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Ductile Failure in High Strength Steel Sheets
Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, The Institute of Technology.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Developments in computer-aided engineering and the rapid growth of computational power have made simulation-driven process and product development efficient and useful since it enables detailed evaluation of product designs and their manufacturing processes. In the context of a sheet metal component, it is vital to predict possible failure both during its forming process and its subsequent usage. Accurate numerical models are needed in order to obtain trustworthy simulation results. Furthermore, the increasing demands imposed on improved weight-to-performance ratio for many products endorse the use of high-strength steels. These steels often show anisotropic behaviour and more complex hardening and fracturing compared to conventional steels. Consequently, demand for research on material and failure models suitable for these steels has increased.

In this work, the mechanical and fracture behaviour of two high-strength steels, Docol 600DP and Docol 1200M, have been studied under various deformation processes. Experimental results have been used both for material characterisation and for calibration of fracture criteria. One major requirement as concerns the fracture criteria studied is that they should be simple to apply in industrial applications, i.e. it should be possible to easily calibrate the fracture criteria in simple mechanical experiments and they should be efficient and accurate. Consequently, un-coupled phenomenological damage models have been the main focus throughout this work.

Detailed finite element models including accurate constitutive laws have be used to predict and capture material instabilities. Most of the fracture criteria studied are modifications of the plastic work to fracture. Ductile tensile and ductile shear types of fracture are of particular interest in sheet metal applications. For these fractures the modification of the plastic work relates to void coalescence and void collapse, respectively. Anisotropy in fracture behaviour can be captured by the introduction of a material directional function.

The dissertation consists of two parts. The first part contains theory and background. The second consists of five papers.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. , 60 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1579
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-105213DOI: 10.3384/diss.diva-105213ISBN: 978-91-7519-389-2 (print)OAI: oai:DiVA.org:liu-105213DiVA: diva2:704817
Public defence
2014-04-11, C3, Hus C, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2014-05-27Bibliographically 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, E-ISSN 1873-4774, 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: 2017-12-12
2. Failure of high strength steel sheets: Experiments and modelling
Open this publication in new window or tab >>Failure of high strength steel sheets: Experiments and modelling
2013 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 213, no 7, 1103-1117 p.Article in journal (Refereed) Published
Abstract [en]

Failure in sheet metal structures of ductile material is usually caused by one of, or a combination of, ductile fracture, shear fracture or localised instability. In this paper the failure of the high strength steel Docol 600DP and the ultra high strength steel Docol 1200M is explored. The constitutive model used in this study includes plastic anisotropy and mixed isotropic-kinematic hardening. For modelling of the ductile and shear fracture the models presented by Cockroft–Latham and Bressan–Williams have been used. The instability phenomenon is described by the constitutive law and the finite element (FE) models. For calibration of the failure models and validation of the results, an extensive experimental series has been conducted including shear tests, plane strain tests and Nakajima tests. The geometries of the Nakajima tests have been chosen so that the first quadrant of the forming limit diagram (FLD) were covered. The results are presented both in an FLD and using prediction of force–displacement response of the Nakajima test employing element erosion during the FE simulations. The classical approach for failure prediction is to compare the principal plastic strains obtained from FE simulations with experimental determined forming limit curves (FLCs). It is well known that the experimental FLC requires proportional strains to be useful. In this work failure criteria, both of the instability and fracture, are proposed which can be used also for non-proportional strain paths.

Keyword
Sheet metal failure, high strength steels, forming limits, ductile fracture, shear fracture
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-77758 (URN)10.1016/j.jmatprotec.2013.01.027 (DOI)000318325100011 ()
Available from: 2012-05-28 Created: 2012-05-28 Last updated: 2017-12-07Bibliographically approved
3. Failure characteristics of a dual-phase steel sheet
Open this publication in new window or tab >>Failure characteristics of a dual-phase steel sheet
2014 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 214, 1190-1204 p.Article in journal (Refereed) Published
Abstract [en]

Failure in ductile sheet metal structures is usually caused by one, or a combination of, ductile tensile fractures, ductile shear fractures or localised instability. In this paper the failure characteristics of the high strength steel Docol 600DP are explored. The study includes both experimental and numerical sections. In the experimental sections, the fracture surface of the sheet subjected to Nakajima tests is studied under the microscope with the aim of finding which failure mechanism causes the fracture. In the numerical sections, finite element (FE) simulations have been conducted using solid elements. From these simulations, local stresses and strains have been extracted and analysed with the aim of identifying the fracture dependency of the stress triaxiality and Lode parameter.

Place, publisher, year, edition, pages
Elsevier, 2014
Keyword
Sheet metal failure; Forming limit; Instability; Ductile tensile fracture; Ductile shear fracture
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-105209 (URN)10.1016/j.jmatprotec.2014.01.004 (DOI)000334006600003 ()
Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2017-12-05Bibliographically approved
4. Prediction of fracture in a dual-phase steel subjected to non-linear straining
Open this publication in new window or tab >>Prediction of fracture in a dual-phase steel subjected to non-linear straining
2014 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 214, no 11, 2748-2758 p.Article in journal (Refereed) Published
Abstract [en]

In this work, selected fracture criteria are applied to predict the fracture of dualphase steel subjected to non-linear strain paths. Furthermore, the effects of manufacturing history are studied. Four fracture criteria were calibrated in three tests using standard specimens. The fracture criteria were first validated in the circular Nakajima test. A second validation test case was included in order to validate fracture prediction for non-linear strain paths. In this test a sheet metal component was manufactured and subsequently stretched until it fractured. All fracture criteria included in this study predict fracture during the Nakajima test with reasonable accuracy. In the second validation test however, the different fracture criteria show considerable diversity in accumulated damage during manufacturing which caused substantial scatter of the fracture prediction in the subsequent stretching. This shows that manufacturing history influences the prediction of fracture.

Place, publisher, year, edition, pages
Elsevier, 2014
Keyword
Sheet metal failure, high strength steels, forming limits, non-linear strain paths, forming history
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:liu:diva-105211 (URN)10.1016/j.jmatprotec.2014.05.028 (DOI)000340300400059 ()
Note

Funders: SSF ProViking project entitled "SuperLight Steel Structures"

Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2017-12-05
5. Anisotropic fracture criteria for a dual-phase steel
Open this publication in new window or tab >>Anisotropic fracture criteria for a dual-phase steel
2014 (English)Manuscript (preprint) (Other academic)
Abstract [en]

The main objective of this work is to examine the use of anisotropic fracture criteria in order to predict fracture in dual-phase steel. The introduction of a material directional function into the fracture criterion was used in order to account for anisotropy observed in experiments. Selected fracture criteria were fist calibrated by ordinary tensile and in-plane shear tests using specimens cut in three material directions. In order to validate the performance, two types of validation tests were conducted. First, plane strain (notched tensile) tests were carried out in three material directions. Second, Nakajima tests with a waist of 130 mm were conducted, also in three material directions. The fit to the calibration tests was improved with all material directional functions compared to the isotropic criterion. Overall best performance was achieved when a material direction function based on the structural tensors was introduced.

Keyword
Sheet metal failure, anisotropy, ductile tensile fracture, ductile shear fracture
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-105212 (URN)
Available from: 2014-03-13 Created: 2014-03-13 Last updated: 2014-03-13Bibliographically approved

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