Surface roughness influences on localization and damage during forming of DP1000 sheet steel

Surface roughness strongly influences the occurrence of edge cracks in metal forming processes. Recently, a sub-modelling approach has therefore been presented that is capable of predicting whether an edge forming process could be performed without failure events, but the macroscopic load-deformation behavior could not be predicted. This approach is extended to consider roughness induced micro damage even on the macroscopic scale. The concept is based on the idea to define an individual set of material parameters for those elements that are located at the sample´s surface. In order to calibrate the required set of parameters for these surface elements, sub-models are created which geometrically represent the roughness profiles that were determined experimentally before the bending tests were conducted. The procedure is demonstrated for the example of bending tests performed on samples made of steel DP1000 that have undergone two different surface treatments to adjust roughness conditions - fine grinding and polishing in the one extreme case and grinding with 80-grit sand paper in the other one. Experimental results reveal significant differences between the two sample configurations: while the samples with smooth surfaces remain free from cracks during the entire duration of the experiment, the samples with rough surfaces show fracture events. For both cases, the new simulation framework allows to reproduce both the macroscopic load-deflection curves and the individual damage and fracture behaviours.