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Mahdi Kazemzadeh's picture

crack instabilities around tips in Molecular Modellings

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Considering the MD (molecualr dynamics) simulation programs, they enable us to define the initial crack and then using different theories they propagate the crack. This process is actually a dynamic feature at least when the sample is going to fail. Here is the question that present in the most modellers assumptions, which will limit the simulation or maybe it is not possible to simulate the process with out these assumptions. One of them which I would like to know your ideas about is the linear velocity which come into conclusions before the simulations start.

Mike Ciavarella's picture

Contact mechanics of rough surfaces: is Persson's theory better than Greenwood & Willamson?

A recent string of papers originated from Persson's paper in the physics literature contain a number of interesting new ideas, but compare, of the many theories for randomly rough surfaces, only Persson's and Bush et al, BGT. These papers often assume the original Greenwood and Williamson (GW) theory [1] to be inaccurate, but unfortunately do not test it, assuming BGT to be its better version. The original GW however is, I will show below, still the best paper and method today (not surprisingly, as not many papers have the level of 1300 citations), containing generally less assumptions than any other model, including the constitutive equation which does not need to be elastic! I just submitted this Letter to the Editor: On "Contact mechanics of real vs. randomly rough surfaces: A Green's function molecular dynamics study" by C. Campaña and M. H. Müser, EPL, 77 (2007) 38005. C. Campaña and M. H. Müser also make several questionable statements, including a dubious interpretation of their own results, and do not even cite the original GW paper; hence, we find useful to make some comments.

Henry Tan's picture

experimentally, stresses cannot be measured directly

Experimentally, loading to a mechanical system can be applied either through the displacement control or the force control.

However, the responses of the system can only be measured in displacements, and hence strains.

Is Strain Gradient Elasticity Relevant for Nanotechnologies?

Determination of Strain Gradient Elasticity Constants for Various Metals, Semiconductors, Silica, Polymers and the (Ir) relevance for Nanotechnologies

Strain gradient elasticity is often considered to be a suitable alternative to size-independent classical elasticity to, at least partially, capture elastic size-effects at the nanoscale. In the attached pre-print, borrowing methods from statistical mechanics, we present mathematical derivations that relate the strain-gradient material constants to atomic displacement correlations in a molecular dynamics computational ensemble. Using the developed relations and numerical atomistic calculations, the dynamic strain gradient constants have been explicitly determined for some representative semiconductor, metallic, amorphous and polymeric materials. This method has the distinct advantage that amorphous materials can be tackled in a straightforward manner. For crystalline materials we also employ and compare results from both empirical and ab-initio based lattice dynamics. Apart from carrying out a systematic tabulation of the relevant material parameters for various materials, we also discuss certain subtleties of strain gradient elasticity, including: the paradox associated with the sign of the strain-gradient constants, physical reasons for low or high characteristic lengths scales associated with the strain-gradient constants, and finally the relevance (or the lack thereof) of strain-gradient elasticity for nanotechnologies.

Srinivasan Sivakumar's picture

Generalized Thermoplasticity

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Thermodynamically consistent plastic deformations at macro and microlevels under thermomechanical loading conditions.

L. Roy Xu's picture

Intersonic interface crack propagation (two shock waves)

This high-speed photography image recorded a very special fracture mechanics phenomenon: two fast cracks (as demonstrated by two shear shock waves) just met at the specimen center. After a steel projectile hit a model sandwich plate (steel/transparent Homalite -100 polymer/steel), stress wave propagation was observed in the form of photo-elasticity fringe movement. Two interfacial cracks from the two ends of the model sandwich plate, entered the field of view with very high speeds (> 1400m/s) and formed two shock waves (since the crack tip speed exceeded the shear wave speed of the polymer). For further technical details and more photos, click here to read the related paper (Xu and Rosakis, IJSS, 2002) For more real movies recorded from a high-speed camera( click here). It will take a few minutes to access my movie site since the size of each movie is quite large. But the movie resolution and layout from my site is much better than the movie from YouTube (below). © Dr. L. R. Xu (Vanderbilt University) and Dr. A. J. Rosakis (California Institute of Technology)

Best load cell for testing small structures - what have you found?

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For all of the small-scale mechanics testing to date, there seems to be a lack of load/force cells that combine a number of desirable features including vacuum compatibility, high resonant frequency, and sub-microNewton resolution.

L. Roy Xu's picture

Biologically inspired design—natural convex joints reduce stress concentrations

Finite element stress analysis and corner optimization of a tree-steel railing interface/joint (Mattheck, 1998) showed that the naturally formed tree/railing joint was very effective in reducing stress concentration.

L. Roy Xu's picture

Interaction between an Interface and a Dynamic Incident Mode-I Crack

This high-speed photography image shows a mode I crack (representing by a symmetric photo elasticity fringe pattern) is approaching a weak interface in a brittle polymer (Homatel-100). The crack tip speed is around 300-400 m/s. There will be three possibly situations for dynamic interfacial failure mode transitions : 1) crack kinks at the interface, 2) crack directly penetrates the interface and 3) interface debonding occurs before the incident crack reaches the interface. Which case will occur?

