Formability of Automotive H240LA Steel Sheets in Stretch– Bending Processes

This paper discusses the influence of punch radius on the formability of 1.2 mm thick H240LA steel sheets. A series of Nakazima tests with hemispherical punch and stretch–bending tests with cylindrical punches of variable diameter were performed in order to assess the influence of the strain gradient on sheet failure. Necking limit strains were obtained by using a recently reported time-dependent methodology that can be used not only in conventional Marciniak and Nakazima tests, but also in the presence of a severe strain gradient across the sheet thickness. Based on the results, formability in H240LA steel sheets increases with decreasing t0/R ratio. Fracture limit strains in both test series were evaluated by using an optical strain measurement system and measuring specimen thickness after fracture.