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Pradeep Sharma's blog

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Paper by Ellad Tadmor & Nikhil Admal---Atomistic Definition of Stress

For both mechanicians and physicists, linking continuum concepts to the underlying microscopic characteristics of materials has remained a major preoccupation. Certainly, with the recent advent of the so-called "multiscale" modeling approaches, this topic has been brought to a sharp focus. Cauchy stress is one such continuum concept that has courted a fair amount of controversy.

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Mechanical Engineering Department Chair Opening at University of Houston

The Cullen College of Engineering of the University of Houston invites nominations and applications for the position of Chair of the Department of Mechanical Engineering (ME). As part of a major drive for excellence, the college seeks innovative and enthusiastic leadership to build upon the strengths of the research and educational programs of the ME Department. The successful candidate must possess academic skills and credentials of the highest caliber, with an established national and international reputation in research, and academic and professional leadership in Mechanical Engineering or related fields. Credentials appropriate to the rank of Professor are required.

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The origins of electromechanical indentation size effect in ferroelectrics

The attached paper was recently accepted for publicaiton in Applied Physics Letters. Here we try to speculate on the origins of a type of electromechanical indentation size-effect. Although we have decent supporting evidence, I think that our assertions are still on the speculative side and hope that future experiments can shed further light on this.

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Revisiting Quantum Notions of Stress

I plan to submit the attached paper on quantum mechanical definition of stress in the next few weeks. Comments and feedback are welcome. Fair amount of work has been done on stress definition in the context of classical molecular dynamics (also attracting some controversies). In contrast, there appear to be several open issues in the quantum case. Hopefully, the attached paper provides a starting point.

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Book Review

Micromechanics---loosely speaking, is the study of heterogeneities in materials and its consequences for material or continuum behavior. This encompasses studies of inclusions, dislocations, cracks or more generally defects. A related problem is that of "coarse-graining" or in other words the effective homogenized properties of a heterogeneous material. The latter is a recurring theme in all of physical sciences not just solid mechanics. Micromechanics, a formidable subject by all means, dominated a substantial part of the history of solid mechanics. Several of our Timoshenko awardees have been associated with this subject, e.g. Eshelby, Hill, Keller, Irwin, Rice among others.

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Journal Club Theme of March 2009: Mechanics Issues in Nanocapacitors and Ramifications for Energy Storage

Next generation advances in energy storage for nanoelectronics, micro and nanosensors among others, require capacitors fabricated at the nanoscale. High dielectric constant materials such as ferroelectrics are important candidates for those. Consider the following: the expected capacitance of a 2.7 nm SrTiO3 thin film is 1600 fFmicro-m-2. What is the likely value in reality? 258 fFmicrom-2! This dramatic drop in capacitance is attributed to the so-called "dead layer" effect.

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Simple strategies to produce perfect long range order in self-assembly

In a recent rapid communication (see attached paper), using principles of pattern formation, we expose some simple stategies to reliably produce perfect long range order in self-assembling systems. Most self-assembling systems exhibit short ranged order. This imperfection is detrimental to several practical applications. It is almost always possible to produce perfect patterns in small domain sizes but self-assembly over a larger areal span results in defects.

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Review paper by Professor Jerry Ericksen on Cauchy-Born rule; Special issue on scale effects---freely available for a month

Xin-Lin Gao and I had the pleasure of guest-editing a special  issue on "scale effects in mechanics" for the journal, Mathematics and Mechanics of Solids (editor: Professor David Steigmann , UC Berkeley).

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How shell-like is a carbon nanotube?

(Carbon) Nanotubes have attracted considerable attention from the mechanics community; probably second to none when it comes to nanotechnologies. Although I personally have done very little in this particular topic, I have enjoyed reading about the many developments made by mechanicians in terms of modeling the behavior of nanotubes and the applicability of standard continuum mechanics notions. A post on this subject on iMechanica, which received a fair amount of attention from many mechanicians involved in this topic, may be found here .

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Open Faculty Position at University of Houston in Nanomechanics, Computational Materials Science

University of Houston
Department of Mechanical Engineering
Cullen College of Engineering
Faculty Position-Nano Mechanics, Nano Materials

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Why is the reported elastic modulus of carbon nanotube so scattered? “Yakobsons Paradox” and Perspective from Huang et. al.

For many mechanicians and materials scientists one of the most confounding things (in the ever increasing literature on carbon nanotubes) is the reported theoretical value of the nanotube elastic modulus. Depending upon the specific paper at hand, the reported numerical values range from 1 -6 TPa!

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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.

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Nonlinear Stability Analysis of Self-assembling Nanoscale Patterns

I thought I should take advantage of iMechanica and obtain feedback on some recent work that we did on nonlinear stability analysis of patterns.

A paradigmatic model that governs monolayer self-assembly was constructed a few years back by Wei Lu (Michigan) and Zhigang Suo. Apart from obtaining several physical insights they also conducted a linear stability analysis of their model. Borrowing technqiues from the nonlinear physics community, our work presents nonlinear stability analysis i.e. the initial state is no longer homogeneous and stable states beyond the transition are calculated. This allows a detailed construction of stability maps for various patterns without extensive numerical calculations.

This work is currently under review and I am attaching a pre-print with this post. Any comments and suggestions would be well-appreciated.

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How "hot" is a research topic?

A student pointed me to a recent article on physicsweb. This article discusses a new (scientific) ranking system developed by a German student (Michael Banks) in Max Planck Institute of Solid State Physics to characterize the "hotness" of the scientific subject. If, after reading the popular physicsweb article linked above, you are interested in more details you may wish to read the attached original article posted by Banks. "Carbon nanotubes" emerges at the top of the list.

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Journal Club: Response/Feedback requested

Hello everyone,

I had previously posted this entry on the AMD blog and perhaps it worthwhile to post it again on this forum. I would like to solicit feedback and comments on an idea to further enhance the role and utility of iMechanica.

This inspiration comes from Bell labs and the physics community.....

They started a journal club (year 2003). Each month ONLY 2-3 already published recent journal papers are reviewed and commentary posted in the form of a newsletter. Since only 2-3 papers are reviewed, the selection is much more stringent and careful. The contribution is regular and periodic (monthly). Hence, this newsletter is taken seriously by physicists.

In our case, this can be done within iMechanica. I suspect we could achieve the same kind of interest if we restrict "notable" papers to 1-3 per month and make it a regular monthly feature. In principle anyone could submit a commentary but the blog moderators will select the top 2-3.

The operational rules are open for discussion. Briefly though, I am thinking on the lines of rotating 1-2 moderators with a term of say 2 months. The moderator will receive commentaries on recently published papers RELATED to mechanics area. The moderator will highlight 1-3 notable commentaries in the journal club newsletter. A key requirement must be that the commentaries/paper highlighted are related to mechanics in some form or the other. The concept of rotating moderator is to provide breadth and prevent bias of any one individual. Rotation of journal club moderators will also keep the "work-load" well distributed.

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Collected Works of J.D. Eshelby

Perhaps a post has already been made in this regard; A book containing all the papers by J.D. Eshelby was recently released by Springer. This book is compiled by Markenscoff and Gupta. Congratulations to both of them for such a great idea!

I bought this book last week and it is fascinating to read all of Eshelby's papers in chronological order. Furthermore, I found a few papers that I had not even been aware of. The price, at roughly $195 on Amazon is a bit steep but (in my opinion) well worth it. The book also contains forewords by several researcher who knew Eshelby personally.

Here is the amazon link to this book


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