Numerical and experimental investigation of the bending zone in free U-bending

With the increasing application of the advanced high strength steel material in the automobile industry, the thickness reduction of the bending area has attracted more and more attention since the product strength is highly influenced by the quality of the bending region. In this paper, three major factors: the thickness reduction, the variation of the local bending radius within the bending zone and the tooling mark on the product's surface are investigated through three different loading patterns for a free U-bending profile numerically and experimentally. The results demonstrate a thinning pattern consists of three peaks over the bending region for large bending ratio (R/t=2.14) and only one peak for small bending ratio (R/t=0.5). Corresponding valleys for the local radius are found to match the thinning pattern. Further, use of finite element simulation can successfully predict the location and the severity of the wear on the product. From the experiment results, even if the metal blank only experienced one stroke, the tooling mark contains both adhesive and abrasive wear. A better understanding of the characteristics of the bending zone is achieved and the findings can help in improving the design process for forming strategies.