bluuebunny/crossref_metadata_2025_split huggingface dataset:
Characterizing axial crash foldability of AHSS & UHSS sheets by means of L-profile compression tests
Authors: Wagner L. , Larour P. , Lackner J. , Schauer H. , Berger E. 
Published: November 2019

Abstract
Small-scale crash tests of crash boxes have established themselves as an assessment tool for high strength materials for their application in crash-relevant structures in the automotive body. However, when investigating UHSS grades it can be seen that the performance outcome is highly susceptible to details of the crash box geometry, e.g. shape of cross section or position of spot welds, and the test setup. This may even lead to opposing crash performance ratings for the same grade if different setups are compared. In order to rule out any side effects of specimen geometry and test setup, a simpler test method is desired. If a crash box sustains plastic folding in its first fold it often also sustains the subsequent plastic folding events. Accordingly, the L-profile compression test is presented where a specimen, mimicking a quarter of the crash box cross-section, is compressed, forming a single plastic fold. The crash performance assessment is carried out by means of the so called crash index, a weighted relative measure of crack and fracture lengths within the sample, and correlated to the results of corresponding small scale tests, i.e. bending, notched tensile and hole expansion tests.
Multi-Coefficient Optimization of Homogeneous Anisotropic Hardening Model for Ahss
Authors: Lee Shinyeong , Kim Jin-Hwan , Barlat Frédéric 
Published: November 2019

Abstract
The goal of this research is to find out which test is best to predict the hardening of advanced high strength steel (AHSS) sheets and what kind of optimization method is most efficient. In this work, tension-compression and cyclic simple shear tests were conducted on AHSS sheets. Both experiments were carried out in the format of several cycles. Based on the experimental results, the distortional plasticity model (HAH) coefficients were optimized through the Nelder-Mead simplex algorithm and a genetic algorithm. The results were discussed to assess which test is best for the prediction of the stress-strain curve during forward-reverse loading and which method leads for lower computing time and higher accuracy.
Improved bendability characterization of UHSS sheets
Authors: Wagner L. , Schauer H. , Pauli H. , Hinterdorfer J. 
Published: November 2019

Abstract
Bendability characterization of UHSS sheets is often limited by punch-sheet-liftoff during standardized bending tests. This phenomenon is characterized by a smaller radius on the bent sheet than the desired tool radius and might lead to a misinterpretation of the admissible bending radii from such tests. An alternative bending setup is proposed suppressing punch-sheet-liftoff. The proposed setup aims at imitating bending in multistage stamping processes and to some extent also roll forming. The target bending radius to sheet thickness ratios of the investigated UHSS sheets were chosen to be 2.0. Alternative tool geometries are investigated as well. Short segments of an Euler spiral between the tool parts exhibiting the target curvature and the linear neighbor regions are used to avoid triggering strain localizations at these curvature discontinuities, further decreasing the admissible bending ratios. Finite element simulations of the proposed bending test were conducted in order to identify the optimal tool shape. All results were compared to corresponding results from standardized bending tests of the respective UHSS sheets. The proposed setup succeeded in suppressing punch-sheet-liftoff for the desired bending ratios and was able to provide smaller bending ratios for the investigated UHSS sheets as compared to the ratios from standardized bending tests.
Investigation of non-proportional load paths by using a cruciform specimen in a conventional Nakajima test
Authors: Norz R. , Volk W. 
Published: November 2019

Abstract
The prediction of necking in sheet-metal forming is one of the most important tasks in the simulation of forming processes. The most common way to predict the necking for linear load paths is the Forming Limit Diagram (FLD) according to ISO 12004-2. This criterion is valid only for linear load paths. For non-proportional loading paths, Volk introduced a Generalized Forming Limit Concept (GFLC). This model enables the prediction of the formability for any non-proportional loading paths. The experimental investigation of those loading paths is still very challenging and complex. It involves a wide range of testing equipment for the pre- and post-strain of specimens. Jocham introduced a new method that allows the investigation of various non-proportional loading paths by using a cruciform specimen and a draw bead tool in a conventional Nakajima test setup. The height of the draw beads can be varied to create different load paths in the specimen centre. Nevertheless it is necessary for the thinnest part of the specimen to be in the middle of the specimen. This is achieved by milling an indentation into the centre. This mechanical procedure affects the formability of the material. In this paper, the cruciform specimens are laser weld from three single metal sheets in order to minimize the influence of the manufacturing process on the specimen. With this procedure, it is possible to test unmanufactured sheets. However, the crack still occurs in the centre of the specimen. With the new cruciform specimen, we are able to create arbitrary non-proportional strain paths.
Influence of single hat crash box flange triggering and impactor top plate welding strategy on axial crash foldability of AHSS & UHSS sheets
Authors: Larour P , Lackner J , Wagner L 
Published: November 2019

