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Towards Virtual Tryout and Digital Twins: Enhanced Modeling of Elastic Dies, Sheet Materials, and Friction in Sheet Metal Forming
Blekinge Institute of Technology, Faculty of Engineering, Department of Mechanical Engineering. Volvo Cars.ORCID iD: 0000-0002-6526-976x
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Competition and complexity in the global car market are continuously increasing. To gain an edge in the market while making a profit, each competitor in the market needs a strong focus on technology development. Such a focus will increase the value of the product for the customers. On top of the challenges of developing the technical content of the cars, it is equally important to develop the manufacturing processes. This will enable the production of technically advanced and attractive products at a competitive cost.

An important manufacturing process within the automotive industry, and the focus of this PhD thesis, is sheet metal forming (SMF). The SMF process has been simulated for the past few decades with finite element (FE) simulations, whereby factors such as shape, strains, thickness, springback, risk of failure, and wrinkles can be predicted. A factor that most SMF simulations do not currently include is die and press elasticity, limiting the reliability of SMF simulations for virtual die tryout and digital twins. This factor is instead handled manually during the die tryout, which is time-consuming and expensive.

The importance of accurately representing press and die elasticity in SMF simulations is the focus of this research project. The objective is to achieve virtual tryout and production support through SMF simulations that consider elastic die and press deformations. Accurate simulations of die tryout, or dies in running production, are found to be impossible without the inclusion of reliable representations of elastic dies and presses.

Accurate CAD models of stamping dies are common. Strategies are presented for how to include them in SMF simulations models that are easy to set up and fast to solve. Full representations of presses are rarer. Methods for measuring and inverse modeling the elasticity of presses are therefore presented together with an overview from the literature of other methods. A unified method that is reliable and cost-efficient for the stamping industry is of high interest.

SMF simulations with elastic dies will only yield accurate predictions if reliable models of sheet materials and lubrication systems are included. Friction in particular is highly dependent on die deformations since contact pressure is an important parameter in determining the friction level. Reliable models for both friction and sheet materials are dependent on accurate experimental data and characterization methods.

Including elastic die and press deformations in SMF simulations, together with reliable representations of friction and sheet materials, has immense potential to reduce the lead time of stamping dies. The simulation methods that include elasticity will also be vital for more accurate digital twins of press lines.

Abstract [sv]

Konkurrens och komplexitet ökar ständigt inom den globala bilmarknaden. För att skapa konkurrensfördelar behöver företag som verkar på marknaden fokusera på teknikutveckling, vilket i sin tur ökar produkternas värde för konsumenterna. Samtidigt som man utvecklar det tekniska innehållet i sina produkter så är det lika viktigt att utveckla sina tillverkningsprocesser. Välutvecklade tillverkningsprocesser möjliggör produktion av tekniskt avancerade, och attraktiva produkter, till en konkurrenskraftig kostnad.

En viktig process, som denna avhandling fokuserar på, är pressning av plåt. Plåtformningsprocessen simuleras sedan ett par decennium tillbaka med Finita Element (FE) simuleringar. Man kan på så sätt prediktera form, töjningar, tjocklek, återfjädring, rynkor, risk för försträckning och sprickor m.m. En faktor som för tillfället inte inkluderas i näst intill alla plåtformningssimuleringar är elastiska press- och verktygsdeformationer. Detta begränsar möjligheterna att använda plåtformningssimulering för virtuell inprovning och att skapa pålitliga digitala tvillingar. Elastiska deformationer hanteras istället manuellt under, den oftast långa och dyra, inprovningsfasen.

Detta projekt visar på vikten av att inkludera press- och verktygsdeformationer genom simuleringar av verkliga pressverktyg. I de simulerade fallen är det inte möjligt att uppnå bra resultat för inprovning och produktionssupport utan att inkludera verktygsdeformationer i modellen.

CAD-modeller finns för nästan alla pressverktyg idag. Strategier för att inkludera elastiska verktygsgeometrier i simuleringar presenteras. Modellerna är enkla att skapa och snabba att lösa. Kompletta CAD-modeller av pressar är inte lika vanligt. Metoder för att mäta upp pressar och inkludera i simuleringar är därför också presenterade, tillsammans med exempel på andra metoder från vetenskaplig litteratur. En standardiserad metod för plåtformningsindustrin är av intresse.

