Prediction of S-N fatigue curves using various long-crack-derived ΔKeff fatigue crack growth curves and a small crack life prediction model

fn this study, stress-life (S-N) fatigue curves are predicted for high-strength aluminum alloy 7055 for open-hole specimens at two stress ratios. R = 0.1 and 0.5, and smooth specimens at R = 0.1 using the small-crack growth model of Brockenbrough et al. [1,2] and closure-free FCG curves obtained from long-crack tests by the following methods: (1) high R testing at R = 0.7; (2) constant K-max testing at K-max of 11 and 24.7 MPa root m; (3) a Delta K-eff curve obtained at the appropriate stress ratio (R = 0.1 or 0.5) by the ASTM method. and (4) a Delta K-eff curve obtained at the appropriate stress ratio by the adjusted compliance ratio (ACR) method. The predictions were compared to experimental S-N fatigue data. The objective of the study was to determine which method of obtaining closure-free FCG curves from long-crack tests provided the best estimates of fatigue life for the three combinations of specimen type and stress ratio in conjunction with the small-crack growth model employed. The Delta K-eff curves obtained by the ACR method yielded the closest and most consistent fatigue predictions for all three conditions. This was attributed to this method being able to account for K-max sensitivity of fatigue crack growth rates in aluminum alloys that could not be accounted for by the other methods.