A thermal model based on explicit time integration is developed and implemented into the explicit finite element code DYNA3D to model simultaneous forming and quenching of thin-walled structures. A staggered approach is used for coupling the thermal and mechanical analysis, wherein each analysis is performed with different time step sizes. The implementation includes a thermal shell element with linear temperature approximation in the plane and quadratic in the thickness direction, and contact heat transfer. The material behaviour is described by a temperature-dependent elastic-plastic model with a non-linear isotropic hardening law. Transformation plasticity is included in the model. Examples are presented to validate and evaluate the proposed model. The model is evaluated by comparison with a one-sided forming and quenching experiment

A computational model for quenching simulations of thin plates has been developed. The model is examined by comparisons with experiments with one-sided water spray cooling. With this experiment, the thermomechanical behavior of the material can be monitored throughout the time history of the test. Experiments have been performed with a material that undergoes only martensitic transformation during quenching. For the conditions examined the plate exhibits permanent deformation after quenching. In the stress calculation, transformation plasticity is included in the effective-stress-function (ESF) algorithm as an additional strain component related to the stress and to the progress of transformation. Analyses of the present experiments show that the permanent deformation of the plate is exclusively due to transformation plasticity

A database for finite element models and related data is developed and incorporated into a prototype system for integration of non-linear finite element codes with a product design system. In the prototype system, the database is used as a link for integrating commercial, public domain as well as in-house codes. In the present system, the public domain finite element codes NIKE2D, NIKE3D, DYNA2D, DYNA3D and TOPAZ2D are integrated with the CIM-system I-DEAS. The prototype system is primarily intended as a platform in research projects for development of integrated environments tuned for simulations of specific manufacturing processes such as quenching, welding, hot rolling, metal powder compaction and hot isostatic pressing

Crashworthiness is a relevant engineering property for car parts. However it is not easy to measure at laboratory scale and complex impact tests have to be carried out to determine it. Crash resistance for high strength steel is commonly evaluated in terms of cracking pattern and energy absorption in crashed specimens. Accordingly, the material resistance to crack propagation, i.e. the fracture toughness, could be used to rank crashworthiness. It has been proved in a previous work by the authors, so the measure of fracture toughness, in the frame of fracture mechanics in small laboratory specimens, would allow determining the best microstructure for crash resistance parts. Press hardening offers the possibility to obtain a wide range of microstructural configurations, with different mechanical properties. So the aim of this work is to evaluate the fracture toughness following the essential work of fracture methodology for ferrite-pearlite, bainite, ferrite-bainite, martensite and martensite-bainite microstructures. Results showed that bainitic microstructures have high fracture toughness, similar to TWIP and CP steels, which allows pointing them as potential candidates for obtaining high crash resistance in parts manufactured by press hardening.

Tailored tempering can produce different amounts of martensite, upper and lower bainite and ferrite depending on the press hardening conditions are usually obtained. The tensile properties of such microstructures are quite well known, but the intrinsic fracture properties, as fracture toughness, have not yet been characterized. This is mainly related the experimental difficulty to measure fracture toughness in thin sheets. Recently, the authors have proposed to measure it through the application of the Essential Work of Fracture methodology. The knowledge of facture toughness would give valuable information to increase their applicability in automotive components and would also allow a further understanding of fracture and crack propagation mechanisms in B steel hardened at different cooling conditions. Thus the aim of this paper is to determine the fracture toughness in microstructures of a 22MnB5 steels obtained by tailored tempering. Results indicate that the EWF methodology can be applied to measure fracture toughness and the energy for crack initiation in ferrite-pearlite and ferrite-bainite microstructures. Fracture initiation energy is the same in ferrite-pearlite and ferrite-bainite microstructures, which will indicate that ferrite has a clear effect on fracture initiation. Moreover, fractography results reveal the effect of inclusion content on fracture energies