Simuleringar med elastiska verktyg ger endast pålitliga resultat om plåt och friktion representeras på ett bra sätt. Speciellt friktion är starkt beroende av verktygsdeformationer då kontakttryck är en viktig faktor i friktionsmodeller. Material- och friktionsmodeller vilar på väl fungerande metoder för karaktärisering utifrån god experimentell data.

Plåtformningssimuleringar med elastiska verktyg, tillsammans med pålitliga modeller av plåtmaterial och friktionssystem, har en stor potential att reducera ledtiden för pressverktyg. Dessa metoder kommer också att vara viktiga i skapandet av digitala tvillingar av verktyg och pressar.

Place, publisher, year, edition, pages
Karlskrona: Blekinge Tekniska Högskola, 2020.
Series
Blekinge Institute of Technology Doctoral Dissertation Series, ISSN 1653-2090 ; 7
Keywords [en]
Sheet Metal Forming, Stamping Die, Virtual Tryout, Digital Twin, Stamping Press, Structural Analysis, Finite Element Simulation, Optimization, Digital Image Correlation, Inverse Modeling
National Category
Mechanical Engineering
Research subject
Mechanical Engineering
Identifiers
URN: urn:nbn:se:bth-20047ISBN: 978-91-7295-406-9 (print)OAI: oai:DiVA.org:bth-20047DiVA, id: diva2:1460472
Public defence
2020-09-22, J1630, 13:00 (English)
Opponent
Supervisors
Funder
Vinnova, 2016-03324Vinnova, 2018-03331Available from: 2020-08-25 Created: 2020-08-24 Last updated: 2020-12-14Bibliographically approved
List of papers
1. Friction and lubrication modeling in sheet metal forming simulations of a Volvo XC90 inner door
Open this publication in new window or tab >>Friction and lubrication modeling in sheet metal forming simulations of a Volvo XC90 inner door
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2016 (English)In: IOP Conference Series: Materials Science and Engineering, 2016, Vol. 159, no 1, article id 012021Conference paper, Published paper (Refereed)
Abstract [en]

The quality of sheet metal formed parts is strongly dependent on the tribology, friction and lubrication conditions that are acting in the actual production process. Although friction is of key importance, it is currently not considered in detail in stamping simulations. This paper presents a selection of results considering friction and lubrication modeling in sheet metal forming simulations of the Volvo XC90 right rear door inner. For this purpose, the TriboForm software is used in combination with the AutoForm software. Validation of the simulation results is performed using door inner parts taken from the press line in a full-scale production run. The results demonstrate the improved prediction accuracy of stamping simulations by accounting for accurate friction and lubrication conditions, and the strong influence of friction conditions on both the part quality and the overall production stability. © Published under licence by IOP Publishing Ltd.

Keywords
Drawing (forming), Friction, Metal forming, Sheet metal, Stamping, Tribology, Friction conditions, Full-scale production, Lubrication condition, Lubrication models, Part quality, Prediction accuracy, Production process, Stamping simulations, Lubrication
National Category
Metallurgy and Metallic Materials
Identifiers
urn:nbn:se:bth-14041 (URN)10.1088/1757-899X/159/1/012021 (DOI)000402734500095 ()2-s2.0-85014714526 (Scopus ID)
Conference
International Deep Drawing Research Group Conference on Challenges in Forming High-Strength Sheets, IDDRG , Linz
Available from: 2017-03-22 Created: 2017-03-22 Last updated: 2021-01-12Bibliographically approved
2. Introduction of elastic die deformations in sheet metal forming simulations
Open this publication in new window or tab >>Introduction of elastic die deformations in sheet metal forming simulations
2018 (English)In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 151, no S1, p. 76-90Article in journal (Refereed) Published
Abstract [en]

