iMechanica

I wanted to share some our work on the deformation behavior of metal nanowires that was recently published in Advanced Functional Materials. In this work, we considered the tensile deformation of three experimentally observed silver nanowire geometries, including five-fold twinned, pentagonal nanowires. The manuscript abstract and urls to videos of the tensile deformation of the three nanowire geometries are below. A copy of the manuscript is attached.

Abstract. This letter addresses the dependence of homogeneous dislocation nucleation on the crystallographic orientation of pure copper under uniaxial tension and compression.  Molecular dynamics simulation results with an embedded-atom method potential show that the stress required for homogeneous dislocation nucleation is highly dependent on the crystallographic orientation and the uniaxial loading conditions; certain orientations require a higher stress in compression (e.g., <110> and <111>) and other orientations require a higher stress in tension (<100>).  Furthermore, the resolved shear stress in the slip direction is unable to completely capture the dependence of homogeneous dislocation nucleation on crystal orientation and uniaxial loading conditions.

Might also be useful for simulating dislocation motion in a finite body.

Several sets of boundary integral equations for two dimensional elasticity are derived from Cauchy integral theorem.These equations reveal the relations between displacements and resultant forces, between displacements and tractions, and between the tangential derivatives of displacements and tractions on solid boundary.Special attention is given to the formulation that is based on tractions and the tangential derivatives of displacements on boundary, because its integral kernels have the weakest singularities.The formulation is further extended to include singular points, such as dislocations and line forces, in a finite body, so that the singular stress field can be directly obtained from solving the integral equations on the external boundary without involving the linear superposition technique often used in the literature. Body forces and thermal effect are subsequently included. The general framework of setting up a boundary value problem is discussed and continuity conditions at a non-singular corner are derived.  The general procedure in obtaining the elastic field around a circular hole is described, and the stress fields with first and second order singularities are obtained. Some other analytical solutions are also derived by using the formulation. 

Myfeeling is that what we're trying to find are elastic constants of a continuum structure whose response in several (ideally all) deformations is the same as that of the carbon nanotube subjected to the same boundary conditions as the continuum structure.  We (A. Sears and R. C. Batra, Macroscopic Properties of Carbon Nanotubes from Molecular-Mechanics Simulations, Physical Reviews B, 69, 235406, 2004) have simulated simple tension and torsional deformations of a SWNT and its equivalent continuum structure defined as the one whose strain energy density is the same as that of the SWNT.  For an isotropic structure, the thickness of the equivalent structure was found to be~0.21 and it depends upon the MM potential used.  This has been validated by performing bending, buckling and combined loading tests on the SWNT and the equivalent continuum structure.

Here is a link to a 1996 book by C.H. Wang on Fracture Mechanics from the DSTO Aeronautical and Maritime Research Laboratory in Melbourne.

http://www.dsto.defence.gov.au/publications/1880/DSTO-GD-0103.pdf

In this chapter of "Hankbook of Theoretical and Computational Nanotechnology", I will provide an overview of the progress made in the last decade on theoretical modeling and computer simulation of strain-mediated formation of nanostructures on surface, focusing on strain-induced self-assembly and self-organization of two-dimensional (2D) patterns and structures. As part of a handbook, the main objective of the chapter is not to provide an extensive literature review on the topic. Instead, I will try to provide a general introduction and overview of the basic concepts and physical models along with some relatively detailed discussion of mathematical derivations and technical treatments so that readers (especially graduate students) who are interested in this topic can use this chapter as a guide and reference to start their own modeling and simulation.

In the brief presentation attached, I am summarizing my lab's recent work in the field of adhesion and work-of-adhesion measurements, and hoping to see who else is working in the field.  Here is some intro to the topic (by no means, it is complete - maybe we can add some recent work to this list as discussions develop)

By Martijn Feron, Zhen Zhang and Zhigang Suo

The mobility of charge carriers in silicon can be significantly increased when silicon is subject to a field of strain.In a microelectronic device, however, the strain field may be intensified at a sharp feature, such as an edge or a corner, injecting dislocations into silicon and ultimately failing the device. The strain field at an edge is singular, and is often a linear superposition of two modes of different exponents. We characterize the relative contribution of the two modes by a mode angle, and determine the critical slip systems as the amplitude of the load increases. We calculate the critical residual stress in a thin-film stripe bonded on a silicon substrate.

J.R.BARBER: INTERMEDIATE MECHANICS OF MATERIALS

Many of you may know my book on Elasticity, but may not be aware that I also wrote an undergraduate book on Intermediate Mechanics of Materials (Published by McGraw-Hill - ISBN 0-07-232519-4). This picks up from the typical elementary Mechanics of Materials course and deals with the next range of topics such as energy methods, elastic-plastic bending, bending of axisymmetric cylindrical shells and axisymmetric thick-walled cylinders. A full Table of Contents and the Preface are given below.

