Torben Gade Faurholdt

Inverse modelling of constitutive parameters for elastoplastic problems

To obtain an improved identification of constitutive parameters to be used in finite element method simulations of elastoplastic deep-drawing processes an inverse method was applied using an explicit finite element code to simulate material tests. This problem was addressed by formulating the constitutive parameter identification as an optimization problem. The method was to minimize the objective function defined as the error between the result from the material test and the result from the finite element simulation. The optimization technique is based on the Levenberg-Marquardt method. The objective function was established in a least-squares sense where the design variables were the constitutive parameters of the material. The inverse method was started and, when a global optimum was reached, a set of constitutive parameters were identified. This was performed for both a linear hardening model and a power-law hardening model. It is shown that the inverse method predicts two models which qualitatively show the same overall characteristics for the investigated material.

Characterization of residual stresses generated during inhomogeneous plastic deformation

Abstract Residual stresses generated by macroscopic inhomogeneous plastic deformation are predicted by an explicit finite element (FE) technique. The numerical predictions are evaluated by characterizing the residual elastic strains by neutron diffraction using two different (hkl) reflections. Intergranular residual elastic strains between subsets of grains are predicted numerically and verified by neutron diffraction. Subsequently, the measured residual strain profiles in the test samples are modified by the intergranular strains and compared to the engineering predictions of the FE technique. Results compare well and verify the capability of the numerical technique as well as the possibilities of experimental validation using neutron diffraction. The presented experimental and numerical approach will subsequently be utilized for the evaluation of more complicated plastic deformation processes resembling forming operations.