iMechanica - crack - Comments

crack, fracture, failure

Sandip Haldar — Tue, 06 Nov 2007 11:50:31 -0500

We have got three terms 'crack', 'fracture' and 'failure' . I think we may consider them as-

'crack'
as a fault or flaw or discontinuity, which we can see in an
object/solid from whether it is loaded or not it is increasing in size
or not;

'fracture' as a process to take place over time to
create/increase the 'crack' ( we consider crack as a limit of ellispe,
I dont know if any geometric estimation is there like we have for
slenderness ratio to check a column for short or long).

'failure' as an extreme of fracture (hence, a process) when the body fails to resist the load/ serve the purpose.

Also, as Mark mentioned, the difference is clear from '2 mm crack NOT 2 mm fracture'.

Sandip Haldar

Crack Analysis at Aero Engine (Rotor Disc)

prashant sharma — Sat, 03 Nov 2007 13:55:16 -0400

hi handa

good to know that u r also looking crack initiation and progation of crack in rotor..

well i would like to which FEM tool u r using , as i am using ABAQUS for my application.

so can u share your experience..

regards

prashantsharma8@gmail.com

allsel before solve:

Gopinath Venkatesan — Mon, 29 Oct 2007 22:42:25 -0400

Hi kvdalmisli

Guess you might have figured out by now. Issue allsel before solve to include the nodes left out. It solves. As such I did not go through the program, so I don't know if the analysis is correct.

Vgn

Graduate Student

University of Oklahoma

Andrew, I think that you

Liang Xue — Fri, 26 Oct 2007 20:56:07 -0400

Andrew,

I think that you have already made a good distinction between "crack" and "fracture" in that"crack" is the usually result of some "fracture"mechanisms or processes. However, sometimes, people use them interchangeablly, like in "crack formation" and "fracture initiation".

fracture versus failure

Mark E. Walter — Fri, 26 Oct 2007 10:10:01 -0400

I think there is a third term that is more important. That term is "failure." I really dislike it when people talk about the "fracture strength" of a tensile specimen. To me "fracture strength" is reserved for bodies that have cracks. If a body does not have a crack when initially loaded, then the value at which it breaks should called the "failure strength".

With regard to "fracture" versus "crack", I would never say something like "The body has a 2 mm fracture." I would say "The body has a 2 mm crack." The word "fracture" could be a noun, but I never use it that way.

Dear Colleague.

Dusan_Boocz — Thu, 25 Oct 2007 04:18:37 -0400

Dear Colleague.

try this page, in case you already didn´t . Maybe you will find related topics , books , references or tips for your problem http://www.cfg.cornell.edu/index.htm

Crack Analysis at Aero Engine (Rotor Disc)

handa — Wed, 24 Oct 2007 10:22:42 -0400

Dear Sir..

I'm student of Aerospace Engineering, ITB, Indonesia. I put my concern in Crack analysis at Rotor Disc Rolls Royce Engine Tay-650-15 as my final assignment. But I found some difficulties to search related topics to do that in detail (i.e book refferencse, engineeering journal, tips and trics, etc.)

Would you please to help me????

Thanks in advance,

Best Regards,

Handa

e-mail : catiav5user@yahoo.com

Hi Xiao, I am trying to

Biswas — Mon, 02 Apr 2007 03:18:33 -0400

Hi Xiao,

I am trying to implement a method to compute K for interfacial crack using ANSYS APDL. But my problem is to get the nodal force data at the crack tip (I am trying to use virtual crack closure technique). Could you please help me regarding this matter? If possible, please let me know the APDL command to get nodal force data.

Thanks.

Regards,

Biswas

"steady state"

Xiao Hu Liu — Fri, 22 Dec 2006 18:35:48 -0500

Hi Nanshu,

Thank you for the comments. As the delamination elongates infinitely, the energy release rate diminishes in all cases. Although it is hardly discernable in Fig. 7 for small w/h but it can be seen for w/h=1.5. The "steady-state" term is misleading; I should have used other one, "plateau" is probably appropriate, since the curves tail off to zero slowly as compared to the variation of the curves at small crack length.