If you are interested in nano-calorimetry or combinatorial analysis, you might also find the following paper interesting. It was published as part of the MRS spring ‘06 meeting proceedings (http://www.mrs.org/s_mrs/sec_subscribe.asp?CID=6447&DID=175796&action=de...). This paper describes the parallel nano-differential scanning calorimeter (PnDSC), a new device for measuring the thermal properties of nano-scale material systems using a combinatorial approach.

The IUTAM Symposium "150 Years of Vortex Dynamics" will be held October 12-17, 2008, at the Technical University of Denmark (DTU) in Lyngby, and in Copenhagen, Denmark. The rationale behind the title is that H. von Helmholtz published his seminal paper enunciating his three laws of vortex dynamics in 1858.

The Scientific Committee, consisting of Profs. Keith Moffatt (Cambridge), Paul Newton (Los Angeles), Slava Meleshko (Kiev), Morten Brøns (Copenhagen), GertJan van Heijst (Eindhoven), Shigeo Kida (Kyoto), and H. Aref (Copenhagen and Blacksburg) has been constituted.

By Michael H . Suo and Zhigang Suo

While browsers have improved greatly in recent years, we have noticed that many academics are still missing out on important functions. In this post, we will focus on Firefox, an open-source browser that has recently gained popularity. Note: this is not a Microsoft-bashing article. Internet Explorer 7 is a very functional modern browser, but for the reasons below, we like Firefox better.

I think that the note by Zhigang Suo puts forth a pressing issue. Coming from remotely related areas in physics (quantum mechanics and electromagnetic theory), my own interests and work have recently evolved to topics in both solid mechanics (crystal surfaces) and aspects of fluid mechanics (advection-diffusion and a most recent start on biomembranes). Hence, practically, I cannot see any boundaries between these disciplines.

The Department of Energy and Refrigerating Air-Conditioning Engineering at National Taipei University of Technology (NTUT) invites applications for one tenure-track faculty position in the area of ENERGY. Ph.D. in engineering is required. The position starts on August 1, 2007. Duties include teaching and research. The due day for application is on March 1, 2007.

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!

Initially posted on Applied Mechanics News on 28 April 2007.

Hot rolling from ingot is the dominant fabrication method of producing plate, sheet, and foil aluminum products. It is a striking fact that the total rolling-plant recovery of aluminum process from ingot to final products is typically about 50%. This recovery loss causes enormous amount of energy waste both as remelt energy and energy to process material that is just recycled. Assuming the annual US domestic net shipments of sheet and plate products being 10,500 million lb, 10% improvement of the hot rolling recovery will result annual savings of $126 million per year for the US domestic aluminum industry. The annual domestic energy savings would be 2.54 trillion Btu. The environmental benefits include annual reduction of 2.32 million lb SOx , 1.01 million lb NOx, 303.2 million lb CO2, 0.67 million lb of particulate, and 11000 lb VOCsd .

The fundamental inability to reduce or eliminate these recovery losses is “lack of the integrated models that relate structural properties to manufacturing processes”. Currently, processing parameters are determined by trial and error and largely based on experience. This makes it difficult to optimize the process even on the macroscale level, and almost impossible from microstructure level. Research in the following areas are desirable:

The number of registered iMechanicians has just passed 1000 today. iMechanica went "officially" online on 9 September 2006. See other numbers of iMechanica recorded today, and our evolving history.

The following are links to the FEMA (Federal Emergency Management Agency) documents:

Mitigating the threat of terrorist attacks against high occupancy buildings is a challenging task.

Chapter 1: ASSET VALUE, THREAT/HAZARD, VULNERABILITY, AND RISK
This chapter presents several methodologies for architects and engineers to quantify risk and to identify the most effective mitigation measures to achieve a desired level of protection against terrorist attacks at an acceptable cost.

C. Eberl, D. S. Gianola, R. J. Thompson (in alphabetic order)

With this contribution we would like to point to a free MATLAB tool which uses digital image correlation and tracking techniques to measure strain from a series of digital images. The code can be found on the ‘MATLAB central file exchange’ as well as the documentation, example images and some slides. We use the code on a daily basis for micro- and nanoscale measurements and present it here to be used and further developed by the community. Since it was posted at the end of september the code is now ranked place one or two in google and has been downloaded about 1000 times.

Post-doctoral position - Stochastic computational techniques to deal with uncertainties on the geometry in structural analysis

The post-doctoral student will join the pole "Structures and Couplings" of the Research Institute en Civil Engineering and Mechanics (GeM), Nantes, France (Nantes University, Ecole Centrale Nantes, CNRS UMR 6183)

We have recently been awarded a Research Project by the French National Research Agency. This project addresses theorical and numerical developments in the field of stochastic computational mechanics. The main goal of this project is to develop a robust computational technique to deal with uncertainties on the geometry in structural analysis. The proposed methodology lies on the extension of the Extended Finite Element Method (X-FEM) into the stochastic framework and the development of efficient computational techniques for solving stochastic systems.