Simulations of sheet metal forming (SMF) with finite element models (FE-models) for stamped parts in the car industry are useful for detecting and solving forming problems. However, there are several issues that are challenging to analyze. Virtual tryout and analyzes of stamping dies in running production are two important cases where many of these challenging issues are present. Elastic deformations of dies and press lines and a physically based friction model is often missing when these types of cases are analyzed. To address this, this research aims to develop a method wherein the results of two separate FE-models are combined to enable SMF simulations with the inclusion of elastic tool and press deformations. The two FE-models are one SMF model with two-dimensional (2D) rigid tool surfaces and one structural model of the die and press. The structural model can predict surface shapes and pressure distributions for a loaded stamping die. It can also visualize relatively large and unexpected deformations of the die structure. The recommended method of transferring the deformations from the structural model to the 2D surfaces is through an FE technique called submodeling. The subsequent SMF simulations show that the method for calculating and using the deformed surfaces together with the TriboForm friction model yields a result that matches measured draw-in and strains. It is verified that the ability to virtually deform a die and include the resulting geometry in forming simulations is of high importance. It can be used for the virtual tryout and optimization of new dies or analyses of existing dies in running production. It is suggested that future research focus on a more efficient and automated workflow. More experimental data and simulations are also needed to verify the assumptions made for the simulation models. This will enable the method to be adopted in a reliable way for standard SMF simulations. © 2017.

Place, publisher, year, edition, pages
Elsevier Ltd, 2018
Keywords
Elastic tooling, Friction, Sheet metal forming, Structural analysis, Surface compensation, Automotive industry, Computer supported cooperative work, Deformation, Dies, Metal forming, Presses (machine tools), Sheet metal, Stamping, Tribology, Automated workflow, Forming simulations, Friction modeling, Physically based, Research focus, Structural modeling, Two Dimensional (2 D), Finite element method
National Category
Materials Engineering
Identifiers
urn:nbn:se:bth-14475 (URN)10.1016/j.ijsolstr.2017.05.009 (DOI)000447577700007 ()2-s2.0-85019444887 (Scopus ID)
Note

The authors are grateful for the financial support from Volvo Cars and wishes to thank Tata Steel, AutoForm Engineering, and TriboForm Engineering for valuable cooperation and support. The authors also wish to express their appreciation of their management team at Volvo Cars and Mats Walter, Head of Mechanical Engineering Department at Blekinge Institute of Technology, for enabling an eminent cooperation.

Available from: 2017-06-13 Created: 2017-06-13 Last updated: 2020-08-24Bibliographically approved
3. Framework for Simulation-Driven Design of Stamping Dies Considering Elastic Die and Press Deformations
Open this publication in new window or tab >>Framework for Simulation-Driven Design of Stamping Dies Considering Elastic Die and Press Deformations
2017 (English)In: PROCEEDINGS OF THE 20TH INTERNATIONAL ESAFORM CONFERENCE ON MATERIAL FORMING (ESAFORM 2017) / [ed] Brabazon D.,Ul Ahad I.,Naher S., American Institute of Physics (AIP), 2017, Vol. 1896, article id 010001Conference paper, Published paper (Refereed)
Abstract [en]

