iMechanica - Spring 2007 - Comments

"I have taken a class that

Steve Mccallion — Fri, 05 Dec 2008 11:51:52 -0500

"I have taken a class that the professor made slides with some missing
information that you add in as the lecture progresses. This definitely
keeps you following the lectures."

I like this format of lecturing, for the same reasons as stated. Also, some lecturer's suffer from a terrible case of can't-write-very-neatly-itis. This is not a personal attack on any lecturers (I know there are many here!), it's just a fact of life that not everyone has legible hand writing. Leaving spaces to be filled in definitely helps keep the students' attention, and reduces the hand work required by the lecturer.

I am definitely not in favour of lectures that are conducted solely on power point. First of all, something ALWAYS go wrong. Also, with chalk/pen in hand the lecture tends to become more dynamic, particularly when the lecturer has developed a class to ask many questions, afterall a picture is worth a thousand words.

A larger history can be displayed over several boards, allowing several 'slides' to be in the viewing range of the students. The lecturer also retains the ability to quickly jump back to prior board work when a student asks a question, which is particularly important with many students puzzling over a question for a minute or so before making themselves heard.

I could criticise powerpoints for quite some time, but they do of course have some fantastic advantages when used effectively. I'm about to complete a masters level course in material mechanics with a strong focus on the computational aspects of the subject. My lecturer has created some excellent presentation modules in mathematica which were invaluable for showing the effects of hardening in a much clearer manner than he could have done by hand (perhaps he just downloaded those modules, I know wolfram makes a lot of stuff freely available).

My friends keep talking of

johannabartley — Mon, 10 Nov 2008 12:22:50 -0500

My friends keep talking of COD's and CTOD's and though they learn at the Ohio Dominican University they couldn't tell me the difference between the two notions. Thanks for asking and thanks to all of you who answered.

my test

Manfred H Ulz — Mon, 10 Nov 2008 05:27:17 -0500

Thank you for the hint.

Blogging with LaTeX!!

HsiaoYing Shadow Huang — Thu, 21 Aug 2008 17:20:09 -0400

Hi Alex,

Thanks for this info. Finally I am able to type math entries in the blogs. I've tried it out and it worked perfectly.

Thanks again!!

thanks Zhigang Suo

Muller Abera — Thu, 08 May 2008 10:33:24 -0400

share what we have

Comprehensive Distance Education

Noel M. Dioyan — Fri, 11 Apr 2008 09:56:43 -0400

That is very interesting idea sir,if imechanica can provide a distance education from all field of engineering mechanics.The whole world is watching all the comments and suggestion in this site. Maybe, We here from asia can be benefited in your great proposal " Education without boundary" reagardless of distance,races,ethnics and culture..

Hi aswani kumar i am

Ravitej K — Sat, 03 Nov 2007 00:24:44 -0400

Hi aswani kumar

i am ravitej, doing my mtech in IIT kanpur. from ur posting i can guess that u had great knowlwdge in theoritical concepts of strength of materials.i want to ask a doubt regading strength of materials.

i want the mid point deflection in case of "four point bend test" on a
specimen of length 140mm, width of 40mm and thickness of 10mm.

Problem is that, beam theory is valid if the cross-sectional
dimensions are less than axial dimensions. Here this condition is
violated. i had modeled the specimen in Abaqus and the diffection at
mid point is almost double that of the actual diffection calculated
from beam theory.

how can i find the theoritical difflection at mid point in this case?

i had one option, using timoshenko beam theory and applying larger
deflection theory. will it be a reasonable approximation in this case?

please help me in this aspect.

Ravitej Kommana
IIT Kanpur
INDIA.

hi Jamilla

prashant sharma — Wed, 03 Oct 2007 14:27:33 -0400

i am MSc. Engineering by research student with mechancis.

my research works include. study of aero engine turbine disk for protential instabilty using ABAQUS, i have to perform analysis to simulate and predict the bursting phenomenon of disk for which i have to do elastic-plastic analysis and finally bursting of disk in which crack initiation and finally frature have to taken in to account.

so i was wondering if i have some tips regarding the Fracture mechanics particualy crack intiation phenomenon using Cohesive zone model techique from you.

thanks

prashant sharma

prashantsharma8@gmail.com

Updates on hydrogels and poroelasticity

Zhigang Suo — Tue, 25 Sep 2007 20:00:56 -0400

Jerry Qi has led a Theme of the Month on the mechanics of hydrogel.
I have posted notes on polyelectrolyte gels.
Several members of my group and I have extended the lecture notes into a paper, which has been submitted to a journal. The preprint of the paper has been posted on iMechanica, and has initiated a thread of good discussion.

COD and CTOD:

Gopinath Venkatesan — Mon, 23 Jul 2007 17:18:48 -0400

To my knowledge, COD is first introduced by Boyle [1962] for measuring cracklength directly (I mean without measuring from microscope readings). This is essentially a strain gage reading for compliance (inverse of stiffness, COD/Load). I thought CTOD is the displacement of the crack tip and is different from COD. All these days, I convinced myself that CTOD is developed for elastoplastic while COD can take care of LEFM area but now confused. I will go through the references cited by Wu. Thanks.