Abstract
Axial crash tests are usually performed in order to assess the crash foldability of AHSS & UHSS sheets in the framework of a material homologation process. With a real component like single hat column geometry with RSW flange welded and MAG welded top/down head plates crash box design, it is getting more and more difficult to assess properly the crash performance of AHSS & UHSS steel grades with tensile strength Rm ⩾ 1000 MPa (RSW: resistance spot welded, MAG: metal active gas welded). The crash box flange is namely excessively stiff and prevents the initiation of a proper crash-folding pattern, resulting in a high amount of material failure, which makes some judgement about material crash ability difficult. Therefore the present experimental investigation focuses on the one hand on the triggering strategy in the top flange area to break the excessive stiffness of the spot-welded flange. On the other hand some effort has been put in the improved joining strategy of the top impactor head plate to the crash box itself. The top head plate is usually necessary in real components for load transfer to the crash box and fixation to the surrounding body in white structure. Some excessive welding of the top impactor plate, especially in the already largely stiffened spot-welded flange region, has however a strong detrimental effect on crash fold initiation. The welding of the top head plate in the flange and back crash box areas is avoided, thus increasing the degrees of freedom between top impactor plate and crash box column along with an effective reduction of the initial crash box bending stiffness. Some minimized welding of impactor plate to the crash box column, especially avoiding flange areas, allows a successful initiation of the crash folding process but only in combination with pressed round triggers in the flange area near the first spot weld. With these modifications it is first possible to assess and rank the intrinsic axial crash foldability of the material with Rm ⩾ 1000 MPa by means of a crack length based crash index.
Digital process support in toolmaking by using optical metrology
Authors: Essig Peter , Liewald Mathias , Bolay Christian , Schubert Thomas 
Published: November 2019

Abstract
Increasing demands on the quality of outer skin car body panels, shortening product development processes of new car models in the future fundamentally challenge manufacturing companies in international competition. Such challenges emerge, for example, in the field of toolmaking, which plays a decisive role in the valueadded chain due to its high time and costs share. Here, the high proportion of manual, experiencebased operations in tool tryout often leads to unstable processes requiring many iteration steps until approval. Blue, non-dry colour is applied on sheet metal which creates a spotting image that makes the contact areas between the forming die surface visible. In die tryout, spotting images provide a visual indication of the current phase and also serve as quality criteria of sheet metal components. For this reason, research work reported about in this paper deals with the development of a procedure for generating digital spotting images by using optical 3D coordinate-measuring technology to keep the spotting image permanently available during die tryout phase, for example on smart devices. In fundamental investigations, different influences on the spotting image due to light conditions and STL-mesh size of the scanned geometry are initially examined in order to quantify both the physical and the digital spotting image. The knowledge gained is subsequently validated and further improved by means of test with series press tools. The results demonstrate how digital and physical spotting images are generated and quantified for a reduced time in sheet metal simulation and die tryout phase of large press tools. This is achieved by the systematic provision of process parameters from the series production, such as the force transmission between the forming die surfaces, the local sheet thicknesses and the documentation of process variation in the blank holder by using digital spotting images.
Prediction of flow curves of very thin brass sheets incorporating size effect in hardening model
Authors: Anand Dhruv , Kumar D Ravi 
Published: November 2019

Abstract
Due to the size effect, the plastic deformation behavior of very thin sheets is dependent on the ratio of specimen thickness to grain size. The general hardening models used for bulk materials and thick sheets are not applicable for representing the plastic flow behavior of very thin sheets accurately. In the present work, the grain size effect (ratio of sheet thickness to grain size t/d) on flow behavior in uni-axial tension has been studied. Very thin sheet specimens of CuZn36 brass of thickness 50μm, 100μm, 150μm and 200μm have been used in this study. The brass sheets were annealed with different combinations of annealing temperature and time to achieve a large range of t/d ratio (1 to 17). A new hardening model has been developed incorporating the grain size effect on flow stress by determination of strength coefficient as a function of t/d. The predicted curves using the developed hardening model agreed well with the experimental results.
Investigation of the influence of forming parameters on the springback of hot-stamped hat-shaped parts
Authors: ChiuHuang Cheng-Kai , Wang Shi-Wei , Lee Ping-Kun 
Published: November 2019