Press hardening is widely utilized to form ultra-high-strength steels characterized by a high strength-to-weight ratio for automotive components. Press hardening processes include heating boron–manganese steels to austenite phase, forming the steels at a high temperature, and cooling the formed blanks until the martensite phase is reached . However, press hardening processes lead to severe contact conditions between the blank and the tools including contact pressure, relative sliding, and high temperatures, which result in tool wear and increased maintenance cost. The contact conditions that occur in the stamping tool are difficult to study on site. Additionally, simplified tests, such as pin on disc and ball on disc, are insufficient to reproduce press hardening conditions in laboratory environments . The aim of this study includes developing a tribological test with press hardening conditions in which tool steel pins continuously slide on fresh and hot boron–manganese steel strips. The test programme mimics press hardening conditions with respect to sliding distance, sliding velocity, contact pressure, and surface temperature that were studied based on finite element (FE) simulations of a press hardening experiment. Furthermore, a FE simulation of the tribological test is established and it provides contact temperature in the pin tip with a high accuracy. A tribological test is used to study friction and mass loss with variational pressures and velocities that represented typically variational contact conditions in the press hardening. The tribological test is also used to obtain correlations between the tribological behaviours and process parameters in press hardening including pressure and sliding velocity.

Tool wear occurring in press hardening processes receives insufficient attention since its corresponding measurements and full-scale experiments are complicated and expensive. This paper presents a study of tool wear in press hardening based on laboratory experiments and FE-simulations. Two experimental laboratory setups depending on the contact conditions in press hardening build the base for the wear models implemented in the FE-simulation to predict wear depths. The highest wear depth is found at the radius of the stamping tool and the discrepancies in wear predictions based on the two different laboratory test setups are analyzed.

In the press hardening industry, industrial and academic efforts are being directed toward predicting tool wear to realize an economical manufacturing process. Tool wear in press hardening is a tribological response to contact conditions such as pressure and sliding motion. However, these contact conditions are difficult to measure in-situ. Furthermore, press hardening involves high temperatures, and this increases the complexity of the tribo system. The present work investigated the contact conditions of press hardening with a commercial FE code (LS-DYNA) as a base for tool wear simulation. A press hardening experiment was established in industrial environments and evaluated through FE simulations. The numerical model was set up so as to approximate the manufacturing conditions as closely as possible, and the sensitivity with respect to the friction coefficients was examined. The influence of numerical factors such as the penalty value and mesh size on the contact conditions is discussed. The implementation of a modified Archard’s wear model in the FE simulation proved the possibility of tool wear simulation in press hardening. Finally, a comparison between the tool wear simulation and the measured wear depth is presented. 

To extend the service life of stamping tools for press hardening processes, tool wear prediction gradually becomes an important topic of concern in industry. However, wear simulations based on the finite element method are mainly developed in laboratory and the lack of validation of full-scale experiments restricts the proposal of an accurate wear model. The work presented in this paper aims at validating the used wear models for stamping tools through a full-scale press hardening experiment. The wear model in conjunction with finite element (FE) simulations are dependent on contact mechanics and the corresponding wear data is obtained from laboratory tests, where the test parameters are specified in ranges that accord with the contact conditions of press hardening. The full-scale press hardening experiment producing a dog-bone shaped part is run for 200 continuous strokes. Geometry updating of the stamping tool is used to investigate the influence of shape change on the pressure occurring on the tool. The results have shown that the geometry change of the stamping tool, after producing a large number of parts, causes changes in the contact pressures and therefore affects the wear simulation. In the end, the wear simulation results are compared to the preliminary result of the full-scale press hardening.

Tool wear in press hardening has attracted researchers' attention and recently several papers have focused on the wear observations in the laboratory tests mimicking the press hardening conditions. However, the wear observations and quantification in full-scale press hardening tests are rare in view of high costs and longtime requirements. In this work, the wear behavior in a full-scale press hardening wear test has been studied in order to understand the wear mechanisms occurring in the stamping tool. Furthermore, the wear depths in the real stamping tool were measured by using a coordinate measurement machine (CMM) that can provide the quantified wear result and serve as a base to validate the possible wear model. The present study also includes the wear observations in both the counterparts i.e., the blank (workpiece) and the stamping tool with the aim of studying the transfer material/wear particles. Two common tool steels were used in the full-scale press hardening wear test and the differences in the wear severities were observed. Since abrasion and adhesion are major wear mechanisms previously in many laboratory tests, the present study identifies the wear evolution and mechanisms on uncoated tool and workpiece surfaces in the press hardening wear test.

