iMechanica - suo group research - Comments

Hi Chunguang, I will be

Wei Hong — Fri, 10 Oct 2008 09:57:26 -0400

Hi Chunguang,

I will be in Champaign next week, too. Let three of us find a time to meet.

Tuesday during lunch time or in the evening will be good for me.

Wei

Hi Wei, I quite agree with

Chunguang Xia — Thu, 09 Oct 2008 22:52:00 -0400

Hi Wei, I quite agree with you on the current theories on hydrogel. And I am desperately looking a field theory that can explain our experiments. And that is why I am excited about yours and hanqing's work. I am very happy to see if your theory can explain and predict my experiments.

thanks

chunguang

let us meet

Chunguang Xia — Thu, 09 Oct 2008 22:51:39 -0400

Hi Hanqing, i am not good at thermodynamics and thank you and wei for helping me out the confusion. I am free all the next Tuesday. Let me know your plan.

thanks

Chunguang

Solvent concentration and deformation

Wei Hong — Thu, 09 Oct 2008 20:48:34 -0400

Hi Chunguang,
There are two things I might need to comment a little bit more:

Our theory does not need 1+aC=det(F) at the first place. The condition is just a simplification based on the incompressibility of the material. For incompressilbe material, even when it is dry, C=0, det(F)=0 should be satisfied. So there is no problem at all applying the theory to dry network.
At the current stage of research on hydrogels, I don't think there is any theory that could "predict" the behavior of a gel. It is more or less a curve fitting or extracting of material constants from observations. The theory presented in our recent paper is rather a theoretical framework than a material model. It would not predict anything without experimental input. In terms of material model, we merely adopted Flory-Huggins theory for the static behavior, and linear kinetic law for time-dependent behavior, as an example in that paper. If fitting a single curve is regarded as prediction, then all existing theories could "predict" equally well.

Hope these comments will resolve your concern. Wei

thanks for the comment

Hanqing Jiang — Thu, 09 Oct 2008 18:11:57 -0400

Dear Chunguang:

Thanks. Firstly of all, unlike biphasic theory or coupled solid/fluid mixture theory, Prof. Suo's theory does not have "dray area". The deforamtion gradient F is for the mixture (i.e. gel). If a dry gel, which has no solvent molecules or C =0, is taken as the reference configuration, det(F) will be zero. Also, I thank you for pointing our the time scale issue. The framework proposed by Prof. Suo does not depend on material model, namely free energy function and mobility tensor. To more accurately predict the time scale, a more accurate material model is needed.

PS. By the way, I will be at UIUC next Tuesday. Maybe we can meet

Thanks.

Hanqing

Nice work

Chunguang Xia — Thu, 09 Oct 2008 14:21:30 -0400

Hi Hanqing,

This is Chunguang Xia from UIUC. How are you? Thanks for sharing your nice work. In your paper, you applied Prof. Sou's theory to dry sample, but the incompressbity condition in his paper, 1+aC=det(F) is not for dry sample. Because there is deformation in dry area, but the concentration C=0. The phenomenons in your simulation examples can also be predicted by most of others. In order to test Prof.Suo's theory, I think another very important aspect is the time scale, can his theory predict the swelling time scale more accurate than others(Li-Tanaka,Wang-Li-Hu,Tatsuya-Doi,etc).

thanks

Chunguang

ABAQUS UEL Fortran source code

Hanqing Jiang — Thu, 25 Sep 2008 11:44:20 -0400

Dear Rui:

I just posted the Fortran source code.

Dear Rui:

Hanqing Jiang — Thu, 25 Sep 2008 01:00:10 -0400

Dear Rui:

Regarding ABAQUS UEL, we follow the standard procedure to formulate element stiffness matrix, starting from the shape function, the integration scheme, material matrix, to stresses. In fact, it does not have big difference between steady state or transient problem, if you use UEL. The main difference is that chemical potential is a field varibale in transient analysis, while it is a constant (or uniform parameter) for steady state analysis. For the examples in the manuscript, yes, we do need to use some "tricks", such as the modificatioin of the stiffness matrix.

Thanks.

Hanqing

User-Element in ABAQUS

Rui Huang — Wed, 24 Sep 2008 23:51:49 -0400

Hi Hanqing

Thanks for posting your very interesting work! Could you give some details about the User-Element subroutine you used in ABAQUS? As I remember, in an earlier work by Wei Hong et al., they used UHYPE (user-defined hyperelastic material) subroutine to analyze the equilibrium deformation of hydrogels. Something different has to be done for the transient analysis as shown by the numerical examples in this manuscript.

Best,

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