bluuebunny/crossref_metadata_2025 huggingface dataset:
On the way towards a comprehensive failure modelling for industrial sheet metal stamping processes
Authors: Manopulo N. , Carleer B.
Published: November 2019

Sheet metal stamping processes are nowadays designed with a high level of standardization. The resulting short development cycles bring by the need for highly efficient simulation software, which are able of delivering unambiguous and reliable formability predictions. This contribution aims making a snapshot of the present state of industrial needs related to this topic and to discuss the possibility for a comprehensive failure modeling approach, able of matching the high standardization requirements. For this purpose the most common failure mechanisms in stamping applications are reviewed and discussed in the context of state of the art modelling approaches. Furthermore the overall qualities required for a comprehensive failure modelling approach in the context of stamping simulation are elucidated and discussed.
Non-linear strain path experiment and modeling for very high strength material
Authors: Wi M S , Lee S Y , Barlat F
Published: November 2019

The goal of this work is to develop experimental methods and modeling of very high strength steel sheets subjected to non-linear strain paths. A 1.6 mm thick TRIP steel sheet with a tensile strength of 1180 MPa was investigated. A suitable test for the uniaxial tension pre-strain was developed. A constraint was to achieve an as uniform as possible strain field over the entire gauge area. The pre-strain was conducted on a 500kN MTS tensile testing machine. The geometry of a new specimen was optimized through finite element simulations and experiments on the TRIP1180 material. Then, different types of subsequent specimens were extracted from the pre-strained material by electro-discharge machining (EDM) for the second deformation step leading to a strain path change. The subsequent plastic behavior was measured and modeled using the HAH distortional plasticity model. Comparisons between the experimental and predicted behavior are discussed.
Accurate sheet metal forming modeling for cost effective automotive part production
Authors: Chezan A.R. , Khandeparkar T.V. , ten Horn C.H.L.J. , Sigvant M.
Published: November 2019

Recent implementations of accurate material and tribology models in finite element codes for sheet metal forming process development have the potential to reduce development time and the associated development costs significantly. Adoption of new models requires validated material parameters and assessments of the overall accuracy. The paper presents a study aimed at accuracy estimation by comparing strain measurements and finite element simulation results for a laboratory flat bottom hole expansion test and an industrial automotive component produced at Volvo Cars. The use of the tensile test based Tata Steel Vegter yield locus model results in accurate prediction of dimensions and plastic deformation distribution in sheet metal forming applications.
Determining a threshold Strain Nonuniformity Index (SNI) to predict failure in sheet metal components
Authors: Date P P , Kulkarni Pratik , Jamadar K D
Published: November 2019

The Strain Nonuniformity Index (SNI) was introduced as a strain distribution based failure criterion about 10 years ago and its application to prediction of failure in industrial sheet metal components was demonstrated in the previous Conferences. Quantitatively, the SNI represents the rate of change of spatial strain gradient. This means that higher the SNI, sharper the neck, and hence greater is the likelihood of failure. It is immaterial as to how the neck / strain gradient came into existence, and failure is linked with the threshold value of this quantity, beyond which failure could be expected. Since phenomenological parameters are not linked with the process of establishing a threshold SNI, it does not seriously matter whether it was aluminium that was formed or whether it was steel, it does not very much matter if it was hot forming or cold forming. The present paper describes the methodology of estimating the threshold SNI for each of the numerous critical planes in the deforming sheet metal component. Interestingly, the relationship between SNI of major strain and SNI of the thickness strain was found to lie in a narrow band for a diverse set of materials. This therefore helps in finding the critical planes on which the SNI at a given punch travel has crossed the threshold value and failure locations so determined have correlated well with the shopfloor predictions as presented earlier.
Effect of pre-strain on stretch flange deformation limit of steel sheets
Authors: Iizuka E , Higai K , Yamasaki Y
Published: November 2019

The effect of pre-strain on the stretch flange deformation limit of steel sheets was investigated by hole expansion tests under various pre-strain conditions. Pre-strain was given by stretch forming with a cylindrical punch. The strain ratio and amount of pre-strain were controlled by changing the blank shape and forming height. The hole expansion tests were carried out with a conical punch and cylindrical punch in order to clarify the effect of the strain gradient. The effect of the strain ratio of pre-strain on the stretch flange deformation limit can be expressed by equivalent strain. In the case of higher strain gradients, the stretch flange deformation limit increased with increasing pre-strain. However, at lower strain gradients, the stretch flange deformation limit was almost constant. These results suggest that the formability of the stretch flange deformation area given by pre-strain should be determined from not only the total equivalent strain but also the strain gradient and the amount of pre-strain expressed by equivalent strain.
Virtual die spotting: Compensation of elastic behavior of forming presses
Authors: Zgoll F , Kuruva S , Götze T , Volk W
Published: November 2019

