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dislocations

On structured surfaces with defects: geometry, strain incompatibility, internal stress, and natural shapes

Given a distribution of defects on a structured surface, such as those represented by 2-dimensional crystalline materials, liquid crystalline surfaces, and thin sandwiched shells, what is the resulting stress field and the deformed shape? Motivated by this concern, we first classify, and quantify, the translational, rotational, and metrical defects allowable over a broad class of structured surfaces. With an appropriate notion of strain, the defect densities are then shown to appear as sources of strain incompatibility.

Chiqun Zhang's picture

On the relevance of generalized disclinations in defect mechanics

Chiqun Zhang            Amit Acharya

The utility of the notion of generalized disclinations in materials science is discussed within the physical context of modeling interfacial and bulk line defects like defected grain and phase boundaries, dislocations and disclinations. The Burgers vector of a disclination dipole in linear elasticity is derived, clearly demonstrating the equivalence of its stress field to that of an edge dislocation. We also prove that the inverse deformation/displacement jump of a defect line is independent of the cut-surface when its g.disclination strength vanishes. An explicit formula for the displacement jump of a single localized composite defect line in terms of given g.disclination and dislocation strengths is deduced based on the Weingarten theorem for g.disclination theory at finite deformation. The Burgers vector of a g.disclination dipole at finite deformation is also derived.

mesarovic's picture

Short course: Mesoscale models: From micro-physics to macro-interpretation

Short course at Centre International de Sciences Mechanique (CISM), Udine.

May 22-26, 2017

Lecturers:  S Forest, I Groma, D McDowell, S Mesarovic, J-N Roux, H Zbib

The flyer is attached.  Register at:   http://www.cism.it/courses/C1703/ 

Summary 

Amit Acharya's picture

A microscopic continuum model for defect dynamics in metallic glasses

Amit Acharya                       Michael Widom

To appear in Journal of the Mechanics and Physics of Solids

Motivated by results of the topological theory of glasses accounting for geometric frustration,
we develop the simplest possible continuum mechanical model of defect dynamics in metallic
glasses that accounts for topological, energetic, and kinetic ideas. A geometrical description
of ingredients of the structure of metallic glasses using the concept of local order based on
Frank-Kasper phases and the notion of disclinations as topological defects in these structures is
proposed. This novel kinematics is incorporated in a continuum mechanical framework capable
of describing the interactions of disclinations and also of dislocations (interpreted as pairs of
opposite disclinations). The model is aimed towards the development of a microscopic understanding
of the plasticity of such materials. We discuss the expected predictive capabilities of
the model vis-a-vis some observed physical behaviors of metallic glasses.

Cai Wei's picture

Postdoctoral Position Opening at Stanford University

Prof. Wei Cai at the Mechanical Engineering Department of Stanford University is seeking a postdoctoral researcher to lead a project on the modeling of grain structure evolution in the surface layer under friction.  The major task of this project is the development of a phase field model for grain/sub-grain structure evolution under friction.  It is likely that molecular dynamics and dislocation dynamics simulations would be required as well to obtain a good physical understanding of the process.

Rouzbeh1's picture

Two Post-doctoral Associate Positions Available at Rice University

Position Description: Two postdoctoral associate positions, one in computations and one in experiments, are available at Rice University. The first position is in the area of atomistic and molecular modeling of nanoscale defects, dislocations and mechanics of complex, low symmetry compounds. The computational techniques will be integrated to approaches from statistical mechanics and physical chemistry to provide quantitative predictions of properties of complex materials across different time- and length-scales. 

Tuncay Yalcinkaya's picture

Full scholarships available at European Commission funded workshop on physics based material models in Izmir / Turkey

European Commission' s JRC (Joint Research Centre) is organizing the 3rd workshop on physics based material models and experimental observations (http://iwpmeo.org/) in collaboration with University of Oxford, Max-Planck-Institut für Eisenforschung and Middle East Technical University. The workshop will be held in Izmir/Turkey on 2-4 June 2014.

Amit Acharya's picture

Continuum mechanics of the interaction of phase boundaries and dislocations in solids

Amit Acharya         Claude Fressengeas

Springer Proceedings in Mathematics and Statistics on Differential Geometry and Continuum Mechanics, Vol. 137, pages 123-165. Ed: G. Q Chen, M. Grinfeld, R.J. Knops (Proceedings of  Workshop held at the Intl. Centre for Mathematical Sciences in Edinburgh, 2013.)

dabiao liu's picture

search for Petch's classic paper entitled "The cleavage strength of polycrystals"

Recently, I am reading several classic works on crystal plasticity. But, it seems to be difficult for me to obtain several very old papers in this area. For example, one of the classic papers on Hall-Petch relation,

Petch, N.J., The cleavage strength of polycrystals. The Iron and Steel Institute, 1953, 174: 25-28. I have tried many ways to get the original paper, but I finally failed. Could someone please send me a copy of this paper? Thanks a lot. Here is my email: dbliu2009@gmail.com 

Post-Doctoral Opening on Multiscale Modeling of Materials

We are seeking to hire outstanding candidates for a post-doctoral research position in

the Koslowski’s group at Purdue University to work on dislocation dynamics and/or

continuum modeling of plasticity. The project involves developing and applying

numerical methods to study creep and plastic deformation in nano and ultrafine

crystalline materials. Successful candidates will have the opportunity to interact with a

wide range of experimentalists and theoreticians in academia, industry and national labs.

