Microscopic Damage Growth with Hydrogen-induced Longitudinal Cracks in Tensile-shear Test on Mechanical Clinching Joint of Mild Steel Sheet and Aluminum Alloy Sheet

<p>To address environmental problems such as global warming, improvements in fuel efficiency, motor performance, and collision safety are required. Therefore, weight reduction, high strength, and rigidity have been prioritized in automobile bodies. In this regard, a multimaterial body composed of steel and aluminum alloys has been developed. To evaluate the reliability of a multimaterial body, the effect of hydrogen, which significantly contributes to delay failure, on its strength characteristics must be clarified. In this study, we investigated the evolution of mesoscopic and microscopic damages during tensile shear tests on clinched joints, with and without hydrogen charging. Subsequently, cross-sectional observations and fractography were conducted. The cross-sectional observations showed that cracks initiated at lower loads in the hydrogen-charged specimen, as compared to those in the uncharged specimen. Additionally, longitudinal cracks appeared only on the cross section of the hydrogen-charged specimen. At lower loads where crack initiation was confirmed, submicro voids were observed on the cross section. The fractography results indicated a layered fractured surface with submicro dimples on the hydrogen-charged specimen and a general ductile fractured surface with voids on the uncharged specimen. These results indicate that hydrogen causes the damage to evolve from primarily void coalescence to longitudinal crack coalescence. Additionally, prior to the formation of a main crack and longitudinal cracks, submicro voids are initiated. These initiation and growth mechanisms indicate that even on plastic deformation joints, hydrogen enhances localized plasticity and damage evolution, thereby reducing tensile shear strength.</p>