iMechanica - suo group research - Comments

Sorry for the late reply.

Ying Li — Tue, 24 Jun 2008 04:56:10 -0400

Sorry for the late reply. EBSD is a good method to monitor the texture of the film. By the EBSD, the grain size of the Cu could be clearly investigated. Especially, the in-situ SEM loading with the EBSD could check the grain growth when the Cu film is under tension. Such a in-situ SEM with EBSD method has been successfully applied to investigate the equal channel angular extruded Mg-3Al-Zn magnesium alloy. However, as you say, the indexing rate is very low for nano grains.

Ying Li Department of Engineering Mechanics Tsinghua University Beijing, 100084, P. R. CHINA

The size effect on failure modes of metal thin films

Nanshu Lu — Thu, 12 Jun 2008 13:26:26 -0400

Hi, Teng,

Thanks for pointing this out. Actually it is what I am doing now.

Basically the film failure mode exhibits a ductile-brittle transition as film thickness reduces. But we found that neither the thickness nor the grain size alone will be sufficient to understand the phenomenon. I am trying to take both factors into consideration to give better explanations now.

I'll send you some preliminary results through email.

Nanshu

Difficulty with grain size distribution

Nanshu Lu — Thu, 12 Jun 2008 13:10:00 -0400

Ying, thanks for your interest and comment.

We haven't found any efficient way to determine the statistical distribution of nano grains so that we are trying to show straightforward micrographs at this point.

TEM is not suitable because the film microstructure is very unstable and easy to change during sample preparation.

EBSD works well for annealed films with micron-sized grains but the indexing rate is very low for nano grains.

XRD does not show much broadening because our grain size is not small enough.

I will try AFM to see if it can give reasonable results.

Manually sketching grain boundaries will always work but I will keep it as the last choice.

Do you have any experience or suggestions on this to share with us? Many thanks.

Nanshu

Effect of film thickness

Teng Li — Thu, 12 Jun 2008 11:08:49 -0400

Nanshu,

Neat experiments! The paper reveals another critical mechanism governing the failure of metal films on polymer substrates.

The films reported in paper are about 1 micron thick. I'm curious about the effect of film thickness on the deformation-induced grain growth. For example, does similar phenomenon occur in a thinner film (say, a few hundred nm thick) on a polymer substrate.

Interesting work!

Ying Li — Thu, 12 Jun 2008 08:26:24 -0400

Have you done the detailed work on the grain size distribution according to your SEM or FBI imagines? I think the statistical data may be more helpful to understanding the deformation-induced grain growth.

Ying Li Department of Engineering Mechanics Tsinghua University Beijing, 100084, P. R. CHINA

Dear Wei, Thanks for

marc — Thu, 29 May 2008 22:17:55 -0400

Dear Wei,

Thanks for your explanation for the free-energy function. As you said,we must do some experiments on soft materials which have no corresponding free energy funtion. the acquirement of the free-energy function for a new soft material may be very difficult, and the experimental test is very importrant. For a new soft material without free-energy function,experimental tests may be the only way to describe its mechanical behavior!

THANKS

Lianhua

Limitation of the current work

Wei Hong — Wed, 28 May 2008 16:01:38 -0400

Dear Lianhua,

Thank you for your interest in our work!

As you mentioned, the current implementation does have its limitations: it is only suitable for the final equilibrium state of diffusion. Although the chemical potential is not required to be constant in the simulation, it is a predefined field. In other words, we can not solve for the chemical potential, it must be given. Therefore a steady-state calculation of a complex domain might not be possible either.

No theory could ever predict a general free-energy function, although some theoretical abstraction might give insights to some specific material behavior, Florry-Huggins, for example. The right way to investigate the mechanical behavior of a material would always be experimental. Instead of doing one experiment on one material, one should do a series of experiment on a same material, using different loading conditions, different sample shape/sizes. Instead of starting from nowhere, I think it is always better to start from a theoretical model, and see the deviation. If there is no deviation, good, we extract the material parameter; if, most likely, there is deviation, we either modify the theory to say why, or just use the test result numerically, if a result is important. Also instead of testing the static/equilibrium behavior together with the kinetic properties, I suggest to do separate tests for a same material. These are just my general thoughts. Let's keep on the discussion if you have further interest.

