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Arash_Yavari's picture

A Theory of Anharmonic Lattice Statics for Analysis of Defective Crystals

This paper develops a theory of anharmonic lattice statics for the analysis of defective complex lattices. This theory differs from the classical treatments of defects in lattice statics in that it does not rely on harmonic and homogeneous force constants. Instead, it starts with an interatomic potential, possibly with in¯nite range as appropriate for situations with electrostatics, and calculates the equilibrium states of defects. In particular, the present theory accounts for the differences in the force constants near defects and in the bulk. The present formulation reduces the analysis of defective crystals to the solution of a system of nonlinear difference equations with appropriate boundary conditions. A harmonic problem is obtained by linearizing the nonlinear equations, and a method for obtaining analytical solutions is described in situations where one can exploit symmetry. It is then extended to the anharmonic problem using modified Newton-Raphson iteration. The method is demonstrated for model problems motivated by domain walls in ferroelectric materials.

Surface Roughness and Electrical Contact Resistance

J.R.Barber The contact of rough surfaces Surfaces are rough on the microscopic scale, so contact is restricted to a few `actual contact areas'. If a current flows between two contacting bodies, it has to pass through these areas, causing an electrical contact resistance. The problem can be seen as analogous to a large number of people trying to get out of a hall through a small number of doors.

Classical treatments of the problem are mostly based on the approximation of the surfaces as a set of `asperities' of idealized shape. The real surfaces are represented as a statistical distribution of such asperities with height above some datum surface. However, modern measurement techniques have shown surfaces have multiscale, quasi-fractal characteristics over a wide range of length scales. This makes it difficult to decide on what scale to define the asperities.

Robert Gracie's picture

A new finite element method for dislocations based on interior discontinuities

Comments and feedback of the following paper would be appreciated.

Abstract:

A new technique for the modelling of multiple dislocations based on introducing interior discontinuities is presented. In contrast to existing methods, the superposition of infinite domain solutions is avoided; interior discontinuities are specified on the dislocation slip surfaces and the resulting boundary value problem is solved by a finite element method. The accuracy of the proposed method is verified and its efficiency for multi-dislocation problems is illustrated. Bounded core energies are incorporated into the method through regularization of the discontinuities at their edges. Though the method is applied to edge dislocations here, its extension to other types of dislocations is straightforward.

Luis Dorfmann's picture

New Directions in Large Deformation Solid Mechanics

We write to invite your participation in the symposium on New Directions in Large Deformation Solid Mechanics to be held at Texas A&M University campus in College Station as part of the 44th Annual Technical Meeting of the Society of Engineering Science, October 21–24, 2007. The purpose of this symposium is to address and outline new directions in large deformation solid mechanics and to furnish a forum for discussions on a wide range of research in all fields comprising modern mechanics.

Luis Dorfmann, Ray Ogden, Alan Wineman

5 PhD positions in solid mechanics

There are 5 vacant PhD positions with good stipend in the field of solid mechanics, constitutive modelling and numerical simulations. We are looking for very competent, creative students all over the world. It is a rare opportunity for a young scientist to develop his versatile skills and this is your chance. As some people know, SIMLab (Structural Impact Laboratory) is internationally reputed group working on problems related to Crashworthiness and Structural Impact. Our group recently got a Centre for Research based Innovation (CRI). All these positions are filling under CRI. Please find the details in the links here:

Ken P. Chong's picture

Call for proposals on advanced high strength steel

The Division of Civil, Mechanical and Manufacturing Innovation (CMMI) of NSF and the DOE Office of Freedom CAR and Vehicle Technologies intend to co-sponsor proposals addressing fundamental research issues in advanced high strength steels (AHSS). Specifically, proposals focused on

  1. AHSS materials development and characterization,
  2. predictive modeling that integrates AHSS material structure and product performance, and
  3. fundamental research in the area of processing and manufacturing of AHSS, are of interest. This collaborative effort is a direct outcome of the Advanced High Strength Steel Workshop.

Interested PIs should consider submitting an unsolicited proposal to the core programs of the CMMI Division namely, (1) Materials Processing & Manufacturing (MPM), (2) Materials Design & Surface Engineering (MDSE), (3) Applications & Structural Mechanics, or (4) Mechanics & Structures of Materials (MSM), during the January 15, 2007 to February 15, 2007 submission window. Unsolicited proposals in response to this letter should have titles beginning with "AHSS:".  Proposals from the March-April 2007 panel review will be eligible for co-funding, pending availability of funds.