Severe adhesion, also referred to as galling, is a critical problem in press hardening, especially in stamping tools used for hot forming of Al–Si-coated ultra-high strength steel. Galling is known to develop rapidly on the tool surface and it negatively affects the quality of the formed products. Earlier research on this topic has focused on the galling initiation. However, studies on the galling development during extended sliding and the corresponding quantitative measurement still lack depth. In the present study, a tribological test is established to study the galling development under press hardening conditions. The tribological test set-up aims to simulate extended sliding between the Al–Si-coated boron steels and the tool die material. The contact conditions in the interface are studied by a numerical model of the tribological test. The friction coefficients and material transfer are discussed taking into account the variation of the different test conditions. Using the results from the tribological tests, the galling simulation is performed in the numerical model. A geometry-updated sample based on the galling (transferred material build-up) height is simulated and the consequent pressure fluctuation is obtained in the numerical model. This contributes to the explanation of the severe transferred material accumulation during the test.

Press hardened steels are commonly used as a lightweight choice for manufacturing car components because of the high ratio of strength to weight. The use of ultra-high-strength steels for the design of lightweight vehicles contributes to the reduction of emissions of carbon dioxide while maintaining passenger safety. Stamping tools used in press hardening processes suffer harsh contact conditionsin terms of dramatic temperature changes, cyclic loadings, and complex interactions between coatings and oxidation. In mass production, tool wear is an inevitable problem that increases maintenance costs. Severe adhesive wear, also called galling, substantially occurs in the stamping tool used against Al—Si-coated workpieces. The galling that takes place during press hardening not only degrades the production quality but also shortens the service life of the tool. In order to properly arrange tool maintenance and minimize galling through adjusting process parameters, engineers need to know when and where galling occurs, based on modelling of the galling in press hardening simulations. In order to implement a galling simulation for press hardening, a modified Archard wear model is employed in the present study, which is a contact-mechanics-based model. The specific wear rate in the model is calibrated by the quantitative galling measurements of a high-temperature tribometer test. The tribological test is designed to mimic the press hardening conditions, where the correlations between galling and process parameters such as temperature, pressure, and sliding distance are outlined. The galling simulation is implemented in a full-scale press hardening experiment, and the predicted galling is validated in terms of severe galling positions and galling profiles. The galling profile evolution is correlated to variations in the contact conditions. Uncertainties in the numerical model, such as the choice of penalty scaling factor and friction coefficient, are analysed with a parameter study and discussed. This study demonstrates finite element (FE) simulations involving galling prediction in press hardening so as to improve product development and production efficiency.

Two constitutive models for the simulation of fracture and fragmentation processes in rapidly loaded rocks are studied. The models were included in a wave propagation finite element code. The results obtained from the two models were compared by a study of the intensity and extent of fracturing obtained from two different configurations of explosive charge and reflecting surfaces in a plane strain problem. The evolution of the time dependent fracture pattern and fragmentation process is also presented.

In this work plastic strain localization, also referred to as necking, of press-hardened ultra-high strength steel is observed using digital speckle correlation. The region of the neck is studied during tensile tests of specimens specially designed to facilitate strain localization at an inner point of the material, thus avoiding edge effects on localization and fracture. By using measurements with a length scale small enough to properly resolve the neck, its growth and shape can be studied. Furthermore, the anisotropy of the material is investigated by examining specimens cut out at different angles to the rolling direction. It is seen that the local fracture strain of specimens cut out along the rolling direction is approximately twice as high as it is for specimens cut out perpendicular to the rolling direction.

Light weight structures with high structural performance are one of the most important goals for automotive and transportation applications. One manufacturing technology, aiming to enable low weight design, is the combined forming and quenching of tubular thin-walled profiles of high strength steel. For optimal utilisation of this technology it is necessary to simulate and analyse the processes involved in a fast and efficient way. In this work, experiments of high temperature bending of thin walled profiles are performed and the forming response force is compared with results from finite element simulations. The analysed forming is modelled as a constant temperature forming and the material data for the specified temperature is evaluated from experiments and literature. The simulations and experiments are conducted to study the ability of the finite element model to predict high temperature forming characteristics and simulate the influence of profile and tool geometry. The need for further improvements and developments in the simulation technology is however identified. This work is part of a research project LOWHIPS (Low Weight High Performance Steel structures) aiming to obtain new knowledge concerning the involved forming and quenching processes and how they will affect the performance of the product.

