bluuebunny/crossref_metadata_2025_split huggingface dataset:
Compensating the springback of ultra-high-strength steel parts by specific stress superposition during sheet metal forming
Authors: Radonjic R , Liewald M 
Published: June 2021

Abstract
Ultra-high-strength steel sheets offer considerable advantages in car body construction regarding the permanent requirements for vehicle weight reduction as well as the increase in crashworthiness of sheet metal parts. However, when forming such sheet metal materials, the main problem relates to their distinctive springback behaviour, which occurs after the formed part has been released. This paper shows a methodology for springback compensation based on the superposition of stresses acting in the meridian direction of the formed sheet metal part. These stress superposition effects are induced via the tool radii by alternating blank draw-in during deep drawing. As a result, the workpiece areas contacting the punch radii during the deep drawing process are subjected to repeated bending and unbending. In order to analyse occurring stress superposition effects and their influence on springback appearance, simulation was performed using the FE-code LS-Dyna. Furthermore, an optimal blank draw-in kinematic could be defined using the forming simulation, which significantly reduced springback of a double-curved hat-shaped component made of DP 980 steel sheet after forming. Finally, this numerically determined methodology for springback compensation has been experimentally proved using tool that enables alternating blank draw-in during sheet metal forming.
Structural springback analysis of car body closure assemblies using finite element process chain simulations
Authors: Schuler F. , Liewald M. 
Published: June 2021

Abstract
In recent decades, numerical simulation methods have become increasingly important in car body engineering to achieve the high dimensional quality requirements for closure assemblies like bonnets, doors and tailgates. In order to model the entire manufacturing process, latest research works have been focusing on combining the finite element-based simulation models. These complete process chain simulations are able to predict the springback behaviour of the assembly before the physical realization of the tools and processes. The purpose of this study is to introduce a method to use finite element process chain simulations to analyse the structural springback behaviour of car body closure assemblies. Therefore, the numerical process chain simulations are combined with statistical and stochastic tools in order to develop a meta-model, which represents the dimensional accuracy of the investigated assembly. The presented method will then be schematically advanced to be ultimately used in different application areas like robustness and tolerance analyses.
A critical assessment of notched tensile tests for formability mapping of AHSS sheets
Authors: Wagner L. , Larour P. , Sonnleitner F. , Felbinger A. , Angeli J. 
Published: June 2021

Abstract
The influences of specimen geometry during the determination of local ductility have recently been investigated, using the fracture surfaces of standardized tensile tests specimens. Among the shortcomings of these specimens was the rather undefined strain path to failure, i.e. uniaxial tension followed by a deviation towards a more or less pronounced plane strain path, and the varying fracture angle, often hampering accurate image acquisition of the fracture surface. Notched tensile tests are currently used during damage model calibration for sheet metal for crash simulation. These samples exhibit a defined strain path to fracture and the notch triggers fracture predominantly perpendicular to the tensile direction. These kind of specimens have therefore been suggested as alternative to standardized tensile test specimens. However, several issues need to be addressed for notched tensile specimens as well, in addition to being an undesired additional test during e.g. quality control. These include again geometric considerations as well as the effect of different fracture types on the resulting measures of local fracture strain.
Alternative characterization method for the failure behavior of sheet metals derived from Nakajima test
Authors: Kohl D , Merklein M 
Published: June 2021

Abstract
One sustainability approach in sheet metal forming is to reduce costs by increasing the reliability of the forming process by applying numerical simulations. These are based upon the finite element analysis and require specific characteristic values of the material. To improve the accuracy of the numerical model regarding the failure behavior, forming limit curves (FLC) which display the conduct and failure boundaries of sheet metals are characterized. Optical measurement systems are used to analyze the strain distribution and to generate a material dependent FLC. To reduce the amount of testing, the tensile test, the plane-strain test and the hydraulic bulge test are combined to characterize the uniaxial, plane, and the biaxial strain area and the forming limit. The results are summarized in the presented FLC, which is consequently compared with results from the standardized Nakajima test. The method is validated with different materials to compare a ductile (DX54D), a brittle (DP800) and a lightweight sheet metal (AA5182). That approach is used for the validation of the alternative and cost-effective FLC characterization. This eliminates the need for the Nakajima testing facility and the high utilization of a testing machine can save costs and time in creating the specific material data.
Study on the impact of temperature on the warm bending of aluminium alloy sheets
Authors: Mauduit A , Maillard A 
Published: June 2021

