iMechanica - Tan's hot topic series - Comments

Let me share my view on

Milli — Fri, 31 Oct 2008 07:51:11 -0400

Let me share my view on your question.

Bulk modulus,K, is often assumed time-indepenent. The reason behind this lies on the assumption that the material in subject is highly incompressible and has a high bulk modulus value.If the material is incompressible, then it implies that the possions ration is nearly 0.5 always and thus can be taken time-indepenent. The main point here is, the material has to be highly incompressible for the assumption to remain valid.

How can we use the concept of temperature for few atoms

Teng zhang — Tue, 21 Oct 2008 05:35:48 -0400

Very interesting topic!

I read the first parts of the book Thermal physics written by C.Kittel under the suggestion of Prof. Suo. I post this comment to explain what I learn form that book.

First, the thermal contact plays a signifcant role in the definition of the temperature and entropy. As for two systems, we can obtain the total degeneracy of all the accessible configurations after the thermal contact. If the number of particles in at least one of the two systems is very large, the numbers of that total configurations can be replaced by the number of the states in the most probable configuration. Only in this case, the additivity of the entropy is valid.

As defined in the lectures of Prof. Suo:1/T = change in the logarithm of the number of quantum states divided by the change in the energy of the system, everything else being fixed. The introduction of temperature is to describe the equilibrium state of two systems under thermal contact. It is noted that this equilibrium state is just the most probable configuration. The formalism of T is also derived from the maximum of total degeneracy of all the accessible configurations. In this sense, we can think T is corresponding to the most probable configuration. However, the states expect the most probable configuration can be observed only when the number of particles in at least one of the two systems is very large. If the two systems are both small, then we can see that many different states expect of the most probable configuration which can be represented by the temperature.

Thus, for a small system with only few atoms, we can define the temperature of this kind system via letting it contact with a very large system. However, when we make two small systems together, how can we obtain the final temperature of these two systems, even though we know the temperature of them before contact. If we make they contact with a large system, this may destroy the states of the real systems and make them have the same temperature of the large system itself.

I am not sure whether my understanding is reasonable or not, however, I hope this can make any help for this topic.

Best regards

Teng Zhang

VAMUCH

Wenbin Yu — Fri, 26 Sep 2008 12:42:39 -0400

Yes. You can calculate the exact value of effective stiffness of fully periodic medum by one unit cell. If you use VAMUCH , a general-purpose micromechanics code, you will always get the same effective properties no matter whether you use one unit cell or multiple unit cells.

Effective stiffness of composite

hamanh — Tue, 27 May 2008 11:03:43 -0400

Dear mechanician,

Thank you for useful discussion. I would have one question that: Is it possible to calculate the Exact Value of Effective Stiffness of Fully Periodic medium by One Unit Cell ?

Thank you very much

Regards

Accuracy when volume fraction approaches 1

Henry Tan — Thu, 22 May 2008 12:24:15 -0400

How accurate is the micromechanics estimation when the particle volume fraction approaches 1?

Tensorial LAW

Chidanand Kadakol — Fri, 16 May 2008 12:04:18 -0400

Can i know more about tensorial law for viscoelastic.., as i have developed material modeling and shift factor estimation.., for viscoelastic materials ..,

so than i can contribute my work for u..,

Regards
Chidu

prepare CNTs by using CVD process

chau pai tung — Fri, 09 May 2008 12:22:00 -0400

Hi all,

This is Bankim J Sanghavi. i am doing research on CNTs. i want to prepare CNTs by using CVD process. But the reproducibility by this CVD method is not very good.

Could someone please suggest me some way how to get good reproducibility.

Are there any other methods which can be used in a normal laboratory to prepare CNTs?

I shall be highly obliged if I could get a reply soon

Thanking You,

Bankim J Sanghavi

email: bankim_j_sanghavi@yahoo.com

vol. 140 (2003), 1-5

Henry Tan — Tue, 11 Mar 2008 12:36:59 -0400

vol. 140 (2003), 1-5

viscoelasticity

yoshiaki yamada — Sun, 09 Mar 2008 23:52:53 -0400

When I submitted a review article "Time Dependent Materials" to a Monograph Shock and Vibration, Ed. W and B Pilkey, University of Virginia, SAVIAC, 1995, pp.253-284, I found that ABACUS included a well organized capability for viscoelasticity with also a well prepared manual and believe that ABACUS being remained as the most proficient software for the subject by the finite element method.