The rapid change in the automotive industry towards an electrified and digitalized future necessitates companies to shorten product development cycles. To meet customer needs regarding design and quality, the requirements on the production processes are constantly increasing. As additive manufacturing cannot provide large enough batch sizes yet, conventional stamping of car body parts will be the technology in the foreseeable future. The most time-consuming step during the production of stamping dies is the manual spotting during try-out of the tools. To achieve a homogenous pressure distribution on the parts a precise tool closure is necessary. The behavior of the press has huge influence on the tool closure, especially the elastic deformations of ram, bolster and cushion during forming. Current compensation methods are solely based on experience from previous tools. This contribution presents a methodology for designing a virtual die-compensation to account for press machine behavior based on experimentally determined measurement data of the try-out press. The data is used to calibrate FE-substitute-models of ram and bolster. A forming simulation using rigid tools is coupled with structure simulation using elastic tools and machine. By evaluating the deflection of the active surfaces at the bottom dead center of the press, the necessary die-compensation can be determined.
Coating effects on the galling behaviour of aluminium metal forming processes
Authors: Yang Xiao , Hu Yiran , Zheng Yang , Politis Denis J. , Wang Liliang
Published: November 2019

This paper investigates the effect of coatings on the galling behaviour in metal forming processes. To investigate this phenomenon, pin-on-disc tests were conducted on the Anton Paar THT tribometer with discs manufactured from tool materials and pins manufactured from aluminium to represent the workpiece. Three different coating conditions were tested: uncoated contact, AlCrN and CrN coatings, at three different temperatures: room temperature, 300°C and 450°C. The investigation indicated that advanced coatings have a significant effect on coefficient of friction and galling behaviour, while these effects may rely on the selection of substrate and forming temperature. In general, compared with uncoated contact conditions, lower friction coefficients were generated with coated tools. In addition, the selection of aluminium grade for the pin affected friction coefficient, with AA6082 generating higher friction coefficients compared to AA7075.
Characterization of Heat Transfer Coefficient of Lightweight Alloys in Kirksite Dies
Authors: Omer Kaab , Butcher Clifford , Worswick Michael
Published: November 2019

The heat transfer coefficient (HTC) is an important parameter in the finite element (FE) modelling of warm and hot forming operations. The HTC, among other parameters, governs the FE model predictions for the cooling rate within the blank and the resulting constitutive behaviour and formability. In the current work, the HTC of two aluminum alloys (AA5182-O and AA7075-T6) and one magnesium alloy (ZEK100) is characterized. Blanks were heated in a convection furnace and subsequently quenched in a set of kirksite dies under various contact pressures. Kirksite is a zinc-based alloy commonly used in prototype tooling. The temperature-time (T-t) profile of the blanks and die were measured during each quenching experiment. The resulting T-t profiles were input into a Matlab script, which calculated the HTC using an iterative regression technique.
Modelling of dynamic loads during series operation for optimisation of part holder design
Authors: Burkart M. , Liewald M. , Wied J.
Published: November 2019

Energy and resource efficiency today are fundamental challenges in production technology, which also arise in the production of sheet metal body parts when using state-of-the-art press technology. Since sheet metal forming processes are becoming more and more complex due to modern design of car body shape, the tools for producing body parts are increasing in size and modern press technology is developing correspondingly faster. Simultaneously, produced tools for new car models must withstand applied process related loads emerging during lead time of model to ensure a robust production process. Based on the increase in productivity supported by servo press technology of today and resulting higher demands on required tool strength, part holders assembled in large line dies in particular belong to critical tool components. In this paper, an optimization concept for part holder design is presented. Higher acceleration of press ram and upper tool results into higher levels of dynamic impacts, which can lead to critical load cases originating from spatial mass distribution of part holder. Occurring loads are reduced by use of damping elements. The distribution of damping elements into part holder structures is optimised according to the mass distribution of the part holder and the design of the upper die. For this purpose, dynamic effects of the tool were modelled based on analytical models and verified by multi-body simulations and corresponding stress calculations. The application of dynamic load characteristics of tool components thereby allow realistic modelling of distributed mass and force effects. Finally, simulation results were validated on the basis of tests under series production conditions.
Improvement of formability for hot stamping of aluminum alloy sheets by press motion control
Authors: Ota E , Yogo Y , Iwata N
Published: November 2019

