Measurement and prediction methods for forming limit strain of stretch bending deformation in sheet metal forming

In this study, a new test method using protruding punch was proposed, that can separate bending and stretching deformations. In this test, the blank makes simultaneous contact with the punch head and shoulder to ensure a constant bending angle during the test. After the simultaneous contact, increasing the punch stroke increases the amount of stretch deformation without changing the bending deformation. In addition, this test method can control the inflow of materials by controlling the BHF, and it is possible to evaluate cracks during stretch bending deformation even for materials with low forming limits. This test can control the distribution of strain through the thickness. As a result, the measurable surface formation limit strain in this test was higher than FLC with uniform strain along the thickness. It is suggested that this is because the strain gradient through the thickness delayed the timing of necking occurrence. There is a linear relationship between this strain gradient and the maximum principal strain on the sheet surface, which can be applied to the prediction of stretch-bending cracks in FEM analysis.