iMechanica - Comments

Very interesting

Anurag Gupta — Sun, 11 May 2008 23:14:45 -0400

Prof. Suo,

I find the idea of such a group very useful. It can differ from (and thereby enriching it) the contents of jclub, by laying more stress on the fundamental ideas onto which solid mechanics rests. Our discussions can range from classroom problems to classical works.

Anurag

there are many discussions

wenlong — Sun, 11 May 2008 20:48:34 -0400

there are many discussions lately about fund, research and the system in academic field, which are very interesting and significative. like in numerical field, there are always commercial software and open source software. both of these are made by our scientists and researchers(some from industry), and both of these are used by and made for our scientists and researchers. so the process going through is making some money in this field among these people. and the money goes somewhere we do not know, but mostly out this field, out these people, or not related to this field, not related to these people. somehow like researchers doing research for selling the research to other researchers for doing research, and recycle. many people who do not want to pay or cannot pay for this repeat the same work done by others. the work mostly by students. it could not be expected that everything is free today. but at least it is wasting the time and energy in academic field , which is not good, but the truth. i believe the achievement of the field as an entity should be much higher without all this repetition than now. then this work and mechanicians would play more important role in society and attract more attention. grand and fund may be an easier issue for mechanicians.

as said above, the system is old and maybe not suitable for today's development and speed. but why do researchers publish papers? first, students need publications for graduation. for academic staff publications are needed in order to find a good job or get promoted in academic field.this has been posted in some post in imechanica. in conclusion, we need publishing papers to make ourself look strong, good and recognised by other people also from this field. so it seems the problem is actually from ourself, it should be easier to correct something we have ourselves.

it seems that publications are a symbol today, in the introduction or resume of researchers, people always list how many publications they have like titles and journals to show the glory. and these are mostly reviewed and considered by our other researchers also in academic field.

secondly, the work published could be considered approved, and mostly correct. not like many things today not correct on web or somewhere without responsibility cannot be trusted. but this reason seems not the most important one.

we definitely need to work and make the work right and known by others colleagues if they are contributive. people more use internet for searching papers today, not book, wait for delivery and read the jounals themselves. we can input some key words in google or other engines, then may find what we need, the paper found might be useful, maybe not, but you may be not really read which journal it is from sometimes. if it's like this, the journal itself maybe not so much important, but the impact factor.

is that possible that researchers who have made some work done, not bothered by which journal should it be sent to, but send it here imechanica. since lots of authoritative scientists are here, and lots of researcher maybe not so authoritative, but also have lots of knowledge and lots of enthusiasm here. they can make the work correct and approved, then put the approved work by many people's review and discussion in a section in imechanica (e.g.paper section). and the work cannot be approved with even one single rejective voice.

maybe then in ideal, for researchers at least from engireering, we are not going anywhere else for researching papers and the newest work, just come to imechanica, since here is the most fast undated, comprehensive and authoritative place to be. google may not be so dominant at least for this field, maybe it is still, but come back here finally anyway. journals may die, since all papers come here,not them, just in different specifc field entry in this section. and the thing happend to paul paris or above may not be happening again after examination by millions eyes.

in rejected papers, there are definitely some papers which are not good enough, there also might be some papers with some novel or genius ideas buried, because they are not well decorated or ahead of the times or like the reasons from the above post. which is shamed and better avoid this happening.

linear connection and dislocation densitry

Amit Acharya — Sun, 11 May 2008 17:31:45 -0400

17:10:0917:10:09

Dear Arash,

If one does not want to think about the intermediate configuration, that is fine - it is a conceptual crutch, for those that find it useful to visualize things.

Given the field F^e-1 on the current configration, one can define a linear connection and then a torsion tensor (you know about all this) - all kosher geometrically - and of course what is defined as the two-point dislocation density in terms of curl Fe^-1 on the current configuration can be precisely related to the torsion.

In my paper with John Bassani in 2000, we work out these relations between the geometric and continuum mechanics points of view. I'll upload it to my original post just in case some people are interested.

best,

Amit

intermediate configuration

Arash_Yavari — Sun, 11 May 2008 16:41:23 -0400

Dear Amit:

Thanks for your detailed and informative response.

Just one comment.