In this work, the formability of Al-Mg-Si aluminum alloy sheets during hot stamping has been improved by controlling the press motion. In the hot stamping, a heated blank sheet is cooled partially by contacting with the dies; therefore, its deformation proceeds non-isothermally. The resulting temperature distribution leads to cracking at low forming depths during deep drawing despite the excellent tensile properties of the alloy at high temperatures. Local deformation, i.e. necking, tends to occur in a high-temperature area that is not in contact with the dies until the press reaches the bottom dead center. To suppress this deformation, the potential cracking area near the walls was cooled and hardened by holding the heated blank sheet between the blank holder and the upper die before the forming step. The proposed technique was applied to the square cup forming of a 1.0 mm thick 6000 series aluminum alloy sheet. As a result, the forming limit depth was increased to twice depth with 1.0-s die-holding duration before the forming step as compared to that obtained for the standard hot stamping procedure without the press motion control.
Material characterization of high strength sound-deadening sheets and its application on a square cup drawing simulation
Authors: Park Hyeonil , Kim Se-Jong , Lee Jinwoo , Kim Daeyong
Published: November 2019

In this study, the mechanical properties of a high strength sound-deadening sheet (SDS) in the stretch deformation mode was characterized, and it successfully apply to the formability prediction of the square cup drawing tests. The high strength SDS was fabricated with dual phase (DP) 590 steel sheets as outer skins and polymeric adhesive layer as a core by a roll bonding process. Uniaxial and biaxial mechanical characteristics were evaluated by the uniaxial tension tests and the in-plane biaxial tension tests, respectively, while the forming limit was investigated by the punched dome tests. Then, based on the experimental data, the mechanical properties of SDS was characterized, viz., hardening curve, yield function and forming limit curve. Square cup drawing was carried out using the SDS, and its deformation and failure behaviour were successfully predicted via the finite element simulation performed by inputting characterized mechanical properties.
Hot stamping steel grades with increased tensile strength and ductility - MBW-K 1900, tribond 1200 and tribond 1400
Authors: Rosenstock D , Banik J , Gerber T , Myslowicki S
Published: November 2019

The use of hot formed 22MnB5 steel with an aluminium-silicon (AS) coating is well established for high strength automotive parts to combine weight reduction and passenger safety. Recent developments aim to increase the material strength for further weight saving and to improve the ductility. Especially an improved ductility makes hot formed parts suitable for additional applications, such as parts with higher energy absorption or even axially loaded parts. thyssenkrupp Steel Europe AG has developed new hot stamping grades. With an increased strength of about 1900 MPa, the MBW-K 1900 allows the application of even lighter parts compared to 22MnB5. Due to the increased strength level, delayed fracture has to be evaluated. Furthermore, the recommended parameters for processing MBW-K 1900 are regarded.

A high crash performance of a material implies a high strength, while the material still shows no cracking during folding or heavy bending of the automotive part. This is the basis for the development of tribond 1200 and tribond 1400. As a new concept for hot stamping, tribond 1200/1400 are composite materials, which bond two outer layers of a ductile 500 MPa steel grade with a high strength 1500 MPa steel core layer. Both grades are already well known as monolithic steels MBW 500 and MBW 1500 for hot stamping. An aluminium-silicon coating allows tribond 1200/1400 to be processed in the same way as AS coated 22MnB5. Nevertheless tribond 1200/1400 show a reduced susceptibility to hydrogen embrittlement in the 4-point bending test compared to 22MnB5. By their improved crash behaviour, tribond 1200/1400 enable the manufacturing of more kinds of hot formed parts such as longitudinal beams. An advanced simulation method takes the composite structure of tribond 1200/1400 into account to evaluate the crash behaviour.
Formability of AlSi and Zn coating during hot stamping
Authors: Venema Jenny , Botman Gerben , Phan Tu , Kop Theo
Published: November 2019

Hot stamping is widely used in the automotive industry due to the combination of high strength (±1500 MPa), low springback and good formability. The sheet is austenitized in a furnace transported to a press and formed at high temperatures (600-800°C). Forming at lower temperatures could be interesting with respect to multi-step hot stamping. Coatings are used to prevent scaling during the austitisation. AlSi is the most common protection. Zn coatings are also applied but are susceptible to microcracks. These microcracks can be avoided by using pre-cooling. In this investigation the formability and fracture of AlSi and Zn coating at several temperatures are investigated. Cross dies are formed and extensively analysed. At low temperatures the AlSi coating is very brittle and results in large amount of tool pollution. With respect to fracture and formability the Zn coating is stable in the whole temperature range and could be suitable for multi step hot stamping process.
Evaluation of the stress vs strain curve using a high temperature bulge test device
Authors: Boyer A , Laurent H , Oliveira M C
Published: November 2019