Finite element modelling and simulation is becoming an increasingly important tool in the development process for structural automotive components, manufactured using thermo-mechanical forming techniques. Accurate and reliable analysis of coupled thermo-mechanical processes requires efficient simulation tools as well as good quality and relevant material data, usually obtained by experimental testing of the mechanical and thermal properties. The work present in this paper concerns methods for obtaining and evaluating the mechanical properties, required for modelling the high-temperature forming of a high-strength boron-alloyed steel. The material data was obtained from high temperature compression tests and dilatometric measurements made using a Gleeble 1500 thermo-mechanical simulator. Two examples of finite element simulations using the data obtained are also presented. The first example is an isothermal finite element simulation of a thin-walled tubular beam subjected to high-temperature bending. The predicted press force showed acceptable agreement with experimental results in the initial part of the process. In the second example, a cylindrical specimen compressed during continuous cooling was simulated, and the press force and radial displacement were compared with experimental results. Again the simulations showed acceptable agreement with experimental results but indicated the need for further improvements in the simulation technology and methods used for material parameter evaluation.

This paper presents a benchmark study on the use of simulation models to represent the powder compaction process. A two level part that represents a synchroniser hub is used as detailed experimental information for this part is documented in the literature. The models used incorporate different representations of friction and material yielding behaviour together with different fill densities. The study shows that computer modelling is capable of predicting density distributions and tool set force levels for the powder compaction process. It has highlighted the sensitivity of the output with respect to friction, fill density, and material yield models. It has also shown that models that incorporate material behaviour using different material models yield surprisingly reasonable results. Based on best performance, it is now possible to predict density to within 0·05 g cm^-3 and tool set force within 10%for an iron powder. The computational requirements indicate a simulation time of typically 1 h for a two level component. This makes simulation a practical tool for this industry sector.

One of the most critical characteristics of welding and press hardening of advanced high strength steels is a low ductility related to a martensitic transformation due to high cooling rate and/or plastic deformation. The present work proposes the application of quenching and partitioning (Q&P) processing to welding and press hardening in a single production step. Using this methodology will not only improve the ductility without losing the ultra-high strength but also accelerate the whole process rate significantly in comparison with austempering treatment in connection to hot pressing and decrease the cost. In this regard, Gleeble simulation of different Q&P cycles beside simulation of deformation at different rates at different temperatures were applied to a medium carbon, Si- alloyed Q&P steel. Samples were characterized using OM, SEM, XRD, hardness, compression and tensile tests. The aim of the project is to establish manufacturing strategies for obtaining components with extreme properties.

The present work investigates the fracture toughness of a thick press hardened 22MnB5 steel sheet, in order to assess its applicability for truck safety parts. The fracture toughness is evaluated in the frame of the elastic plastic fracture mechanics (EPFM), in terms of essential work if fracture (EWF) and J-integral. Digital Image Correlation (DIC) measuring techniques are used to measure crack tip opening displacement and crack extension. It is shown that EWF presents some limitations for thick high strength metal sheet applications, making necessary the use of conventional EPFM procedures. Digital image analysis has shown to be very useful in fracture toughness characterization of thick press hardened steel sheets. The differences observed between different methodologies are discussed and results are compared to toughness values of different AHSS grades.

Any numerical model needs to be evaluated in order to perform as accurately as possible. The aim of the present study is to develop an FE model of a seat structure with integrated safety belts evaluated to full-scale experiments. Simplified seat structures with 3-point integrated safety belt configurations and corresponding FE models were established. The dimension and the material states of the seat back frame were varied. A 50th percentile Hybrid III dummy was used as occupant. A number of biomechanical and mechanical responses of both experiments and simulations were compared and evaluated. The majority of the simulated responses showed good agreement with or slightly underestimated the corresponding experimental responses during belt loading but differed during belt unloading in some cases. Some inadequacies of the FE model were discovered and areas for further development are suggested. The FE model developed and evaluated in the present study may well be used in future studies