Gopinath

COD & CTOD (Reply to Ling Zhu)--Fracture Mechanics class

Xiangfa Wu — Mon, 23 Jul 2007 15:34:02 -0400

Dear Ling, COD as a criterion is used for crack growth in elastoplastic materials, for which the K-criterion does not hold due to the finite plastic zone near crack tip. However, within the framework of SSY, COD-criterion is equivalent to K-criterion (Kanninen and Popelar, 1985). COD-criterion is considered as a supplementary fracture criterion for elastoplastic materials before the evaluation of J-integral (A.A. Wells, 1979) to be discussed in this class. The typical definition of CTOD for cracking in ductile materials can be found in the following link: http://www.efunda.com/formulae/solid_mechanics/fracture_mechanics/fm_epfm_CTOD.cfm

Based on my understanding, COD and CTOD indicate the same term though they may have a little difference in literature here and there. For example, the COD-criterion was discussed in detail in the classic textbook: Advanced Fracture Mechanics (Kanninen and Popelar, 1985), in which the term COD is actually also the CTOD. While in the textbook Cracks and Fracture (Broberg, 1999, pp. 575), it says that COD and COA (crack opening angle) are often written as CTOD (crack tip opening displacement) and CTOA (crack tip opening angle). Also, in some literature, it says that COD is the total displacement at crack tip (usually mixed-mode crack), while CTOD indicates the crack tip opening displacement due to pure mode I loading [e.g. Sha et al., Int. J. Fracture 104 (2000) 409-423]. There may be many others. Therefore, when we use COD or CTOD, we need to take care of the definition used in context. Hope this would make it clear.

thanx for the lecture notes

Zoeb Kaizar Lakdawala — Thu, 14 Jun 2007 09:15:52 -0400

Respected Sir,

I am a M. Tech Student doing my dissertation on Coding on fracture mechanics your lecture notes proves to be exceptional for the begineers like me. Please keep doing the good work for the future aspirants like me

Regards

Zoeb Lakdawala

Biphasic vs Poroelastic (on behalf of Dr. Van C. Mow)

Ed Guo — Mon, 14 May 2007 13:25:45 -0400

1) Highlights in the historical development of porous media theory:
toward a consistent macroscopic theory. de Boer, Reint, Applied
Mechanics Review, 49:201-262, 1996.

2) Mow VC, Kuei SC, Lai WM, Armstrong CG: Biphasic creep and stress
relaxation of articular cartilage in compression: Theory and Experiment,
J Biomech Engng, Trans ASME, 102:73-84, 1980.

3) Lai WM, Hou JS, Mow VC: A Triphasic theory for the swelling and
deformational behavior of articular cartilage. J Biomech Engng, Trans
ASME, 113:245-258, 1991.

4) Lai WM, Mow VC, Zhu WB: Constitutive modeling of articular cartilage
and biomolecular solutions. J Biomech Engng, Trans ASME,115:474-480, 1993.

5) Gu WY, Lai WM, Mow VC: A mixture theory for charged hydrated soft
tissues containing multi-electrolytes: passive transport and swelling
behavior, J Biomech Engng, Trans ASME, 120:169-180, 1998.

6) Mow VC, Ateshian GA, Lai WM, Gu WY: Effects of fixed charges on the
stress-relaxation behavior of hydrated soft tissues in confined
compression problem. In: Poroelasticity: Maurice Biot Memorial Issue,
edited by AHD Cheng, E Detournay, Y Abousleiman, Pergamon Press,
pp4945-4962, 1998.

7) /Porous Media, /edited by W Ehlers, J Bluhm, 2002, Springer, pp459.

Thanks for the change

Nanshu Lu — Fri, 04 May 2007 13:15:06 -0400

Thank you, Zhigang.

Mechanics: accumulative and infinitely extensible

Zhigang Suo — Fri, 04 May 2007 07:46:15 -0400

Dear Jay: Good to hear from you! Thank you so much for your kindness. I'm sorry that you had to return to Korea early in the semester. I've just given the last lecture of ES 241 Advanced Elasticity yesterday. Most other sit-in people stayed with me till the end.

This is a second-year graduate course in solid mechanics. The students have all taken ES 240 Solid Mechanics, so that I can assume that they have some adult experience of the subject. In addition, we have separate courses on fracture mechanics, plasticity, mechanical behavior of materials, and kinetics. They are all taught at advanced levels.

There is no other constraint on what I have to cover. From very beginning, I knew I would not be able to cover everything that I'd love to cover. Instead of trying to cover everything, I have tried to uncover a few things that are interesting to me and that seem to have lasting value. The notes are rather terse and contain typos and, I'm sure, errors and bad judgments. Often I would go over two pages of the notes in a 90-minute lecture. Many students made perceptive comments in class.

This was the first time that I taught the course. It was both an exhilarating and exhausting experience. I have tried to build ideas from scratch, and tried to trivialize them (i.e., making them transparent in hind sight). I was constantly delighted by the insights and ingenuities of great mechanicians of the past. It was humbling, too. I had to admit in class so many times that I could not have discovered this or that even if I were there!

But that is precisely the beauty of our subject: great depth and breadth. All we are hopeful is to learn great ideas of the past, make a few interesting applications, and add a few new ideas that will make a future professor say, "Ah, that is clever. I wouldn't be able to come up with this even if I were there."

Our subject is accumulative and, I believe, infinitely extensible.