Von Mises isn't always good enough by its self? 5

[Crompto29]

Example: If I am modeling a complex object like a weld neck flange, there can be areas where the three principle stresses are positive, similar and well above yield, however according to Von Mises Criterion the Equivalent Von Mises Stress is close to zero.

i.e. if s1=s2=s3~800MPa then Svm~0. Material yield is 260MPa. (the ultimate strength is 515MPa).

If the area of high principle stresses is large enough the flange will fail, wouldn't it? Yet the Von Mises Stress is well below its allowable value. How is this dealt with or even predicted in an FEA model? There must be an additional test.

This issue seems to arise as the Von Mises stress surface is an infinitely long cylinder.

ASME VIII Div 2 requires the additional test: S1+S2+S3 < 4Sy to which my example will fail. This effectively puts a lid on the Von Mises cylinder. Is this the most common method in the wider FEA design industry?

Am I right in saying that Von Mises Criterion is for yielding only and that even if stresses are large and similar (i.e. Svm=0) the object will theorically never yield. (But will at some point catastrophically fail which is a material issue and has nothing to do with Von Mises.)

My main question is how do you deal with the Von Mises infinitely long stress cylinder when Analysing FEA results?

(Von Mises seems to work fine by its self with thin shell analysis as s3 is about equal to 0.)

 

[EdR]

Crompto29:

Typically, plasticity models assume yielding is independent of J1 (J1=sigx+sigy+sigz). Therefore if the principal stresses are approximately equal there will be no yielding i.e. a body subjected to hydrostatic stress does not yield. If the stresses are tensile then a different yield criterion is applicable as we cannot apply equal, increasing, tensile stresses to a body without failure. In your question about the VM cylinder this is consistent with the infinitely long cylinder and yield is associated with the radius normal to the cylinder axis.

Yes you are correct in saying that typically VM criterion is for yielding only, although in some cases a defined yield surface can be used as a failure surface and the results interpreted in light of this fact.....

Last point is there is nothing to do (or interpret) regarding the infinitely long VM cylinder, however, as I noted above it may be that you should be using a different failure criterion depending on the system......

Ed.R.

 

[johnhors]

Crompto29, why do you think the three principal stresses in a complex object will be similar?

In a homogenous structure you can expect the maximum stress to be on a free surface. If this surface is not loaded or restrained, then one of the principal stresses is normal to the surface and is zero (in FEA never exactly zero but of very small magnitude compared to the other two). It is then very unlikely that the two inplane surface principal stresses will be similar.

http://www.Roshaz.com

 

[Crompto29]

Cheers EdR,

You have made things a lot clearer. FEA seems far vaster and more widely used, with many failure criterions.

If I focus on ASME VIII FEA linear elastic design, the failure criterion is Von Mises, however the infinitely long cylindrical failure surface is capped off in the tensile direction with S1+S2+S3 < 4×Sy.

I hope I used the correct terminology.

And Johnhors, you make a lot of sense. So below the components free surface the Von Mises Equivalent Stress would drop as the 3rd principal stress will come into play.

Its pronunciation depends on which corner of the globe you come from. In my corner there is no difference, hence the spelling error. And as I didn't memorise a dictionary as a kid and have no plans to do so in the future, I tend to spell phonetically.

rb, Now that you have learnt that there is more than one location in the world, is there really a need to write a special message to point out a spelling mistake when the correct meaning is obvious. It only brings down the quality of this great forum. How about adding something useful as well.

P.S. does anyone know if the word "Principle" can be deleted from a Word spell checker?

 

[johnhors]

Crompto29, below the free surface most components of stress will tend to drop since bending stress drops as distance from the section neutral axis decreases. Similarly any torsional stress also drops off as the radius from the section centre (or effective centre - sorry center to those on the far side of the pond!) also decreases. Hence the term "extreme fibre (fiber)" to describe the position of maximum stress in a traditional hand calculation. There will be all three principal stresses present below the surface but you they will in general have well spread values and will almost certainly not be similar.

In about every other issue of Professional Engineer in the UK, job adverts appear for a "Principle Engineer". I cringe when I see this. If a prospective employer can't get their spelling correct on a senior job position, what does it say about their standards and the quality of staff already employed? Similarly I take my daughter to a dance school where the lady in charge name appears on a notice board followed by "Principle", I cringe. Lastly, my current boss thinks I'm a "Principle Engineer"!

http://www.Roshaz.com

 

[EdR]

Crompto29:

I must come to the defense of both rb1957 and johnhors regarding the use of "principal" vs. "principle" in dealing with stresses. While not written down anywhere that I know of it has long been a standard among structural engineers (and indeed any engineer that has taken Mechanics of Materials from me!) that use of the proper term is an indication of the level of competence of the user. So in the opinion of many that have been around for a while improper usage is an indication of the lack of understanding of the subject....

A further point in this regard is that among engineers accurate communication is a significant requirement in order to prevent really bad screw ups. Such casual and improper communication (word usage) is just one of a number of things that may lead to misunderstandings. Also consider how significant it might become should you find yourself on the witness stand trying to convince a judge and/or jury that your really, really meant "principal" stresses.....

Ed.R.

P.S. In your comment to rb you seemed to indicate that usage may be different in different parts of the world...I seriously doubt this as I have never seen any text that uses this term improperly.....

 

[Crompto29]

Once upon a time a group of people started saying 'centre' instead of 'center' or when 'Aluminium' finally was named by Humphry there were a group who didn't hear about it and used the word 'Aluminum' etc, etc, etc. I've never heard of anyone standing in court for making this type of phonetic spelling error. The dictionary isn't a post in the ground; it is a gauge for the ever changing English language. Words go into it. Words are deleted from it.

Type "Principal Stress" and "Principle Stress" into Google and look at the page numbers for each. Both are used regularly in all sorts of published books and technical websites.

There are many Engineers and Scientists that are/were Dyslectic or something similar or there are practically minded greats that could barely write. I find it old fashioned to judge Engineers practical/academic understanding of complex design principles by their ability to memorise the dictionary. (How many of you try to make your left handed children write with their right hands?)

EdR, the reason it hasn’t been written down is because it is only used by those good with language skills. It may filter out the 10% of incompetent engineers that can’t spell, however it also filters out the 40% of competent engineers that are not naturals with language. And what you are left with are 40% competent engineers that can spell and the final 10% are great report writers but otherwise are a waste of space. It is an old wives tale. Only a person in one of the 40% groups could understand this. Guess which group.

We can’t all spell things however we want, however for an online forum and a commonly miss spelt term I think you all know what "Principle Stress" means.

 

[IRstuff]

Seems like the point has been made sufficiently.

We should now return to the originally scheduled broadcast...

TTFN

FAQ731-376

 

[Crompto29]

Sorry IRstuff,

I should have noticed the inapropriate post link.

Thanks Johnhors,

Your post describes the distrubution of stresses in terms of bending and principal stress theory, which I am more familiar with. With this I can understand Von Mises a little better.