Bending Limit Prediction of an Aluminum Thin Sheet

The aim of this work is to predict the occurrence of rupture during bending of an aluminum alloy thin sheet by considering ductile damage. The first step consists in characterizing the mechanical behaviour of the material under different strain paths, such as tension of straight and notched samples and equibiaxial tension, up to necking and final rupture. The parameters of Gurson-Tvergaard-Needleman model are then identified for different triaxiality ratios ranging from 0.3 up to 0.67. Inverse identification is performed, by coupling the optimization software with a finite element code. A value of 0.15 for the critical void volume fraction corresponding to the void coalescence is obtained. In order to determine experimentally the onset of rupture in bending for this material, square specimen of length 60 mm are bent over a small radius of 0.2 mm, with a designed-on-purpose device. The area just beneath the bending tool is observed with a scanning electron microscope, for different tool displacements. The bending test is then simulated and a good correlation between the numerical onset of rupture, defined when the void volume fraction equals its critical value, and the occurrence of cracks on the sample surface is found.