Experimental results and predictions on fatigue crack growth in structural steel

Fatigue crack growth, the methodology of estimating crack closure from compliance records, and calibrating a strip yield type model for crack growth predictions using constraint factors are studied for structural steel. Trends in fatigue crack growth observed for two structural steels with distinct mechanical properties under constant amplitude and variable amplitude load histories of various types are outlined. An algorithm for evaluating crack closure based on the analysis of open crack compliance variations is proposed. Compared to standard procedures assuming a constant compliance value for the fully open crack, this approach yields an improved correlation between the measured closure behaviour and the observed crack growth behaviour. It is shown that adequate crack growth predictions for structural steel cannot be obtained from the strip yield model incorporating a constraint factor on tensile yielding only, as usually practised for Al-alloys. To correlate the observed trends in crack growth, constraints on compressive yielding both ahead of the crack tip and in the crack wake are also required. A methodology for selecting the constraint factors is proposed, which offers the means to calibrate the strip yield model for structural steel based on a physical foundation, namely through matching the observed and predicted stress-local cyclic strain behaviour.