The increasing amount of hot and warm stamping parts in the automotive industry shows that high temperature forming is critical to reach the emission goal fixed by regulation authorities, due to its contribution in reducing the vehicle’s weight. Hot stamping and warm forming take advantage of the material softening with temperature to reduce the forming forces and springback. The use of different temperature conditions also enables exploring the phase transformation occurring with in-tool quenching in hot stamping boron steels or artificial aging in heat treatable lightweight aluminum alloys. These new forming conditions require either new material characterization methods or the current methods need to be adapted to high temperature conditions. The equibiaxial expansion test allows the evaluation of the mechanical behavior of materials on a large strain range. However, the use of high temperature conditions gives rise to difficulties when using Digital Image Correlation (DIC) systems. This study analyses the possibility of using a scanner laser to obtain the stress vs. strain curve from an equibiaxial test performed with an aluminum alloy of the 6xxx series, at 150°C. The aim is to validate an experimental procedure that can be used to evaluate the stress vs. strain curve from high temperature bulge tests performed with other metallic sheets, including quenchable boron steel.
Local ductility – key parameter for predicting formability of AHSS
Authors: Westhäuser S , Schneider M , Teschner M , Denks I A
Published: November 2019

In order to be able to make targeted use of the advantages of AHSS grades in forming processes it is necessary to determine local material properties. Hence, in addition to edge crack sensitivity measures, such as hole expansion ratio according to ISO 16630 (HERISO), it is currently intensively discussed whether measures, that assess the local ductility of the material, should also be specified in material specifications. One representative is given by the parameter true fracture strain (TFS) that is based on the measurement of the fracture area of a cracked flat tensile specimen. By means of 16 different cold- and hot-rolled steel batches, the relation between the parameters TFS and HERISO was investigated. The outcome is that these parameters do not correlate to a satisfying degree when considering all materials and consequently are regarded as representatives of different material properties. Thus, the issue of the reliability of the measures is considered separately. For the regarded steel grades, the overall scattering of TFS, i.e. neglecting the sample orientation, is found to be 13 – 44 % and the scattering of HERISO is within 5 – 35 %. The high scattering of TFS is mainly caused by the anisotropy of the materials. In case of parallel measurements of the same batch the relative standard deviations of TFS and HERISO are on the same level for cold-rolled strips, whereas in case of hot-rolled strips the relative standard deviation of HERISO is significantly higher. By investigating the influence of the tensile specimen shape for a bainitic hot-rolled strip numerically, it could be shown that the width-to-thickness ratio, that takes the hole fracture area into account, influences the TFS value directly.
Basics for inline measurement of tribological conditions in series production of car body parts
Authors: Hansen B , Hoebler M , Purr S , Meinhardt J , Merklein M
Published: November 2019

The quality of car body parts in series production is strongly dependent on the tribological behavior. Fluctuating material properties such as the sheet roughness and the amount of lubricant have an influence on the forming process. On the basis of large amounts of data it is possible to investigate the friction behavior in series production and to make process adjustments if required. Therefore, inline measurement systems have a great potential to detect the sheet roughness during the cutting process of blanks in the coil line. Furthermore, contactless systems are advantageous as they do not damage the surface. Nevertheless, the optical measuring is influenced by the lubricant layer on top of the surface. Therefore, the previously unknown impact of the lubricant on the measuring result is investigated.

Within this study, stationary optical roughness measurements have been conducted using different amounts of lubricant on hot-dip galvanized EDT steel. The results demonstrate the influence of different amounts of lubricant on the sheet roughness measurement. Hence, it is possible to correct the inline measuring results and gain knowledge of fluctuating surface roughness. In addition, strip drawing test has been carried out to investigate the effect of fluctuating tribological conditions.
Temperature dependency of material constitutive behaviour: a simple model
Authors: Atzema E H
Published: November 2019

The realisation that the temperature in the stamping process plays an important and underestimated role is gaining recognition. In the ASPECT project [1] the implications of temperature on friction and material behaviour are modelled and used to design a control algorithm. Although the influence of temperature on the process is mainly via friction the change in material behaviour cannot be neglected. Tata Steel have long been advocates of dislocation physics based constitutive models and in this conference the results of the current modelling approach with temperature influence in the dynamic stress are presented [2] as well. For even higher accuracy a temperature influence on the static hardening part of the model can be taken into account [3] but those are in differential equation form and need to be continuously evaluated in the FEA algorithm which means adaptations in the source code. In this paper a simplified approach based for temperature influence on static hardening is presented that can be written in a closed form and thus be supplied as (tabular) hardening law to any desired FEA code.
A plane stress yield surface using Bézier curve interpolation in two directions
Authors: Vegter H , Abspoel M , Mulder J
Published: November 2019

