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On Material Modelling of High Strength Steel Sheets
Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, The Institute of Technology.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The work done in this thesis aims at developing and improving material models for use in industrial applications.

The mechanical behaviour of three advanced high strength steel grades, Docol 600DP, Docol 1200M and HyTens 1000, has been experimentally investigated under various types of deformation, and material models of their behaviour have been developed. The origins of all these material models are experimental findings from physical tests on the materials.

Sheet metal forming is an important industrial process and is used to produce a wide range of products. The continuously increasing demand on the weight to performance ratio of many products promotes the use of advanced high strength steel. In order to take full advantage of such steel, most product development is done by means of computer aided engineering, CAE. In advanced product development, the use of simulation based design, SBD, is continuously increasing. With SBD, the functionality of a product, as well as its manufacturing process, can be analysed and optimised with a minimum of physical prototype testing. Accurate numerical tools are absolutely necessary with this methodology, and the model of the material behaviour is one important aspect of such tools.

This thesis consists of an introduction followed by five appended papers. In the first paper, the dual phase Docol 600DP steel and the martensitic Docol 1200M steel were subjected to deformations, both under linear and non-linear strain paths. Plastic anisotropy and hardening were evaluated and modelled using both virgin materials, i.e. as received, and materials which were pre-strained in various material directions.

In the second paper, the austenitic stainless steel HyTens 1000 was subjected to deformations under various proportional strain paths and strain rates. It was experimentally shown that this material is sensitive both to dynamic and static strain ageing. A constitutive model accounting for these effects was developed, calibrated, implemented in a Finite Element software and, finally, validated on physical test data.

The third paper concerns the material dispersions in batches of Docol 600DP. A material model was calibrated to a number of material batches of the same steel grade. The paper provides a statistical analysis of the resulting material parameters.

The fourth paper deals with a simple modelling of distortional hardening. This type of hardening is able to represent the variation of plastic anisotropy during deformation. This is not the case with a regular isotropic hardening, where the anisotropy is fixed during deformation.

The strain rate effect is an important phenomenon, which often needs to be considered in a material model. In the fifth paper, the strain rate effects in Docol 600DP are investigated and modelled. Furthermore, the strain rate effect on strain localisation is discussed.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2012. , 59 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1478
Keyword [en]
material modelling
National Category
Applied Mechanics
Identifiers
URN: urn:nbn:se:liu:diva-80115ISBN: 978-91-7519-791-3 (print)OAI: oai:DiVA.org:liu-80115DiVA: diva2:545669
Public defence
2012-10-26, C3, Hus C, Campus Valla, Linköpings Universitet, Linköping, 10:15 (Swedish)
Opponent
Supervisors
Projects
SFS ProViking Super Light Steel Structures
Available from: 2012-09-25 Created: 2012-08-21 Last updated: 2012-09-25Bibliographically approved
List of papers
1. A study of high strength steels undergoing non-linear strain paths—Experiments and modelling
Open this publication in new window or tab >>A study of high strength steels undergoing non-linear strain paths—Experiments and modelling
2011 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, Vol. 211, no 1, 121-131 p.Article in journal (Refereed) Published
Abstract [en]

This paper presents an evaluation of the constitutive behaviour, including plastic anisotropy and mixed isotropic-kinematic hardening of two high strength steels, Docol 600DP and Docol 1200M, during strain path changes. A series of tensile and shear tests was performed on both virgin and pre-strained materials. The initial anisotropy and work hardening parameters were obtained from tensile tests, shear tests and a bulge test of the virgin material, whereas the kinematic hardening parameters were identified by comparing numerical predictions to experimental results related to the pre-strained materials. Numerical predictions using the obtained parameters agree well with the experimental results, both in the case of proportional, and under non-proportional strain paths.

Place, publisher, year, edition, pages
Elsevier, 2011
Keyword
Non-linear strain paths, Plastic anisotropy, Mixed isotropic-kinematic hardening, Shear test
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-60772 (URN)10.1016/j.jmatprotec.2010.09.004 (DOI)
Available from: 2010-12-01 Created: 2010-10-26 Last updated: 2014-03-13
2. On the modelling of strain ageing in a metastable austenitic stainless steel
Open this publication in new window or tab >>On the modelling of strain ageing in a metastable austenitic stainless steel
2012 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 212, no 1, 46-58 p.Article in journal (Refereed) Published
Abstract [en]

The plastic hardening of metastable austenitic stainless steel is partly governed by martensitic transformation, the occurrence of serrated plastic flow, and plastic strain ageing phenomena. In this paper an elasto-viscoplastic material model with isotropic distortional plastic hardening is developed. The model accounts for static and dynamic strain ageing as well as the martensitic transformation. An experimental programme has been conducted in order to fit the model parameters to an austenitic stainless steel within the EN 1.4310 standard. The identification of the dynamic strain ageing was based on so called jump tests, where a sudden strain rate increase was shown to result in an instantaneous positive strain rate sensitivity followed by negative steady state strain rate sensitivity. Furthermore, the static strain ageing was identified by unloading tensile test specimens at specified plastic strains and then reloading these specimens after different periods of time. The observed material behaviour in the test situations can be predicted by the developed model. Lastly, the model was validated by predicting the force-displacement relation of the material in a shear test: the prediction agrees well with experimental results.