AbstractWarm forming of aluminum is a recent process and its developments have been mainly focused on drawing. This study takes a look at bending, a process widely used in industry. After the 2017A T4 and 6061 T6 aluminum alloys are characterized at both room and warm temperatures, bending of these materials is carried out at different temperatures. Various bending strategies are studied. The findings point to the value for these materials in heating the bend during the forming operation to avoid cracking, in some cases, and to minimize springback. An analysis of the sheet metal distortion mechanism is carried out based on micrographic sections sampled in the bend. The analysis highlights the formation of “macro-bands” indicating the location of the distortion. Asperities are observed on the outer surface. This could be the fracturing site of certain grains located in the grooves of the asperities, with possible intergranular or transgranular cracking. A potential follow-up to this study could be to assess the changes in the mechanical characteristics after the aluminum alloy sheets have cooled to room temperature.
Cryogenic deep drawing of aluminum alloy AA6014 using macro-structured tools
Authors: Tulke M , Wolf A , Brosius A 
Published: June 2021

Abstract
For functional lightweight construction, 6xxx aluminum alloys are essential materials for exterior components due to their low weight to stiffness ratio and acceptable formability. The formability of the 6xxx alloys is lower compared to other aluminum alloys but they are free of flow lines. MOUSAVI ET AL. [1] demonstrate an alternative method of improving the forming limit with a deep drawing process using macro-structured tools. Another approach increasing the formability of aluminum alloys is forming at cryogenic temperatures [2]. This paper presents an advancement of deep drawing on macro-structured tools at cryogenic temperatures. This assumes a lower heat flux due to the reduction of the contact area between the blank and deep drawing tools, enabling forming at cryogenic blank temperature. After characterization of the material, experimental investigations are carried out on a cup test geometry. In addition to the required punch force at different temperatures, this paper presents the influence of macro-structured tools at cryogenic temperatures on springback and hardness distribution. Finally, the extension of the process limits and options by cryogenic macro-structured deep drawing are discussed.
Influence of synthetically generated inclusions on the stress accumulation and concentration in X65 pipeline steel
Authors: Fehlemann N. , Sparrer Y. , Pütz F. , Konemann M. , Münstermann S. 
Published: June 2021

Abstract
The use of a simulative approach with representative volume elements (RVE’s) is particularly well suited to investigate the influence of different microstructural parameters on the damage behavior of a material. In order to statistically analyze the individual components of the microstructure (e.g. geometric structure of grains and inclusions), well-known distribution functions such as logNormal/Gamma are normally used, but these do not take into account the interdependencies between the different parameters. However, newer approaches like machine learning techniques can only describe one phase of a single material at a time. Therefore, in this study, we extended an existing Wasserstein Generative Adversarial Network (WGAN) to a Conditional Wasserstein GAN with Gradient Penalty (CWGAN-GP), with which it is possible to process multiple materials/phases simultaneously. Training this algorithm on different steels and associated inclusions showed that a single trained network can generate synthetic microstructure for all different phases and materials with very high quality. A newly implemented evaluation method using the regularized Wasserstein-distance confirmed the excellent agreement of the real data with the synthetic data for all phases/materials. As a use case for our algorithm, the influence of different inclusions on the stress accumulation and concentration of X65 pipeline steel was investigated to find initiation sites for damage in the material. These investigations showed a pronounced correlation between stress concentration and inclusion parameters, thus confirming the usefulness of the CWGAN-GP as an input-generator for RVE’s.
Experimental research of formability limits in different thicknesses of polycarbonate sheets
Authors: Rosa-Sainz A , Magrinho JP , Silva M B , Centeno G , Martínez-Donaire A J , Vallellano C 
Published: June 2021

Abstract
This experimental investigation evaluates the methodologies for determining the forming limits by necking and fracture in polycarbonate (PC) sheets. The proposed approaches are based on two methodologies that are commonly utilized in sheet metal forming. To that end, Nakajima tests for polymer specimens with different strain loading paths were carried out for 1 mm, 2 mm and 3 mm thickness. The evaluation of the principal strains was accomplished by means of Digital Image Correlation (DIC) and thickness measurements to obtain failure strains were performed. The experimental results highlighted the different behaviour attained in polymers and metals, and the need of establishing a suitable adaptation of the methodologies used in sheet metal forming, especially due to the elastic recovery of polymers that cannot be neglected. This analysis allowed obtaining accurate formability limits by necking and by fracture for the PC sheets of 1, 2 and 3 mm of thickness, additionally these results were compared with the formability limits for PVC sheet of 3 mm of thickness. The representation of these formability limits within the principal strain space allowed establishing a general framework for analysing the fracture limit, which is the relevant forming limit in incremental sheet forming processes.
Influence of the strain dependent material behaviour under plane strain on the yield locus modelling
Authors: Lenzen M , Merklein M 
Published: June 2021

