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Micromechanical modeling of cleavage fracture in polycrystalline materials
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Cleavage fracture in ferritic steels can be defined as a sequence of few critical steps. At first nucleation of a microcrack takes place, often in a hard inclusion. This microcrack then propagates into the surrounding matrix material. The last obstacle before failure is the encounter of grain boundaries. If a microcrack is not arrested during any of those steps, cleavage takes place. Temperature plays an important role since it changes the failure mode from ductile to brittle in a narrow temperature interval.

In papers A and B micromechanical models of the last critical phase are developed (cleavage over a grain boundary) in order to examine the mechanics of this phase. An extensive parameter study is performed in Paper A, where cleavage planes of two grains are allowed to tilt relative each other. It is there shown that triaxiality has a significant effect on the largest grain size that can arrest a rapidly propagating microcrack. This effect is explained by the development of the plastic zone prior to crack growth. The effect of temperature, addressed through a change in the visco-plastic response of the ferrite, shows that the critical grain size increases with the temperature. This implies that with an increasing temperature more cracks can be arrested, that is to say that less can become critical and thus that the resistance to fracture increases.

Paper B shows simulations of microcrack propagation when the cleavage planes of two neighboring grains are tilted and twisted relatively each other. It is shown that when a microcrack enters a new grain, it first does it along primary cleavage planes. During further growth the crack front is protruded along the primary planes and lags behind along the secondary ones. The effect of tilt and twist on the critical grain size is decoupled with twist misorientation offering a greater resistance to propagation.

Simulations of cracking of a particle and microcrack growth across an inclusion-matrix interface are made in Paper C. It is shown that the particle stress can be expressed by an Eshelby type expression modified for plasticity. The analysis of dynamic growth, results in a modified Griffith expression. Both findings are implemented into a micromechanics-based probabilistic model for cleavage that is of a weakest link type and incorporates all critical phases of cleavage: crack nucleation, propagation over particle-matrix interface and into consecutive grains.

The proposed model depends on six parameters, which are obtained for three temperatures in Paper D using experimental data from SE(B) tests. At the lowest temperature, -30° , the model gives an excellent prediction of the cumulative failure probability by cleavage fracture and captures the threshold toughness and the experimental scatter. At 25º  and 55º  the model slightly overestimates the fracture probability.

In Paper E a serie of fracture experiments is performed on half-elliptical surface cracks at 25º in order to further verify the model. Experiments show a significant scatter in the fracture toughness. The model significantly overestimates the fracture probability for this crack geometry.

Place, publisher, year, edition, pages
Stockholm: KTH , 2008. , ix, 31 p.
Series
Trita-HFL. Report / Royal Institute of Technology, Solid mechanics, ISSN 1654-1472 ; 0469
Keyword [en]
cleavage fracture, probabilistic modeling, brittle to ductile transition, cohesive zone, visco-plastic material
National Category
Mechanical Engineering Other Engineering and Technologies not elsewhere specified
Identifiers
URN: urn:nbn:se:kth:diva-9773OAI: oai:DiVA.org:kth-9773DiVA: diva2:128023
Public defence
2008-12-19, Sal D2, KTH, Lindstedtsvägen 5, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20100910

Available from: 2009-06-23 Created: 2008-12-11 Last updated: 2013-01-14Bibliographically approved
List of papers
1. Influence of grain size on arrest of a dynamically propagating cleavage crack in ferritic steels-micromechanics
Open this publication in new window or tab >>Influence of grain size on arrest of a dynamically propagating cleavage crack in ferritic steels-micromechanics
2009 (English)In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 158, no 1, 51-71 p.Article in journal (Refereed) Published
Abstract [en]

Cleavage fracture in ferritic steels is controlled by several critical steps. First a microcrack must nucleate, grow and overcome barriers, such as grain boundaries. The latter is examined here by use of a periodic, axisymmetric model representing two grains. A microcrack nucleated at the center in one grain is driven by a constant remotely applied stress towards the second grain. The cleavage planes of the grain in which the microcrack is nucleated coincide with the principal loading direction. In the adjacent grain, due to misalignment in possible cleavage planes, the propagation direction changes and separation occurs in mixed mode, involving both normal and shear separations. The temperature dependence of the mechanical properties of the material is accounted for by use of a temperature dependent elasto viscoplastic material model. The largest grain size that can arrest a rapidly propagating microcrack is defined as the critical grain size. The effects of stress state and temperature on the critical grain size are examined. The influence of mismatch in lattice orientation between two adjacent grains in terms of a tilt angle is both qualitatively and quantitatively described. It is shown that the critical grain size is influenced by plastic geometry change and prestraining, which depend on the applied stress state. The results also show that a microcrack can be arrested in an adjacent grain under specific conditions.