Sheet metal forming (SMF) simulations are used extensively throughout the development phase of industrialstamping dies. In these SMF simulations, the die and press are normally considered as rigid. Previous research has howevershown that elastic deformation in these parts has a significant negative impact on process performance. This paperdemonstrates methods for counteracting these negative effects, with a high potential for improved production support anda reduced lead time through a shorter try-out process. A structural finite element model (FE-model) of a simplified die isstudied. To account for elastic deformation, the blankholder surfaces are first virtually reworked by adjusting the nodalpositions on the die surfaces attaining a pressure distribution in accordance to the design phase SMF simulations with rigidsurfaces. The elastic FE-model with reworked surfaces then represents a stamping die in running production. The die isnow assumed to be exposed to changed process conditions giving an undesired blankholder pressure distribution. Thechanged process conditions could for example be due to a change of press line. An optimization routine is applied tocompensate the negative effects of the new process conditions. The optimization routine uses the contact forces acting onthe shims of the spacer blocks and cushion pins as optimization variables. A flexible simulation environment usingMATLAB and ABAQUS is used. ABAQUS is executed from MATLAB and the results are automatically read back intoMATLAB. The suggested optimization procedure reaches a pressure distribution very similar to the initial distributionassumed to be the optimum, and thereby verifying the method. Further research is needed for a method to transform thecalculated forces in the optimization routine back to shims thicknesses. Furthermore, the optimization time is relativelylong and needs to be reduced in the future for the method to reach its full potential.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2017
Series
AIP Conference Proceedings, ISSN 0094-243X
National Category
Applied Mechanics
Identifiers
urn:nbn:se:bth-15392 (URN)10.1063/1.5008124 (DOI)000419825000167 ()9780735415805 (ISBN)
Conference
20th International ESAFORM Conference on Material Forming, Dublin
Funder
VINNOVA, 2016-03324
Available from: 2017-10-30 Created: 2017-10-30 Last updated: 2020-08-24Bibliographically approved
4. Characterizing the Elastic Behaviour of a Press Table throughTopology Optimization
Open this publication in new window or tab >>Characterizing the Elastic Behaviour of a Press Table throughTopology Optimization
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2017 (English)In: Journal of Physics: Conference Series / [ed] Volk W., Institute of Physics Publishing (IOPP), 2017, Vol. 896, article id 012101Conference paper, Published paper (Refereed)
Abstract [en]

Sheet metal forming in the car industry is a highly competitive area. The use ofdigital techniques and numerical methods are therefore of high interest for reduced costs andlead times. One method for reducing the try-out phase is virtual rework of die surfaces. Thevirtual rework is based on Finite Element (FE) simulations and can reduce and support manualrework. The elastic behaviour of dies and presses must be represented in a reliable way in FEmodelsto be able to perform virtual rework. CAD-models exists for nearly all dies today, butnot for press lines. A full geometrical representation of presses will also yield very large FEmodels.This paper will discuss and demonstrate a strategy for measuring and characterizing apress table for inclusion in FE-models. The measurements of the elastic press deformations iscarried out with force transducers and an ARAMIS 3D optical measurement system. The presstable is then inverse modelled by topology optimization using the recorded results as boundaryconditions. Finally, the press table is coupled with a FE-model of a die to demonstrate itsinfluence on the deformations. This indicates the importance of having a reliable representationof the press deformations during virtual rework.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2017
Keywords
Automotive industry, Finite element method, Flanges, Fracture, Fracture testing, High strength steel, Materials testing, Metal forming, Metal testing, Metals, Strain
National Category
Applied Mechanics
Identifiers
urn:nbn:se:bth-15394 (URN)10.1088/1742-6596/896/1/012068 (DOI)000424196000068 ()
Conference
36th IDDRG-2017 - MATERIALS MODELLING AND TESTING FOR SHEET METAL FORMING, MUNICH
Funder
VINNOVA, 2016-03324
Note

open access

Available from: 2017-10-30 Created: 2017-10-30 Last updated: 2020-08-24Bibliographically approved
5. Simulation of Sheet Metal Forming using Elastic Stamping Dies
Open this publication in new window or tab >>Simulation of Sheet Metal Forming using Elastic Stamping Dies
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2019 (English)In: Proc. of the 12th European LS-DYNA Conference 2019, 2019, 2019Conference paper, Published paper (Other academic)
Abstract [en]

Simulation of sheet metal forming is one of the major applications of LS-DYNA. Today, a majority of the forming industry is using Finite Element models to design the stamping dies in order to prevent excessive thinning, wrinkling and producing parts within tolerance by compensating for springback deformation. All these simulations are made using the assumption of rigid forming surfaces. Depending on the type of press, tool design and sheet metal part, this assumption could prove to be incorrect which yields a forming result that depends on the elastic deformation of the stamping die and in some cases the entire stamping press. Such deformations are usually compensated during die try-out by manual rework which is costly and time consuming.

