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L. Roy Xu's picture

Tensile strength and fracture toughness of nanocomposite materials

Are not as high as we expected although very stiff and strong nanotubes or nanofibers (Young’s modulus E~1000GPa) are added into soft polymer matrices like epoxy (E~4GPa).  In our early investigation on the  systematic mechanical property characterizations of nanocomposites (Xu et al., Journal of Composite Materials, 2004--among top 5 in 2005;and top 10 in 2006 of the Most-Frequently-Read Articles in Journal of Composite Materials.) have shown that there was a very small increase (sometimes even decrease) of critical ultimate tensile/bending strengths, and mode-I fracture toughnesses in spite of complete chemical treatments of the interfacial bonding area, and uniform dispersions of nanofibers (click to view a TEM image). Similar experimental results were often reported in recent years. Therefore, mechanics analysis is extremely valuable before we make these “expensive” nanocomposite materials. Our goal is to provide in-depth mechanics insight, and future directions for nanocomposite development. Till now, nanocomposite materials are promising as multi-functional materials, rather than structural materials. Here we mainly focus on two critical parameters for structural materials: tensile strength and fracture toughness. We notice that other mechanical parameters such as compressive strengths and Young’s moduli of nanocomposite materials have slight increase over their matrices.

What are the current research areas in computational mechanics?

Hello mechanicians,

What are the current research areas in computational mechanics? What is the future of CM? Where can one find such information on the net? Is there a central location?

Dean Eastbury's picture

2nd International Conference on Mechanics of Biomaterials & Tissues

In December 2007 Elsevier will organise the 2nd International Conference on Mechanics of Biomaterials & Tissues (www.icmobt.elsevier.com). The aim of the conference is to provide a forum for the discussion of the modeling and measurement of deformation and fracture behavior in biological materials and in those materials which are used to replace them in the human body.

Ashkan Vaziri's picture

"Persistence of a pinch in a pipe" by L. Mahadevan, Ashkan Vaziri and Moumita Das

The response of low-dimensional solid objects combines geometry and physics in unusual ways, exemplified in structures of great utility such as a thin-walled tube that is ubiquitous in nature and technology.

MichelleLOyen's picture

Viscoelastic Contacts

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I was a little bit surprised in the introduction of this new forum to see mention of elastic and plastic contacts but no specific mention of viscoelastic contacts.

In the era of commercially-available instruments for indentation testing, the examination of viscoelastic contact mechanics, both in the context of polymers and biological tissues, seems to have taken on new life. To a first approximation, for indentation testing in the time domain, the fundamental mechanics has not much advanced beyond a few classic papers of the 1960s: Lee and Radok, J. Appl. Mech. 27 (1960) 438 and Ting TCT, J. Appl. Mech. 88 (1966) 845. However, the implementation of techniques for analysis of experimental data has progressed substantially. With spherical indenters the use of linearly viscoelastic models for characterization of a material creep or relaxation function is straightforward. Recent experimental studies have confirmed this, while more lingering questions remain for sharp contacts including Berkovich pyramidal indenters (most commonly shipped with commercial indenters). Sharp contacts seem to give rise to nonlinearly viscoelastic responses. Other topics of recent interest include frequency-domain measurements and examination of oscillating contacts and adhesion. (Although not mentioned in the listing of KLJ's most-loved topics in contact mechanics, viscoelastic contact has been the subject of several recent KLJ publications!)  Although research in viscoelastic contact mechanics has been strong in recent years, perhaps a challenge remains in the dissemination of information and the establishment of approachable experimental techniques for use by non-experts.

Interfacial toughness and mode mixity

When I was a graduate student, I spent several months to measure interfacial toughness between metalic (Cu and Au) films and thick substrates(Si and Polycarbonate). My methods were bulge test (blistering test) and 4-point bending test. I had many problems such as making an initial crack(pre-cracking), changing load phase angle applied to specimens, preparing/patterning thin films, constructing my own test apparatus, etc. The biggest problem was to measure the interfacial toughness over a wide range of loading phase angle. For a bimaterial with a non-zero oscillatory index(epsilon), we don't know the phase angle for a minimum interfacial toughness beforehand. Therefore, we need to measure the interfacial toughness over a wide range of phage angle. For engineering purpose, we need a minimum interfacial toughness value for reliability design because this value will lead to a conservative design of systems.

arindam.chakraborty's picture

A paper on developing stochastic micromechanical model for elastic properties of functionally graded material (FGM)

Given link is for a stochastic micromechanical model developed for predicting probabilistic characteristics of elastic mechanical properties of an isotropic functionally graded material (FGM) subject to statistical uncertainties in material properties of constituents and their respective volume fractions.

Robert Gracie's picture

2007 NSF Summer Institute on Nano Mechanics and Materials

Please find below the announcement for the NSF Summer Institute on Nano Mechanics and Materials:

Pradeep Sharma's picture

iMechanica get together at the next mechanics conference?

Regarding iMechanica.......how about having an "iMechanica get-together" during one of the upcoming conferences, either McMat Austin or IMECE Seattle? Many of us will attend these two events.The purpose of this get-together would be:

Zhigang Suo's picture

Journal publishers are pioneers of Web 2.0

Eric Mockensturm has just posted a publication agreement proposed by provosts of several universities. In structuring iMechanica, we have tried to avoid the question of open access, and simply asked the question what if all papers are already openly accessible. Many mechanicians have discovered iMechanica, and the registered users have recently passed 1000. Recent discussions of copyright on iMechanica have prompted Eric to post his entry, which has just led to this one.

Is there a shear instability in metal foams?

Last year I spent three months modeling the compressive behavior of aluminum alloy foams. I had hoped to find some evidence of the banding instability that is often observed in elastomeric foams [1]. Lakes writes that this sort of banding instability provides indirect experimental evidence for negative shear modulus [2].

Deformation of Top-Down and Bottom-Up Silver Nanowires

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.

Mark Tschopp's picture

Tension-Compression Asymmetry in Homogeneous Dislocation Nucleation

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.

Honghui Yu's picture

Integral Formulations for 2D Elasticity: 1. Anisotropic Materials

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. 

rbatra's picture

Elastic Modulus of a Carbon Nanotube/Yacobson's Paradox

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.

C.H. Wang, "Introduction to Fracture Mechanics"

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

George G Adams's picture

Welcome to Contact Mechanics Forum

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Welcome to a new forum on iMechanica which is specifically designed as a "Contact Mechanics Forum". You do not need to register to view the postings on this forum, but you must register (it's free) in order to post something. The posting can be an idea, an announcement of an upcoming conference, an announcement of a journal article (see node/265), the pdf of the manuscript form of the paper can be posted in many cases (see http://romeo.eprints.org/ for a list of journals which permit it), or almost anything related to contact mechanics.

fengliu's picture

Modeling and Simulation of Strain-mediated Nanostructure Formation on Surface

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.

Rolling Moment Resistance of Particles on Surfaces

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)

Martijn Feron's picture

Split singularities and dislocation injection in strained silicon

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.

IINTERMEDIATE MECHANICS OF MATERIALS

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.

Patrick J McCluskey's picture

An introductory paper on thermal combinatorial analysis of nano-scale materials

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.

Hassan Aref's picture

150 Years of Vortex Dynamics

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.

Michael H. Suo's picture

Why We Use Firefox

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.

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