FE-Weld Fatigue mesh insensitive- Battelle
FE-Weld Fatigue mesh insensitive- Battelle
(OP)
I was wondering if anyone has had any experience or completed work using the Structural Stress FE-Weld Fatigue mesh insensitive approach as defined from Battelle?
Continuing from an old previous thread - Fatigue assessment without the fudge thread 727-155500 - the topic was discussed but since there has been a commercial package released, (Verity) and also included in the ASME Section VIII, Division.2 , anyone found good results from it?
Has anyone had success with applying the theory to their weld fatigue, or even defined they own Master S-n Curve?
What are your thoughts
Continuing from an old previous thread - Fatigue assessment without the fudge thread 727-155500 - the topic was discussed but since there has been a commercial package released, (Verity) and also included in the ASME Section VIII, Division.2 , anyone found good results from it?
Has anyone had success with applying the theory to their weld fatigue, or even defined they own Master S-n Curve?
What are your thoughts





RE: FE-Weld Fatigue mesh insensitive- Battelle
corus
RE: FE-Weld Fatigue mesh insensitive- Battelle
Can you please explain the reasoning for your statement "the mesh density effects the relative stiffness of the finite elemnt model" ?
RE: FE-Weld Fatigue mesh insensitive- Battelle
The one caveat that I would put on the mesh-insensitive part, is that the mesh needs to be sufficiently refined to avoid the issues that corus refers to. However, beyond an "adequately" refined mesh, because the method calculated the membrane and bending stresses using nodal forces, it has less sensitivity. The way I understand it is that forces are primary variable in the F=kx equations, whereas stresses are derivatives of x.
RE: FE-Weld Fatigue mesh insensitive- Battelle
It's my understanding from early days looking at the theory that the coarser the mesh then the stiffer the overall model, such that convergence with increasing mesh density tends from only direction, rather than oscillitating about the true solution.
To me the assumption that the method is not mesh sensitive will result in people having as few a number of elements in a model as they can get away with. I'd have serious concerns about results from such 'insensitivity'.
corus
RE: FE-Weld Fatigue mesh insensitive- Battelle
For accurate stress results a fine mesh is certainly a requirement. However to see a degradation in overall stiffness you would have to generate an absurdly coarse mesh! The whole basis behind sub-modelling for instance is that the refined sub-model has the same stiffness as the coarsely meshed complete model, otherwise it simply wouldn't work. Models made purely for dynamics and natural frequency analysis can be relatively crudely meshed and still produce accurate results, because their stiffness and mass distributions remain correct with a coarse mesh.
RE: FE-Weld Fatigue mesh insensitive- Battelle
Of course if an absurdly coarse mesh produces a degradation in overall stiffness then perhaps a mildly silly coarse mesh would produce less of a degradation in overall stiffness, but still a difference in stiffness that this method relies on. To say that the method is not mesh sensitive seems a little dangerous to me and I'd prefer to see mesh sensitiveity tests to verify the results before relying on it.
corus
RE: FE-Weld Fatigue mesh insensitive- Battelle
Stiffness changes will only start to occur when curved surfaces become poorly defined as they take on a facetted appearance, but even then the difference will be very minimal indeed.
Thus any method which relies on the stiffness in the surrounding structure and not stress, is by definition going to be mesh insensitive.
RE: FE-Weld Fatigue mesh insensitive- Battelle
I still have my doubts on the method and would prefer to see sensitivity studies on the method before accepting it.
corus
RE: FE-Weld Fatigue mesh insensitive- Battelle
Thus the fine mesh shows a 0.12% error reduction over the extremely crude mesh. This change in error being less than a quarter of the total error for the fine mesh.
I think that it is fair to say that the overall stiffness of the block is practically insensitive to mesh density.
RE: FE-Weld Fatigue mesh insensitive- Battelle
Do you know what the error would be for the five element case if using linear elements (8-node) instead of quadratic (20-node)?
Thanks
RE: FE-Weld Fatigue mesh insensitive- Battelle
I get a displacement of 0.4959 using five eight node bricks (with Calculix) , thus giving an error of 0.082%
RE: FE-Weld Fatigue mesh insensitive- Battelle
Thus the FE process itself can be viewed as applying additional restraint to the structure, albeit a mathematical restraint ("you will deflect according to this set of equations") rather than a physical one.
RE: FE-Weld Fatigue mesh insensitive- Battelle
The method in british standards, such as BS7608, is to classify each type of weld, it's location, and principal stress direction. This method seems to disregard that and imply that you can get away with any kind of mesh to get results. I have my doubts.
corus
RE: FE-Weld Fatigue mesh insensitive- Battelle
As I said before, the method is an accepted method in ASME Section VIII, Division 2 (2007), with the standard caveats that the FE mesh needs to be adequately refined so as to avoid the issues that corus is referring to.
And my recollection, too, is that convergence is monotonic from below. If your model is too coarse, then whatever results you take from it are lousy. That said, with this method, you don't need 10 quadratic elements through the thickness and 1000 elements around the circumference of the nozzle weld either...
RE: FE-Weld Fatigue mesh insensitive- Battelle
Whilst the error does decrease with mesh density, the accuracy of the very crude mesh is still very good.
The displacement functions used in these elements are very good at describing deflections, the better they are the fewer elements are required for displacement analysis (not stress !). For instance, beam elements which use a cubic displacement function are a perfect match with simple beam bending theory in linear statics, one element of length 250 gives the same result as 50 elements of length 5 at the end of a cantilever (unfortunately this doesn't apply to natural frequency and buckling mode shapes)
RE: FE-Weld Fatigue mesh insensitive- Battelle
Of course the method relies on the reaction/nodal forces which must equate to the input forces. It's the distribution of such forces in a more complex model I'd be concerned about and at least ASME has included the necessary caveats to maintain standards of modelling techniques. It'll be interesting to see if the european standards include the same method.
corus
RE: FE-Weld Fatigue mesh insensitive- Battelle
The key points about the method are:
» It allows stresses predicted by an appropriately modelled FE analysis to be used directly against an S-N curve.
» The S-N curve to be used is given in the document. It is
For N<10^7 log10(N) = 13.358 - 3.0*log10(S)
For N>10^7 log10(N) = 17.596 - 5.0*log10(S)
where S is in MPa.
» Reentrant corners in the weld must be modelled as "notches" with a root radius of 1mm.
» For quadratic elements (which are preferred) element spacing around the root radius must not be greater than 22.5 degrees.
» Elements of approximately that size must be used for at least three elements in from the notch surface.
» The analysis should assume linear elastic behaviour.
» The stresses to be taken from the analysis should be surface stresses rather than "integration point" stresses, calculated by extrapolation if necessary.
The S-N curve given (and reproduced above) is based on "mean minus two standard deviations", and thus embraces 97.6% of the experimental data considered.
The application of this method is quite straight forward, and able to be done using pretty much any FE program.
As for my results, I'll have to wait forty years to find out whether I got it right.
Does anyone have any experience of / views on this method?
RE: FE-Weld Fatigue mesh insensitive- Battelle
corus