I don't think one can identify tangent spaces in the intermediate
configuration the way you're describing it. This is fine as long as
you're dealing with Euclidean spaces. In the case of non-Euclidean
spaces (and as far as I can understand it the intermediate
configuration is not Euclidean) you cannot make such an identification
of tangent spaces; you would need a "connection" to be able to parallel
transport vectors (or tensors in general). Maybe the way Burger's
vector is defined traditionally works for practical problems but as far
as I can tell the definition in its present form is not mathematically
consistent. I don't think this is a matter of taste; if one realizes
that a given space is not Euclidean and defines it only locally (and
calls it an "anholonomic space", a "collection of pieces", etc.) then
integration of a vector field would be meaningless (this integration is
meaningless even for a smooth curve in the intermediate configuration).
You're correct that we integrate tractions in continuum mechanics but
that's because continuum mechanics is formulated on Euclidean spaces.
And of course, this is reasonable as Euclidean space is where we
observe the deformed body and is where our deformed bodies live. But as
soon as you define something like an intermediate configuration, things
are completely different. I agree that "rigor" is not always necessary
in engineering problems and many important problems can be solved by
engineering/physical intuition, but I think in the works that argue a
geometric (or mathematical) treatment of defect mechanics, Burger's
vector is ill-defined and intermediate configuration seems to be more
or less like a mysterious object.

Regards,
Arash

As a student and daily user

Xuanhe Zhao — Sun, 11 May 2008 14:26:00 -0400

As a student and daily user of iMechanica, I find the idea on Study Group quite attractive. I would like to add one more suggestion. Besides general ideas, the study group may also consider some specific technical questions, sush as "how to solve a type of boundary value problems with ABAQUS". As we can see, many technical problems raised by students on imechanica are relatively unattended. Examples include:

For engineers and engineering students, the answer to these questions is of essential importance to their current projects. A wise clue may save them months of struggle in mud. Therefore, a study group focused on technical questions would attract more attention from student side.

Study Group of ideas of lasting values

Zhigang Suo — Sun, 11 May 2008 11:47:57 -0400

I've thought more about Dhruv Bhate's idea of Study Group. His idea is appealing:

the mission of the Study Group can be simply to discuss ideas of lasting value to mechanics.

We learn difficult ideas by discussing and by teaching one another. We appreciate these ideas better by relating them to other ideas, and to problems of current interest. Some sample ideas can be

The Griffith paper.
Singular stress field
Bifurcation.
Koiter's thesis
Biot's 1941 paper on poroelasticity.
Kinematics of deformation, polarization, growth, etc...

The Study Group can run like the jClub, with an Editor recruiting Discusiion leaders, each Discussion Leader initiates a Theme, and users discuss. As Biswajit Banerjee pointed out, many of us are perpetual students. The Discussion Leaders can be recruited among students of all ages.

I'd love to hear from you about this and related ideas.

note

Huai Huang — Sun, 11 May 2008 08:21:26 -0400

changed some incorrect notes in ppt, maybe need more polish

a letter and my reply from Prof. Andrea Carpinteri from Parma U

Mike Ciavarella — Sun, 11 May 2008 07:47:46 -0400

Dear mciava@lms.polytechnique.fr,

I would like to know:
(1) WHICH are your name and surname, please?
(2) WHY did you send such a kind message to me, please?

I look forward to receiving a kind reply.
Andrea Carpinteri

______________________________________________

Professor (Dr Ing Mr) Andrea CARPINTERI
Full Professor of Structural Mechanics
Professore Ordinario di Scienza delle Costruzioni
(Structures, Materials, Fatigue and Fracture)
Director of the Testing Laboratory
Direttore del Laboratorio Prove Materiali e Strutture
Dept. of Civil Engineering
University of Parma
______________________________________________

dear prof. Andrea Carpinteri

thanks for your letter. I guess you are in a rush and do not have time to read the details. I hope when you act as reviewer you don't do the same.

I wrote to you as Member of Editorial Board of Int J Fat. to inform you of my reaction to the rejection.

I am not complaining against you, not even against Neil James whom I like very much. I complain about the middle-aged system called scientific journals with only 2 referees, which in the times of WIKIPEDIA and GOOGLE is saturated and dying.

I invite you to read Zhigang Suo proposal to NSF (also that not accepted by referees); which originated imechanica. Read the original proposal, too

Zhigang very interesting that your proposal was not funded...
http://imechanica.org/node/2908#comment-7392

An old and unfunded proposal to NSF to create iMechanica
http://imechanica.org/node/1484

Since my paper is suggesting to return to the pre-Paul Paris era, and Paris himself ound very difficult to enter with his innovation, I am finding the same
resistance again, even to return where we were!!! Moreover, it is harder for me alone, than it was for Paul Paris. Paris in 1957 had only 3 sets of data, now

what the reviewers ask me to do is to take a HUGE LITERATURE of millions of data points, and return back to my model, with pre-Paris.