A methodology using finite element (FE) modelling and simulation with a property-based model (PBM) is presented. A generic 3-D FE model of a seat structure with a three-point seat-integrated safety belt configuration was established. A 50th percentile Hybrid III FE dummy model was used as occupant. Metamodelling techniques were used in optimisation calculations performed in two steps. Step 1: Six separate optimisations minimising biomechanical responses of the FE dummy model. Step 2: Four separate optimisations with different start values of the design variables, with the total mass of the seat structure as objective function and with the minimised biomechanical responses from Step 1 as constraint values. Six design variables were used in both Step 1 and Step 2. The four optimisations performed in Step 2 generated four different results of the total mass. Thus, different local minima were found instead of one single global minimum. The presented methodology with a PBM may be used in a concept design phase. Some issues concerning the FE model suggest further improvement.

The aim of the present study is to investigate how the physical properties influence the interaction of the seat back frame and the safety belt. Seat-integrated 3- and 4-point configurations with both non-rigid and rigid seat back frames were compared with common 3-point configurations with anchor points on the car body. The LS-DYNA FE-analysis software was used in order to perform frontal crash simulations with a belted 50th percentile Hybrid III FE-dummy model as occupant. The belt-webbing distribution between the lap and the torso belts via a slip-ring and in combination with a non-rigid seat back frame increases the ride-down efficiency compared to a system with no belt-webbing distribution. No tendencies of pelvis submarining were observed regardless of belt configuration. The dynamic response of the seat back frame has some influence on the ride-down efficiency.

Mitigation of neck and head injuries is critical in automotive occupant protection. The aim of the present study is to investigate how the physical properties influence the interaction of the seat back frame and the safety belt. Seat integrated 3- and 4-point configurations with both non-rigid and rigid seat back frames were compared with 3-point configurations with anchor points on the car body. The LS-DYNA FE-analysis software was used in order to perform frontal crash simulations with a belted 50th percentile Hybrid III dummy model as occupant. The belt-webbing distribution between the lap and the torso belts via a slip-ring and in combination with a non-rigid seat back frame had an advantageous influence concerning the loads of the upper neck and injury criteria compared to a system with no belt-webbing distribution.

During the last decades, the use of ultra-high strength steel (UHSS) has increased as its favorable ratio between strength and mass allows the design of lighter body-in-white while maintaining passenger safety. Modeling impact loads of components made of UHS steel requires reliable descriptions of the material deformation and fracture behavior.

Traditional stress or strain based fracture criteria are used in finite element modeling. A different approach in modeling fracture in components uses the fracture energy as a model parameter.

Fracture toughness is difficult to measure in thin sheets; a method termed Essential Work of Fracture (EWF) provides the possibility to determine the fracture toughness in sheet metal. With knowledge of the fracture toughness the understanding of fracture behavior and crack propagation in ultra-high strength steel can be increased. The obtained EWF is related to the fracture energy and can be used in numerical models as a material parameter.

In the present work results from preliminary testing are shown and a discussion on cross-head speed and strain rate in the critical specimen cross section is given. The use of digital image correlation provides information about the displacement field in the vicinity of the notch and hence about the strain- and strain rate distribution. Furthermore, the difficulties in reliable measurement of force and elongation in high speed tensile testing machines are elucidated. Issues encountered during the development of the high-speed DENT specimen are not limited to the specific geometry presented in this paper.

The present work aims at the development of a test specimen to obtain the Essential Work of Fracture (EWF) at high test speed. This work contributes to the overall goal to model fracture behavior and crack propagation, dependent on the strain rate. For the investigation, a high-speed tensile testing machine equipped with an in-house developed load cell and an optical elongation measurement system was used with a high-speed camera to obtain data for digital image correlation.