As an self-introduction, please visit: Journal of Material Processing Technology, Articles in vol.140(1903), 1-5 Y.Yamada, Mechanics of Materials, I saw and participated: a reminiscence.

yoshiaki yamada

viscoelastic

TarekNinouh — Thu, 06 Mar 2008 16:56:56 -0500

Hi I am looking for the development of the law tensoriel viscoelastic
Thank you

toughness in viscoelastic materials

MichelleLOyen — Sun, 13 Jan 2008 07:43:46 -0500

I think this is still quite an open question. Historically it is typical to lump plastic deformation in with toughness such that plasticity is a toughening mechanism. However, since viscoelastic processes are recoverable--just perhaps on a time-scale long relative to the experiment itself--we chose to separately account for viscoelastic dissipation explicitly when looking at fracture of soft tissues. In this case, we subtracted off the viscoelastic part of a hysteresis loop in which energy was dissipated by both viscoelastic and fracture modes. There are relatively few papers on fracture in soft tissues and I think it will take some time before we understand fully whether this is a more useful approach than lumping the two parameters together. But in some unpublished preliminary results we found that the viscous deformation and fracture resistance did not directly correlate, supporting the idea that these are two very different things. For details on our method see the paper linked here .

Re:Particle with inertia in a viscoelastic medium

Biswajit Banerjee — Fri, 04 Jan 2008 13:46:31 -0500

Hi Honglai,

I think the problem of viscoelastic flow around a rigid sphere was simulated by Sugeng and Tanner, 1986,J. Non-Newtonian Fluid Mech., 20, 281-292. There are probably more recent results.

The best text on that sort of thing is, in my opinion, "Dynamic of Polymeric Fluids: Vol.1: Fluid Mechanics", by Bird, Armstrong, and Hassager published by Wiley-Interscience in 1987.

-- Biswajit

Viscoelastic Fracture

vicky.nguyen — Fri, 04 Jan 2008 10:34:40 -0500

Assuming that the failure process is rate-independent (that is the intrinsic fracture energy is constant) you can calculate the increase in the energy release rate from bulk viscous dissipation. For a 3 parameter (standard solid) model, the energy release rate for a fast steady-state growing crack (no inertia) scales with E0/Einfity where E0 and Einfity are the instantaneous and equilibrium Young's moludus.

My paper below includes a bunch of references on this topic.

T.D. Nguyen and S. Govindjee (2006) Numerical study of geometric constrain and cohesive parameters ini steady-state viscoelastic crack growth, Int. J. Fracture, 141, 255-269.

Thao (Vicky) Nguyen
Assistant Professor
Mechanical Engineering Department
Johns Hopkins University
http://me.jhu.edu/~tnguy108

viscoelasticity

vicky.nguyen — Fri, 04 Jan 2008 10:04:44 -0500

Hi Henry,

I just stumbled on your blog regarding viscoelasticity. I work on modeling viscoelasticity, most recently anisotoropic nonlinear viscoelasticity of fibrous tissues. I'd like to address a few of your points:

1) I like Ferry's book Viscoelasticity in Polymers. It's a wonderful mechanics and materials treatment that combines experiments and modeling.

2) Bulk modulus does indeed change with time for a lot of polymers. It's often neglected because for most materials, it changes by a factor of 2 or so while the shear modulus can change by orders or magnitude. Plus it's more difficult to characterize experimentally.

3) If you're dealing with large deformation or with a material where the creep/relaxation rate is dependent on stress/strain you should consider a fully nonlinear treatment. These treatments do not assume a separable time-dependence and strain-dependence of the stress response. These can't be solved analytically, but let me know if you're interested and I can point you to some references.

Thao (Vicky) Nguyen
Assistant Professor
Mechanical Engineering Department
Johns Hopkins University
http://me.jhu.edu/~tnguy108

A viscoelastic material is tougher

Henry Tan — Fri, 04 Jan 2008 09:39:53 -0500

Toughness is the resistance to fracture of a material when stressed. It is defined as the amount of energy per volume that a material can absorb before rupturing.
A viscoelastic material causes energy dissipation during fracture, and therefore, increases the amount of energy before rupture.
So, what you said is right. A viscoelastic material is tougher than the a material with the same properties but without viscoelasticity.