It is particularly paradoxical and reminds me of when Science and the Inquisition invented system to put smoke and barriers for people to understand science, like the use of Latin.

That my idea works is for SURE, since there are recent BOOKS about the 2 limits from which I interpolate (DAVID TAYLOR AND BAZANT). So my innovation is only conceptual, but I DO NOT do much! Maybe the reviewers are BAZANT and TAYLOR, and they are upset that with only 1 equation I condensed 2 books?

To me, this is very likely. Sorry for them.

Thanks for your signature, which is very nformative as you are Director of many things I didn't know of. One nformation you don't write in your signature is that you are member of Editorial Board of Int J Fatigue, and if you take more time than a few seconds to read my E-mail, you will understand the rest. You can ask
also your collaborator Andrea Spagnoli about me. Finally, I can write you an old-fashion letter on ink and paper if that helps you. Is is ok on A4 format, or you prefer some pergamena (plural pergamene)? Is it ok English, or you prefer Latin, or ancient Greek, for explainig the matter?

For now, best regards
Mike

and my reply to Prof. James

Mike Ciavarella — Sun, 11 May 2008 07:31:44 -0400

From: mciava
To: M.James@, david.mcdowell@,
CC: D.Lovegrove@elsevier.com[+]

dear Neil

Indeed, as I say, I like you! It is the old system of journals that is
dying. 2 referees are not enough, and too random choice. Also, if you
took for example, David Taylor and Luca Susmel for example, they would
like to stop me, as they may fear I go 10 times faster than me, despite I work on 100
more things than them, and have no time for details, as Leonardo had no
time for details. So by posting to imechanica, one day people will help
me to improve the paper for details, like presently WIKIPEDIA has
already surpassed BRITANNICA in only 4 years.

So, I am loosing interest to write papers. It is slow and not rewarding.

Also, these days, we need collaborative efforts. It is impossible for me,
given fatigue is 1% of my interests, to make tests like the reviewers
ask (they are NOT honest, as if they were, the VERY DAVID TAYLOR BOOK
does already this). So I am only INTERPOLATING between equations that
are VERY CONSOLIDATED. THESE PEOPLE WANT TO STOP ME!

I am not upset, I am creating my own system, it will NOT be with ELSEVIER
I am afraid, as ELSEVIER is too old-fashion too.

I wrote to Dan Lovegrove some time ago about putting together a new
journal, he didn't not even reply!!! They are going to dye very soon.

They have not read Al Gore and his CURRENT TV, they haven't read GOOGLE
and how it will move into everything. Elsevier will soon collapse and
Dan Lovegrove will loose his job for not being clever to respond to my
emails.

Maybe you can take part of my new role, which I partly describe here:

A paper rejected by Int. J. Fatigue Persistent Nepotism in Peer Reviews,
and why traditional journals are dying very fast!!
Submitted by Mike Ciavarella on Sat, 2008-05-10 17:02.
http://imechanica.org/node/3170

Zhigang very interesting that your proposal was not funded...
http://imechanica.org/node/2908#comment-7392

The American Paradox: Is the TOP science moving silently to ASIA ??
Submitted by Mike Ciavarella on Wed, 2008-04-30 07:54.
http://imechanica.org/node/3144

Regards
Mike

the reply from Neil James, Editor IntJFat, and Pro-Vice Chancell

Mike Ciavarella — Sun, 11 May 2008 07:29:00 -0400

From: Neil James[+]
To: mciava@lms.polytechnique.fr[+], David McDowell[+]
CC: Lovegrove, Dan P (ELS-OXF)[+]

Dear Mike

I am sorry that you are unhappy with the outcome of the review process, but I can assure you that your paper went to two very respected fatigue researchers, whose views coincided quite closely. Having had papers declined myself, I can understand your disappointment, but I personally always view referees comments as a positive opportunity to improve my understanding and manuscripts.

Yours sincerely

Professor M Neil James
Pro-Vice Chancellor and Dean: Faculty of Technology
Co-Editor: International Journal of Fatigue
Web Pages: http://www.plymouth.ac.uk/si http://www.elsevier.com/locate/ijfatigue
Faculty of Technology
University of Plymouth
Drake Circus, Plymouth, PL4 8AA
ENGLAND

dear friend, thanks for your suggest to be prudent, but....

