Delayed fracture stress thresholds degraded by the shear punching process in ultra-high-strength steel sheets: Analysis using an in-plane bending test with numerical assimilation

This study established delayed fracture stress thresholds for punched surfaces of two ultra-high-strength steel sheets, JSC1180Y and 22MnB5. Punched surfaces exhibit deterioration in ductility and high residual stresses, thereby lowering delayed fracture thresholds. Consequently, identifying such degraded thresholds is essential for the early detection of delayed fractures in industrial applications. To address this challenge, we newly developed a special in-plane bending test that imposes arbitrary stresses on punched surfaces while simultaneously subjecting them to cathodic hydrogen charging. Our findings revealed the delayed fracture stress thresholds in JSC1180Y for the first time, as 1003 MPa for 10%t and 1124 MPa for 15%t clearance. Conversely, in the case of 22MnB5, punched surfaces with 10 and 15%t clearances exhibited delayed fractures solely due to hydrogen charging. Notably, at 0.8%t clearance, we identified a delayed fracture stress threshold of 643 MPa, with X-ray diffraction (XRD) analysis showing an unexpected 800 MPa margin in the initial residual stress. To further analyze these results, we proposed the use of observation-assimilated finite element (FE) simulations. These simulations estimated the circumferential stress near the boundary under the bending load, revealing that the delayed fracture in JSC1180Y occurs at approximately 1 GPa, regardless of the clearance. This analysis sheds light on the variation mechanism of stress thresholds with punch-die clearance in JSC1180Y. Moreover, our observation-assimilated FE simulation approach was demonstrated to be a promising practical and effective method for proactively detecting delayed fractures, thereby reducing the reliance on XRD measurements of punched surfaces.