Abstract
Modern lightweight materials mostly exhibit a low formability especially in the plane strain state. Having in mind that over 80% of the occurred failures in the press shop in the automotive sector can be related to this strain state, the characterization under plane strain load should be further intensified. The evaluation of the plane strain flow curve is commonly done by notched tensile tests or biaxial tensile tests with optimized strain path. Both tests have some disadvantages in the flow curve evaluation especially at a high degree of deformation. With an elliptic hydraulic bulge test, the material behaviour under near plane strain condition can be evaluated up to fracture. The aim in this investigation is the analysis of the resulting material behaviour under plane strain of commonly used alloys AA5182, DC06 and DP600 with respect to the material anisotropy. Therefore, hydraulic bulge tests with a plane strain load path are performed in rolling direction and transversal direction. The resulting flow curves are used to analyse the material hardening with respect to the anisotropy. The results show differing hardening mechanisms for the investigated materials which will lead to different strategies for the material modelling.
Evaluation of Simple Shear Test Geometries for Constitutive Characterization using Virtual Experiments
Authors: Narayanan A , Abedini A , Weinschenk A , Worswick M J , Butcher C 
Published: June 2021

Abstract
In-plane simple shear tests have become commonplace in the fracture characterization of automotive sheet metals but have received less attention for constitutive characterization. Unlike tensile tests, simple shear tests do not have any tensile instability and remain in a state of plane stress until fracture. From plastic work equivalence, an isotropic hardening model can be readily constructed from the tensile and shear test data without inverse finite-element analysis. The success of the methodology hinges upon the shear specimen geometry and how the local strains in the gage region are measured using digital image correlation (DIC). In this study, finite-element simulations of seven shear test geometries were evaluated for an isotropic material in a series of virtual experiments by varying the input hardening response. The data from the simulations was extracted from the surface as if DIC was employed and used to determine the hardening behavior in comparison with the exact solution. Shear geometries without a notch eccentricity in the gage region appear to be best suited for characterizing low hardening materials with an error of less than 1% in the stress response for an n-value of 0.02. Conversely, for higher hardening materials corresponding to an n-value of 0.20 or greater, the geometries with a notch eccentricity performed best.
Modeling continuous dynamic recrystallization of lightweight alloys by coupling polycrystal plasticity approach
Authors: Chen S F , Zhang S H , Song H W , Lee M G 
Published: June 2021

Abstract
In the thermo-mechanical process of lightweight alloys, such as aluminum alloys, complex microstructure evolutions are inevitably involved via the continuous dynamic recrystallization (CDRX) featured with extensive formation of subgrains followed by progressive transition into new grains. In this study, a novel fully integrated CDRX-VPSC (visco-plastic self-consistent) model is established. First, numerical formulations for formation and rotation of subgrains are addressed by reasonably tackling the grain boundary characteristics, and also the concept of generation for CDRX grains is newly introduced. By considering these two aspects, the grain population is updated according to the complex grain boundary evolutions. In the VPSC module of the integrated modeling, the shear strain rate, dislocation density, and orientation are calculated to update the grain populations and grain sizes in the CDRX module. The validation of the proposed model is conducted by simulating the hot compression of extruded AA7075 sheet, in which the DRX mechanism is controlled by the subgrain related dynamic recovery (DRV) and CDRX. The simulated flow stresses, microstructural evolutions (grains size and DRX fractions), and texture features agree well with reported experimental observations.
Influence of the orthotropic behaviour on defects prediction in cup drawing, reverse redrawing and expansion
Authors: Oliveira M C , Neto D M , Alves J L , Menezes L F 
Published: June 2021

Abstract
Finite elements analysis is commonly used in the cans tool design since it allows predicting different sheet metal defects and instabilities that significantly affect the efficient production of beverage cans. These defects include earing, due to the material orthotropic behaviour, thinning and fracture. The numerical prediction of these kind of defects is more complex when different combinations of strain paths are involved. The process conditions considered in this work involve the drawing of a cylindrical cup, the reverse redrawing and the expansion. The two materials under analysis are an AA5352 aluminium alloy and a TH330 steel. The materials orthotropic plastic behaviour is modelled with a non-quadratic yield criterion. The results show that changes in the yield surface lead to slight changes in the earing profile and in the location were strain localization occurs. Moreover, the numerical model must describe in detail the process conditions, since they may affect the formability prediction.
Development of an automatic crack detection method for cupping tests on sheets and foils with a wall thickness below 0.1 mm
Authors: Schneider J-L , Staupendahl D , Wahlers L 
Published: June 2021

