Damage Kinetics of Pre-Cycled Low-Carbon Steel with Coarse-Grained and Ultrafine-Grained Structure

The kinetics of microcracks accumulation and fracture at various stages of tension of grade 20 steel samples with coarse-grained and ultrafine-grained (UFG) structure obtained by equal-channel angular pressing (ECAP) has been studied. To research the effect of mechanical degradation, a part of the samples were subjected to preliminary cycling to a relative lifetime of 50%, followed by tensile tests, during which the parameters of acoustic emission and deformation fields obtained by digital image correlation were evaluated, and the intensity of the residual magnetic field was measured. The length and density of surface microcracks were measured by optical microscopy using computer image analysis. It is established that pre-cycling causes hardening of the material and a decrease in its plasticity. The fracture stages were revealed, and the origins of fatigue cracks were found on the internal delaminations of samples with a UFG structure after preliminary cyclic loading. It is shown that the formation of the UFG structure after the ECAP of samples made of grade 20 steel leads to hardening and reduction of the fracture work, the area of the plastic zone, maximum major deformations and damage, as well as to an increase in the number of AE signals and the intensity of the residual magnetic field.