Experimental investigation of material failure during bending of pre-deformed sheet metal

Many sheet metal parts go through a bending operation during the manufacturing process. Compared to deep-drawing operations, failure in bending operations cannot be predicted accurately with a forming limit curve from the Nakajima or Marciniak experiment, especially in a pre-deformed state. Due to the small bending radii and the associated strong curvature, the failure only occurs with significantly higher strains for states without pre-deformation. Likewise, the failure is not caused by a localization, but by damage to the outer surface of the sample. The introduction of pre-deformation in the sheet material leads to development of texture and damage, where these mechanisms depend on the loading direction. If such pre-deformed sheet material is subsequently bent, the sample may fail unexpectedly early compared to the initial forming limit curve. The present experimental work aims at investigating the influence of pre-deformation and subsequent loading direction for different materials. Therefore, specimens have been pre-deformed in different orientations, followed by bending tests in different orientations. Different pre-deformation levels and loading directions combinations on three sheet materials were investigated. Based on the experimental results a so called bending forming limit curve (BFLC) can be derived enabling enhanced prediction of failure for bending processes after pre-deformation.