Thanks,

Wei

good job.

Lianhua Ma — Wed, 28 May 2008 07:12:27 -0400

Dear Wei Hong,Your investigation on mechanics of gel is very good job.

you had implemented the theory in the finite element package, ABAQUS. the simulation of large deformation of similar hyperelastic material requires the satisfaction of quasi-static mechanical behavior. so , the FEM simulation in your paper did not reflect the diffusion process of solvent in gel. In other words, you assumed that the chemical potential is constant in one simulation, In fact, the chemical potential is variable in different position of gel. the theory in your paper may be only suitable for final equilibrium state of diffusion.

another question: the key of your paper is the free energy funtion W . For other soft materials, If we have no corresponding free energy funtion put forward by predecessor, How can we investigate the mechanical behavior of soft materials? can you give me any advice? thanks for your paper! Hope to keep in touch with you.

THANKS

L.H. MA

Re: additional questions

Wei Hong — Tue, 27 May 2008 10:58:10 -0400

We don't need to specify the thermal expansion coefficient. We use the "T" just as a general field parameter, not as temperature, so it has nothing to do with thermal expansion.

Due to the definition of the chemical potential, it is always negative. It can be negative infinity to 0. (-0.05 is just an arbitrary number we picked.) As we start from -0.05 and end in 0, so it is still swelling instead of contracting. Just don't read it as temperature.

Please feel free to let me know if you have further concerns.

Wei

Re:Re: A very good topic

Chang-Guo Xue — Sat, 24 May 2008 00:48:19 -0400

Thanks!Your answer give me prompting.The "onion structure of natural hydrogels was a promising work.I am very interested in it.But this time , I must do the research on microcantilever biosensor ,so I only study the hydrogels in my spare time . In the next step, I will fabricate a simple onion using Chitosan in plan.I prepare to learn more Knowledge for my research on the hydrogels.If you have any idea or good articles, Please give me in the email. I wish I can do some work in this field

additional questions

Minkyoo Kang — Fri, 23 May 2008 18:31:18 -0400

Thank you for your prompt response.

I have additional questions. In defining the temperature in the predifined fields, we usually need to define thermal expansion coefficient in the property module, otherwise we don't see any deformation. I wonder in your simulation whether you also need to specify thermal expansion coefficient, and if that's the case, how does the thermal expansion coefficient relate with the chemical potential?

Your chemical potentials are in the range of -0.05 to 0. Does this means we have to use the same values for the temperture input in the predefiend field which actually means contraction not swelling in the point of temperature?

I'm bothering you with many questions and I really appreciate your help.

Thanks,

Min Kyoo

setting chemical potential

Wei Hong — Fri, 23 May 2008 14:42:40 -0400

Dear Min Kyoo,

Thank you for your interest in our work.

Yes, you should specify the initial chemical potential according to the given initial free swelling ratio (3rd material parameter)

The chemical potential is specified using pre defined fields (temperture) in abaqus input.

Let me know if you have further questions.

Wei

Question about the chemical potential

Minkyoo Kang — Thu, 22 May 2008 19:16:58 -0400

Dear Wei Hong,

Thank you for sharing your source code.

I'm trying to use it for a swelling deformation problem. I have a question about the chemical potential. Since the initial free swelling is an input parameter, shoud we specify the initial chemical potential accoroding to the free swelling equation? If so, since the chemical potential is mimicked by a temperature-like variable in ABAQUS as stated in your paper, How do we specify the increment of the chemical potential or temperature?

Thanks,

Min Kyoo Kang

Re: A very good topic

Zhigang Suo — Thu, 22 May 2008 12:36:59 -0400

Dear Chang-Guo: Thank you very much for your interest. My group has only started working on polymers recently. These lectures mainly described what we have learnt so far, from reading papers, running calculations, and talking to experts in the field. Our recent papers have been posted on iMechanica, filed with the tag Suo Group Research. It would be great that people like you, with more background in polymers, are interested in this line of work. We hope to hear more feedback form you. Incidentally, we also liked the "onion paper" in Nature.

thanks a lot

zhan-sheng guo — Thu, 22 May 2008 00:20:56 -0400

gone with the wind