Keyword
Dynamic fracture, Cleavage fracture, Grain boundary, Crack arrest, Elasto viscoplastic material, low-alloy steel, fracture initiation, cohesive elements, growth-resistance, plastic solids, temperature, boundaries, strength, shear, iron
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-18672 (URN)10.1007/s10704-009-9374-z (DOI)000268789400005 ()2-s2.0-67749105971 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2011-01-14Bibliographically approved
2. Micromechanical modeling of grain boundary resistance to cleavage crack propagation in ferritic steels
Open this publication in new window or tab >>Micromechanical modeling of grain boundary resistance to cleavage crack propagation in ferritic steels
2009 (English)In: International Journal of Fracture, ISSN 0376-9429, E-ISSN 1573-2673, Vol. 160, no 2, 151-167 p.Article in journal (Refereed) Published
Abstract [en]

In ferritic steels a propagating cleavage microcrack changes its propagation direction as it advances from grain to grain. This is due to differences in the orientation of the cleavage planes of two neighboring grains. In order to reach a cleavage plane in a new grain, a microcrack must first penetrate the grain boundary. Grain boundaries therefore act as natural barriers in cleavage fracture. The influence of a grain boundary and the associated misorientation in cleavage planes on crack arrest is here examined using a 3D finite element model with axisymmetric periodicity, representing two grains whose cleavage planes are tilted and twisted relative to each other. The temperature dependent mechanical properties of ferrite are modeled using a temperature dependent viscoplastic response. The development of the crack front as the microcrack penetrates through a grain boundary is here presented. The influence of the twist misorientation on the critical grain size, defined as the largest grain size that can arrest a rapidly propagating microcrack, is examined in a temperature range corresponding to the ductile to brittle transition (DBT) region. It is shown that when both tilt and twist misorientation are present, the influence of tilt and twist, respectively, on crack growth resistance can be decoupled.

Keyword
Dynamic fracture, Cleavage fracture, Grain boundary resistance, Crack, arrest, Elastic viscoplastic material, fracture initiation, cohesive elements, alloy, shear, iron, temperature, toughness, strength, region, strain
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-18960 (URN)10.1007/s10704-009-9415-7 (DOI)000271749500004 ()2-s2.0-75949121805 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2011-01-14Bibliographically approved
3. Micromechanics and probabilistic modeling of cleavage microcrack nucleation and growth caused by particle cracking
Open this publication in new window or tab >>Micromechanics and probabilistic modeling of cleavage microcrack nucleation and growth caused by particle cracking
2008 (English)In: Journal of the mechanics and physics of solids, ISSN 0022-5096Article in journal (Other academic) Submitted
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-24475 (URN)
Note
QS 20120314Available from: 2010-09-10 Created: 2010-09-10 Last updated: 2012-03-14Bibliographically approved
4. A micromechanics based probabilistic model for cleavage fracture: Application to SE(B) experiments
Open this publication in new window or tab >>A micromechanics based probabilistic model for cleavage fracture: Application to SE(B) experiments
2008 (English)In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315Article in journal (Other academic) Submitted
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-24477 (URN)
Note
QS 20120315Available from: 2010-09-10 Created: 2010-09-10 Last updated: 2012-03-15Bibliographically approved
5. Validation of two probabilistic cleavage fracture models for half elliptic surface crack
Open this publication in new window or tab >>Validation of two probabilistic cleavage fracture models for half elliptic surface crack
2008 (English)In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315Article in journal (Other academic) Submitted
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-24478 (URN)
Note
QS 20120315Available from: 2010-09-10 Created: 2010-09-10 Last updated: 2012-03-15Bibliographically approved

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