These bands varied systematically with R , that is, the higher the stress ratio at a given value of ΔK , the higher the rate of fatigue-crack growth. This spread in rates was small at the lower stress-intensity range, but became progressively larger as the stress-intensity range was increased. Semiempirical equations developed by Forman et al ., by Broek and Schijve, and by Paris were fitted to the data from this investigation using a least squares fit. Forman's equation gave the best fit to the data. The fracture surfaces of the specimens changed from the normal mode to a shear mode within a reasonably narrow range of crack growth rates for all R values. The stress-intensity range at transition was approximately a constant for tests at negative stress ratios. Forman's equation adequately represented the variation between the stress ratio and the stress-intensity range at positive stress ratios.