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Influence of grain size on arrest of a dynamically propagating cleavage crack in ferritic steels-micromechanics
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
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.

Place, publisher, year, edition, pages
2009. Vol. 158, no 1, 51-71 p.
Keyword [en]
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
URN: urn:nbn:se:kth:diva-18672DOI: 10.1007/s10704-009-9374-zISI: 000268789400005ScopusID: 2-s2.0-67749105971OAI: diva2:336719
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2011-01-14Bibliographically approved
In thesis
1. Micromechanical modeling of cleavage fracture in polycrystalline materials
Open this publication in new window or tab >>Micromechanical modeling of cleavage fracture in polycrystalline materials
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.
Trita-HFL. Report / Royal Institute of Technology, Solid mechanics, ISSN 1654-1472 ; 0469
cleavage fracture, probabilistic modeling, brittle to ductile transition, cohesive zone, visco-plastic material
National Category
Mechanical Engineering Other Engineering and Technologies not elsewhere specified
urn:nbn:se:kth:diva-9773 (URN)
Public defence
2008-12-19, Sal D2, KTH, Lindstedtsvägen 5, Stockholm, 10:15 (English)

QC 20100910

Available from: 2009-06-23 Created: 2008-12-11 Last updated: 2013-01-14Bibliographically approved

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