On the use of the strip-yield model to predict fatigue crack growth under irregular loading

In this paper, the behaviour of the LEFM strip-yield model proposed by Newman and implemented in the FASTRAN II computer program is analyzed. The capabilities of the model to predict crack growth life under variable amplitude loading is considered. Special attention is paid to the effect of the constraint factors used to consider the stress condition (plane stress to plane strain), the effect of the finite length loading sequence and the effect of overloads into an irregular loading history. The results of simulation for 30 different loading histories obtained from the same stationary random process are analyzed and compared with the experimental results obtained for 2024-T351 aluminium alloy. The simulated lives present a fairly good fit with the experimental results, with a strong influence of the constraint factor selected and of the maximum peak in the loading history. Although predictions are usually good, it has been found that for any constraint factor producing good life predictions (with respect to the mean value of the life obtained with the 30 loading histories) the results of each particular simulation may be over- or under-conservative depending on the maximum peak in the loading history used.