Fatigue crack growth acceleration in S355 steel under a single and periodic underload

To consider the acceleration effect induced by the underloads in the design stage, the fatigue crack growth rates in S355 steel were measured based on the compliance method. Three different load sequences, constant amplitude loading (CAL), single underload (UL), and periodic underloads (PULs), were tested. Moreover, several methods were used to study the relevant mechanisms from the macroscopic and microscopic perspectives, including the digital image correlation technique, X-ray diffraction, transmission electron microscopy, and finite element method, etc. Compared with the crack growth during CAL, the UL induced a notable crack growth acceleration in subsequent cyclic loadings. When the amplitude of UL was constant, the effect of UL enhanced with the lower stress ratio of the fatigue baseline loading. In the macroscopic view, the acceleration effect was primarily attributed to the decreasing plasticity-induced crack closure and induced tensile residual stress at the crack tip. A higher amplitude underload led to an increase in the crack acceleration, which gradually degenerated within the following cycles. The PULs were also affected by the amplitude of underloads. The relevant crack growth acceleration mechanisms were closely related to the cycles affected by UL and the crack opening level. Another main parameter affecting PULs was associated with the number of cycles between two consecutive underloads: the frequency of PULs, which reflects the degree of interaction and accumulation. In the microscopic view, the accumulation of dislocation around the crack tip induced by PULs contributed to the crack acceleration, along with the corresponding periodic striations in the fractography. This study showed the crack growth acceleration effect caused by underloads and demonstrated the interactions between the UL and PULs, offering a better understanding of the acceleration mechanisms.