Developments of the hot stamping technology have enabled the production of components with differential microstructure composition and mechanical properties. These can increase the performance of certain crash-relevant automotive structures by combining high intrusion protection and energy absorption. This paper presents a comprehensive experimental investigation on the flow and ductile fracture properties of boron-alloyed steel with a wide range of different microstructure compositions. Three types of dual phase microstructures at three different volume fractions, and one triple phase grade, were generated by thermal treatment. Flow curves extending beyond necking and the equivalent plastic strain to fracture for each grade was determined by tensile testing using full-field measurements. The influence of phase composition and microstructural parameters were further investigated by means of a multi-scale modeling approach based on mean-field homogenization in combination with local fracture criteria. Inter-phase and intra-phase fracture mechanisms were considered by adopting two separate fracture criteria formulated in terms of the local average stress field. The micromechanical model captures with useful accuracy the strong influence of microstructure and processing conditions on the flow and fracture properties, implying promising prospects of mean-field homogenization for the constitutive modeling of hot stamped components.

Hot stamping is an industrialized technique with the aim of improving material properties by heat treatment and forming of a component in a single production step. Within the field of hot stamping the method of tailored material properties evolved. Components with tailored material properties possess different mechanical properties in designated areas. The mechanical properties in a blank are modified by the formation of different microstructures. Martensite is a microstructure with high strength but low ductility, ferrite has lower strength but higher ductility. Using special tooling tough martensite and soft ferrite can be placed in adjacent sections in a blank. Between those sections a transition zone consisting of a mixed microstructure exists with mechanical properties between martensite and ferrite. Transition zones possess intermediate cooling rates, hence formation of bainite and composites of bainite and another phase can from.

This paper presents an approach of modelling the complete process from austenitized blank to fracture. The method presented relies on the prediction of phases formed during cooling using an austenite decomposition model. In the course of ferrite formation the carbon content in the remaining austenite increases, the carbon content in austenite influences formation of additional daughter phases. The estimated phase composition is used in a homogenization scheme to predict the hardening of the material during plastic deformation. Fracture in the different microstructural phases is predicted using the strain decomposition provided by the homogenization and a fracture criteria. The homogenization scheme and the fracture criteria use measured data from single phase microstructures, i.e. ferrite, bainite and martensite.

A heat treatment process for tensile test specimens is used to produce samples with different volume fractions of the microstructures ferrite, bainite and martensite. The pre-cut specimens are austenitized, ferrite is formed in a second furnace with lower temperature, bainite and martensite are formed by the use of a temperature controlled plane tool.

Prediction of the phase content in mixed microstructures showed good agreement with microstructural characterization and therefore results can be used as input value for the homogenization. Comparing experimental and numerical results for a variety of different mixed microstructures good agreement in the prediction of hardening and fracture is found.

It is concluded that the use of a homogenization method combined with a fracture model can be used to predict the mechanical response of mixed microstructures. The method described in the present work can be applied in the development of hot stamped components.

Hot stamping is a sequential process for formation and heat-treatment of sheet metal components with superior mechanical properties. By applying different cooling rates, the microstructural composition and thus the material properties of steel can be designed. By controlling the cooling rate in different sections of a blank, the material properties can be tailored depending on the desired toughness. Under continuous cooling, various volume fractions of ferrite and bainite are formed depending on the rate of cooling. This paper focuses on the ductile fracture behavior of a thin sheet metal made of low-alloyed boron steel with varying amounts of ferrite and bainite. An experimental setup was applied in order to produce microstructures with different volume fractions of ferrite and bainite. In total, five different test specimen geometries, representing different stress triaxialities, were heat treated and tensile tested. Through full field measurements, flow curves extending beyond necking and the equivalent plastic strain to fracture were determined. Experimental results were further investigated using a mean-field homogenization scheme combined with local fracture criteria. The mean-field homogenization scheme comprises the influence of microstructure composition and stress triaxiality with usable accuracy, connoting auspicious possibilities for constitutive modeling of hot-stamped components