Abstract
The application of hot stamping parts has become a dominant approach to achieve lightweight design and increase crashworthiness performance in the automotive industry. Compared to the cold stamping parts, the main feature of the hot stamped component is its high strength and good size stability. However, recent studies have shown that the springback or size deviation of hot stamped parts caused some assembly problems. This paper aims to investigate the influence of forming parameters on the part’s springback via a hot stamping hat-shaped die. Three different forming parameters with the same blank heating temperature (930°C) were conducted in the study: (1) transfer time 8 sec/die quenching 15 sec (2) transfer time 8 sec/die quenching 8 sec and (3) transfer time 18 sec/die quenching 15 sec. The selected forming parameters are used to evaluate the effect of the before-forming blank temperature and die-opening blank temperature on the springback of the hot stamping parts. The results show that the case 3 with a prolonged transfer time causes a small portion of ferrite phase formation in the hot-stamped part, resulting in the decrease of the tensile strength and a more significant springback is observed.
On the Failure Prediction of Dual-Phase Steel and Aluminium Alloys Exposed to Combined Tension and Bending
Authors: Barlo Alexander , Sigvant Mats , Endelt Benny 
Published: November 2019

Abstract
The interest in accurate prediction of failure of sheet metals in the automotive industry has increased significantly over the last two decades. This paper aims to evaluate two failure prediction approaches implemented in the commercial Finite Element code AutoFormplus R7.04; (i) the standard Forming Limit Diagram (FLD), and (ii) the Non-linear Forming Limit Diagram. The evaluation will be testing the two approaches accuracy on predicting failure of both an AA6016 aluminium alloy and a CR440Y780T-DP dual-phase steel alloy specimen exposed to combined tension and bending. Based on the findings of this study, it is concluded that neither of the evaluated approaches is able to accurately predict failure in both cases presented.
Sensitivity analysis of process and tube parameters in free-bending processes
Authors: Beulich N , Spoerer J , Volk W 
Published: November 2019

Abstract
Free-bending processes are used to manufacture highly-complex profile shapes for different kinds of industry sectors. This paper focuses on the enhanced version of the MOS bending technique (6DOF), which is used by BMW to produce semi-finished products for the tube hydroforming process. In order to achieve a better process understanding, the bending process was analyzed with regard to process and tube parameters. In the first experimental bending series, the process precision was measured and evaluated by standard deviation and process capability. In the second bending series, the analysis focuses on the weld seam position and the wrinkling behavior of the tube. The correlations shown between process parameters and the resulting bending geometry are the basis for the validation of a process simulation model developed in LS-Dyna. Therefore, the comparison of experimental data and data measured numerically represents a good agreement. The FEM model was used to perform a numerical based sensitivity analysis of the bending process in relation to process and tube parameters. The result identifies the main factors contributing to the free-bending process.
Crystal plasticity based predictions of mechanical properties from heterogeneous steel microstructures
Authors: Kok P.J.J. , Spanjer W. , Korver F. , An Y. , Aarnts M. 
Published: November 2019

Abstract
An accurate understanding of the influence of the microstructure on mechanical properties as well as how the material behaves in forming tests is vital for product development and process improvement. The heterogeneity of the microstructure is very different for the various families of steel grades. As a consequence spatial stress and strain distributions on the micro scale will also differ a lot in these materials during forming operations. In the important family of Dual Phase (DP) steel grades. the combination of ductile ferrite phase and hard martensite particles enables an excellent hardening behaviour. However these micro constituents also result in a strong locally heterogeneous behaviour caused by the high material property contrast. To enable an accurate prediction of the materials response in forming processes advanced microstructure models as well as fast crystal plasticity code is essential. Within Tata Steel both are available. A Multi-Level Voronoi microstructure generator, capable of generating very complex Representative Volume Elements, is directly connected to the DAMASK code of the Max Planck Institüt für Eisenforschung. This crystal plasticity software is based on an ultrafast Fourier Spectral Solver. Crystal plasticity simulations with different loading cases (e.g. uni-axial, plane strain, simple shear) study various effects of microstructure heterogeneity on stress and strain distributions as well as on global hardening behaviour. Outcome is that not only the volume fraction but also the spatial distribution of the martensite particles has a large impact on local stresses and strains as well as on average hardening behaviour. Another important heterogeneity parameter is the carbon content, which influences strongly the hardness and hardenability of the martensite particles. An attempt is made to incorporate also this effect in crystal plasticity simulations.
Warm Forming Simulation of a ZEK100 Magnesium Door Panel
Authors: Omer Kaab , Butcher Clifford , Worswick Michael , Skszek Tim 
Published: November 2019

Abstract
The warm forming of a door panel made of commercial grade ZEK100 magnesium alloy sheet was modelled using finite element techniques (Autoform). The operation consisted of a heating step, a forming step and a cooling/springback step. The material properties of the ZEK100 blank were modelled using a set of temperature- and strain rate-dependent stress-strain curves, which were derived based on a Zerilli-Armstrong constitutive model. In order to simplify the material model to enable its complex response to be represented within a commercial finite element code, the anisotropy of the magnesium sheet was approximated using a Banabic-2005 yield surface and yield asymmetry was neglected, a reasonable approach for warm forming. Necking was predicted using a set of forming limit curves obtained at different isothermal temperatures. The entire forming model was run at a punch speed of 160 mm/s, and two initial blank temperatures: 215 and 230 °C. The tooling was initially at room temperature. The model predicted that the blank cracked when its initial temperature was 215 °C due to excessive cooling. The best formability (lack of wrinkling and necking) was predicted when the blank was heated to 230 °C. These predictions agree well with the forming trial outcomes.
Improved Spring-Back Compensation Strategy through Location Optimized Part Position in the Dies
Authors: Birkert A , Hartmann B , Lepple F , Straub M , Zimmermann P 
Published: November 2019