Rouzbeh1's picture

Post-doc Position in Atomistic Modeling at Rice University

A postdoctoral associate position is available at Rice University in the
area of atomistic and molecular modeling of nanoscale friction and defects in an assembly of
nanoparticles. Computational techniques will be integrated to approaches from statistical
mechanics and physical chemistry to provide quantitative predictions of properties of complex
oxide materials across different time- and length-scales. The research will be carried out at Rice
University's Multiscale Materials Modeling Lab.

Royalty Free Illustrations

Does anyone know of a good source for free science and engineering illustrations? I'm particularly interested in crystallography, crystal defects/dislocations, and mechanical properties illustrations.

Thanks 

Amit Acharya's picture

An observation on the experimental measurement of dislocation density

Amit Acharya and Robin J. Knops

(to appear in Journal of Elasticity)

The common practice of ignoring the elastic strain gradient in measurements of geometrically necessary dislocation (GND) density is critically examined. It is concluded that the practice may result in substantial errors. Our analysis points to the importance of spatial variations of the elastic strain field in relation to its magnitude in inferring estimates of dislocation density from measurements.

Problems with numerical integration of discontinuous functions

Choose a channel featured in the header of iMechanica: 

Hi everybody,

 I am a very beginnerin doing research :-) and my topic is about "micro indentation analysis using continuum dislocation theory". I am applying high-order finite element method for this nonlinear problem.

My plan is first writing a subroutine for the element. However, when I intend to compute the internal force by using Gauss integration, I see a problem with the integrand function of some index of the internal force vector. This integrand is discontinuous function. It is therefore, I cannot get a good approximation with the standard Gauss integration. 

Problems with numerical integration of discontinuous functions

Hi everybody,

 I am a beginner in doing research :-) and my topic is about "Micro Indentation Analysis using Continuum Dislocation Theory". I am applying high-order finite element method for this nonlinear problem.

 My plan is first writing a subroutine for the element. However, when I intend to compute the internal force  by using Gauss Integration, I see a problem with the integrand function of some components of the internal force vector. This integrand is discontious function. It is therefore, I cannot get a good approximation with the standard Gauss integration.

Amit Acharya's picture

Jump condition for GND evolution

Acharya, A., Jump conditions for GND evolution as a constraint on slip transmission at grain boundaries, Philosophical Magazine, 87(8-9), 1349-1359, 2007 (see attachment)

Tuncay Yalcinkaya's picture

Full scholarships available at European Commission funded workshop on physically based material modeling

European Commission' s JRC (Joint Research Centre) is organizing a workshop on physics based modeling of material properties and experimental observations in collaboration with METU (Middle East Technical University). The workshop will be held in Ankara on 22-23 October 2012.

EC FUNDED WORKSHOP ON PHYSICS BASED MODELING OF MATERIALS, Ankara, October 22-23

The objective of the workshop, co-organized by Joint Research Centre of the European Commission and Middle East Technical University, is to discuss the present status of physics based constitutive modelling of metallic materials and associated experimental methods.

The Workshop is funded by the European Commission's Integration and Enlargment Programme.  There is no cost for attending the Workshop. A limited number of scholarships is available for participants from the target countries (see attached flyer).

Jonathan Zimmerman's picture

Submit an abstract for IMECE-2012 sympoisum "Multiscale Perspectives on Plasticity in Metals"

My co-organizers and I invite you to submit an abstract to the upcoming symposium “Multiscale Perspectives on Plasticity in Metals” at the ASME 2012 International Mechanical Engineering Congress & Exposition (IMECE), to be held November 9-15, 2012 in Houston, Texas. This symposium will explore the unique challenges to understanding plastic deformation in metals and formulating material models that can accurately predict thermo-mechanical deformation.

Postdoctoral positions in statistical modeling of fracture and plasticity

We anticipate the opening of four postdoctoral positions in 2012 to work on statistical aspects of fracture and plasticity,  and in particular on size effects in the fracture of  disordered media and in microplasticity.  The positions will be funded by the ERC Advanced Grant 2011, SIZEFFECTS.

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