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Ductile failure and rupture mechanisms in combined tension and shear
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
2006 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

This licentiate thesis is generally concerned with the ductile failure and rupture mechanisms encountered under combined tension and torsion loading. In the first part entitled Paper A, an experimental investigation of the rupture mechanisms in a mid-strength and a high strength steel was 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 was 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.

In the second part entitled Paper B, a micromechanics model based on the theoretical framework of plastic localization into a band introduced by Rice is developed. The model employed 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. 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.

Place, publisher, year, edition, pages
Stockholm: Hållfasthetslära , 2006. , 12 p.
Series
Trita-HFL. Report / Royal Institute of Technology, Solid Mechanics, ISSN 1654-1472 ; 0407
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-4064OAI: oai:DiVA.org:kth-4064DiVA: diva2:10603
Presentation
2006-06-15, Sal E3, KTH, Osquars backe 4, Stockholm, 10:15
Opponent
Supervisors
Note

QC 20101109

Available from: 2006-06-27 Created: 2006-06-27 Last updated: 2013-01-15Bibliographically 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: 2017-12-14Bibliographically 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: 2017-12-14Bibliographically approved

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