Bengt Wikman

Material parameter estimation for boron steel from simultaneous cooling and compression experiments

In order to increase the accuracy of numerical simulations of the hot stamping process, reliable material data is crucial. Traditionally, the material is characterized by several isothermal compression or tension tests performed at elevated temperatures and different strain rates. The drawback of the traditional methods is the appearance of unwanted phases for some test temperatures and durations. Such an approach is also both time consuming and expensive. In the present work an alternative approach is proposed, which reduces unwanted phase changes and the number of experiments. The isothermal mechanical response is established through inverse modelling of simultaneous cooling and compression experiments. The estimated material parameters are validated by comparison with data from a separate forming experiment. The computed global response is shown to be in good agreement with the experiments.

A combined material model for numerical simulation of hot isostatic pressing

In modelling of hot isostatic pressing (HIP) of powder materials the constitutive model should be able to describe different deformation mechanisms during the consolidation process. In the early stage, the consolidation is dominated by granular behaviour. As temperature and pressure increase in the powder the deformation can be described by a viscoplastic model. Experimental observations show substantial time-independent deformation in the early stage. At this stage of the densification process, pores in the powder are still interconnected. This cannot be described properly by a viscoplastic model. The inconsistency between the deformation mechanisms can be treated by a combined elasto-plastic and elasto-viscoplastic model. Here a granular plasticity model is combined with a viscoplastic model. In previous works the viscoplastic model, power-law breakdown, has been used to describe the entire deformation process. The combined model is implemented into an in-house finite deformation code for the solution of coupled thermomechanical problems. The simulation of a hot isostatic pressing test with dilatometer is performed in order to compare calculated results with the experimental measurement. The results from previously performed analysis carried out with a viscoplastic model only are also compared. Analysis with the combined material model shows good agreement with the experiment for the whole densification process.

Estimation of material parameters at elevated temperatures by inverse modelling of a Gleeble experiment

A method for estimation of inelastic material parameters from Gleeble experiments is presented. Material parameters at elevated temperatures are estimated by inverse modelling of a Gleeble compression test during continuous cooling. The direct problem is solved using the implicit finite element program NIKE2D. The material behaviour is described by a rate independent thermo-elastic-plastic model with a nonlinear isotropic hardening law. The objective function is formed based on the discrepancy in force-displacement data between the numerical model prediction and the experiment. Minimisation of the objective function with respect to the material parameters is performed using an in-house optimisation software shell which is built on the subplex method. A Gleeble experiment with different deformation and temperature histories has been used for validation of the computed parameters. The validation experiment is simulated using both the material parameters achieved through inverse modelling and traditional estimation methods. The analysis showed better agreement when using the parameters estimated by the numerical method presented in this paper.