Accurate computer simulations require the selection of suitable material models and precise prediction of their parameters. In the fields of impact engineering and plastic working, stress–strain relations that include the post-necking regime up to fracture are crucial for predicting the behavior correctly. However, obtaining suitable stress–strain relations after necking requires some form of correction and adjustment for stress and/or strain. This study applies a stepwise modeling method for post-necking characterization that only utilizes the local strain field obtained from tensile experiments to precisely measure stress–strain relations at high strain rates. The effects of the notch radius of specimens on stress–strain relations were examined to measure stress–strain relations with large strain near the stress triaxiality of 1/3. The study also discusses adequate resolution for precise stress–strain measurements. Subsequently, specimens with suitable notch radius were used to measure stress–strain relations of plate specimens of aluminum alloy 2024-T3 at high strain rates. The study also examined the effects of strain rate on the flow stress and fracture strain of aluminum alloy 2024-T3.

Sheet metals are often used in automotive and aerospace applications for safety-relevant components. Weight reduction is one possibility to reduce fuel consumption or increase the payload capacity and therewith reduce the carbondioxide emission of these trans-portation vehicles. The weight reduction can be achieved by using new sheet metal alloys and thereby reducing the sheet metals thickness. Advanced material process-ing technologies like for example the press hardening process to manufacture ultra high strength steels (UHSS) are an important contribution to weight reduction. Furthermore, the usage of many different sheet metal materials and grades, like the new generation of advanced high strength steels (AHSS) and aluminium alloys will replace further low strength steel components.To challenge the balance between safety and weight reduction, while maintaining safety, reliable and efficient engineering tools are needed. Finite Element (FE) simulations are commonly used to prove a maintained safety for parts with a decreased sheet thickness and weight. This leads to a high demand on the simulation precision of sheet metals, where an accurate prediction of the failure behaviour and the post-necking hardening of materials is needed. Therefore, an approach on material model calibration for modelling of sheet metal deformation and failure is developed. The ability for companies to predict the performance envelop of all these new sheet metal alloys and components is of great importance for the metal manufacturer as well as for the automotive industry.In this thesis work a method to characterize the elasto plastic post necking behaviour of sheet metal materials, the Stepwise Modelling Method (SMM), is presented. The method uses full field measurements of the deformation field on the surface of tensile specimen. The hardening relation is modelled as a piecewise linear relation in a step by step procedure. The linear hardening parameter is adapted to reduce the residual between experimental and calculated tensile forces. The SMM is used to characterize the post necking behaviour of a ferritic boron steel and the results are compared with the commonly used inverse modelling method. It is shown that the stepwise modelling method characterizes the true stress, true plastic strain relation in an effective and com-putational efficient way. Furthermore, the SMM is used to characterize the stress state evolution during tensile testing, which is an important factor for failure and fracture mod-elling. This method is shown in an aerospace application for the nickel based super alloy Alloy 718. A study on simulating the whole comments lifespan from blank to fractured component is presented by producing a laboratory scale UHSS-component and testing it until fracture. The component performance simulation is based on results obtained by SMM for paint baked fully hardened boron steel. To enable the post necking characterization of anisotropic sheet metals like aluminium alloys an updated SMM version based on an anisotropic plasticity model is presented and evaluated for the aluminium alloys AA6016 and AA5754. Finally, the fracture behaviour of an automotive 6000 series alu-minium alloy in different directions is presented. In this study a GISSMO failure model is calibrated based on full field measurements under different stress states and evaluated on a multi triaxiality tensile specimen.The results shown in this thesis are that the presented Stepwise Modelling Method is an effective and efficient alternative method to characterize the deformation and failure of sheet metals. Based on the results of this method plasticity and fracture models can be calibrated and used for advanced forming and component performance simulations. This can lead to reduce time and costs during the development processes of new materials and products.

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.

In the last decade, hot metal forming of advanced high strength steel (AHSS) have improved passenger safety and open possibilities for lightweight design. Hot metal forming can be applied to locally tailor the microstructure of components and gradual vary mechanical properties to improve crash resistance behaviour and optimized weight for e.g. safety related parts. Sometimes post punching or trimming must be done on hardened parts. Such conditions induce damage and fractures in the trimmed edge. Another issue is that high pressures are required in cutting operations due to the high yield stress of press hardened parts, which accelerate wear and produce premature fracture in tools. Optimizing cutting operations to minimize damage and wear are essentials and numerical simulations of cutting operations can be of good assistance. One of the main challenges in the numerical modelling consists of numerically be able to treat the extremely large deformation occurring in the cutting zone. A second challenge is to find suitable failure models. In this work, the punching process of soft and hard microstructures obtained by press hardening is experimentally studied, but also modelled with a combination of smoothed particle Galerkin (SPG) method and finite element method (FEM). Laboratory punching tests with different clearance values were carried out using sheets of different fracture strengths. All experimental cases are numerically modelled. Validation is conducted by comparing numerical results with experimental measurements of punch force and displacement. In addition, morphology of the final cutting edges from both real and virtual are compared. Numerical results show good agreement against experimental measurements. Furthermore, the combined method gives robust-ness and stability as it can handle large deformations efficiently.