Xiaoyan Li's picture

Simulating Fullerene Ball Bearings of Ultra-low Friction

We report the direct molecular dynamics simulations for molecular ball bearings composed of fullerene molecules (C60 and C20) and multi-walled carbon nanotubes. The comparison of friction levels indicates that fullerene ball bearings have extremely low friction (with minimal frictional forces of  5.283×10-7 nN/atom and  6.768×10-7 nN/atom  for C60 and C20 bearings) and energy dissipation (lowest dissipation per cycle of  0.013 meV/atom  and  0.016 meV/atom  for C60 and C20 bearings). A single fullerene inside the ball bearings exhibits various motion statuses of mixed translation and rotation. The influences of the shaft's distortion on the long-ranged potential energy and normal force are discussed. The phonic dissipation mechanism leads to a non-monotonic function between the friction and the load rate for the molecular bearings.

Micromechanical Exfoliation and Graphene: 1999 papers and brief discussion of them

The discovery of a new material type, graphene and extremely thin platelets of graphite, was discussed in several articles from my research group published in 1999:

Lu XK, Huang H, Nemchuk N, and Ruoff RS, Patterning of highly oriented pyrolytic graphite by oxygen plasma etching, APPLIED PHYSICS LETTERS, 75, 193-195 (1999).

A good beginning of 2007

In the very beginning of 2007 I have four papers published or accepted (one is independent research and others are collaborated). All of them are the work done in my doctoral period. The topic is focusing on the enhancement of creep resistance of polymers by incorporating of nanofillers including particles and CNTs.

Teng Li's picture

Journal Club Theme of March 2007: Mechanics of Flexible Electronics

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Flexible electronics is an emerging technology with an exciting array of applications, ranging from paper-like displays, skin-like smart prosthesis, organic light emitting diodes (OLEDs), to printable solar cells. These potential applications will profoundly impact various facets of our daily life, and excite our curiosity on: what's the future of newspapers and books? Will OLEDs replace light bulbs and fluorescent lamps, and emerge as future lighting source? Can we power electronic devices everywhere cordlessly? Significant progress has been made in the past several years, especially as sizable investments flux in. For example, Polymer Vision just released the first commercial product of rollable display (as shown in the figure) after secured $26M investment in January 2007. The future success of this emerging technology largely relies on:

Three-dimensional anisotropic elasticity - an extended Stroh formalism

Tom Ting and I have recently developed a method of extending Stroh's anisotropic formalism to problems in three dimensions. The unproofed paper can be accessed at .

Vikram Gavini's picture

Quasi-continuum orbital-free density-functional theory : A route to multi-million atom electronic structure (DFT) calculation

I would like to share the research work I have been pursuing over the past four years. I believe, through this forum, I will be able to reach researchers with various backgrounds and expertise. Suggestions and comments from members will be very useful. I am also attaching links to preprints of manuscripts describing this work. Please follow these links:

Ashkan Vaziri's picture

Multi-Axial Failure Models for Fiber-Reinforced Composites

The increasing use of fiber-reinforced composites accentuates the need for developing multi-axial fatigue failure models for these materials. In this article (attached), we proposed several multiaxial fatigue failure models for fiber-reinforced composites considering the contribution of mean and cyclic normal stress/strain and shear stress/strain at the plane of failure and examined their capability for predicting the fatigue life of the E-glass/epoxy composite materials.

Accuracy and error estimation in extended finite element methods

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Stephane Bordas, Marc Duflot and Pierre-Olivier Bouchard announce the WCCM8 mini-symposium Link to detailed pdf description 3d error estimation by extended moving least squares

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Which phenomenological flow stress model is the best?

A couple of years ago a colleague who wanted to simulate high-speed machining asked me: " Which is the best phenomenological flow stress model for metals?" I wasn't able to give an answer right away and decided to look in the literature.

What I found was, every ten years or so, a new model appears in the literature that tries to solve some of the problems of older models. However, a clear ranking of models has not been established yet.

Roozbeh Sanaei's picture

what's most advanced open source program to multi-scale simulation?

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I recently interested in multi-sclae modelling problems. and i want to know what's most advanced open source program to multi-scale simulation?

L. Roy Xu's picture

TEM image showing uniformly distributed nano-fibers inside epoxy matrix

We observed that all nanofibers are curved not straight. So the Young’s modulus of the nanocomposite material is always lower than the model prediction using traditional micro-mechanics theory assuming straight nanofibers/nanotubes, as reported by Prof. Y. Huang’s group at UIUC.

Markus J. Buehler's picture

Mesoscale modeling of mechanics of carbon nanotubes: Self-assembly, self-folding and fracture

Using concepts of hierarchical multi-scale modeling, we report development of a mesoscopic model for single wall carbon nanotubes with parameters completely derived from full atomistic simulations. The parameters in the mesoscopic model are fit to reproduce elastic, fracture and adhesion properties of carbon nanotubes, in this article demonstrated for (5,5) carbon nanotubes. The mesoscale model enables one to model the dynamics of systems with hundreds of ultra-long carbon nanotubes over time scales approaching microseconds.

2007 Melosh Competition Finalists Announced

The six finalists for the 19th Annual Robert J. Melosh Medal Competition for the Best Student Paper in Finite Element Analysis were announced last Friday. They are

Baskar Ganapathysubramanian, Cornell University


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