Von Mises Vs Max Principle
Von Mises Vs Max Principle
(OP)
Given the legal design requiremnt: The Stress level, under load condition, at any point in the structure shall be limited to a level that provides a safety factor of 3 against permanent deformation.
Whould you use Von Mises or Max Principle in your FEA modle?
Whould you use Von Mises or Max Principle in your FEA modle?





RE: Von Mises Vs Max Principle
First point, there is no such thing as "Principle Stress".
Von Mises is a theoretical measure of stress used to estimate yield failure criteria in ductile materials and is also popular in fatigue strength calculations (where it is signed positive or negative according to the dominant Principal stress), whilst Principal stress is a more "real" and directly measurable stress.
Since you have a legal design requirement, then you would be best advised to cover yourself using either stress values.
RE: Von Mises Vs Max Principle
RE: Von Mises Vs Max Principle
Where does this requirement come from (ASME or similar)? What are the loads? What is the structure? Which industry (Civil, Nuclear, Marine...) is the structure designed for? Etc.
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RE: Von Mises Vs Max Principle
Thanks for the replies and sorry for the spelling. This is a general question for our engineers in the design of aircraft support equipment. Currently some our engineers use Von Mises while other’s use the max/min principal stress. Clearly we all should be designing the same way. Since the requirement to design to is as stated in my 1st post, my question is based on the interpretation of the stated requirement. Could you interpret “The Stress level, under load condition, at any point in the structure shall be limited to a level that provides a safety factor of 3 against permanent deformation” to mean Yield Stress of the material must be greater then 3 times the calculated Von Mises Stress? Or do I need to use the max/min principal stress? Follow up question: how would you define ductile material? Is there a maximum % elongation or some other material property which could define weather a material would be ductile enough to use Von Mises?
RE: Von Mises Vs Max Principle
hope this helps,
RE: Von Mises Vs Max Principle
Given possible legal implications I would use the safest criteria and clearly state it
RE: Von Mises Vs Max Principle
RE: Von Mises Vs Max Principle
principal stress? If there are 3 principal stresses, and one is larger than the other 2, doesn't that make it the maximum principal stress? (I assume the dispute wasn't over the spelling of 'principal' but over the concept of max. principal stress.)
Yield criteria are a completely different discussion. The main reason in my experience for engineers using different yield criteria (of which there are many: max. principal stress, von Mises yield, max. shear stress, etc.) to define failure is that one criterion seems to have worked better in the past than others. Assuming that this experience is based on reliable test data, it is hard to argue against a particular choice of yield criteria. The selection of Yield Criterion seems to be mainly experience based.
Since the criterion given "The Stress level, under load condition, at any point in the structure shall be limited to a level that provides a safety factor of 3 against permanent deformation" seems to be a purely static strength criterion, I agree with rb1957 that this means limit load is 1/3rd of the yield stress for the material.
If the object being designed undergoes fatigue loading, then a completely different method must be used to ensure design safety (for instance, crack initiation or crack propagation).
I am interested in finding out TrapperJohn would propose to 'unify' the selection of Yield criterion (after having made statement "Clearly we should all be designing the same way") given the huge amount institutional bias (prior experience, design tool documentation, informal "best practices" that are defined in most organizations, etc.) in the selection of Yield criterion for design optimization. Define a test or a series of tests that could be used to test criteria?
RE: Von Mises Vs Max Principle
RE: Von Mises Vs Max Principle
RE: Von Mises Vs Max Principle
prost you're perfectly correct ... max principal and vn Mises are both accepted ways of describing the complex stress state of a structure in terms that are comparable to uni-axial test (strength) data.
RE: Von Mises Vs Max Principle
(myself, I rarely do static strength analysis, being more often concerned with fatigue, fracture and damage tolerance).
RE: Von Mises Vs Max Principle
Sounds like a good idea!
RE: Von Mises Vs Max Principle
i think everyone listing their static strength criteria and how they apply them would be next to pointless.
first, we represent many different industries which naturally focus on different things.
second, if you're working in an industry, you should know common practice
third, if you don't know, then these forums (sorry, i refuse to say fora) really aren't the place to educate your self (on such a general topic).
i'll go now.
RE: Von Mises Vs Max Principle
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Von Mises Vs Max Principle
Similar informat can be found with a google search, such as "von mises stress shear failure criteria"
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In the past, for metals, I have generally used Von Mises to identify the highly stressed areas, and then looked at normal and shear in that area.
Also need to check buckling, cripling, etc.
Also, I've experienced more difficulties in getting agreement on the which failure criteria to use with composites.
I would like to see a list of failure criteria also. Everytime I write a stress report, it is a topic of discussion.
RE: Von Mises Vs Max Principle
RE: Von Mises Vs Max Principle
RE: Von Mises Vs Max Principle
there is not a "better" or "worse" criteria to combine stresses. There are plenty, which apply for ductile materials, or for fragile materials, or for laminar composites, and so on.