This paper presents the result of a study performed at Volvo Cars press shop in Olofström, Sweden, aiming at determining computational methods to introduce elastic stamping dies in the sheet metal forming simulations in order to minimize manual rework by performing a virtual tryout of the stamping die. The methodology to model the stamping die and the forming surfaces in LS-DYNA are presented and a simulation model is gradually improved from using nominal rigid CAD surfaces through scanned tool surfaces and finishing with an elastic model of the stamping die assembly. The part used in the study is the side door inner for Volvo XC90 and comparisons are continuously made between simulations results and measurements on parts from running production.

National Category
Mechanical Engineering
Identifiers
urn:nbn:se:bth-20040 (URN)
Conference
The 12th European LS-DYNA Conference 2019, 14-16 May, 2019, Koblenz, Germany
Funder
Vinnova, 2016-03324
Available from: 2020-06-30 Created: 2020-06-30 Last updated: 2020-08-24Bibliographically approved
6. A Complete and Rapid Simulation Method for Virtual Try-out of Stamping Dies Considering Elastic Deformations
Open this publication in new window or tab >>A Complete and Rapid Simulation Method for Virtual Try-out of Stamping Dies Considering Elastic Deformations
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(English)Manuscript (preprint) (Other academic)
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:bth-20046 (URN)
Funder
Vinnova, 2016-03324
Note

Manuscript submitted to International Journal of Material Forming.

Available from: 2020-06-30 Created: 2020-06-30 Last updated: 2020-08-24Bibliographically approved
7. BBC05 with non-integer exponent and ambiguities in Nakajima yield surface calibration
Open this publication in new window or tab >>BBC05 with non-integer exponent and ambiguities in Nakajima yield surface calibration
2021 (English)In: International Journal of Material Forming, ISSN 1960-6206, E-ISSN 1960-6214, Vol. 14, no 4, p. 577-592Article in journal (Refereed) Published
Abstract [en]

Reliable sheet metal forming simulations depend on accurate descriptions of real process conditions. These conditions include material behavior, lubrication systems, tool deformations, press dynamics, and more. Research on material models is the most mature area for describing these conditions in a reliable way. Several advanced and flexible models exists. This study focuses on two versions of yield criteria for sheet materials that are assumed to follow the plane stress assumption: the BBC05 model with integer exponent and the BBC05 model with non-integer exponent. The literature has previously described the BBC05 model with integer exponent. This paper elaborates on a modified version with non-integer exponent that offers more flexibility in the mathematical description. Furthermore, it outlines the implementation of this material model and similar yield criteria as user subroutines in finite element software. As mathematical flexibility increases, it enables more physically correct material approximations. However, it also becomes more challenging to calibrate because of ambiguities due to a larger number of mathematical variables. These ambiguities is demonstrated by using a Nakajima test without lubrication during inverse modeling of parameters for the BBC05 model. It shows that it is impossible to accurately identify the physically correct combination of friction coefficient and the yield surface exponent, k, using strain distributions and punch force. It is suggested to use two Nakajima tests in the inverse modeling process where friction can be neglected due to testing conforming to ISO12004-2. One test that corresponds to equi-biaxial strain of the sheet, and one that corresponds to plane strain in the transverse direction of the sheet. By utilizing these samples in the inverse modeling it is possible to separate friction from the exponent k. A non-integer value of k is found to yield the most reliable prediction of strains and forces in the simulations, thereby also demonstrating the need of flexible yield surface models such as BBC05 with non-integer exponent, YLD2000, Vegter and more advanced yield criteria.

Place, publisher, year, edition, pages
SPRINGER FRANCE, 2021
Keywords
Sheet metal forming, Yield criteria, Non-integer exponent, Yield surface calibration, Friction, Inverse Modelling
National Category
Other Mechanical Engineering Other Mechanical Engineering
Identifiers
urn:nbn:se:bth-19330 (URN)10.1007/s12289-020-01545-0 (DOI)000518077200001 ()2-s2.0-85081629634 (Scopus ID)
Projects
Reduced Lead Time through Advanced Die Structure Analysis
Funder
Vinnova, 2016-03324
Note

open access

Available from: 2020-03-19 Created: 2020-03-19 Last updated: 2025-02-14Bibliographically approved

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