Digital image correlation and optical strain measuring in bendability assessment of ultra-high strength structural steels

Air-bending is a widely used forming process for ultra-high strength steel because it is fast, cost-effective and flexible way to form material. The bendability of a material can be expressed by minimum bending radius R/tmin, which is the relation of the smallest inner radius to the sheet thickness the material can be bent without damage. Damage usually occurs on the outer surface of the bend in the form of intense strain localization that further progresses to cracking. The minimum bending radius contains no other information that affects the bendability such as the lower tool width or the desired bending angle. Hence, developing more detailed test procedure is critical to better describe the behavior of ultra-high strength steel sheet in bending. In this paper, a method for more detailed assessment of bendability for ultra-high strength structural steel is presented. Using optical strain measuring techniques and digital image correlation coupled with bending tests in a universal tensile test machine one can measure the strain evolution at the outer surface of the bend and determine the critical strains that limit the bendability of these materials