Of course, if you trace the sigma-tau plot of all these criteria, you may find that some are more "conservative" than others, BUT the real point is that you may have to choose the most appropriate for YOUR kind of material (see Drej's questions...).
Another point to pay attention to is: do you have to respect a NORM? If so, you don't have any choice: you MUST use the specified criteria. For example, if you decide that you will design a pressure vessel according to ASME, because you think ASME is recognized "de facto" worldwide, you will have to use Trescà-Guest. In the same scenario, if you decide to use European "PED" with the appropriate EN norm, you will have to use Von Mises.
Regards
RE: Von Mises Vs Max Principle
RE: Von Mises Vs Max Principle
My question may be a little away from the original discussion and I appologize. I also appreciate if you experts can cast some lights on my question.
I am working on Aluminum casting process of automotive suspension parts such as control arms and knuckes. It seems to me that the part design is mainly focusing on stress requirement (von mises)instead of cyclic fatique.
My question is:
If the casting alloy's fatique strength is increased while the other properties (tensile, elongation, etc) remain the same, will it be helpful to reduce the part weight in the part design?
Thanks again.
YY
RE: Von Mises Vs Max Principle
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Von Mises Vs Max Principle
RE: Von Mises Vs Max Principle
corus
RE: Von Mises Vs Max Principle
RE: Von Mises Vs Max Principle
RE: Von Mises Vs Max Principle
In a uniaxial stress state (where P1 = Von Mises and P2 = 0) , yes.
In a fully biaxial stress condition where P1 = -P2, the Von Mises stress value is higher than P1, and is thus a safer value to use. After all Von Mises is an energy based yield criterion. What does puzzle me though is why some people (are they really people?) insist on using a "signed" Von Mises in fatigue analysis!!
RE: Von Mises Vs Max Principle
as for signed vM stresses ... actually that makes sense to me, to distinguish between tenisle conditions and compressive conditions for fatigue analysis (but then i'd use principal stress anyways).
RE: Von Mises Vs Max Principle
If S1=S2=S3 then the von Mises stress is zero. I don't think you can say that it would not yield is not quite right. If you could produce a stress state where Si are all equal, and tensile, the material would yield. I'm not sure what a real material would do if you applied Si compressive to extremes. You could try this by dropping a bowling ball sized sphere of chewing gum into the Mariannas Trench (35,000 ft deep water) and observe what happens.
A general state of stress can be divded into the hydrostatic and deviatoric stresses. Hydrostatic stresses change the volume of the solid element while deviatoric stresses are changing its shape. Von Mises theory assumes that damage is caused by this deviatoric stress. In the case of a uniaxial tension test, the von Mises stress and the maximum principal stress are equal and both can be used to predict the onset of yield in a ductile metal.
Doug
RE: Von Mises Vs Max Principle
A steel ball submerged in oceanic trench is the same example cited on Kachanov’s “Plasticity” text. According him, it will not yield, it just will be elastically compressed.
Yielding (change of shape) is produced just by the deviatoric component of the stress tensor. Von Mises stress is a measure of its magnitude; it coincides with the second invariant of the deviatoric component of the stress tensor.
Regards.
RE: Von Mises Vs Max Principle
This can be visualized via Mohr's Circle.
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RE: Von Mises Vs Max Principle
To re-iterate my previous response:-
there is no such thing as "Principle Stress".
In the English language Principle and Principal have very different meanings and cannot be interchanged. Sorry if I appear over bearing on this point, but I believe this to be very important. There has already been more than enough deterioration in the quality of work produced by engineers during my career.
RE: Von Mises Vs Max Principle
RE: Von Mises Vs Max Principle
Johnhors, you're right, but English is not the only language in the universe, and I know some extremely good engineers who know it only approximately, so please let's concentrate on the concept and not so much on terminology when all the context is clear... This said, of course this forum is anglo-saxon so everybody should make an effort to "speak" correctly...
The given definition is 100% correct, though perhaps it doesn't add so much to the topic. I feel this thread is now beginning to turn round and round in circles when, historically, different failure theories have been formulated for the simple reason that some are more appropriated for some materials, some for others.
And, from the given definition of "principal stress", IMHO it is obvious that "transverse stress" was written for "shear stress"...
RE: Von Mises Vs Max Principle
My intention was not to criticize anyone's English, I know how I struggle with comprehension of the two other languages I sometimes use as well as English; nevertheless, there are many ways to use 'transverse stress' and just because it's obvious some doesn't mean it's obvious to all. I sometimes find it very difficult to establish up front what particular terms mean, depending on the client; sometimes defining 'obvious' terms is one of the most important steps you can take on a project.
RE: Von Mises Vs Max Principle
prost,
<if you think...stop [adding to it]> : yes, I agree. This is, by all means, the last post of mine in this thread unless it takes a well-defined direction.
<there are many ways... doesn't mean it's obvious to all> : once again, I agree: my previous was not a critic to what you wrote. Saying "transverse" is vague almost every time. Nevertheless, in THIS case, the poster gave a definition which didn't leave any space open to interpretation... Simply my opinion, of course.
Regards