perpendicular shells
perpendicular shells
(OP)
Hi,
I have four shell elements connected perpendicular to one another (in a cross). One face of the cross is subject to a bending load, the other is fixed at one end. A stress concentration is calculated at the interface. Is this a valid stress? It seems very high.
Any comments are welcome.
I have four shell elements connected perpendicular to one another (in a cross). One face of the cross is subject to a bending load, the other is fixed at one end. A stress concentration is calculated at the interface. Is this a valid stress? It seems very high.
Any comments are welcome.





RE: perpendicular shells
RE: perpendicular shells
Good luck.
RE: perpendicular shells
Can you post of figure showing the real part/structure? And a figure showing the loads and boundary conditions applied to the model?
RE: perpendicular shells
RE: perpendicular shells
RE: perpendicular shells
As has been said, you can't class the stresses there as peak and assess for fatigue damage, but you could class them as being secondary stresses (occuring at a structural discontinuity) with a limit on the stress range of twice yield.
corus
RE: perpendicular shells
That being said, I would also guess that refining the mesh would just increase the max. stresses to infinity, since this looks to be a singularity at this connection point.
The question I have is are you modeling reality adequately? Are these pieces going to be welded together? Bolted together? If they were connected as shown, with loads that cause the stress to go to infinity at this connection, then that could indicate problems with the modeling of the connection itself. Is the way you have modeled it realistic? If you really need the stress at the connection point, then you have to do a better job of modeling what is really going on at that connection point. If you are interested only in stiffness (that is, how much it deflects for a given load), what you have might be adequate but you can't trust the stresses at that connection point.
RE: perpendicular shells
I'm with Stringmaker and Prost when they say that refining the mesh more and more will show higher and higher stresses. In fact, the hot-spots you are seeing are in geometric "edge-discontinuities" (triple- or multiple-corner in one case, T-corner in the other. These are the typical cases for "mesh-paradoxes".
The stress distribution, imho, can makes sense (as far as I can guess because there is no indication about the restraints and the loads). If you plot S1 instead of SEQV, you should find that the stress at the "external appendix" T-joint is in compression, while the hot-spot in the I-beam is in traction.
Where I see a possible problem, as Corus says, is in the discontinuity web/flange/appendixes. You should check that the nodes are merged etc...
Anyway, as "the" solution to your problem if you do are interested in the "realistic" stresses at the junction, I'm with Stringmaker in proposing "Submodeling" (with Ansys which you are using, you can even perform shell-to-solid submodeling without problems). In your solid submodel, include all realistic features and, possibly, avoid any sharp edge discontinuity between concurring elements.
Regards
RE: perpendicular shells
The structure being assessing is a reel, composed of various steel sections which is why it has been modelled with beams and shells. A stress assessment is to be performed to determine the capacity.
I understand that the mesh is coarse, however this is just a small segment of the model, and unfortunately due to the loading conditions, I cannot exploit symmetry. I have performed a mesh sensitivity check and the stresses away from the concentration are unaffected by further refinement.
I need to consider the stresses at the concentration to ensure they are within the allowable of the code. The structure is to be used offshore and therefore I am performing the assessment according to DNV Rules for Marine Operations.
The shells are joined at common nodes, however they are composed of different thicknesses which adds to the discontinuity. It is all the same material. In reality, the shells are welded together.
I will try submodelling using solid elements; in order to quantify the peak.
Thanks for your help, I will keep you posted.
I have attached a figure of the model. I shall add additional posts showing further detailed plots.
RE: perpendicular shells
RE: perpendicular shells
RE: perpendicular shells
RE: perpendicular shells
RE: perpendicular shells
RE: perpendicular shells
RE: perpendicular shells
tg
RE: perpendicular shells
I will be careful with stress averaging at the intersection. Is there an easy way to stop averaging at the interface? I guess I could set up different materials for each shell.
I have built a 3D submodel, though intial results show peak stresses at the sharp corners, to be expected. I will add appropriate fillets and rerun.
RE: perpendicular shells
Stresses in the submodel away from the cut boundary are similar to the global results, indicating a successful transfer.
The results on the submodel show high stresses at two locations, both are internal corners and I have added appropriate fillet radii. The maximum concentration is ~550MPa, much higher than yield (355MPa). This concentration is a secondary stress localised on the surface of the model.
Corus mentioned that I could get away with assessing localised results against twice yield. I can see that the stresses through the section away from the concentration are less than yield, and therefore will provide support against failure. I know that it is acceptable in some codes to exceed yield locally, as long as the structure is not compromised.
Does anyone have experience of justifying why such a stress concentration would be acceptable, or can point me in the direction of a reference to a section in a code that deals with such stress concentrations?
Any further questions, please ask. Thanks for your help.
Below is a link to a plot showing the global and submodel stresses.
RE: perpendicular shells
a stress peak higher than yield means there is some localised yielding going on. if you can rationalise that this is indeed a localised behaviour then go for it. if you can't, how difficult is it to do a material non-linear analysis ? this would give you a "definitive" answer.
RE: perpendicular shells
RE: perpendicular shells
RE: perpendicular shells
if in the submodel the fillets are all modeled "as built" and their mesh is appropriately fine (now you don't have non-tangent faces sharing an edge, so you won't get the "mesh paradox"), meaning at least three or four parabolic elements along the fillet radius, then I personally would be extremely cautious as to disregard the peaks as you could do in presence of a sharp edge.
Try clipping the stress results, showing only the portion of geometry whose stresses are higher than 0.9*yield. If these portions are "small" with respect to the net section, and the average stress in the net section is below the correct limits (which are these limits depend upon the rules you are following, be it internal rule, ASME, or whatsoever), then I think you can accept them. Otherwise not.
Regards
RE: perpendicular shells
1) you've cut-off the maximum stress plotted to 300MPa. this gives a misleading view on how much is over 300 MPa ... in the upper left solid pic, everything inside the red perimeter is over 300 MPa (it plots as black, which doesn't look so bad !?) ... that means that pretty much the entire surface has yielded. your section shows that this is localised to the surface, which is good news statically. if fatigue is a concern, i'd shot-peen the fillet.
2) the web supporting the yielded flange has a nasty amount of yielded material, but again more a fatigue concern than a static one (i see lots of material that this overload could be redistributed into wihtout changing the global loadpaths).
3) why do you think the solid model "lights up" on the "innocent" corner (what looks to be a free corner ?
4) i have a little trouble seeing how this detail model fits into your overall structure ... the large "wheel" ?