Fatigue crack growth under overload/underload in different strength structural steels

Aiming at the different strength-level structural steels, fatigue crack growth rate (FCGR) tests were conducted under a series of loading conditions, i.e., constant amplitude load (CAL), single overload (OL), single underload (UL), or periodic underloads (PULs). Meanwhile, the effects of various baseline loading stress ratios on the FCGR were considered. To analyze the effects of crack-tip behavior on the crack propagation following an OL/UL, the crack-tip strain fields and crack opening displacement were obtained by digital image correlation (DIC). It was found that when the OL/UL-amplitude was constant, the fatigue crack propagations under the OL/UL were affected by both steel strengths and baseline loading stress ratios. In general, an increase in the steel strength grade indicated a lower crack retardation/acceleration during the OL/UL process, which was more evident under the lower baseline stress-ratio condition. The higher the strength, the lower the crack-opening stress level, the smaller the plastic zone size, and the higher the crack-tip residual strain. Correspondingly, there were different variations in the instantaneous FCGR and OL/UL-affected cycles. When the OL/ULs are considered in the design stage, our work can guide the steel strength grade selection and provide a better understanding of the crack growth retardation/ acceleration mechanisms.