Mike Ciavarella — Sun, 11 May 2008 03:51:00 -0400

You miss the point. In times of wikipedia, it is impossible for me alone to make the counterrevolution from Paris. Most of what I do was already proved by experiments. I clearly cite in the paper that there are books from David Taylor and Bazant who have shown this, so why the reviewers wants to see this proof again? Maybe he is blind, or maybe he looked at this only 1 minute? We are all in the saturated world of the old-type research, and we are all stressed out and we don't do reviews properly.

Or maybe the reviewers are exactly David Taylor (or his very close collaborator Luca Susmel), and they are simply protecting themselves from the ideas that they think will make them publish 200 more papers in the next 10 years? Jumping to the conclusions, I spoil the easy career progress of other people.

This is what I am talking about. Of course people react. I am engaged now in a project to create on the web collaborative places like wikipedia, but for scientists, so if I have an idea, I will post like this, and people will help me. I have so many interests that I cannot spend my entire life to improve this paper I am talking about. Other people will. And because it was rejected, it will work even faster!!

a reaction from a collegue expert in fatigue from south-america

Mike Ciavarella — Sun, 11 May 2008 03:48:18 -0400

Hi Mike
I am sorry you received such a tough review, but you are not the
first nor
will be the last to have good ideas questioned and rejected by
scientific
journals. Remmember that Paris himself could not publish his
classical
paper, the first really original idea since Wöhler days, because it
was
rejected by all the good journals of that time: it really sounds
ridiculous
today, but the fact is that he had to publish it in an obscure
Washington U.
internal publication. Thus, you are not in bad company.
But,
if you permit me to talk as a friend, please do not publicize too much
your
emotional protest, to avoid an institutional payback. The
so-called
"scientific community" is as hypocritical as any other, and reacts
to
general attacks segregating the attacker. I am older than you are, and
I
have passed through similar desapointing experiences, receiving
absurd
reviews which probably even included prejudice against below the
equator
researchers, with observations that almost could be translated by
"who you
think you are to question this point". And I have learned that a
strong but
very polite answer to the reviewers letters, elegantly pointing
out the
fallacies of their arguments, is far more productive than a loud
protest.

Toughness and Crack tip sharpness in nanocomposites

Arun K. Subramaniyan — Sun, 11 May 2008 01:17:18 -0400

The relationship between toughness and the relevant microstructure length scale pointed out in the paper is interesting. We reported a similar behavior in nanoclay reinforced composites and a quantitative method for finding the validity of stress intensity factor as a measure of fracture toughness in one of our papers: Composites Part A: Applied Science and Manufacturing Volume 38, Issue 1,
January 2007,
Pages 34-43, doi:10.1016/j.compositesa.2006.01.021

RE to Anurag continuum defect mechanics

Amit Acharya — Sun, 11 May 2008 00:53:31 -0400

Dear Anurag,

Thank you for your comments. My responses, following your numbering scheme:

1) As you might have noted, I do not imply that the C&G measure is not important, especially kinematically. My partial concern in the paper is purely with its appearance in the free energy function and I provide a concrete argument for saying so. Please also see my response to Arash in this regard.

One can also formulate a dislocation mechanics and plasticity without any physical need whatsoever for a reference configuration, without involving the dislocation tensor in the free energy and, for that matter, without involving the tensor F^p altogether - if you are interested, you can see my JMPS 2004 paper for the situation where the scale of resolution is such that there are no 'statistical dislocations' and the last section of another JMPS paper in 2006 with Roy for the mesoscale situation.

2) I don't understand your point here and whether you have an objection or not. I suggest a local constitutive equation for the free energy and the stress (not including the dislocation tensor) and show that with such constitutive equations, one recovers the stress field prediction of classical dislocation theory related to individual or collections of discrete dislocations with non-singular cores. Moreover, the argument also shows that if you stuck in a dependence on the dislocation tensor as well, you would overestimate the strain energy of the dislocation distribution.

Interestingly the argument also says that if in conventional plasticity theory one puts in a plastic distortion field whose curl corresponds to a given dislocation density field (including a single discrete dislocation), and then one solves the equlibrium equation, classical plasticty would deliver the stress field of that dislocation field. As an aside, to pull this off numerically the equations have to be solved somewhat differently and a JMPS paper paper with Roy in 2005 shows this.

I don't think this point is appreciated widely enough.

Re to Arash, continuum defect mechanics, convected derivative...

