Experiments and modeling of the combined tension-shear ultra-low-cycle fatigue of structural steel sheet regarding loading histories

Abstract

This paper presents the experimental and modeling studies of the combined tension-shear ultra-low-cycle fatigue (ULCF) of Q345 structural steel sheet with thickness of 8 mm, considering the loading histories and stress state dependencies. Butterfly-shaped cyclic shear specimens, along with fixtures designed for adjustable shear angles, are used to explore ULCF damage evolution subjected to various combinations of shear and normal stress. The cyclic test programs consist of loading cases with shear angles of 0°(CS00), 30°(CS30), and 45°(CS45), all subjected to cyclic loading with constant-deformation-range (CR), varying-deformation-range (VR), and random-deformation (RD). Testing results confirm dependencies on deformation range and stress state. The combined tension-shear ULCF life increases as the deformation range decreases, while the higher triaxiality induced by the increase in shear angle reduces the ULCF life. Subsequently, the cyclic shear-void damage model (CSVDM) is proposed to comprehensively characterize the evolution of combined tension-shear ULCF damage. The kinematic equivalent plastic strain is introduced to determine dependency on plastic deformation histories. A new damage locus related to stress state is proposed to consider the matrix-shear mechanism under plane strain and the void-growth-collapse mechanism under axisymmetric loading. Compared to the conventional stress-weighted ductile fracture model (SWDFM) and the Lode parameter-enhanced cyclic void growth model (LCVGM), the proposed model exhibits advantages in accurately predicting ULCF failure under different loading protocols and shear angles, as indicated by impressively low average errors of 0.92 % for CS00, 2.08 % for CS30, and 1.42 % for CS45 specimens.