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

•Effects of the single underload and periodic underloads on FCGR in S355J2W + N steel were studied.•FCGR under a single underload with various amplitudes of underload under different stress-ratio baseline load were compared.•FCGR under periodic underloads with various cyclic numbers in one loading block under different stress-ratio baseline load were compared.•Strain distributions around the crack tip were collected by DIC&FEA and used for analyzing the mechanism of crack growth acceleration.•Crack growth acceleration following the underload were mainly attributed to the introduction of tensile residual stress and reduction of plasticity-induced crack closure.