Here's a puzzle for our readers.  The following image is of the surface of a failed joint followed by a picture of the joint (not the same one but a similar one) before joining.  What material is it? What caused the failure?  All manner of speculation is welcome.

Modern composite structures have a wide spread in their failure stress.  Advanced multiphysics codes can have a wide range of predicted behavior for nominally the same inputs.  How do we certify the design of such structures or the accuracy of such codes?

A few weeks ago I had mentioned in one of my comments that

"Almost every object that is used in modern societies contains some
material that has been extracted from the earth.  A huge amount of
energy is used in crushing rocks to a fine enough size that surface
chemistry may then be used to extract a particular metal.  Assuming
that we would like to use energy more efficiently, is there a way our
knowledge of mechanics can be used to minimize the energy used in
crushing rocks?  A huge number of people have worked on this problem -
yet an optimal solution seems as far away as ever. "

A meeting with my ex-classmate Dr. Barun Gorain of Barrick Gold Company  last weekend showed me that there are still quite a few problems of that nature in the mining industry.  In fact, Barrick is willing to spend as much as $10 million to solve one such problem.  The main site is called Unlock the Value and the bit that they are willing to offer cash for is a particular silver recovery operation.  Barun tells me that proposals are already in and are being evaluated - but I'm not sure whether those are for the silver problem or some other issue.   Can mechanicians help solve the problem?  Some of you might want to give it a try.

-- Biswajit 

I'm leaving the US in a couple of days and will probably take a hiatus of a few months from iMechanica.  Before I leave, I want to tell you about a C++ idiom that eases the implementation and use of multiple material models in a computational mechanics code.

The first book on C++ idioms was by Jim Copilien sometime in the late 1990s called "Advanced C++: Programming Styles and Idioms".  The idea of such idioms has become considerably widespread since then - so much so that there is a Wikibook on the subject .

I had promised to talk about subversion in my post on digitizing and editing figures but never quite got around to it. A recent spate of requests about downloading Uintah reminded me of my promise. So here's the story.

From Jim Guilkey: You are invited to attend the Fourth MPM Workshop.  This year workshop will be held in Salt Lake City, Utah on the campus of the University of Utah, on March 17th & 18th, 2008.  The format will be similar to the previous workshops,  held at University of Utah, Oklahoma State University and Sandia National Laboratory, where relaxed environments facilitated understanding and awareness of the MPM's strengths and  weaknesses via informal talks and discussion.  Topics of interest include, but are not limited to, numerical implementation and properties, algorithm properties, variations and additions, and  applications.  Please feel free to pass this announcement along to other potentially interested colleagues.

The following link brightened my day quite a bit.  Wikipedia can now be tasted!

http://ourfounder.typepad.com/leblog/2007/10/jimmy-wales-gro.html 

Metals have a lustre because electromagnetic radiation in the visible range is almost completely absorbed at an exponential rate close to a metal surface and then radiated back.  This effect and other properties of crystalline materials can be explained by their electronic band structure and the associated dispersion relations. In certain materials, notably semiconductors, distinctive band gaps are found indicating that electrons of certain forbidden energies which cannot propagate through the material.  A concise description of the basics can be found here.  

There is a close analogy between the motion of electrons in periodic crystals and electromagnetic waves in periodic dielectric structures. This analogy and the quest for materials with unusual electromagnetic properties has led to exploration of periodic materials with tunable electromagnetic band structures and band gaps.  Such materials are called photonic crystals and photonic composites.  A good review article on the subject is  E. Yablonovitch, 1993, "Photonic band-gap crystals", J. Phys.: Condens. Matter, 5, 2443-2460,   doi:10.1088/0953-8984/5/16/004.

Those of you who are looking for academic jobs and have not heard back from the places that you applied may find the following wiki useful.

http://wikihost.org/wikis/academe/wiki/mechanical_engineering 

The idea is to let people know whether a position has been filled or not.  You can add the positions you have applied to, whether you have been called for an interview, whether references have been requested, or whether you have got a rejection letter.

