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The effect of stress state in ductile failure
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The industrial application of high strength steels in structural components has increased the demand on understanding the ductile failure behavior of this type of materials. In practical situations the loading experienced on components made out of these materials can be very complex, which may affect the failure behavior.

The objective of this work is to study the effect of stress state on ductile failure and the mechanisms leading to rupture in high strength steels. The stress state is characterized by the stress triaxiality T and the Lode parameter L, which is a deviatoric stress state parameter that discriminates between axisymmetric or shear dominated stress states. For this purpose experiments on two different specimen configurations are performed; a double notched tube (DNT) specimen tested in combined tension and shear and a round notched bar (RNB) specimen tested in uniaxial tension. The two specimens give rise to different stress states at failure in terms of T and L. The failure loci for the DNT specimen show an abrupt change in ductility, indicating a transition between the rupture mechanisms necking of intervoid ligaments and shearing of intervoid ligaments. A clear difference in ductility between the two specimen configurations is also observed, which is closely associated with the difference in stress state at failure.

A micromechanical model is developed, which assumes that ductile material failure occurs when the deformation becomes highly non-linear and localizes into a band. The model, which is applied to analyze the experiments, consists of a band with a square array of equally sized cells, with a spherical void located in the center of each cell. The model, extended with a shear criterion, captures the experimental trend rather well. The model also shows that the effect of the deviatoric stress state (L) on void growth, void shape evolution and coalescence is significant, especially at low levels of T and shear dominated stress state.

Place, publisher, year, edition, pages
Stockholm: KTH , 2008. , 30 p.
Series
Trita-HFL. Report / Royal Institute of Technology, Solid mechanics, ISSN 1654-1472 ; 0443
Keyword [en]
Ductile failure; Rupture mechanisms; Stress triaxiality; Lode parameter; Finite element analysis; Experiments; Localization; Micromechanics; Void growth; Void coalescence
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-4667OAI: oai:DiVA.org:kth-4667DiVA: diva2:13333
Public defence
2008-04-15, F3, KTH, Linstedtsvägen 26, Stockholm, 10:15
Opponent
Supervisors
Note

QC 20100621

Available from: 2000-03-10 Created: 2000-03-10 Last updated: 2013-01-14Bibliographically approved
List of papers
1. Rupture mechanisms in combined tension and shear - Experiments
Open this publication in new window or tab >>Rupture mechanisms in combined tension and shear - Experiments
2007 (English)In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 44, no 6, 1768-1786 p.Article in journal (Refereed) Published
Abstract [en]

An experimental investigation of the rupture mechanisms in a mid-strength and a high-strength steel were conducted employing a novel test configuration. The specimen used was a double notched tube specimen loaded in combined tension and torsion at a fixed ratio. The effective plastic strain, the stress triaxiality and the Lode parameter were determined in the centre of the notch at failure. Scanning electron microscopy of the fractured surfaces revealed two distinctively different ductile rupture mechanisms depending on the stress state. At high stress triaxiality the fractured surfaces were covered with large and deep dimples, suggesting that growth and internal necking of voids being the governing rupture mechanism. At low triaxiality it was found that the fractured surfaces were covered with elongated small shear dimples, suggesting internal void shearing being the governing rupture mechanism. In the fractured surfaces of the high-strength steel, regions with quasi-cleavage were also observed. The transition from the internal necking mechanism to the internal shearing mechanism was accompanied by a significant drop in ductility.

Keyword
ductile fracture, rupture mechanisms, experiments, tension, shear
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-8099 (URN)10.1016/j.ijsolstr.2006.09.031 (DOI)000244576400008 ()2-s2.0-33846486175 (Scopus ID)
Note
Conference: Symposium on Physics and Mechanics of Advanced Materials. Inst Mat Res & Engn, Singapore, SINGAPORE. JAN, 2007 Available from: 2000-03-10 Created: 2000-03-10 Last updated: 2011-09-13Bibliographically approved
2. Rupture mechanisms in combined tension and shear - Micromechanics
Open this publication in new window or tab >>Rupture mechanisms in combined tension and shear - Micromechanics
2007 (English)In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 44, no 17, 5481-5498 p.Article in journal (Refereed) Published
Abstract [en]

A micromechanics model based on the theoretical framework of plastic localization into a band introduced by Rice is developed. The model consists of a planar band with a square array of equally sized cells, with a spherical void located in the centre of each cell. The periodic arrangement of the cells allows the study of a single unit cell for which fully periodic boundary conditions are applied. The micromechanics model is applied to analyze failure by ductile rupture in experiments on double notched tube specimens subjected to combined tension and torsion carried out by the present authors. The stress state is characterized in terms of the stress triaxiality and the Lode parameter. Two rupture mechanisms can be identified, void coalescence by internal necking at high triaxiality and void coalescence by internal shearing at low triaxiality. For the internal necking mechanism, failure is assumed to occur when the deformation localizes into a planar band and is closely associated with extensive void growth until impingement of voids. For the internal shearing mechanism, a simple criterion based on the attainment of a critical value of shear deformation is utilized. The two failure criteria capture the transition between the two rupture mechanisms successfully and are in good agreement with the experimental result.

Keyword
micromechanics; mixed mode ductile fracture; rupture mechanisms; void coalescence
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-8100 (URN)10.1016/j.ijsolstr.2007.01.010 (DOI)000248879300005 ()2-s2.0-34447284814 (Scopus ID)
Note
QC 20100621Available from: 2000-03-10 Created: 2000-03-10 Last updated: 2010-07-02Bibliographically approved
3. Micromechanical analysis on the influence of the Lode parameter on void growth and coalescence
Open this publication in new window or tab >>Micromechanical analysis on the influence of the Lode parameter on void growth and coalescence
2008 (English)Report (Other academic)
Place, publisher, year, edition, pages
Stockholm: KTH, 2008
Series
Royal Institute of Technology, Department of Solid Mechanics, 445
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-8101 (URN)
Note
QC 20100621Available from: 2000-03-10 Created: 2000-03-10 Last updated: 2010-07-02Bibliographically approved
4. Influence of the Lode parameter on ductility in the moderate stress triaxiality regime
Open this publication in new window or tab >>Influence of the Lode parameter on ductility in the moderate stress triaxiality regime
2008 (English)Report (Other academic)
Place, publisher, year, edition, pages
Stockholm: KTH, 2008
Series
Royal Institute of Technology, Department of Solid Mechanics, 446
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-8102 (URN)
Note
QC 20100621Available from: 2000-03-10 Created: 2000-03-10 Last updated: 2010-07-02Bibliographically approved

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