Damage Evolution Characterization of Low Carbon Alloy Steel Based on Multiaxial Fatigue Test and DIC

Fatigue damage and subsequent failure account for the majority of crane structural failure. In this paper, the fatigue damage evolution behavior of low carbon alloy steel Q355B under multi-axial proportional and non-proportional loading is studied. Using the strain field nephogram obtained by digital image correlation, and the indirect damage variable characterized by the unloading stiffness recorded by the tension-torsion composite extensometer, the fatigue damage evolution process is analyzed qualitatively. The fatigue failure process under different loading conditions is uniformly divided into three stages: meso-crack initiation stage, meso-crack propagation (macro-crack initiation) stage, and macro crack propagation stage. According to the damage mechanics, the corresponding turning points at different stages are quantitatively analyzed, and it is found that the damage threshold values corresponding to different loading conditions are different, that is, under the same strain control, the damage threshold values required for non-proportional loading are less than that for proportional loading, and there are differences under different strain control. Based on the damage evolution model and the shear damage model, combined with the test results, it is proved that the damage evolution mode caused by multi-axial non-proportional loading is different from that caused by proportional loading.