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Identification of the plastic material behaviour through full-field displacement measurements and inverse methods

Thursday, 27 November, 2008 - 16:30
Campus: Brussels Humanities, Sciences & Engineering campus
Steven Cooreman
phd defence

Nowadays Finite Element (FE) modelling is increasingly applied in industrial practice to optimize all kinds of metal forming operations. Simulations with FE models reduce the cost and the length of the trial-and-error phase. The reliability and the usefulness of such simulations largely depends on the accuracy of the input data, i.e. the geometry, the boundary conditions and the material behaviour. In this work the focus is on the identification of the plastic material behaviour.

Most commercial FE codes prefer to describe the plastic behaviour by means of so-called phenomenological models. Since plastic deformation is strain path dependent, the validity of phenomenological models is limited to situations that are comparable to the range of experiments based on which they are identified. In general the parameters of those phenomenological models are determined by standard tests, e.g. tensile tests. However, the homogeneous stress and strain fields generated during these relatively simple tests do not resemble the complex stress and strain fields which occur in many metal forming operations. Therefore the material behaviour, obtained from standard tests and described by phenomenological models, is merely an approximation that in many cases doesn't allow the reliable simulation of complex forming processes.

This work evaluates the possibilities of identifying the plastic material behaviour based on the heterogeneous displacement fields generated during complex material tests. Those heterogeneous displacement fields are measured by means of the digital image correlation technique. The unknown material parameters - the parameters of the hardening law and those of the yield surface - are determined by minimization of the discrepancy between the experimental and numerical strain fields. The numerical strain fields are computed by means of a FE code.

The obtained results prove that the proposed FE based inverse method (Figure 1) indeed allows to identify the parameters of an isotropic hardening law and those of an orthotropic yield surface based on one single complex material test. The results also show that it can be advised to develop the material test in accordance with the forming process which will be studied afterwards so as to obtain material parameters which are more suited to simulate that particular experiment.