MichelleLOyen's picture

J-Club participation: advice for students

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Now that the Journal Club is getting started, I wanted to post some advice for students who may not have participated in such a forum in the past, either online or in person. The fun of a J-club includes the opportunity to broaden your understanding of the literature of a broad field (like mechanics) by reading carefully-selected groups of papers on small topics of interest to different sub-communities. The challenge in participating in the subsequent discussion is ensuring that you understand the papers sufficiently to participate confidently in the ensuing discussion. This will also prepare you to be a good reviewer for technical journals some day.

Alexander A. Spector's picture

Magnetic Twisting Cytometry and Cell Mechanical Propertries

Some time ago (12-19-06), Daniel Isabey posted an interesting comment on mechanical responses of cells obtained via magnetic twisting cytometry. While the comment was about the nonlinearity of the bead angular displacement, a broader question is how adequately the bead moment/angle relationship represents the complex cell mechanics. There are different patterns of actin bundles at the whole-cell level.

George Herrmann passed away

(A message from Dave Barnett) George Herrmann passed away yesterday in Switzerland -- quickly, quietly, and peacefully.

MichelleLOyen's picture

Journal Club Theme of January 2007: Biomechanics and Non-Affine Kinematics

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Biological materials are frequently constructed of hydrated biopolymer networks. Examples include fibrous collagen in the extracellular matrix and actin within the cell's cytoskeleton. There are differences in the molecular composition of the biopolymer subunits as well as differences in the network density and organization. Images can be seen here and here for dense collagen networks and for portions of actin networks look at images here and here.

The Energy Blog

I just jointed iMechanica. Great blog site! I thought to bring to your attention another blog that I enjoy, run by a retired engineer, on renewable energy issues. Here is the link: http://thefraserdomain.typepad.com/

Zhigang Suo's picture

Josiah Willard Gibbs and his two shorter papers on thermodynamics

There seems to be tremendous enthusiasm among young mechanicians to master thermodynamics. I have found no better source for enlightenment than Gibbs's own writings on the subject, collected in a paperback, still in print. By common consensus, his masterpiece on the subject is the 300-page paper entitled "On the Equilibrium of Heterogeneous Substances". Although I have returned to the long paper many times for illumination, my own favorites are his two shorter papers, written in 1873, before the long one. In many ways, I think, the longer paper is an elaboration of the ideas in the two shorter ones. The title of the short papers are

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Luis Dorfmann's picture

Nonlinear Electroelastic Deformations

Electro-sensitive (ES) elastomers form a class of smart materials whose mechanical properties can be changed rapidly by the application of an electric field. These materials have attracted considerable interest recently because of their potential for providing relatively cheap and light replacements for mechanical devices, such as actuators, and also for the development of artificial muscles. In this paper we are concerned with a theoretical framework for the analysis of boundary-value problems that underpin the applications of the associated electromechanical interactions. We confine attention to the static situation and first summarize the governing equations for a solid material capable of large electroelastic deformations. The general constitutive laws for the Cauchy stress tensor and the electric field vectors for an isotropic electroelastic material are developed in a compact form following recent work by the authors. The equations are then applied, in the case of an incompressible material, to the solution of a number of representative boundary-value problems. Specifically, we consider the influence of a radial electric field on the azimuthal shear response of a thick-walled circular cylindrical tube, the extension and inflation characteristics of the same tube under either a radial or an axial electric field (or both fields combined), and the effect of a radial field on the deformation of an internally pressurized spherical shell.

Nonlinear elasticity of biological gels

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I would like to propose the recent papers by Janmey, P.A., and coworkers on the nonlinear elasticity behavior of biopolymer gels for "biomechanics" issue in J Club. In their original work, they proposed the biopolymer network model composed of semi-flexible polymers that behave like a worm-like-chain (WLC) model. Their models surprisingly capture the mechanical response of biopolymer gels such as neuro-filaments. The details of their work are as follows:

Rui Huang's picture

Harry F. Tiersten

(Professor Tiersten in his office, behind a pile of files on his desk.)

Harry F. Tiersten (1930-2006), Professor of Mechanics at Rensselaer Polytechnic Institute, passed away suddenly on June 12, 2006 from a heart attack. Professor Tiersten was one of the founders of continuum electrodynamics. In this paper we present a brief summary of Tiersten’s major contributions to the theories of continuum electrodynamics and their applications.