Current advanced yield criteria, like BBC2005, Yld2000 and Yld2004, offer significant flexibility and are proven to be convex. The number of coefficients of the function is fixed, which limits the number of experimental results that can be fitted independently. A common approach is to define an error function. The optimal anisotropy coefficients are established by minimizing the error function. The Vegter yield criterion is very flexible with respect to experimental results. It is in fact defined as a function that smoothly connects experimental results, while ensuring a continuous function for the derivative. Fitting is therefore a non-issue; the experimental results are the coefficients for the yield criterion. Second order Bézier curves are used for the interpolation between experimental results. Extreme values for these results can cause concavity in the definition of the yield locus. Although rarely an issue, convexity needs to be validated as part of the implementation of the yield criterion. Recently we have seen some cases of concavity that could be attributed to the cosine interpolation in the original Vegter yield criterion definition. Cosine interpolation is used between experimental results at different angles to the rolling direction. An improved method is proposed that uses Bézier interpolation not only between biaxial stress states in one orientation, but also between stress states with a different material orientation. The benefits are greater flexibility in defining experimental results and a coherent approach towards a 3D extension. The benefits of this new yield criterion will be demonstrated in a direct comparison with Yld2000 and Yld2004. The comparison will include the definition of the yield locus, showing actual cases, where in particular biaxial stress states like plane strain and shear are difficult to fit accurately. The comparison will also include improved accuracy in forming simulations.
Computationally efficient necking prediction using neural networks trained on virtual test data
Authors: Greve L , Schneider B , Andres M , Martinez J D , Eller T , Weg B v d , Hazrati J , Boogaard A H v d
Published: November 2019

The onset of localized necking under monotonic and non-monotonic loading can be well-predicted by the imperfection-based approach proposed by [1] (MK). However, a large number of virtual imperfections has to be investigated for an accurate necking prediction, making the MK approach computationally expensive and hence preventing the industrial application for full-scale vehicle models. To overcome these issues, a computationally efficient neural network (NN) model is proposed for replacing the MK model in the present work. An extended version of the MK model has been implemented into a User Material for an explicit crash solver. The model continuously computes the “distance to localized necking” as an important engineering quantity. Single shell element simulations are utilized for creating a comprehensive virtual test database for monotonic and non-monotonic loading for a 22MnB5 grade in an as-delivered state. A simple feed-forward NN model, featuring only one hidden layer, is trained and tested against the virtual data, where invariants of the stress and plastic strain tensors represent the input features of the NN and the “distance to necking” represents the output value of the NN. For comparison of the computational cost, the NN architecture has also been implemented in a User Material Routine for shell elements. The predictions of the NN and the MK model are in good agreement, where, due to the simple mathematical structure of the NN, the computational cost of the NN is significantly lower than for the MK implementation.
Springback characteristics of a martensitic steel for warm U-shape bending: Experiments and FE simulation
Authors: Sun Y , Cai Z , Politis D J , Luan X , Chen G , Wang L
Published: November 2019

Martensitic (MS) steel, a modern type of Ultra-high Strength Steel (UHSS), offers a significantly improved strength-to-weight ratio compared to conventional cold rolled steel grades. This is especially important for weight saving applications on crash-relevant automotive structures. However, it is challenging to conventionally cold form MS steel sheet into complex components due to the lower formability and excessive springback. In this present work, both experimental and finite element simulation methods have been used to investigate the springback characteristics of MS steel in U-shape bending at cold and warm stamping conditions. It was found that the amount of springback is greatly reduced by approximately 62.3% at elevated temperature compared to cold forming. Fast forming speeds were also found to be beneficial for reducing the springback. FE models were successfully developed for the U-shape forming process and close agreement between experimental data and FE simulation results has been achieved.
Predicting plasticity and fracture of severe pre-strained EN AW-5182 by Yld2000 yield locus and Hosford-Coulomb fracture model in sheet forming applications
Authors: Camberg A A , Tröster T , Bohner F , Tölle J
Published: November 2019

The objective of this contribution is to evaluate the capabilities of the Yld2000 yield locus and the Hosford-Coulomb fracture model under isothermal conditions for an EN AW-5182 H18 material in sheet metal forming applications. The calibration of the Yld2000 model is based on uniaxial tension tests in 0°, 45° and 90° in respect to rolling direction and additional layer compression tests. Subsequently, the calibrated plasticity model is evaluated under shear stress and plane strain dominant conditions. Stress state dependent fracture strains are obtained from various tensile tests as well as from FLD tests. The implemented failure prediction is based on a generalized incremental stress-state dependent fracture model (GISSMO) combined with a three-parameter Hosford-Coulomb fracture and instability curve. Finally, the plasticity and fracture models are validated at a cross die cup. It is shown that the models are capable to provide an accurate prediction of the onset of fracture and could be used for a non-isothermal extension in future work.
Descriptions of stress-strain responses of non-linear unloading and closure of stress-strain hysteresis loop based on the Yoshida-Uemori model
Authors: Yoshida F
Published: November 2019

