precise prediction of the post-necking behaviour of materials is needed to increase the precision of computer simulations with large deformations. Applications in which this need is encountered include crash, forming, and failure simulations. By using an optical full-field measurement of the localised deformation field, an effective and computationally fast method is presented to determine the relationship between true stress and true plastic strain, including post-necking behaviour. The presented stepwise modelling method is used to characterise heat-treated boron steel using thin sheet metal specimens. These results are validated with the results determined by a method based on inverse modelling. It can be concluded that the stepwise modelling method is considerably faster than the compared inverse modelling method. The method is also suitable for effectively determining element size dependency due to regularisation of the hardening behaviour needed for finite element analysis with strain localisation, e.g., for crash simulations

The demand for weight reduction of cars has increased the number of press hardened sheet metal parts used in the automotive industry. This leads to an increased demand on the precision of simulations of press hardened sheet metals. An accurate prediction of the post-necking behaviour of materials is therefore needed to increase the precision of computer simulations with large deformations, as for example in forming simulations and crash simulations. Especially fracture simulations of press hardened steel parts with tailored properties have a huge demand on precise material models.

Inverse modelling is a common engineering tool to characterize the elasto-plastic behaviour of materials.  Taking experimental data, such as force and displacement data, the material model parameters are optimised until the simulated output reaches a target function.  Then inverse modelling is highly time demanding and needs nonlinear hardening material models. 

Lately a new fast method for post necking characterisation of sheet metals, called the Stepwise Modelling Method (SMM), was presented. This method uses full field measurements to obtain the strain field on the surface of sheet metal tensile specimens.  Furthermore, the stepwise modelling method models an experimental hardening curve in a stepwise process.  This hardening curve is a piecewise linear curve and not restricted to any specific material model.

In this paper SMM is used to characterize the hardening behaviour for thermally treated boron steel.  These results are compared with the results of inverse modelling. Three different material models are used. The comparison shows a minor deviation in the resulting hardening relations between stepwise modelling and inverse modelling. Since the efficiency is an important factor in product development calculation times are taken into account.  Comparing calculation time using SMM is considerably more efficient than using inverse modelling. Furthermore another advantage of SMM is shown in the fact that the piecewise linear hardening curves can be fitted to almost any material model without computational costs.

Sheet metals are often used as safety structures in automotive applications where the fracture behaviour is a key design parameter. Theoretical and experimental observations have shown that the fracture behaviour of many metals depends on the stress state. Modelling the stress state dependency of fracture in Finite Element (FE) simulations has led to the development of advanced stress state dependent fracture criteria. The calibration of advanced fracture models is currently limited by the characterisation methods, which have not developed much during the last decades. Experimental characterisation methods that can determine the stress state accurately are necessary to ensure reliable calibrations of advanced fracture models. In this article, an experimental method to obtain the stress state and its evolution during deformation is presented. The stress state evolution is determined using measured local displacement field data, which were obtained by digital image correlation, coupled with a stepwise modelling method. This article shows that the stepwise modelling method can capture the stress state evolution for three different specimen geometries subjected to tensile loading. The resulting experimentally determined stress state evolutions are compared with the results of FE simulations, and both results are in good agreement. The accurate stress state evolutions characterised directly from experiments using the proposed method enables calibration of advanced fracture models rapidly and reliably

Hot stamping of boron alloyed steel has become a standard in the automotive industry for safety relevant chassis components. Hot stamping of ultra-high strength steel allows the design of complex geometries with superior mechanical properties. In the present work, a laboratory scale test component is followed up from blank to fractured component. The production process starts with a pre-cut blank, which then is austenitized, transferred to the press hardening tool, formed and quenched and ends with post-cooling to room temperature. These components are tested under tensile deformation until fracture, where force, elongation and the strain field on the components surface are measured. The strain field measurements are performed by using digital image correlation (DIC). The laboratory scale test component is evaluated using finite element modelling. The production process is modelled starting with a pre-cut austenitized blank, subsequent transfer and forming operation, and ends with post-cooling. Furthermore, the deformation and fracture under tension/bending is studied using the OPTUS damage model. The as-produced component is measured using a three dimensional scanning system. Shape deviation and thickness change are compared to in the forming simulation predicted geometry after post-cooling. A finite element investigation on the deformation and fracture under tensional/bending loading is conducted applying shape and thickness deviations in the model. The majority of industrial components undergo paint curing before they are included in an assembly. Paint baking is a heat treatment at relatively low temperatures and causes relaxation in a martensitic microstructure. The effect of paint baking on the mechanical response of the laboratory scale test component is investigated. In the present work the reliability of modelling tools from blank to fractured component is shown. The possibility is shown to predict the failure of the component, with the specific phase composition after the hot stamping process obtained from simulations. Furthermore, the influence of the paint baking process on the mechanical properties is presented.

Modelling and simulation are important tools during design and development processes. For accurate predictions of, e.g. manufacturing processes or final product performance, reliable material data is needed. Usually, the applied material models are calibrated by utilising direct methods such as conventional uniaxial tensile/compression tests but also inverse methods are occasionally applied. Recently, an effective inverse method, the stepwise modelling method (SMM), was presented. By using SMM, the flow stress from initial yielding, beyond necking to final fracture, can be determined. However, the method is developed for sheet materials having isotropic von Mises hardening. In this paper the SMM is extended for post necking characterisation of anisotropic sheet metals using the Barlat yield 2000 criterion. The novel method was applied to analyse the post necking plasticity of the widely used aluminium alloy AA6016 in T4 condition and the aluminium alloy AA5754 in H111 condition. The latter alloy has reported to show serrated yielding, also known as the Portevin–Le Chatelier effect. The obtained flow stress curves agree well with the curves form conventional uniaxial tensile tests up to the point of necking and show credible post necking predictions to final fracture. Furthermore, SMM showed that it could handle the effect of serrated yielding for AA5754-H111. Hence, the novel approach can be used to characterise the post necking hardening of a variety of anisotropic sheet metals and thereby contributes to efficient and reliable material model calibration.