Hot stamping of boron alloyed steel has become a standard in the automotive industry for safety relevant body in white components. This process allows the design of complex geometries with superior mechanical properties. Special tool design enables to manufacture components with special properties based on varying microstructures in designated areas. This is a challenge for finite element (FE) simulations of deformation and failure for multi-phase microstructure components.

In the present work, a laboratory scale test component with multi-phase microstructure is studied from blank to fractured component. Using different tool temperatures and adding an air-cooling step before transfer to the press hardening tool, the microstructure of the component is varied. By this, components with four different multi-phase microstructures are produced. These components are tested under tensile deformation until fracture, where force, elongation and the strain field on the components surface are measured.

The laboratory scale test component is evaluated using FE-modelling. The complete production process is modelled starting with the pre-cut austenitized blank, subsequent transfer, air-cooling, forming operation, and the final post-cooling. The resulting multi-phase micro structures are evaluated using manual optical microscope image analysis and compared with the simulated phase composition. Furthermore, the deformation and fracture of the manufactured component under tensional loading is studied using a mean-field homogenization scheme for the multi-phase composition combined with the OPTUS failure model. This finite element investigation is conducted taking the microstructure composition, shape and thickness deviations from the forming simulation into account.

The present work shows the feasibility of modelling methods of the complete process chain for press-hardened components with multi-phase microstructures, from blank to fractured component.

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.

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

Weight reduction is one possibility to reduce fuel consumption and emission of transportation vehicles. Sheet metals are often used in automotive and aerospace applications and therefore the weight reduction achieved by reducing the sheet metals thickness is an important contribution to weight reduction. Increasing the strength of sheet metal materials gives the opportunity to reduce the total weight while maintaining safety. To prove a maintained safety for parts with a decreased weight Finite Element (FE) simulations are commonly used. This leads to a high demand on the simulation precision of sheet metals, where an accurate prediction of the post-necking behaviour of materials is needed. Improved FE simulations are reducing time and costs during the development processes. 

One application to improve the strength of sheet metals in the automotive industry is the usage of ultra high strength steels, which has constantly increased in usage during the last decades. The development of the press hardening process, where sheet metal blanks are formed and quenched simultaneously, brings new design opportunities. Using press hardening tools with zones that uses different cooling rates sheet metal parts can be produced with tailored properties, to improve their performance. Simulating materials based on the microstructure demands high precision on the plasticity modelling for high strain values. 

In this thesis work a method to characterize the elasto plastic post necking behaviour of sheet metal materials, the Stepwise Modelling Method (SMM), is presented. The method uses full field measurements of the deformation field on the surface of tensile specimen. The hardening relation is modelled as a piecewise linear in a step by step procedure. The linear hardening parameter is adapted to reduce the residual between experimental and calculated tensile forces. The SMM is used to characterize the post necking behaviour of a ferritic boron steel and the results are compared with the commonly used inverse modelling method. It is shown that the stepwise modelling method characterizes the true stress, true plastic strain relation in an effective and computational efficient way. Furthermore, the SMM is used to characterize the stress state evolution during tensile testing, which is an important factor for failure and fracture modelling. This method is shown in an aerospace application for the nickel based super alloy Alloy 718. 

The results shows that the stepwise modelling method is an effective and efficient alternative method to characterize the deformation and failure of sheet metals. Based on the results of this method plasticity and fracture models can be characterized in future work.

Background: Strain and stress conditions in sheet metal shearing are of interest for calibration of various fracture criteria. Most fracture criteria are governed by effective strain and stress triaxiality.

Methods: This work is an attempt to extend previous measurements of strain fields in shearing of steel sheets with the stress state calculated from the measured displacement fields. Results are presented in terms of von Mises stress and stress triaxiality fields, and a comparison was made with finite element simulations. Also, an evaluation of the similarities of the stress conditions on the sheet surface and inside the bulk material was presented.

Results: Strains and von Mises stresses were similar to the surface and the bulk material, but the stress triaxiality was not comparable. There were large gradients in strain and stress around the curved tool profiles that made the result resolution dependent and comparisons of maximum strain and stress values difficult.

Conclusions: The stress state on the sheet surface calculated from displacement field measurements is useful for validation of a three-dimensional finite element model.