This paper proposes models to describe a non-linear unloading-reloading stress-strain response and closure of a cyclic stress-strain hysteresis loop based on the Yoshida-Uemori (Y-U) model. The average stress-strain slope given by the non-linear model is consistent with the Y-U model of Young’s modulus degradation. The closure of a cyclic stress-strain curve is described by automatically changing one of the Y-U material parameters. An advantage of the above approaches is that no additional parameters are needed for modeling.
Predictive Analysis from numerical and experimental data in press hardening
Authors: Penter Lars , Link Patrick , Stoll Anke , Albert André , Ihlenfeldt Steffen
Published: November 2019

Machine learning, big data and deep learning are today’s catchphrases for how to improve reliability and productivity of your manufacturing equipment. Production companies implement a large number of sensors to record every activity within their production lines and learn as much as possible about their running processes in order to predict shifting product properties and to prevent stoppage due to failure. The successful application of machine learning algorithms to predict machine and process behavior depends on a reliable and balanced database. Since the foremost goal of every manufacturing business is to make sound parts and to avoid defects, there is a large amount of data available for smoothly running processes but only very little for failure production. One approach to solve this imbalance would be to link the production line data with simulation data. Simulation models allow for computing failure parts with no additional costs and therefore enable the exploration of the entire parameter space. We conducted press-hardening experiments with a variation of process parameters for a structural car body part on the press hardening line at Fraunhofer IWU. As an evaluation criterion, we measured the hardness of the final part at critical spots. In order to expand the experimental data, we applied FE simulations to the entire press hardening process chain. The paper explains limitations of the model and elaborates on its parameterization. As a final task, we applied a basic machine-learning algorithm to both experimental and numerical data as well as to their combination in order to evaluate the data space expansion through simulations. The results obtained through machine learning indicate significant differences in the prediction of part quality for solely experimental data and its combination with simulation data. This is especially true for press hardening because of non-linear system behavior and a large amount of uncertain and hard-to-identify parameters. We found that the most challenging parts of uniting measured and simulated data is not only to create simulations with appropriate accuracy, which allow for a meaningful extrapolation of the parameter space, but also to compare simulation and production data based on the same criterion and to have stable simulation models for the entire parameter range.
Effects of fracture area measurement method and tension test specimen type on fracture strain values of 980 class AHSS
Authors: Hance B M , Link T M
Published: November 2019

With increasing focus on “local formability” and fracture behavior of advanced high strength steels (AHSS), the effects of tension test specimen type and fracture area measurement method on fracture strain values were examined. Three 980 class AHSS and three standard tension test specimens with various widths were evaluated. Fractures types were grouped into three categories according to appearance and thickness profile “shape”—Type 1: Perpendicular to the tensile axis (across specimen width) with a V-shape thickness profile; Type 2: Irregular transition from Type 1 to Type 3 with a W-shape thickness profile; and Type 3: Angled across the specimen width with a U-shape thickness profile. All materials exhibited Type 1 fractures when tested with Subsize specimens. However, as the specimen width-to-thickness ratio increased, the fracture type changed from Type 1 to Type 2 or from Type 1 to Type 3. In contrasting the effects of specimen type and fracture area measurement method, specimen type (width) has a far greater impact on the consequent fracture strain value. For tension testing no clear universal relationship exists between the width-to-thickness ratio and the fracture strain. These observations suggest that, when reporting fracture strain values, the specimen type, the material thickness, and the fracture area measurement method must be indicated.
Control of the servo-press in stamping considering the variation of the incoming material properties
Authors: Kim Hyunok , Gu Jiahui C. , Zoller Laura
Published: November 2019

The variation of incoming material properties and the quality of emerging advanced high-strength steels significantly increases the costs for product design and production. It is not economical to tightly control material chemistry to eliminate the variation of material properties at steel mills. This paper introduces a methodology on how to implement a non-destructive evaluation (NDE) tool to measure the variation of the incoming material properties. The servo-controlled forming process was successfully optimized to adapt to variations in the incoming material properties in a lab-scale stamping cell with a 300-Ton mechanical servo press. Final stamping quality was obtained with three different batch materials by altering servo press parameters, such as the slide motion and blank holder force, to adjust for the variation of the material properties from one batch to another. Both a mid-strength (340-MPa tensile strength) steel and a high-strength steel (980-MPa tensile strength) were evaluated in this study. The study shows the effectiveness of an NDE tool to non-destructively measure the sheet material properties and the servo-press control to adjust the press parameters for the variation of the material properties.
A first step towards an in-line shape compensation for UHSS roll forming
Authors: Abeyrathna B , Rolfe B , Weiss M
Published: November 2019

Roll forming is an important sheet metal forming process and is being used increasingly in the automotive industry for the manufacture of structural and crash components from Advanced High Strength Steel (AHSS) and Ultra High Strength Steel (UHSS). In AHSS and UHSS, the material properties can change from coil to coil or within the same coil and this can lead to varying part quality in roll forming. The effect of changes in material properties on part quality depends on the geometry of the part being formed as well as on the process parameters. There is a strong interest in developing in-line shape compensation for roll forming, and this will only be possible if the relationship between part quality, material properties, process- and part shape- parameters is understood.