Abstract
Further increasing requirements to the quality of car body components represent a huge challenge for the process planner. As the elastic energy is changing in each stamping operation, it is quite challenging to decide how to compensate which stamping operation. The belonging process definition is known as the compensation strategy. A key prerequisite of an appropriate compensation strategy is a proper location of the part in all stamping operations along the process chain. Proper means that the distance of the part to the die must be minimal to prevent any undefined shift of the part in the die and thus unwanted deformations when closing the dies. Both shifting and deformations would cause dimensional deviations and surface defects. Today different strategies are applied to meet these requirements at least approximately. Strategies, which are published in different papers yet, can only provide suboptimal results. Here a new compensation strategy is being presented, which fulfils the requirement to a proper part location not only in the trimming dies, which corresponds to the so-called Drawshell Strategy, but also in the restriking dies. An important advantage of the respective strategy is that it generally can be applied on all multistage car body stamping operations.
Numerical simulation of hydraulic bulging using uniaxial and biaxial flow curves and different yield criteria
Authors: Prakash Ved , Kumar D. Ravi , Merklein Marion , Hagenah Hinnerk 
Published: November 2019

Abstract
In sheet metal forming industry, finite element analysis has become a necessary step before actual production of the parts. It helps in the improvement of part quality as well reduces the effort applied in experimental trials. However, the success of finite element simulation mainly depends on the constitutive model used for defining the plastic deformation behaviour of the sheet material. Material characterization in biaxial stress state helps in selecting advanced constitutive models in numerical simulations. In this work, aluminium alloy AA5083 sheet material is characterized in uniaxial and biaxial tension conditions. Hydraulic bulge tests are performed to obtain the flow curves and material properties which are then used to find the coefficients in different yield criteria. These coefficients and the flow curves, fitted using different work hardening laws, are used in finite element simulation of the hydraulic bulge test and the results are validated by experiments.
Prediction of residual stresses in biaxial stretching of tailor welded blanks by finite element analysis
Authors: Kumar Amit , Kumar D Ravi , Gautam Vijay 
Published: November 2019

Abstract
Residual stresses in sheet metal components play an important role in determining the life of parts especially in automotive industry. In this work, residual stresses have been predicted numerically in biaxial stretching of laser welded interstitial free (IF) steel blanks of different thickness combinations. The effective stress-effective strain curves of the parent sheets obtained from uniaxial tensile tests have been used as an input in finite element (FE) simulations to define the flow behavior of the materials. It has been found out that the residual stresses are tensile and similar on both sides of the weld zone in tailor welded blanks (TWB) of same thickness combination. In TWBs of different thickness combinations, the residual stresses on both sides are tensile and almost equal in the central region of the cup and they became compressive as the distance from the pole increases. The effect of thickness ratio on residual stresses has also been predicted in FE simulations of biaxial stretching of TWBs. It has been found out that with increase in the thickness ratio, the residual stress has slightly increased. Residual stresses have also been determined experimentally by using x-ray diffraction technique and it has been found out that the predicted values agreed well with the experimental results.
Sliding characteristics of hot-dip galvanized steel sheets depending on aging time after production
Authors: Hoshino K , Yamasaki Y , Taira S 
Published: November 2019

Abstract
The effect of aging time after production on the frictional characteristics of hot-dip galvanized steel sheets (HDG) was examined in this study to stabilize and improve the press formability of HDG. Conventional HDG with the Zn coating weight of 67 g/m2 including 0.36 mass% Al were used as test specimens. The specimens were aged for a predetermined period, after which their friction coefficients were measured with a flat sliding test. It was found that an Al-based oxide grew on the HDG surface and the friction coefficient tended to decrease with increased aging time. This suggests that the Al-based oxide reduced the friction coefficient. To understand how the Al-based oxide affects the friction coefficient, surface observation and analysis of both the test specimens and the tools after sliding were carried out. Although the Al-based oxide was not detected on the HDG surface in the area of contact with the sliding tool, the oxide was detected as adhering material on the tool surface after sliding, suggesting that the adhering material on the tool reduced the adhesion force between the metallic Zn of the HDG and the tool. This could lead to a lower friction coefficient because shearing resistance was decreased.