Influence of shear-affected-zone due to punching on tensile characteristics of steel plate

摘要

In punched steel plate, "punched specimen" from here onwards, the region near the edge of the punched hole is called the shear-affected-zone (SAZ), where tensile characteristics are compromised due to the punching process. However, the specific characteristics and influence on the tensile characteristics of the SAZ are unknown. Therefore, it is difficult to predict the effects of punching on the specimen. Here, we evaluated two kinds of specimens: punched and "honed." The hole of the honed specimen is formed by drilling and polishing, thus there is no SAZ on the honed specimens. In these specimens, we focused on the initiation and propagation of cracks under tensile loading and set the following objectives: (1) Determine whether any special events occur in the fracture mechanism of the punched vs honed specimens under tensile loading. (2) Find the reasons for any special events that occur. In our investigation, a punched specimen showed brittle fracture even within the static range of the strain rate. By failure surface observation of the punched specimens, we found that under tensile loading, cracks caused by the shear stress aligned with the tensile direction initiate in the SAZ. These cracks are the origin of the final fracture of the punched specimen. We assumed that whether the break is a ductile failure or brittle fracture depends on whether the crack tip becomes blunt. The differences in the fracture behaviors are investigated via detailed fracture surface observation.