In this paper, the effect of material properties and both process and geometrical parameters on longitudinal bow is analysed. A new technique is introduced for the identification of variations in the longitudinal bow in a roll formed component based on roll load and torque measurements as well as process parameters and part geometry.
Analysis of material behaviour and shape defect compensation in the flexible roll forming of advanced high strength steel
Authors: Ghanei S , Abeyrathna B , Rolfe B , Weiss M
Published: November 2019

Roll forming is a fast and low-cost forming process with the ability to form a wide range of materials, especially advanced high-strength steels (AHSS) that have limited ductility. However, roll forming is only able to form constant cross-sections along the longitudinal direction of the component, while many automotive components are more complex in shape. Flexible Roll Forming (FRF) uses rolls that have the ability to translate or rotate during production, enabling the manufacture of a part that has a variable cross-section potentially useful for the automotive industry.

Deakin University is equipped with a flexible forming facility that can prototype parts with variable cross-sections. The machine has been developed, patented and manufactured by dataM Sheet Metal Solutions GmbH. Deakin’s Flexible Forming Facility (DFFF) consists of a single forming stand with two opposing robotic arms connected to forming rolls. The flexibly controlled arms form the clamped sheet along a CNC defined bend line while the sheet remains clamped. This facility is capable of creating changes in the cross-section in both width and depth along the part. Moreover, the Deakin facility can form the part in both forwards and backwards directions along the longitudinal dimension of the part. In contrast, conventional FRF facilities use multiple stands of forming rolls, and the forming sequence occurs in a single direction along the part.

The first part of this study compares material deformation in the DFFF in contrast to that of the conventional FRF setup. This study will determine the similarities and differences in the final formed part between DFFF and the forming conditions in conventional FRF. The second step of the study is to investigate the forming properties of the final part when the part has variable depth. An experimental part formed on DFFF was compared to a numerical analysis using COPRA RF FEA to investigate the material flow and the forming defects. The results show that the major shape error is wrinkling. A new forming concept using a blank holder tool was analysed, and it shows promising results in regard to wrinkling reduction.
Influence of the preheating strategy on the deep drawing of extruded magnesium alloy ME20 sheets
Authors: Joghan H Dardaei , Hahn M , Traphöner H , Tekkaya A E
Published: November 2019

Due to the hexagonal crystal structure of magnesium alloys, a high forming limit can only be achieved at elevated temperatures. For the material characterization of extruded magnesium alloy ME20 sheets, at elevated temperatures, the in-plane torsion test and a multi-layer upsetting test were conducted. Also, FLCs were determined at elevated temperatures. For the deep drawing, two different heating strategies are investigated. In the first method, specimens are placed in an oven at 400 °C for around 10 minutes and then rapidly transferred to the tool. In the second method the specimens are directly heated in the deep drawing tool. In both methods the specimens are painted with Bornitrid lubricant. The effect of the preheating on the coefficient of friction is investigated by using strip tensile tests. FEM simulations for the deep drawing comparing two different material models (Barlat 2000, CPB06) are executed. The results show that specimens heated in the tool show a better formability than oven-heated specimens. The numerical results present that there is no significant difference between Barlat 2000 and CPB06 in an isothermal deep drawing condition. The numerical results are in good agreement with the deep drawing experiments which also indicates that the warm FLCs allow for a good failure prediction.
Aging behavior of ultra-fine grained AA 6061 alloy subjected to constrained groove pressing followed by cold rolling
Authors: Changela K , Kumar S , Hariharan K , Ravi Kumar D
Published: November 2019

A novel severe plastic deformation (SPD) route to achieve strengthening of AA 6061 alloy sheets by subjecting them to constrained groove pressing (CGP) followed by cold rolling (CR) is studied in the present work. Solutionized AA 6061 alloy sheets of 3 mm thickness have been subjected to CGP with an equivalent plastic strain of 1.16. The CGP samples have been subsequently cold rolled to reduce the thickness to 1mm (67% reduction) to achieve homogeneous microstructure with ultra-fine grains. It has been observed that yield strength increased significantly due to both CGP and CR. However, the ductility of the processed sample is lower. Therefore, an attempt has been made to study the aging behavior of the processed sample to achieve higher strength along with enhanced ductility through precipitation hardening. Aging of the processed samples was done by isothermal treatment at different temperatures. The microstructure of the deformed and aged samples was characterized by optical microscopy and scanning electron microscopy (SEM). Hardness and tensile tests have been studied to understand the mechanical behavior of deformed and aged samples. From the results, it has been found that processing of this alloy through CGP+CR and subsequent aging improves both strength and ductility due to the combined effect of precipitation hardening and recovery.
Fracture toughness measurements to understand local ductility of advanced high strength steels
Authors: Frómeta D. , Lara A. , Casas B. , Casellas D.
Published: November 2019

