AUTOMATED CALIBRATION OF A VOID CLOSURE MODEL FOR HIGH-TEMPERATURE DEFORMATION

Abstract

The so-called Gurson model is a well-established micromechanical model of the ductile fracture of porous materials, where the mechanism is void nucleation, growth and coalescence. Although the Gurson model, and particularly its modified form developed by Tvergaard and Needleman, is widely used, its application to void closure has received relatively little attention. The first objective of the current work is to explore the applicability of the Gurson model to void closure. The fixed parameters characterizing the modified Gurson model are not universal and must be calibrated for a particular material, typically by trial and error fitting of finite element (FE) simulations to experimental data. However, the trial and error approach is expensive and time consuming (one test generally corresponds to only one triaxiality level). A novel approach has been developed in the present work to identify the void closure model parameters using a nongradient based optimization search method (pattern search method). Rather than using experimental data for void closure, a series of finite element analyses, one of a representative volume element (RVE) containing a spherical void, and another with an equivalent cell of Gurson–Tvergaard (GT) material, has been performed. Both models have parametric characterizations, enabling simulations under different triaxialities and initial void volume fractions. The numerical results of the discrete void RVE and the GT cell can then be compared and model parameters identified. The new automated method was applied using material properties obtained from high temperature tensile testing of Telby plus steel at 900°C and 1100°C, temperatures in the range of those experienced during a typical steel rolling process. The effect of strain rate on void closure is also investigated using this approach for Telby plus steel.