Retardation effect on sheet steel under a mixed I–II mode overload revealed by experiment, DIC and FEM methods

•A series of experiments was conducted to investigate the retardation mechanisms under mixed I–II mode overloads with loading angles 0, 30, 60 and 90°.•The overload retardation is weakened with the increasing mode II portion.•A combined model which considers the residual stress, plasticity-induced and roughness-induced crack closure effects was proposed.•This research can provide a reference for fatigue design and structural optimization of components. As a high strength and commercial steel, Q420 is widely employed in many engineering applications. In this paper, a series of experiments was conducted to investigate the fatigue crack growth retardation mechanisms under mixed I–II mode overloads with loading angles 0, 30, 60 and 90°. The baseline of loading spectrum is a pure mode I cyclic loading with a mixed overload applied after pre-cracking. The results show that the crack path is always straight for each case. Both the retardation level and fatigue life decrease with the increasing loading angle. A combined model that considers the residual stress, plasticity-induced and roughness-induced crack closure effects was proposed to predict the fatigue crack growth rate and fatigue life. The predicted results agree well with several experimental results.