The dotted curves in Fig. 7 are not of interest practically, as we discussed it in the paper, together with Fig. 8. Because, for a larger crack length, a significant portion of the crack surfaces behind the crack tip penetrate into each other. To resolve the artifact one need to include the contact in the model but this is not expected to give a larger driving force for the delaminaton. So we have no motivation to go further.

"Steady State" in Fig.7

Nanshu Lu — Thu, 21 Dec 2006 13:27:45 -0500

Hi, Xiaohu,

I read your paper carefully because I am doing research on stiff islands/ stretchable substrate interface delamination in flexible electronics. The paper is very well organized and gives very good analysis. I learnt a lot from it actually.

However, I have one question regarding to Fig. 7 in your paper. We can see a clear "steady state" for 2a/h>4. However, intuition tells us if the crack tip is far away enough from the gap, the gap infuluence will deminish which will result in no driving force. I am wondering if it is proper to call this "steady state" and from which point on the driving force begins to drop and eventually goes to zero.

I am using it now!

Ying Li — Thu, 21 Dec 2006 11:51:11 -0500

Now I am using the ANSYS to solve many questions about the biomechanics . If you have any questions ,you can contact me at the email : Lee2002hu@yahoo.com.cn

I think we can learn many things from our disgussion.

Can we get people from ANSYS involved in this discussion?

Zhigang Suo — Wed, 20 Dec 2006 11:18:34 -0500

Any of you know people working in ANSYS? Please forward this thread (http://imechanica.org/node/429) to them and urge them to participate in the discussion. In another thread about FEM, people from ABAQUS made valuable contributions.

ANSYS UPF

Xiao Hu Liu — Wed, 20 Dec 2006 10:51:51 -0500

You may want to check the Guide to ANSYS User Programmable Features after looking at the excellent introductory article User Programmable Features in The Focus. There is a user forum xansys where you can ask/answer questions. Sheldon's ansys.net is a great resource for ANSYS users.

ANSYS user programmable features not well documented

ruogang zhao — Tue, 19 Dec 2006 20:05:18 -0500

I’ve been recently looking into both Ansys and Abaqus for my research project. When I looked through Ansys manuals, I realized there are some manuals missing on the user defined material (USERMAT) in the standard released documentation. So I contacted the Ansys local support team. Surprisingly, they do have it! I guess those manuals are not included because you’ll be “using ANSYS in a nonstandard way”. I agree with Jie-Hua‘s comments, Ansys want less trouble when providing user programmable features.

More on ANSYS vs. ABAQUS

Jie-Hua Zhao — Tue, 19 Dec 2006 14:39:42 -0500

The other issue is the cost. Historically, ABAQUS charged much more to commercial users. ANSYS had more options on pricing, which cost much less. To universities, it was a different story.

Nowadays, PC with multiprocessors, multicores and multi-threads are widely used in the industry. The price issue is more prominent. ABAQUS charges by number of concurrent processors, ANSYS charges by number of concurrent jobs (when using its sparse and PCG solvers). This makes a huge difference in pricing. Say if I have a Windows XP 64 machine with dual-core, dual processor running with multi-threading on. I have effectively 8 processors to the FEM solver. When I run ANSYS, I pay $6600/year maintainance fee since we have bought perpeptual licenses long time back. If I run ABAQUS, I need 12 solver tokens plus one CAE token. ABAQUS sells their license by leasing. This will cost me about $30000/year. We just can not afford of having 10 simultaneous ABAQUS jobs running. However, we are doing this on ANSYS almost everyday.

In case of non-linear analysis, ABAQUS does have its advantage. But the gap is closing up after ANSYS implemented the 180 series elements.

ANSYS also has its user programable features, however, they tend to de-emphasize the capability. In releases 8.0 and 9.0, they even took the documentation of these features out from the standard CD distribution. You have to specially order the hard copies. I think this is because of their ISO9001 certification. They want less trouble with the auditors.