Experimental and Analytical Approach to Failure Determination of AHSS in Stretch Bending

Abstract

Experimental and analytical studies on stretch bending characteristics of advanced high strength steel (AHSS) sheets, which are mainly used in the automotive industry, were conducted based on the stretch-bending test (SBT). The SBT is a method in which both ends of a specimen are fixed and a punch with a radius is used to press the center of the specimen until failure occurs. Stretch-bendability, the ability of a material to deform in stretch-bending without failure, is characterized by the ratio of the minimum bending radius and the material thickness. It was evaluated experimentally for selected AHSS sheets using a tool set to simulate the stretch-bending mode, and numerically using finite element (FE) simulations with boundary conditions reproducing those of the test. In general, FE simulations using shell elements precede the development of a forming process to reduce cost and increase time efficiency. However, it has been reported that failures in stretch bending with small punch radii have been unpredictable, so this work aims to investigate the cause and propose a solution. FE analyses were conducted on ABAQUS, a commercial software, after a careful examination of the effects of numerical factors such as element type and size, and integration point number. The failure of a material experiencing stretch-bending was assumed to occur when the load of the punch reaches a maximum. The optimal parameters were determined for solid elements before simulating the SBT. The shell elements were first considered for this numerical study, but this approach proves to be limited in predicting the failure of the stretch-bending mode, especially with small punch radii. In the SBT of the sheet, the maximum forming height and the failure location are dependent on the radius of the tool. Assuming that the strain distribution along the thickness direction in the bending region is not uniform and the strain gradient is a function of the bending radius, it has been confirmed by a theoretical approach that the maximum load at which failure occurs depends on the bending radius. In this study, the results of experimental and FE analysis for unusual failures that may occur during the sheet metal forming were reported. A theoretical failure criterion was also proposed based on the mechanics of stretch-bending, and using process variables and mechanical properties of the material. The index quantified as the ratio of tool radius to material thickness (R⁄t)_critical, not only indicate the forming conditions for failures that are difficult to detect with conventional FLC, but also simplify the prediction of unusual failures based on a theoretical approach. In addition, it can provide guidelines for the efficient design and production of automotive components in the application of high strength steels. Finally, the (R⁄t)_critical determined by the proposed criterion was compared with the experimental results.