The demand of ultra high strength steel (UHSS) components increased in the last decade due to their high strength to weight ratio. The driving force in this development is the automotive industry and regulations concerning passenger safety and fuel consumption. The use of ultra high strength steel enables design of lighter car bodies with equal or better passenger safety compared to earlier car generations. The automotive industry and their suppliers need predictive tools in the development of components with tailored material properties. Components with tailored material properties are produced by hot stamping, in this process a blank is austenitized before it is formed and quenched in one step. By use of sequential heated or cooled tools, different mechanical properties distributed within the same component are achieved.In order to develop a constitutive model for components consisting of regions with varying phase content, a suitable method to describe the elasto-plastic part of the yield curve is needed. The focus of this work is on the description of the elasto-plastic constitutive model of an ultra high strength steel depending on the phase content in the material. Different volume fractions of ferrite, bainite and martensite are experimentally formed. In this study the capability of different homogenization methods on the prediction of the material response of a multi-phase steel depending on the volume fraction of formed phases is investigated. The modeling results are compared to experimental results.The prediction of the composite response using the micromechanical based double-inclusion model and pure phase measured data as well as experimentally obtained phase volume fractions of present phases showed good agreement throughout all samples tested in this study.

Hot stamping or press hardening is an industrialized technique with the aim of improving material properties by heat treatment and forming of a component in a single production stage. Within the field of press hardening the method of tailored material properties evolved. Components with tailored material properties possess different mechanical properties in designated areas. This paper presents an approach for modeling the mechanical response of mixed microstructures under different stress states. A homogenization method is used to predict the hardening of the material; the strain decomposition provides the possibility of applying a fracture criterion per phase. To validate the modeling approach for different stress states a set of samples with different notch and hole geometries as well as microstructural composition are produced. The combination of a homogenization method and a fracture criterion show good agreement with experimental results. The homogenization method is suitable to predict the hardening of the material with good accuracy. Fracture for different microstructural compositions is well predicted over a range of stress triaxialities relevant for sheet metal applications. It is concluded that the use of a homogenization method combined with a fracture model can be used to predict the mechanical response of mixed microstructures for a range of different stress states.

The demand for ultra-high strength steel (UHSS) increased in the last decade due to their favourable properties in the process of press hardening. Press hardening is a process where an austenitized blank of boron steel is formed and quenched in one production step to achieve superior mechanical properties compared to the blank in as-delivered condition. Driving force for the development of this technique is the automotive industry and regulations concerning passenger safety and fuel consumption. In this work fracture in spot welds and their proximity in low alloyed boron steel have been investigated using finite element simulation and digital speckle photography. This study is motivated by the use of spot welding as major joining method in automotive components. The investigation includes blanks with three different base microstructures, ferritic, bainitic and martensitic. These types of microstructures are commonly found in press hardened components with tailored material properties. All test samples are spot welded in a centred position with a coupon. The welding process introduces heat into the blank and affects the microstructure in the vicinity of the weld leading to varying mechanical properties. During tensile testing of the spot welded samples the mechanical response of the material has been recorded by digital speckle photography (DSP), extensometer and force measurement. The experimental method of DSP measurement allows to resolve displacement and strain fields in the specimen up until fracture. For finite element analysis the commercial available code LS-Dyna together with a user defined material model is used. The material model predicts the mechanical response of the bulk material, i.e. the composite, depending on the phase volume fractions of present phases.

Ultra-high strength steel (UHSS) has become a common material in the automotive industry during the last decades. The technique of press hardening allows modifying and tailoring the material properties of the blank in accordance with desired performance.

In the present work, a laboratory scale test component is developed. On basis of tests on the component it is intended to investigate the deformation and fracture behavior of a boron alloyed steel after different heat treatments. The tooling is developed to allow the production of single phase microstructures like martensite and bainite as well as mixed microstructures containing ferrite. Testing of the component is performed in a standard tensile testing machine with additional digital speckle measurements to determine the strain to fracture in the critical cross section. The initial geometry shape introduces bending in the critical cross-section during tensile loading of the specimen.

The aim of this work is to compare different material models on a component like level, including the prediction of failure. A finite element model of a laboratory scale component is analyzed using LS-Dyna. To compare different failure modeling approaches a set of damage models is calibrated to full hardened, martensitic steel. The deformation and fracture behavior of the component is presented in terms of load-displacement, plastic strain-stress triaxiality as well as in principal strain space.

Shearing is a fast and inexpensive method to cut sheet metal that has been used since the beginning of the industrialism. Consequently, published experimental studies of shearing can be found from over a century back in time. Recent studies, however, are due to the availability of low-cost digital computation power, mostly based on finite element simulations that guarantees quick results. Still, for validation of models and simulations, accurate experimental data is a requisite. When applicable, 2D models are in general desirable over 3D models because of advantages like low computation time and easy model formulation. Shearing of sheet metal with parallel tools is successfully modeled in 2D with a plane strain approximation, but with angled tools, the approximation is less obvious.

