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-   -   E-Z question about Von Mises stress (http://www.cfd-online.com/Forums/main/1266-e-z-question-about-von-mises-stress.html)

 Andrew Epp September 8, 1999 08:57

E-Z question about Von Mises stress

Hello peers.

Could someone please give me a simple definition of Von Mises stress and how it relates to shear and tensile stress?

Thanks in advance.

RAE

 Ken Light September 8, 1999 14:27

Re: E-Z question about Von Mises stress

From a structural mechanics point of view, the von Mises stress is a criteria used in predicting the onset of yield in ductile materials. It derives from the distortional energy density criterion which states that yielding begins when the distortional strain energy density at a point equals the distortional strain energy density at yield in uniaxial tension (or compression). The distortional strain energy is the energy associated with a change in the shape of a body. The calculated von Mises stress is a combination of principal and yield stresses. For a more detailed discussion try an advanced structures book like "Advanced Mechanics of Materials" by Boresi, Schmidt, Sidebottom.

I am not sure if this is what you were looking for, and depending on your background, it may not be an E-Z answer.

 tom September 10, 1999 12:45

Re: E-Z question about Von Mises stress

Hy,

that is an interresting subject. Did someone calculate the principal stresses and directions from cfd-results ? are the principal directions also rectangular or not ?

 John C. Chien September 11, 1999 00:34

Von Mises and/or Navier-Stokes

(1). A couple of years back, there was this young student from a foreign country looking for a job after Master's degree. (2). The student's major was thermo/fluid. But the job listings available were mostly related to structure analysis. (3). I guess, there's not much difference between the thermo/fluid and the structure, in terms of the equations, mesh generations, etc. So, I gave the student a suggestion to read more structure books and to try out available commercial structure code at school. I was not a teacher then. (4). Although my colleague at the work place usually run structure codes, I had no experience in running commercial structure codes at all. (5). After I have made that suggestion, I thought I was very funny. How could I make such advice? So, I decided to get an educational version of a structure code on my PC. I surfed the Internet for the tutorial lessons, and I was able to follow some examples in the code. So, at least I can now pretend that the advice I made was justified. (6). Well, the funny thing happened again just a few days ago, when a friend told me that his friend who used to run CFD codes, has been doing structure analysis for an auto company for a year now. (7). This also remind me a friend of mine in early 90's. He too was doing structure analysis when I contacted him, after I came back from a consulting assignment. He used to talk to me about the CFD code he was developing. (8). So, in these days with thermo/fluid jobs disappearing in favor of stock values, I suddenly realized that, for a mechanical engineer, one should be able to handle the structure analysis as well as the CFD. (9). A job is a job, and the structure is less non-linear than CFD. When it comes to the feeding of several mouths in a family, structure is definitely less fluid than CFD. Granted, there are more challenging CFD problems to solve, but, in real world it is not very attractive if he can't sleep every night with a peaceful mind. (10). Besides, the trend is to include the structure portion as part of CFD analysis. So, who cares, as long as it will bring one good salary, it really doesn't matter whether it is Von Mises or Navier-Stokes! The matter is what you don't know can be disadvantage to you.

 tom September 17, 1999 02:11

Re: Von Mises and/or Navier-Stokes

I think, beside the knowledge of another numerical area, the most important thing is to realize, what is possible in comfort. If you take ideas (cad and fem in one package) you see, what the future brings also to cfd. If you once worked with such a really professional program (prof. means: oriented at the customer !!!) you really wonder about software like pcube and even gambit and all the scientific people, that are sitting at their expensive workstations programming and editing and exporting and importing different files. In gambit you could not even measure distances !!!!!

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