Effect of crack-tip deformation on fatigue crack growth: A comparative study under overload/underload conditions

With the aim to provide the structural design stage with data on the crack retardation/acceleration effects caused by overload/underload and to explore the corresponding mechanisms, we conducted a series of comparative fatigue crack growth rate (FCGR) tests on S355J2W+N steel. The various amplitudes of overload/underload were investigated comprehensively under different stress-ratio baseline loads for single and periodic overloading/ underloading processes. Moreover, the crack-tip deformation behaviors (i.e., the strain/stress fields and crack closure responses following the overload/underload) were observed using a digital image correlation (DIC) technique and calculated through the finite element analysis. The relevant fracture morphologies and crack-tip dislocation structures were characterized and analyzed. The results showed that the crack propagation cycles could be influenced to different degrees by a single overload (OL), a single underload (UL), and periodic underloads (PULs). Compared with the transient crack acceleration induced by a UL, a lasting crack growth retardation occurred following an OL. The OL & UL sequences had various effects on the changes in FCGR. Furthermore, a higher stress-ratio baseline loading and larger-amplitude OL increased the retardation effect; a lower stress-ratio baseline loading and larger-amplitude UL enhanced the acceleration effect. The degrees of crack retardation or acceleration following the OL/UL were determined by the changes in the crack closure and residual stress state near crack tip, which were closely related to the baseline loading stress ratio and local residual strain characteristics. Correspondingly, the OL and PULs also changed the dislocation density and structures in the microscopic view.