Background

There is a shortage of experimentally determined strains during sheet metal shearing. These kinds of data are a requisite to validate shearing models and to simulate the shearing process.

Methods

In this work, strain fields were continuously measured during shearing of a medium and a high strength steel sheet, using digital image correlation. Preliminary studies based on finite element simulations, suggested that the effective surface strains are a good approximation of the bulk strains below the surface. The experiments were performed in a symmetric set-up with large stiffness and stable tool clearances, using various combinations of tool clearance and clamping configuration. Due to large deformations, strains were measured from images captured in a series of steps from shearing start to final fracture. Both the Cauchy and Hencky strain measures were considered, but the difference between these were found negligible with the number of increments used (about 20 to 50). Force-displacement curves were also determined for the various experimental conditions.

Results

The measured strain fields displayed a thin band of large strain between the tool edges. Shearing with two clamps resulted in a symmetric strain band whereas there was an extended area with large strains around the tool at the unclamped side when shearing with one clamp. Furthermore, one or two cracks were visible on most of the samples close to the tool edges well before final fracture.

Conclusions

The fracture strain was larger for the medium strength material compared with the high-strength material and increased with increasing clearance.

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.

Throughout the industrial processes of sheet metal manufacturing and refining, shear cutting is widely used for its speed and cost advantages over competing cutting methods. Industrial shears may include some force measurement possibilities, but the force is most likely influenced by friction losses between shear tool and the point of measurement, and are in general not showing the actual force applied to the sheet. Well defined shears and accurate measurements of force and shear tool position are important for understanding the influence of shear parameters. Accurate experimental data are also necessary for calibration of numerical shear models. Here, a dedicated laboratory set-up with well defined geometry and movement in the shear, and high measurability in terms of force and geometry is designed, built and verified. Parameters important to the shear process are studied with perturbation analysis techniques and requirements on input parameter accuracy are formulated to meet experimental output demands. Input parameters in shearing are mostly geometric parameters, but also material properties and contact conditions. Based on the accuracy requirements, a symmetric experiment with internal balancing of forces, is constructed to avoid guides and corresponding friction losses. Finally, the experimental procedure is validated through shearing of a medium grade steel. With the obtained experimental set-up performance, force changes as result of changes in studied input parameters are distinguishable down to a level of one percent.

Shear cutting is common within several sheet metal industry processing steps, e.g. in cut to length lines, slitting lines, end cropping. Shearing is fast and cheap relative to competing cutting methods like laser and plasma cutting, but involves large forces on the equipment that increase with increased sheet material strength. Accurate shear experiments are a prerequisite to increase the knowledge of shearing parameters, improve industrial shearing, and provide data for validation of numerical shear models. Here, the two shear parameters clearance and clamp configuration, identified as important to the shear results, were studied in an experimental set-up with well defined tool movement and high measurability of tool position and force. In addition to force measurements, the sheared edge geometry was characterized. Steels of low, medium, and high strength were selected for the study. Throughout the experimental study, the shear tool penetration before fracture decreased with increased material strength. The required shear force decreased and the force attempting to separate the two shear tools increased when one side of the sheet was left unclamped and free to move. Further, the maximum shear force increased with decreased clearance. Clearance changes were small and moreover continuously measured during all shear experiments.

In this work the Smoothed Particle Hydrodynamics (SPH) method is used to simulate iron ore pellets flow. A continuum material model describing the yield strength, elastic and plastic parameters for pellets as a granular material is used in the simulations. The most time consuming part in the SPH method is the contact search of neighboring nodes at each time step. In this study, a position code algorithm for the contact search is presented. The cost of contact searching for this algorithm is of the order of Nlog2N, where N is the number of nodes in the system. The SPH-model is used for simulation of iron ore pellets silo flow. A two dimensional axisymmetric model of the silo is used in the simulations. The simulation results are compared with data from an experimental cylindrical silo, where pellets are discharged from a concentric outlet. Primary the flow pattern is compared.