Micro-mechanical modelling techniques for forming texture, non-proportionality and failure in auto materials

A well-defined programme of biaxial forming tests will be carried out by our collaborators at BMW in which the steel microstructures, both before and after straining, are fully characterised using optical and scanning electron (with EBSD) microscopy in order to quantify micro- and macro-level texture and its evolution and to provide the material morphology and crystallography as input to the computational work. Additionally, TEM and x-ray/neutron diffraction work will be carried out in order to investigate dislocation structures established and whether particular forms develop during non-proportional straining. The non-proportional 'Nakajima' tests will be carried out on two materials; namely, a conventional 'forming grade' steel and a high strength steel for which currently, formability is a problem. 3D representative volume elements, RVEs, with appropriate periodic boundary conditions will enable texture development, non-proportionality of straining and localisation and necking to be studied and direct comparisons may be made with the experimental data. The key features of the crystal slip model - the form of the evolution of statistically stored dislocations, the development of geometrically necessary dislocations due to plastic strain gradients, and the establishment of dislocation structures - can be refined by use of the TEM and the experimental localisation results. Once established and validated, the RVE technique becomes powerful and enables parametric studies of the effects of non-proportionality to be carried out in a way that is simply impossible with an equivalent experimental programme.The computational models will naturally take into account the full range of length scales that occur in this problem: at the dislocation and grain levels as well as length scales related to the formation of the localized band of deformation. The resulting simulations will be used to guide the development of simplified models based on the Marciniak and Kuczynski approach. This will be undertaken at a number of different levels to aid the development of tools that can be readily used within an industrial environment. Also the full range of simulations will be used to aid the development of design rules which account for non-proportional loading and which can be used to guide the initial development of a processing route.

High strength steels are used for a wide range of automobile components and for a typical car, approximately 130 large sheet metal parts are manufactured in this material. The sheet metal components are often shaped using biaxial forming in which large plastic strains are developed. An important tool in the design of the manufacturing process is the forming limit diagram (FLD) in which the processing conditions are given for which successful forming can take place without failure. FLDs are determined in the industry using costly experiments and the introduction of new materials therefore leads to the need for many such tests. Experiments have shown that marked improvements in the process can be achieved by controlling the proportions of the planar strains applied. If the fundamental deformation mechanisms controlling this effect can be identified and incorporated into computer modelling techniques, the possibility exists for a step-change improvement in the forming process for a whole range of auto sheet components and the potential for the use of this production method for a range of new components. It is the modelling techniques which provide the potential for radical improvement since parametric experimental studies for both currently used steels and new materials remains prohibitively large, costly and commercially unviable. We have a significant track record of successfully benefitting our industrial collaborators and we currently hold two major EPSRC programmes in close collaboration with Rolls-Royce. We manage these and other projects to maximise industrial impact and successful transfer in a number of ways and some examples are given in what follows. Considerable impact has been achieved in micro-deformation and fatigue with Rolls-Royce and this was facilitated by a Royal Society industry fellowship secondment (Dunne) for a year to Rolls-Royce in 2006. Close collaborations with Nippon Steel have resulted in research visits to production facilities in Chiba (Tokyo), the funding of a Nippon Steel researcher to join our team in Oxford for two years (06-08), and the funding (by Nippon Steel) of a Kyushu University academic to join our team for a year (08-09) to work on Nippon Steel fatigue problems. Our research group meetings in Oxford are regularly attended by Dr David Rugg (Rolls-Royce) who was recently appointed a Visiting Professor in Oxford by virtue of his close collaborations with our group.