Plastic-deformation-history effects on the cyclic ductile fracture of steel sheet under high triaxiality

Abstract

This paper presents the experimental and modeling studies of the plastic deformation history effects on the cyclic ductile fracture of thin steel sheet under high triaxiality. The center-holed plate specimens with three cutting radiuses are tested respectively under three cyclic loading protocols, i.e., the ascending-descending (AD), the descending-ascending (DA), and the random deformation amplitude. The prestrain-induced enhanced ductility effect is identified through comparisons between AD and DA testing, as the earlier applied large deformation might reduce the damage accumulation rate when the subsequent lower deformation amplitude is applied. A new cyclic void-induced damage model (CVOID model) is proposed to characterize the damage accumulation and succeeding softening evolution. The damage accumulation is attributed to the void growth and shrinkage subjected to cyclic deformation, which will trigger the softening initiation and development due to the internal micro-crack propagation. Furthermore, a new plasticity state variable is introduced to quantify the current plastic deformation state to consider the reduced damage rate dependent on the prestrain effect. Using the proposed CVOID model, the full-range cyclic hardening–softening behaviors can be accurately predicted under all concerned triaxialities and loading protocols, especially under random cyclic loading. It achieves better prediction of the cyclic softening behaviors than three conventional models (i.e., the CVGM, SWDFM, and the JIA cyclic fracture model), as indicated by the profound percentage errors of the accumulated critical deformation (AcD) lower than 2.1% compared with the errors from 20% to 60% via the existing models.