Symmetry BCs in pressured vessel + Non-linear material

[ecFem]

Hello,

I hope you are doing well.


On one hand (Symmetry BCs in presured vessel) :

I have been strugling for a while, thinking about the proper way to define the symmetry conditions for the model I am studying.

It is a simple pressured vessel, then, as it is symmetrical, I am analyzing a quart of it. It has a load of 30 [bar] = 0.3 [MPa], which I apply through a NL analysis in 60 steps. The vessel is made in aluminum, with 6 [mm] thickness in the cylindrical part, which decreases up to 4.5 [mm] in the spherical part (I've applied a smooth transition).

The problem is that I cannot figure out what are the correct symmetry conditions to apply.

The model (image below) is made of the following sections :
1) Cylindric part
2) Toric part
3) Spherical part

For the cylindrical part, I have defined a cylindrical coordinate system and readily specified the symmetric BCs, but then I can't understand how to define the BCs. I have created a spherical coordinate system and assigned some BCs that I thought were appropriate after some thought.

Clearly, the BCs I am applying are not correct since the results I am obtaining are not appropriate (figure below). The torical part is not expanding and I imagine it has something to do with the BCs.

---------------------------------

On the other hand (NL material) :

I am defining a NL material, by introducing the stress-strain curve.

I would like to know which way is better for the nonlinearity type :

i. Nonlinear elastic : loading the curve into the the nonlinear properties part (in the material definition)
ii. Plastic : loading the curve in the same place but specifying the initial yield stress at 270 [MPa] following VM

In both cases, to test it, I take an element and obtain the stress-strain curve but I see that once the yield stress reached, the curve follows a straight line, i.e., strain increasing but stress stuck at 270 [MPa]. Any thoughts on what can be happening ?

Thanks in advance !!

Cordially,
EC

 

[rb1957]

personally I wouldn't use symmetry ... model the whole damned thing !

if you want to use symmetry, I'd start with use the cylindrical portion ... apply pressure, does is respond correctly ?
then add the end domes. Maybe use something simple (hemi-sphere) to start with; again, does it respond correctly ?
then modify to your more complex curved dome.

but something "odd" is happening at the boundary between the cylinder and the dome, as if the curve is overly constrained ??

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[SWComposites]

It seems to me (without a lot of thought) that the same bc can be used for the entire section.

 

[karachun]

If you model 1/8 portion of vessel you can use three symmetry plane conditions. Symmetry must be defined in Cartesian CSys instead of cylindrical. Shape of vessel does not matter until it has three orthogonal symmetry planes.

 

[rb1957]

on the RH end, won't you want to use cylindrical co-ords ? to identify the radial and tangential directions ? The key is to see that RH end deflect the way it should (ie inflate with pressure).

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.