Amit Acharya — Sun, 11 May 2008 00:17:18 -0400

Hi Arash,

Thank you for your comments. The manuscript is accepted for publication in the Proceedings of a recently held IUTAM symposium in Cape Town.

My Responses:

0) On the Kondo, Bilby, Kroner genre - In Kroner's work there are key ideas that when combined with continuum mechanics and homogenization (of dynamical, nonlinear PDE systems!) can result in much progress for saying something about microstructure evolution and its macroscopic effects. Kondo's program is not yet entirely clear to me (although I have read a fair bit of his RAAG memoirs), and as I am 'narrowly' interested in what progress can be made in plasticity with these ideas, Kondo's stuff is too general. I find Bilby et al. too kinematics oriented on this particular topic and, while this is necessary, it is not sufficient for my goals.

I am not a stickler for 'rigor,' so as long as there are good ideas, it suffices for me - but again, this is a matter of taste.

On your comments on C&G:

1) I don't think there is a problem.

Assume it is a good day, and you have a unique motion with all kinds of smoothness - so the deformation gradient at a point is unique. Now if F^p is unique and invertible (as would happen from a crystal plasticity specification of L^p, assuming everything is going well with the ibvp) then F^e is.

2 + 3) I think this is OK too. I will discuss the issue you raise in a minute.

As I understand it, what they are referring to is the fact that the intermediate 'configuration' is really no coherent configuration but a 'collection of pieces' if you will, one piece corresponding to each material point. So you cannot have a nicely defined continuous curve on such a beast for doing a line integral, because to do so you need to define a tangent to the curve etc.

What you are referring to is how legitimate is it to add vectors on different tangent spaces of the target manifold. Basically, one makes the identification that each tangent space is the same vector space represented by the translation space of three dimensional Euclidean space, so one can add vectors on these different tangent spaces without ambiguity.

From the continuum mechanics point of view this is OK, but you might object to it from the geometric point of view - it is a matter of taste. In my opinion, this is fine, as I entirely buy physical notions like statics means 'traction integrated over the surface of a body is zero' and balance of linear momentum and these statements all rely on the abovementioned identification.

4) On Objective rates as Lie derivatives: Actually, there is a little technicality here, if you want to split hairs. Not all objective rates are Lie derivatives w.r.t to the spatial velocity field or any flow for that matter.

The basic point here is as follows: An objective rate (convected derivative in the language of Hill, 1978) at a material point is defined through the existence of a time dependent invertible tensor function of time that allows one to pull back, do a time derivative on the pullback, and then a push forward - it is a purely pointwise operation. It may turn out that the field of invertible tensors over a local patch of material points may not be compatible so that it cannot be written as the two point gradient tensor of a flow. In that case you cannot define an objective rate as a strict Lie derivative w.r.t some flow.

(just definitions, basically).

However, the motivation behind the definition of something as weird as a convected derivative of some sort of (volumetric, areal, linear) 'density' field is physically best understood, in my opinion, in the context when it is a Lie derivative w.r.t. some flow, coupled with the question of determining the time derivative of the integral of the density field over a time-dependent volume/area/curve, respectively.

Basically Reynolds transport theorems for volumes, areas, curves...

And, yes, of course, the covariant, contravariant, and mixed convected derivatives of the same tensor when convected by the deformation gradient are not all the same tensors (but precisely related), and if you took the same tensor and now convected it with an invertible tensor other than the deformation gradient, you would generate many more objective rates. Hill's Invariance in Solid Mechanics (1978) article gets through an awful lot of high powered stuff in a just a few pages in a very simple but beautiful way. I think this treatment is ideal for the student of continuum mechanics who may not have had any exposure to differential geometry. My students and I go through these in their plasticity class.

5) Regarding what should enter the free energy - The main point there is, we can of course start to put whatever we want but in this case there is a good successful standard to go with if we want to talk about dislocations and their energy, which is the classical theory. I do not think this is a matter of geometry or mathematics - ultimately it is a matter of material behavior. I think a gradient can be important to represent core effects but it will be a small contribution to the energy. One might see a dependence arising due to averaging too (but this requires that one does the required averaging), and in the last paragraph of my paper I have recorded some elementary ideas on what I think of this matter.

6) I have worked out the general nonlinear theory in my 2004 JMPS paper, and even there I work only with a dependence of the free energy on the elastic distortion tensor and no gradients. Things work out quite well in the general theory too.

all the best,

Amit