I just remembered another video that I had seen some time ago.  Many of you have probably seen it but here it is for those who have not. (The original page where I found it is http://www.maniacworld.com/Laminar-Reverse-Flow.html.)

Prof. Pat McMurtry's explanation is:

The following video is a nice depiction of how the eigenmodes of a plate change with increasing forcing frequency.

Following Andy's recommendation I have been reading Ellis Dill's Continuum Mechanics[1]. In page 75 of the book, we find the
well known result that the constitutive equation for an isotropic hypoelastic
material can be derived from a stored energy function only if

Via Jeff Weiss :

Our paper on the Mechanical threshold stress (MTS) model for 6061-T6 aluminum has been accepted by JoMMS.  There are several things of interest in the paper:

1) The use of a phonon drag model to predict the sharp increase in flow stress at strain rates above 10,000 /s.  This behavior is seen in a  number of materials and is hard to fit using standard power law plasticity models.  Our model does a good job in this regard.

2) The sharp drop in flow stress at high temperatures.  A slight modification to the MTS model allows us to do better than previous versions of the model.  But there is still some way to go.

If you are a faculty member on the tenure-track or a post-doc it's a good idea to chair a few minisymposia at conferences to buttress your resume.  Here's an opportunity for you:

Via: Lori Graham-Brady 

I've been to two conferences this year and I've again seen the same annoying features in many talks that have been warned against by numerous people over the years.

Here are some tips that might come in handy :

2 Research Associate Positions in
the
Dynamic Properties of Engineering Materials and Structures

 
 
 
An experimental and theoretical post-doctoral post exists to develop
protective materials and structures to withstand dynamic loading such as
land mines and high speed projectiles.
 
The successful applicants will have a PhD and a working knowledge of the
deformation and failure of engineering materials from a solid mechanics
standpoint.
 

In small strain elastoplasticity we start off with an additive decomposition of the total strain into elastic and plastic parts. In terms of strain rates we write

Some of you probably work on problems that involve moderately large strains. An useful strain measure for such problems in the logarithmic or Hencky strain. In particular, if you deal with the numerics of large strain simulations, you will often need to compute the material time derivatives of logarithmic strains.

From Youtube via Mark Chu-Carroll -> Art of Problem Solving:

Recently I have observed that if I don't perform a routine task for a few months, I tend to forget the steps needed in the process. I have to relearn the process and that's a waste of time. The intention of this blog post is to provide a list of tips for my use which can also be of use to other Linux users. I like free software. Most of the software on this list can be downloaded from SourceForge or some such place.

Professor Graeme Milton will be awarded the 2007 Prager medal from the Society of Engineering Science for his work on solid mechanics.  The medal will be handed out at the 44th Annual Technical Meeting of the Society of Engineering Science (SES2007) to be held between October 21 and 24, 2007 at College Station, Texas.

Guru at Entertaining Research points us to an article from Arstechnica on the recent National Academies of Science report on grand challenges in condensed matter and materials physics.  The report is called Condensed-Matter and Materials Physics: The Science of the World Around Us (the link takes you to a draft copy of the report and should not be cited).

I found the following sections of the report interesting:

Sean Carroll at CosmicVariance has an interesting set of slides on the 2nd law of thermodynamics and the arrow of time.  In this age of simulations, one question that came to my mind was whether we could do a molecular dynamics simulation of a glass of ice cubes, allow them to melt, and then simulate the process in negative time to get the ice cubes back.  Probably not.  But the fundamental physical laws do not have any time directionality to them.  So what gives?  More importantly, the universe appears to have had a low entropy beginning but the second law suggests that we are at an entropy minimum at this point in time.

A video that shows how some common couplings work. It's an advertisement but interesting nevertheless.  Does anyone know how the Thomson coupling works and what its main drawbacks are? It seems a bit complex and will probably have a higher rate of failure than simpler geometries - but maybe not?