Elbow

Hi all!
what boundary one should consider when analysing a pipe elbow for out of plane moment?

Thanks in advance

 

INternal pressure is present on the inner faces of the model throughout..in the elbow as well as straight pipes.

I did calculate the axial stress(P_axal) to be applied at the end as follows:

Pi* Ai = P_axial*A
where Ai= hollow circular area of the pipe
A= Net cross-sectional area of the pipe.

As U mentioned,oncce both ends are fixed,it is not possible to analyse the problem for Moment loads.And there is no necessity for capping both the ends once we apply correct boundary conditions as mentioned above.

Best regards,
GoodWishes

 

[prex]

No, your calculation is possibly incorrect!
If, as I suppose, you use shell elements, then you must be careful. Shell elements are mathematically represented in a FEM model as having zero thickness. This means that the pressure applied at one face will in fact act at the model mid surface, and if you modeled the pipe with elements at mid thickness, then your A_i above must be the pipe area at mid thickness!
My suggestion is once again to cap pipe ends: you'll need no calculations of axial load. Otherwise you have still the option of modeling the pipe at its inner face or at mid thickness: the latter is for sure more correct, as bending behaviour will be better represented. Another approach I have already used is to model at mid thickness and then to scale down the pressure by the ratio D_i/D_m.
In all cases you should check that no transverse reaction at the supported end exists under pressure load only.
prex
motori@xcalcsREMOVE.com

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Thnak you prex!

My model is made of shell elements and I will consider your suggestions.

In one of your previous messages you had mentioned that in the case of only pressure loading ,one should cap both ends of the pipe and both the ends should be supported.Why is it necessary to support both the ends?Isnt it sufficient to cap both ends and clamp one end?

regards.

 

[prex]

Yes, you are right: supporting both ends is probably not necessary.
I'm even not sure whether the unsupported end will move: if it doesn't move (with pressure only of course), then supporting both ends would be useless.
If on the contrary it moves, then of course the bend will continue to correctly withstand pressure even with an unsupported end: however this situation does not correspond to a real one, as no real bends are supported on one side only.
That's what I meant in requiring both ends supported, anyway I'm sure the difference should be quite minor: you'll be able to check this and could report here you results...
prex
motori@xcalcsREMOVE.com

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While modeling with shell elements generally one goes for mid surface.I too modelled with mean dia(Dm) of pipe.

But is it necessary to scaledown the pressure(or any surface loads) by Di/Dm?
Doent a general purpose FEM program automatically recognise the inner surface and apply pressure(or surface loads) on it since we input nodal thickness(es)?

In any case,what is the basis for this formula?

Regards.

 

[prex]

No, the code doesn't that. The pressure applied to inner face will be acting on the surface delimited by the nodes (hence the mid thickness surface): this is OK for a plane model, but if the model is curved that surface will generally not correspond in extent to the surface actually submitted to pressure.
Scaling down of pressure by the ratio of diameters has the goal of calculating the correct circumferential membrane stress in the pipe wall: as this is normally the limiting stress, that's why I do that.
If you wanted to correctly represent the longitudinal stress, then you should scale down by the ratio of the squares of the diameters: however the circumferential stress would become underestimated (and that's not good). So it's much easier and correct to cap the pipe in the model. The longitudinal stress will be a little overestimated, but who cares? prex
motori@xcalcsREMOVE.com

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Thanks for giving this insight.
If one wants to apply, longitudinal stress ,I guess it should be PDm/4t where Dm is the mean dia.
Capping both ends would definitely give good results.

I have another question.If my elbow is a solid model and I want to cap it with a shell mesh,how to connect a shell to a solid? Or how else can I cap it?

Thanks in advance

 

[prex]

You can use shells connected to inner nodes of the solid model. Of course local stress distributions won't be real, but that's a zone of the model where stresses shouldn't interest you.
I personally favor the use of the simplest element types adapted to a specific problem: this avoids having to cope with local stress distributions (example: the variation of circumferential stress across the thickness) that are generally unrelevant to the analysis. Hence I strongly suggest the use of shell elements for your problem.
Note however that with solid elements the pressure will be correctly applied to the intended surface of the model! prex
motori@xcalcsREMOVE.com

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Thats right.
More specifically how to connect a shell node having 6dof to a solid node having only 3dof?Can I merge(!)them?Or shall I need to use some manipulation here?What to do with the ROT dof of the shell node?
It is true that I am not bothered about the local stresses near this connection.

Thanks in Advance.

 

[prex]

Leave the rotations unconnected? prex
motori@xcalcsREMOVE.com

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Nopes..I cant leave the rotations free..coz I want to transfer twisting moment from shell to solid.If I leave ROT dof free,this twisting moment wont be transferred!
regards.

GoodWishes

 

[prex]

Why that?
If the shells serve as the flat bottom to a pipe and the moment is a twist for the pipe, apply it as a couple to the center node of the flat and it will be correctly transmitted to the pipe. prex
motori@xcalcsREMOVE.com

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