Experimental study of unstretched fiber shifting during hemming processes for automotive aluminum alloys

A typical processes chain for today’s hang-on parts of passenger car bodies such as hoods, bonnets or doors consists of a deep-drawing and cutting process, followed by flanging and hemming. The trend towards smaller bending radii of hemmed exterior parts in automotive industry and increasing use of lightweight materials, such as aluminum for vehicle hang-on parts, requires a further understanding of bending and hemming processes. In bending theory, the unstretched fiber describes a circular arc in a section of a bent part, which uncoiled length corresponds to length in unbent sheet. The unstretched fiber moves from middle of the section towards inner bending radius during bending operation. In radially outward direction, tensile stresses increase due to the increasing strain. The small bending radii, typical in the automotive industry, lead to relatively large tensile stresses in outer fiber. On the one hand, unstretched fiber shifting mainly affects the radius of the bent or hemmed part; on the other hand, position of the unstretched fiber during bending has a great influence on material loads. In this experimental study, shifting of the unstretched fiber during hemming operations is determined and quantified. Therefore, a new method for determination of present position of different fibers or layers in a bent section is introduced.