Jens Christlein

Improvement of damage prediction by anisotropy of microvoids

Abstract The finite element method is nowadays widely applied to crashworthiness or sheet metal forming. In order to accurately predict the damage and failure evolution occurring in such simulations, a realistic material description is required. For this purpose, this paper presents a damage model for elasto-plastic materials, taking into account both the material and the damage anisotropy. The damage is then expressed as the microvoid volume fraction due to growth, nucleation and coalescence of microvoids. The anisotropy of the material is introduced by means of Hill’s potential in the yield function and the anisotropy of the damage by the shape of the microvoids. The void is defined as an ellipsoid which could evolve in shape and direction according to the loading direction and the geometry of the structure. A radial return algorithm is proposed for the implementation of a new constitutive law for elasto-plastic porous materials for convected shell elements into an explicit finite element framework. To compare numerical and experimental results of bending tests on a non-axisymmetric aluminium extruded section, identification of the damage parameters is required. This is done by using an inverse method by correlating experimental and numerical macroscopic measurements which are strongly dependent on the parameters. Tensile tests on a thin notched specimen are used as a mechanical test to measure the macroscopic response. Due to the anisotropy aspect of the damage parameters, three macroscopic measurements in the three orthotropic directions L, L–T and T are considered. The behaviour law and damage parameters are identified. Bending tests are then performed and give good correlation with experimental data.

Anisotropic damage applied to numerical ductile rupture

ABSTRACT Damage models models are applied for elasto-viscoplastic materials. They take microvoids volume fraction evolution into account by means of the growth, the nucleation and coalescence of microvoids. The anisotropy of the material is introduced with the Hills potential in the modified yield function. The anisotropy of the damage is taken into account in one model with the shape of the microvoids. The void is defined as an ellipsoid which could evolve in shape and direction according to the loading direction and the geometry of the structure. These models are applied on unit cell specimens and on a double non- axisymmetrical V-notched specimen. The influence of the microvoids shape and their orientations on the final ductile rupture is shown.