Place, publisher, year, edition, pages
Elsevier, 2012
Keyword
Plastic anisotropy; Isotropic-distortional hardening; Portevin-Le Chatelier effect; Dynamic strain ageing; Static strain ageing; Martensitic transformation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-73731 (URN)10.1016/j.jmatprotec.2011.08.003 (DOI)000297893000008 ()
Available from: 2012-01-12 Created: 2012-01-12 Last updated: 2017-12-08
3. An evaluation of the statistics of steel material model parameters
Open this publication in new window or tab >>An evaluation of the statistics of steel material model parameters
2012 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 212, no 6, 1288-1297 p.Article in journal (Refereed) Published
Abstract [en]

In robustness studies, variations of material properties are often represented by simple assumptions, such as scaling of stress-strain relations, often due to lack of knowledge or deeper understanding of the material physics and the material model applied. By performing material characterisation tests on several batches of a DP600 steel and fitting a phenomenological material model to each batch, this paper studies the dispersion of material model parameters, as well as correlations between both experimental and model parameters. It is concluded that some of the charcterisation tests may be omitted in the future, due to correlations found between parameters. The results may also be applied in a robustness study by inversely using the retrieved statistics to generate reasonable new sets of material model parameters. The methodology presented may be adopted for any other type of material characterisation process.

Place, publisher, year, edition, pages
Elsevier, 2012
Keyword
Robustness studies, material model parameter variations, dual phase steel
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-70193 (URN)10.1016/j.jmatprotec.2012.01.016 (DOI)000303176900008 ()
Note
funding agencies|SFS ProViking project Super Light Steel Structures||Available from: 2011-08-24 Created: 2011-08-24 Last updated: 2017-12-08Bibliographically approved
4. On isotropic-distortional hardening
Open this publication in new window or tab >>On isotropic-distortional hardening
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Isotropic-distortional hardening allows for individual stress-plastic strain relations in different straining directions. Such hardening can be obtained by allowing the parameters in the effective stress function to depend on anisotropy functions of the equivalent plastic strain. A methodology to calibrate these anisotropy functions is proposed in this work, and is demonstrated on an austenitic strainless steel. A high exponent eight parameter effective stress function for plane stress states is utilised. The anisotropy functions are calibrated by the use of experimental data from uniaxial tensile test data in three material directions and a balanced biaxial test. The plastic anisotropy is assumed to vary piecewise linearly with respect to the equivalent plastic strain, and it is evaluated at a finite number of plastic strains. Thus, it is correctly represented at each level of plastic strain included in the evaluation, even if large increments in plastic strain are used in the calibration. It was found that there are at least two sets of anisotropy functions which satisfy the conditions in the calibration procedure. The resulting uniaxial stress-strain relations from the two sets of anisotropy functions in four additional straining directions, not included in the calibration set, were compared to the corresponding experimental data. From this validation, one of the anisotropy function sets could be discarded, whereas the other one gave a good prediction of the stress-strain relations in all the four additional directions.

Keyword
Plastic anisotropy, distortional hardening
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-81920 (URN)
Available from: 2012-09-25 Created: 2012-09-25 Last updated: 2012-09-25Bibliographically approved
5. Strain rate effects in a high strength dual phase steel
Open this publication in new window or tab >>Strain rate effects in a high strength dual phase steel
(English)Manuscript (preprint) (Other academic)
Abstract [en]

This work focuses on strain rate effects in a high strength dual phase steel, DP600. Attention is given to the mathematical formulation of the strain rate sensitivity, and to its influence on the strain at localisation. Both an experimental and a numerical study are presented. In the experimental study, uniaxial and notched tensile tests, as well as shear tests were conducted at various loading rates. Furthermore, strain rate jump tests were conducted in order to further investigate the strain rate sensitivity. It is clearly shown that the material has a positive strain rate sensitivity, and that a multiplicative contribution to the flow stress is a better approximation than an additive one. A material model including a non-linear plastic hardening, a high exponent yield surface, a thermal softening effect and a variety of multiplicative strain rate sensitivity functions is presented and calibrated. This material model is used in finite element simulations of the mechanical tests. The predicted results are in good agreement with the experimental findings. The stabilising effect of the strain rate sensitivity is shown to be important whenever strain localisation becomes an issue in finite element simulations. It is also shown that the strain rate sensitivity index depends on the plastic strain rate, which thus is the case also for the stabilising effect. From a supplementary study, it is concluded that the assumed thermal softening will affect the onset of localisation.

Keyword
Strain rate, dual phase steel, plastic instability, notched tensile test, shear test
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-81921 (URN)
Available from: 2012-09-25 Created: 2012-09-25 Last updated: 2012-09-25Bibliographically approved

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