The determination of the material parameters that best predict the local ductility of high strength sheet materials has become the focus of active research. Even though several correlations have been proposed, they can sometimes be not accurate enough and discussion is still open on this topic. This paper investigates the suitability of different fracture toughness measurements for local ductility prediction in multiple advanced high strength steels (AHSS). Fracture toughness is characterized by means of essential work of fracture and Khan tear tests. The results show that the essential work of fracture, we, correlates well with different local formability (HER, critical bending angle from V-bending tests and local strain at fracture from uniaxial tensile tests) and crash resistance parameters (energy absorbed in axial impact tests). It confirms that fracture toughness, measured in the frame of fracture mechanics, is a relevant material property to rationalize cracking issues associated to the local ductility of AHSS. On the other hand, it is also shown that Khan tear tests, which are conventionally used to evaluate the fracture resistance of thin metal sheets, can overestimate crack propagation resistance and offer a poor prediction ability for local formability and crash performance.
Enhancement of fracture forming limit by crystallographic texture reformation of AA1050 sheets in Single Point Incremental Forming
Authors: Shrivastava Parnika , Tandon Puneet
Published: November 2019

Single Point Incremental Forming (SPIF) process that results in the formation of sheet metal components leads to higher forming capabilities in comparison to conventional forming strategies. However, the formability in SPIF is critical and limited by the rapid/sudden failure in the sheet material (without material localization) for the components with high wall inclination angles. The state of stresses and strains plays a huge role in determining the forming limits in SPIF, which is complexly governed by various factors such as sheet thickness, lubrication, incremental step depth, tool rotational speed, part geometry, tool dimensions, and geometry, to name a few. The present work focuses on investigating formability of the sheets from its microstructure and crystallographic texture point of view. Preliminary experimentation led to the conclusion that SPIF of the original commercially supplied AA1050 H14 sheets results in early and premature failure. Microstructural investigation of the undeformed sheet showed that the commercially supplied sheets undergo strain hardening due to cold rolling and therefore, results in dislocated and disorientated microstructure and texture. The present work attempts to reform the microstructure of the sheet by preheating at the optimized temperature. Finite element analysis (FEA) of the process led to the observation that the reformed microstructures result in enhanced fracture forming limit strain and reduced stresses in SPIF. This may be attributed to the decrease in the number of dislocations and dislocated grains, and decrease in the surface area of grain boundary per grain with preheating. Improved fracture forming limits due to preheating were experimentally validated by utilizing digital image correlation technique (DIC) for acquiring true strain values subjected to fractured original and different preheated samples.
Experimental study on the mechanical properties of 7xxx aluminium alloy sheet under different heat treatment conditions
Authors: Choi Yumi , Moon Chanmi , Lee Myoug-Gyu
Published: November 2019

Automotive bodies with high strength aluminium alloys have drawn increasing attentions due to their higher strength and lightweight capability. As one of high strength series of aluminum alloys, peak aged 7xxx series aluminum alloys exhibit superior strength of ∼ 600MPa to the conventional lower or moderate strength alloys. However, in spite of the high strength, the lower ductility has been a technical hurdle to be overcome for the material to be applied to press parts at room temperature, which leads to the development of forming technologies at elevated temperature, or namely hot forming. As an alternative to the hot forming, forming trial at room temperature has been made by performing prior solution heat treatment and quenching. This coupled heat treatment and cold forming increases the formability, while the strength needs to be recovered by additional post treatment. The treatment after forming (or deformation) includes natural aging and paint baking process inducing the artificial aging. The objective of the present study is to experimentally investigate the mechanical properties after different heat treatment conditions, which will be utilized for the future finite element simulations for the investigated forming technology.
Forming simulation for TWIP steel
Authors: Gösling M , Thülig T
Published: November 2019

This paper introduces a new generation of industrially available cold rolled TWIP steel. The TWIP900 is a high manganese steel with approx. 19% manganese and shows a stable austenitic structure at room temperature. The focus of the paper is on the forming behavior, which is remarkable compared to other industrially available sheet metal materials. The key figures of this cold rolled narrow strip are a tensile strength of 900 MPa in combination with an elongation at fracture of 40%. The forming behavior of the material is characterized in tensile tests, bulge tests, and Nakazima tests. A material model for forming simulations is set up and a comparison between different material models is done. Then deep drawing experiments are carried out and numerically simulated. Finally, a flanging and springback study is carried out. A comparison between simulation and experiment demonstrates the good quality of currently industrially applied simulation techniques for TWIP steels.