iMechanica - Comments

"I wonder if such

Nimish — Thu, 07 Aug 2008 11:39:43 -0400

"I wonder if such simulations tell me anything useful at all except when the strains are quite small. " - Biswajit

Dr. Banerjee,

My experience is that when the material model (approx. material model) for the elastomer is developed from the modulus obtained from the elastomer hardness, the results are quiet good for comparison purpose. However, in terms of the absolute values, such a material model may result in a considerable different values. E.g. I had compared the peak contact pressure developed by an o-ring at different lubricant pressures using the approx. material model & using the accurate material model developed from the stress-strain data of the elastomer. I found that the percentage increase in the peak contact pressure - due to increased lubricant pressure - obtained by the approx. material model was close to the one obtained from the accurate material model till ~1000 psi. After ~1000 psi, however, results obtained from the two material models could not be correlated.

~ Nimish.

Research Paper Writing Skills

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researchpaperwriting — Thu, 07 Aug 2008 01:10:02 -0400

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Re: Treloar: The Physics of Rubber Elasticity

Biswajit Banerjee — Wed, 06 Aug 2008 23:22:35 -0400

Thanks Sanjay. The book is not in our company library but I've sent my spies to a nearby university to track it down and get it for me. I'll post my feedback as soon as I get my hands on the book.

Also, I have received a copy of the paper on mapping Shored hardness values to elastic moduli from one of our readers on iMechanica. I am truly grateful for that and will comment on the contents soon.

-- Biswajit

Short Cylindrical Shaft Problem

David M. Cooper — Wed, 06 Aug 2008 22:58:05 -0400

Assuming that this is a cantilevered cylindrical shaft, the conventional formula for the shear stress distribution across the section is

Tau = VQ/(Ib)

where in this case,

V = 27514 N

Q = Integrate[Integrate[y,{x,-(r^2-y^2)^(1/2),(r^2-y^2)^(1/2)}],{y,y,r}]

= (2/3)*(r^2-y^2)^(3/2)

I = Integrate[Integrate[y^2,{x,-(r^2-y^2)^(1/2),(r^2-y^2)^(1/2)}],{y,-r,r}]

= (Pi/4)*r^4

b = 2*((r^2-y^2)^(1/2))

2r = 15.05 mm

which results in a parabolic shear stress distribution

Tau = (4/3)*(V/A)*(1-(y/r)^2)

that assumes a maximum of 206.22 MPa halfway through the section (at y=0).

This problem sits between two extremes: a beam-like variation of the stress distribution (ref. Euler-Bernoulli, Timoshenko theory) and a uniform stress distribution (ref. Mohr's Circle). A computer-aided mechanical analysis would verify this.

The criteria for failure depends on the material and is typically in terms of the principal stresses (ref. Tresca & von Mises Envelopes). The above shear stress (assuming no axial stresses at y=0) is the same as the absolute value of the principal stresses at that point (which are equal in magnitude and opposite in sign/direction).

Treloar: The Physics of Rubber Elasticity

Sanjay Govindjee — Wed, 06 Aug 2008 18:34:02 -0400

Have you looked in Treloar's classic monograph The Physics of Rubber Elasticity? It is nicely explained there.

Prof. Dr. Sanjay Govindjee
University of California, Berkeley

It depends

Sanjay Govindjee — Wed, 06 Aug 2008 18:25:30 -0400

Not to sound too fuzzy but it really depends. if you are working exclusively on a certain class of materials then a company with good practices will over time have a knowledge database on the correlations of the fast easy tests to the more involved tests. The correlations will never be perfect but will give you a certain amount of confidence. If this database does not exist then it is your job (over time) to compile it. Otherwise you can not even say that your computations are even meaningful in the small strain regime.

-sanjay

Prof. Dr. Sanjay Govindjee
University of California, Berkeley

thanks for the link

Rui Huang — Wed, 06 Aug 2008 11:02:14 -0400

Kodanda,

Thank you for the link to OpenDx. Will check it out later.

Re: program for mesh and results visualization

Kodanda Ram Mangipudi — Wed, 06 Aug 2008 05:06:48 -0400

Dear Prof. Rui Huang,

There is one free, actively developed, feature rich, easy to use program for visualizatio: OpenDx. This was initially developed by Intel and then given out for open development. www.opendx.org is the link. It can serve the purpose of any scientific resercher for visualization.

It has an easy interface for creating visualization macros which is just like drag, drop and connect the the modules with mouse.

Kodanda

Write A Dissertation like A Research Paper

researchpaperwriting — Wed, 06 Aug 2008 02:26:26 -0400

In addition of Jack Smith....I want to added
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meshfree code for non-linear lastoplastic equation

aswani006 — Wed, 06 Aug 2008 02:11:29 -0400

my name is aswin and i am doin my m.tech 2nd yr in IIT ROORKEE,INDIA

now i am doin project basedc on meshfree technique.

i am develop[ing code for elastoplastic problem

i have code for elastic pert.but i am not getting how to dvelop code for plastic part and how to solve that non-linear equation

please help me for my dessertation.

hi

aswani006 — Wed, 06 Aug 2008 02:03:46 -0400

my name is ASWIN and i am doing my M.Tech 2nd yr in I.I.T.ROORKEE,INDIA.

i am working on application of meshfre techniuqe to elastoplastic materials.

upto now i have developed code in Mat-Lab for elastic.

now i am getting problem with plasticity.can anybody help me in this aspect how to add this plasti relation to elastic p[roblem and how to solve it

please help me so i will be very gratefull to them.

AFM measurements in cell adhesion

Xiaodong Li — Tue, 05 Aug 2008 21:57:30 -0400

Thanks. This is very interesting in terms of experiment. To measure cell adhesion is very challenging. This requres calibration of AFM lateral force. First one needs to image the cell using AFM and then in situ touch and pull the cell. It is better to find a calibration sample to accuartely measure this. I would like to see more papers or experience how to get the calibration well done. Any suggestions are welcome.

Re: Mechanical Design Principles of Ancient Fish Armor

Xiaodong Li — Tue, 05 Aug 2008 21:38:00 -0400

Thanks Christine for posting this exciting work. The paper reports lots of new insights about the toughening mechanisms. This is very helpful for my current research on biomaterials.

More about rubber: Negative thermal expansion?

Biswajit Banerjee — Tue, 05 Aug 2008 20:53:09 -0400

Since I'm now into rubber I won't be able to rest until I've figured out another issue that's been bothering me.

I distinctly recall being told in a materials science class that natural rubber has a negative coefficient of thermal expansion. However, a search of the literature throws up only positive values of the CTE with a few interspersed papers claiming a negative thermal expansion coefficient. There also appears to be a "thermoelastic inversion effect" which separates the positive and negative expansion regimes. A couple of papers on the effect claim that negative CTEs are observed only at strains greater than 10%.

Can anyone clear up this mess for me?

Thanks in advance.

-- Biswajit