Early crack initiation mode during tensile fracture process in a punch-processed precipitation-hardened steel plates and evaluation method to treat the crack as an equivalent pre-crack

Abstract

For partially worked metals such as punching-processed steel plates, a process-affected zone designated “shear-affected zone” (SAZ) is formed. In this study, tensile tests of punched specimens were conducted to determine the strength of partially worked metals. In the fracture surface of the specimen, a semi-elliptical crack originating in the SAZ, or “shear crack,” was observed, inclined at 45° to the remote tensile stress direction. This shear crack changed the behavior of the metal during tensile loading, e.g., ductile-brittle transition. In specimens exhibiting macroscopic brittle fracture, microscopic brittle fractures cause shear cracks before the specimen reaches its macroscopic yield point. The shear cracks in ductile fractured specimens include a similar brittle-fractured region that can be considered a pre-crack; the punched material can be considered a pre-cracked material. This study highlights the requirement for a novel mechanical model based on linear fracture mechanics to evaluate and predict the fracture mode of punched materials.