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Material Characterization for Modelling of Sheet Metal Deformation and Failure
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials. (Hållfasthetslära)ORCID iD: 0000-0001-9626-5406
2017 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Weight reduction is one possibility to reduce fuel consumption and emission of transportation vehicles. Sheet metals are often used in automotive and aerospace applications and therefore the weight reduction achieved by reducing the sheet metals thickness is an important contribution to weight reduction. Increasing the strength of sheet metal materials gives the opportunity to reduce the total weight while maintaining safety. To prove a maintained safety for parts with a decreased weight Finite Element (FE) simulations are commonly used. This leads to a high demand on the simulation precision of sheet metals, where an accurate prediction of the post-necking behaviour of materials is needed. Improved FE simulations are reducing time and costs during the development processes. 

One application to improve the strength of sheet metals in the automotive industry is the usage of ultra high strength steels, which has constantly increased in usage during the last decades. The development of the press hardening process, where sheet metal blanks are formed and quenched simultaneously, brings new design opportunities. Using press hardening tools with zones that uses different cooling rates sheet metal parts can be produced with tailored properties, to improve their performance. Simulating materials based on the microstructure demands high precision on the plasticity modelling for high strain values. 

In this thesis work a method to characterize the elasto plastic post necking behaviour of sheet metal materials, the Stepwise Modelling Method (SMM), is presented. The method uses full field measurements of the deformation field on the surface of tensile specimen. The hardening relation is modelled as a piecewise linear in a step by step procedure. The linear hardening parameter is adapted to reduce the residual between experimental and calculated tensile forces. The SMM is used to characterize the post necking behaviour of a ferritic boron steel and the results are compared with the commonly used inverse modelling method. It is shown that the stepwise modelling method characterizes the true stress, true plastic strain relation in an effective and computational efficient way. Furthermore, the SMM is used to characterize the stress state evolution during tensile testing, which is an important factor for failure and fracture modelling. This method is shown in an aerospace application for the nickel based super alloy Alloy 718. 

The results shows that the stepwise modelling method is an effective and efficient alternative method to characterize the deformation and failure of sheet metals. Based on the results of this method plasticity and fracture models can be characterized in future work.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2017.
Series
Licentiate thesis / Luleå University of Technology, ISSN 1402-1757
National Category
Materials Engineering Mechanical Engineering Applied Mechanics
Research subject
Solid Mechanics
Identifiers
URN: urn:nbn:se:ltu:diva-62477ISBN: 978-91-7583-845-8 (print)ISBN: 978-91-7583-846-5 (electronic)OAI: oai:DiVA.org:ltu-62477DiVA: diva2:1081257
Presentation
2017-05-12, E231, Luleå University of Technology, Luleå, 10:00 (English)
Available from: 2017-03-15 Created: 2017-03-13 Last updated: 2017-11-24Bibliographically approved
List of papers
1. Post necking characterisation for sheet metal materials using full field measurement
Open this publication in new window or tab >>Post necking characterisation for sheet metal materials using full field measurement
2016 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 238, 315-324 p.Article in journal (Refereed) Published
Abstract [en]

precise prediction of the post-necking behaviour of materials is needed to increase the precision of computer simulations with large deformations. Applications in which this need is encountered include crash, forming, and failure simulations. By using an optical full-field measurement of the localised deformation field, an effective and computationally fast method is presented to determine the relationship between true stress and true plastic strain, including post-necking behaviour. The presented stepwise modelling method is used to characterise heat-treated boron steel using thin sheet metal specimens. These results are validated with the results determined by a method based on inverse modelling. It can be concluded that the stepwise modelling method is considerably faster than the compared inverse modelling method. The method is also suitable for effectively determining element size dependency due to regularisation of the hardening behaviour needed for finite element analysis with strain localisation, e.g., for crash simulations

National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-7653 (URN)10.1016/j.jmatprotec.2016.07.036 (DOI)000383291500035 ()60ea60e7-bbdb-410a-ad81-db321eb80b2c (Local ID)60ea60e7-bbdb-410a-ad81-db321eb80b2c (Archive number)60ea60e7-bbdb-410a-ad81-db321eb80b2c (OAI)
Note

Validerad; 2016; Nivå 2; 20160816 (andbra)

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2017-11-24Bibliographically approved
2. A comparison between Stepwise Modelling and Inverse Modelling methods for characterization of press hardened sheet metals
Open this publication in new window or tab >>A comparison between Stepwise Modelling and Inverse Modelling methods for characterization of press hardened sheet metals
2017 (English)In: 6th International Conference Hot Sheet Metal Forming of High-Performance Steel CHS2: June 4-7 2017, Atlanta, Georgia, USA : proceedings / [ed] Mats Oldenburg, Braham Prakash, Kurt Steinhoff, Warrendale, PA: Association for Iron & Steel Technology, AIST , 2017, 281-288 p.Conference paper, Published paper (Refereed)
Abstract [en]

The demand for weight reduction of cars has increased the number of press hardened sheet metal parts used in the automotive industry. This leads to an increased demand on the precision of simulations of press hardened sheet metals. An accurate prediction of the post-necking behaviour of materials is therefore needed to increase the precision of computer simulations with large deformations, as for example in forming simulations and crash simulations. Especially fracture simulations of press hardened steel parts with tailored properties have a huge demand on precise material models.

Inverse modelling is a common engineering tool to characterize the elasto-plastic behaviour of materials.  Taking experimental data, such as force and displacement data, the material model parameters are optimised until the simulated output reaches a target function.  Then inverse modelling is highly time demanding and needs nonlinear hardening material models. 

Lately a new fast method for post necking characterisation of sheet metals, called the Stepwise Modelling Method (SMM), was presented. This method uses full field measurements to obtain the strain field on the surface of sheet metal tensile specimens.  Furthermore, the stepwise modelling method models an experimental hardening curve in a stepwise process.  This hardening curve is a piecewise linear curve and not restricted to any specific material model.

In this paper SMM is used to characterize the hardening behaviour for thermally treated boron steel.  These results are compared with the results of inverse modelling. Three different material models are used. The comparison shows a minor deviation in the resulting hardening relations between stepwise modelling and inverse modelling. Since the efficiency is an important factor in product development calculation times are taken into account.  Comparing calculation time using SMM is considerably more efficient than using inverse modelling. Furthermore another advantage of SMM is shown in the fact that the piecewise linear hardening curves can be fitted to almost any material model without computational costs.

Place, publisher, year, edition, pages
Warrendale, PA: Association for Iron & Steel Technology, AIST, 2017
Series
CHS2-series, 6
Keyword
Stepwise modelling; Inverse modelling; Material characterisation; Boron steel; Press hardening; Material modelling
National Category
Manufacturing, Surface and Joining Technology Vehicle Engineering Metallurgy and Metallic Materials Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-62459 (URN)978-1-935117-66-7 (ISBN)
Conference
6th International Conference Hot Sheet Metal Forming of High-Performance Steel CHS2, Atlanta, Georgia, 4-7 June 2017
Available from: 2017-03-13 Created: 2017-03-13 Last updated: 2017-11-24Bibliographically approved
3. Experimental Characterization of Triaxiality Stress State Evolution for Sheet Metal Materials
Open this publication in new window or tab >>Experimental Characterization of Triaxiality Stress State Evolution for Sheet Metal Materials
(English)Manuscript (preprint) (Other academic)
National Category
Applied Mechanics
Research subject
Solid Mechanics
Identifiers
urn:nbn:se:ltu:diva-62474 (URN)
Available from: 2017-03-13 Created: 2017-03-13 Last updated: 2017-11-24

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