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Xi Chen's picture

A new website has been created for Prof. Raymond Mindlin, including funding solicitation for the Mindlin Medal

A new website has been recently created for the centennial of Professor Raymond Mindlin. In addition, the Engineering Mechanics Division of ASCE has launched an effort to establish the Mindlin Medal of Applied Mechanics. The goal is to raise about $30,000 to setup an endowment at ASCE.

Teng Li's picture

Delocalizing Strain in a Thin Metal Film on a Polymer Substrate

Teng Li, Zhenyu Huang, Zhichen Xi, Stephanie P. Lacour, Sigurd Wagner, Zhigang Suo, Mechanics of Materials, 37, 261-273 (2005).

Under tension, a freestanding thin metal film usually ruptures at a smaller strain than its bulk counterpart. Often this apparent brittleness does not result from cleavage, but from strain localization, such as necking. By volume conservation, necking causes local elongation. This elongation is much smaller than the film length, and adds little to the overall strain. The film ruptures when the overall strain just exceeds the necking initiation strain, εN , which for a weakly hardening film is not far beyond its elastic limit. Now consider a weakly hardening metal film on a steeply hardening polymer substrate. If the metal film is fully bonded to the polymer substrate, the substrate suppresses large local elongation in the film, so that the metal film may deform uniformly far beyond εN. If the metal film debonds from the substrate, however, the film becomes freestanding and ruptures at a smaller strain than the fully bonded film; the polymer substrate remains intact. We study strain delocalization in the metal film on the polymer substrate by analyzing incipient and large-amplitude nonuniform deformation, as well as debond-assisted necking. The theoretical considerations call for further experiments to clarify the rupture behavior of the metal-on-polymer laminates.

Related posts and discussions

Tension of Cu film on Pi substrate
Local thinning of Cu film
High ductility of a metal film adherent on a polymer substrate

High ductility of a metal film adherent on a polymer substrate

In recent development of deformable electronics, it has been noticed that thin metal films often rupture at small tensile strains. Here we report experiments with Cu films deposited on polymeric substrates, and show that the rupture strains of the metal films are sensitive to their adhesion to the substrates. Well-bonded Cu films can sustain strains up to 10% without appreciable cracks, and up to 30% with discontinuous microcracks. By contrast, poorly bonded Cu films form channel cracks at strains about 2%. The cracks form by a mixture of strain localization and intergranular fracture.

Konstantin Volokh's picture

Griffith controversy

Using the Griffith energy method for analysis of cavitation under hydrostatic tension we conclude that the critical tension tends to infinity when the cavity radius approaches zero (IJSS, 2006, doi: 10.1016/j.ijsolstr.2006.12.022). The conclusion is physically meaningless, of course. Moreover, if we assume that the failure process occurs at the edge of the cavity then the critical tension should be length-independent for small but finite cavities while the Griffith analysis always exhibits length-dependence. The main Griffith idea - introduction of the surface energy - is controversial because it sets up the characteristic length, say, surface energy over volume energy. By no means is this approach in peace with the length-independent classical continuum mechanics.

Pradeep Sharma's picture

Is rest of the world catching up with us? Perspective from Physical Review Letters...

I had posted this on the amd blog...I am posting it here as well:

Last year I attended the annual American Physical Society conference held in Baltimore (during the week of March 13th). One of the non-technical sessions included presentations by the APS journal editors--Physical Review A/B/C/D/E and Letters---and a panel discussion related to these journals. Since many of our mechanics and materials colleagues nowadays are interested in publishing in these journals, I thought I should post a link to some of the slides (from the editors presentation) that I found interesting. Many of the slides presented at APS are in the linked pdf file that also includes additional (humorous slides!) regarding reviewer issues.

The Ranking of Mechanics Related Journals (2004)

Based on a survey from Journal Citation Report (JCR), we listed below the 2004 Journal Impact Factors (IF) for some mechanics, material science, and solid state physics related scientific journals. Our list and information may not be complete. We welcome readers' input, comments, and information. We also caution readers that using IF as the sole criterion to rank scientific journals' academic reputation may not be objective nor true to a journal's actual scientific merits.

Rui Huang's picture

Nanoscience Initiative Workshop

The Air Force Office of Scientific Research (AFOSR) of the U.S. Air Force Research Laboratory (AFRL) and National Science Council (NSC) in Taiwan are pleased to announce that the 4th U.S. Air Force/Taiwan Nanoscience and Nanotechnology workshop will be held on February 8-9, 2007 at the main campus of the University of Houston. We invite you to join us at the workshop.

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