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Rui Huang's picture

Thin films: wrinkling vs buckle-delamination

H. Mei, J.Y. Chung, H.-H. Yu, C.M. Stafford, and R. Huang, Buckling modes of elastic thin films on elastic substrates. Applied Physics Letters 90, 151902 (2007).

Two modes of thin film buckling are commonly observed, one with interface delamination (e.g., telephone cord blisters) and the other with no delamination (i.e., wrinkling). Which one would occur for your film?

Roozbeh Sanaei's picture

what's the status of Discrete-to-continuum scale bridging.

what's the status of Discrete-to-continuum scale bridging. and it's real application?

Bin Liu's picture

Woven Nano-Structure of Carbon Nanotubes

We have studied the above woven nano-structure of carbon nanotubes as one of potential designs for ballistic-resistance materials via the atomic-scale finite element method (AFEM). Our study shows that this structure is insensitive to structure defects. More details can be found in our paper.

Henry Tan's picture

metal foam

cellular metal: space is divided into distinct cells. The boundaries of these cells are made of solid metal, the interior are voids. Ideally, the individual cells are all separated from each other by metal but often this restriction is relaxed

A new type of bubble raft--challenge for clever students

17 years ago, while a postdoc at IBM meant to be doing other things, I thought about the following. Then recently I visited Ali Argon at MIT, and we discussed conventional bubble rafts and how useful they had been in studies of some problems in mechanics...such as of defects and so on.

Henry Tan's picture

Plastic bonded energetic materials

plastic bonded explosives are composites containing energetic grains, ranging in size from less than one to a few hundred micrometers, embedded in a matrix of high-polymer binder.

Mike Ciavarella's picture

Does anyone know a collection of Lennard Jones potential material constants??

I am trying to find out the theoretical adhesive strength limit of a few materials, or more precisely the ratio adhesive strength limit to elastic modulus. I think this is after all part of the Lennard-Jones constants potential - theoretical adhesive strength limit is simply the maximum of the curve.

Markus J. Buehler's picture

Large-scale hierarchical molecular modeling of nanostructured biological materials

There have been several posts recently discussing new directions in computational mechanics. Here is a review article that appeared recently that may be of interest.

Large-scale hierarchical molecular modeling of nanostructured biological materials

MichelleLOyen's picture

Poroelasticity references

Given the growing interest in poroelasticity within this forum, I thought I would post the link to "Poronet" -- the poromechanics internet resources network.  In particular, there is a nice long pdf chapter on the fundamentals of poroelasticity from Detournay and Cheng, 1993, which has become one of the standard references in the field. 

Research directions in computational mechanics

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Dear all,

I just joined this group last week. And, I'd like to share some of reading material that I found regarding research directions in computational mechanics. The paper was published in 2003, written by Tinsley Oden, Belytschko, Babuska and Hughes. It's entitled "Research Directions in Computational Mechanics" (Computer Methods in Applied Mechanics and Engineering, 192, pp 913-922, 2003). They outlined six areas with significant research opportunities in CM:

Mike Ciavarella's picture

clever load normalization parameters for hip joint prosthesis

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A normalising loading parameter useful in summarising the mechanical response of plane, pin in plate-like contacts is extended to axisymmetric, ball in socket-like contacts. Various diagrams reporting

Henry Tan's picture

Graphene-based composite material

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Graphene is the world's thinnest material.

The one-atom-thick carbon layers have shown remarkable strength and stiffness.

The so-called "graphene-based sheets" can be mixed into polymers, glasses and ceramics, to produce novel composite materials with useful thermal, electrical and mechanical properties.

Cycle Sequence of Fatigue Crack Initiation and Growth at the Submicron Scale

I recently used focused ion beam to fabricate some small structures, such as free-standing micro-beams, in LIGA Ni thin films and applied cyclic loads to those small micro-beams. In such a way, dynamics of fatigue crack initiation and growth can be revealed. Part of my results has been attached with this post.

Fatigue Crack Nucleation and Growth

High magnification micrographs for the micro-beam with a coarse grain structure: (a) grain structure in the near-notch region; (b) dislocation slip band structure at the load cycles of 60,000; (c) dis

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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?

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