Abstract
In deep drawing and stretch forming, in-situ crack detection is usually performed by detecting the drop in drawing force, by analyzing structure-borne sound or by reflected light methods. Friction, vibration, and machine noise are disturbance variables that prevent crack detection in thin sheets and foils using the first two methods and highly reflective as well as inhomogeneous surfaces prevent crack detection in the last method. In order to avoid the above-mentioned disadvantages of the existing crack detection methods, a method based on transmission-illumination was developed. Key to this new development is the use of a translucent punch, which incorporates a light source, and a camera connected to an image-processing unit. The detection process works as follows: the luminous punch penetrates the sheet and forms it accordingly. As soon as a crack is formed, light penetrates the crack and falls onto the image sensor of the camera. The image processing unit detects the jump in illumination and triggers the sudden stop of the forming process. Forming tests were performed that show the stable and operator independent performance of the newly developed detection process using sheet samples made from steel and aluminum.
Deformation and thinning field prediction for HFQ® formed panel components using convolutional neural networks
Authors: Attar H R , Zhou H , Li N 
Published: June 2021

Abstract
The novel Hot Forming and cold die Quenching (HFQ®) process can provide cost-effective and complex deep drawn solutions through high strength aluminium alloys. However, the unfamiliarity of the new process prevents its widescale adoption in industrial settings, while accurate Finite Element (FE) simulations using the most advanced material models take place late in design processes and require forming process expertise. Machine learning technologies have recently been proven successful in learning complex system behaviour from representative examples and have the potential to be used as design support tools for new forming technologies such as HFQ®. This study, for the first time, presents a novel application of a Convolutional Neural Network (CNN) based surrogate to predict the deformation and thinning fields for variable deep drawn geometries formed using HFQ® technology. A dataset based on deep drawn geometries and corresponding FE results is generated and used to train the model. The results show that near indistinguishable full field predictions in real time are obtained from the surrogate when compared with HFQ® simulations. This technique can be adopted in industrial settings to aid in both concept and detailed component design for complex-shaped panel components formed under HFQ® conditions, without underlying knowledge of the forming process.
Prediction of forming limit diagrams from tensile tests of automotive grade steels by a machine learning approach
Authors: Finamor F P , Wolff M A , Lage V S 
Published: June 2021

Abstract
Steels’ formability is extremely important to the automotive industry. This property is usually assessed through the Forming Limit Diagrams (FLDs), which comprise the regions of maximum strains, where neither necking nor fracture of the material is observed. In this work, the use of Machine Learning algorithms was evaluated for the FLDs’ construction, based on the tensile properties of a group of steels. A set of historical data, consisting of tensile tests values and measured FLDs, based on Nakajima method of various AHSS, HSLA and IF steels, was used to build the model. In order to evaluate the predictive capacity of the model, experimental and Keeler-Brazier FLDs were compared. The proposed method was able to predict with great correlation and low mean error the FLDs of those materials. The results show a viable route for predicting FLDs based on tensile properties.
Autoencoder based Wear Assessment in Sheet Metal Forming
Authors: Niemietz Philipp , Unterberg Martin , Trauth Daniel , Bergs Thomas 
Published: June 2021

Abstract
The amount of information contained in process signals such as acoustic emission and force signals has proven vital for the detection of changes in physical conditions or quality feature prediction in sheet metal forming applications. Both signal types have also been researched in the context of wear detection, yet systems that reliably identify the wear state at a given time in sheet metal forming processes based on these signals do not exist. This paper proposes an architecture to assess the wear increase within a given time frame in an experiment based on an autoencoder. The ability of autoencoders to encode and decode signals has been widely studied and this approach leverages the fact that autoencoders are more likely to learn representative encodings on stable and homogeneous signals than on heterogeneous signals with high fluctuations. This approach utilizes the circumstance that high tool wear leads to changes in the signal and signal fluctuation. In consequence, autoencoders can be utilized to track tool wear progression without the need for labelled data. The findings show a strong similarity to physical models for the wear progression of tool components, indicating the validity of this approach. Additionally, an analysis of the signals yields characteristic effects of the considered force signals that could specifically represent wear resistance.