On Material Modelling of High Strength Steel Sheets

Abstract: The work done in this thesis aims at developing and improving material models for use in industrial applications.The mechanical behaviour of three advanced high strength steel grades, Docol 600DP, Docol 1200M and HyTens 1000, has been experimentally investigated under various types of deformation, and material models of their behaviour have been developed. The origins of all these material models are experimental findings from physical tests on the materials.Sheet metal forming is an important industrial process and is used to produce a wide range of products. The continuously increasing demand on the weight to performance ratio of many products promotes the use of advanced high strength steel. In order to take full advantage of such steel, most product development is done by means of computer aided engineering, CAE. In advanced product development, the use of simulation based design, SBD, is continuously increasing. With SBD, the functionality of a product, as well as its manufacturing process, can be analysed and optimised with a minimum of physical prototype testing. Accurate numerical tools are absolutely necessary with this methodology, and the model of the material behaviour is one important aspect of such tools.This thesis consists of an introduction followed by five appended papers. In the first paper, the dual phase Docol 600DP steel and the martensitic Docol 1200M steel were subjected to deformations, both under linear and non-linear strain paths. Plastic anisotropy and hardening were evaluated and modelled using both virgin materials, i.e. as received, and materials which were pre-strained in various material directions.In the second paper, the austenitic stainless steel HyTens 1000 was subjected to deformations under various proportional strain paths and strain rates. It was experimentally shown that this material is sensitive both to dynamic and static strain ageing. A constitutive model accounting for these effects was developed, calibrated, implemented in a Finite Element software and, finally, validated on physical test data.The third paper concerns the material dispersions in batches of Docol 600DP. A material model was calibrated to a number of material batches of the same steel grade. The paper provides a statistical analysis of the resulting material parameters.The fourth paper deals with a simple modelling of distortional hardening. This type of hardening is able to represent the variation of plastic anisotropy during deformation. This is not the case with a regular isotropic hardening, where the anisotropy is fixed during deformation.The strain rate effect is an important phenomenon, which often needs to be considered in a material model. In the fifth paper, the strain rate effects in Docol 600DP are investigated and modelled. Furthermore, the strain rate effect on strain localisation is discussed.