Hello, dear colleagues,

I am currently investigating non-circular, thin-walled cylindrical shells under external pressure. Typical parameters are: mean of the two main axis dimensions 1000 mm, wall thickness 22 mm, out-of-roundness 0.015. Out-of-roundness is defined as the quotient of the difference between the two main axis dimensions and their mean.

I am calculating with linear-elastic material (E = 188000 MPa, nu = 0.295) and geometrically non-linear. I am using two different models: Model A with 20-node hexahedral elements, Model B with 8-node shell elements.

I have learned that Calculix converts 8-node shell elements internally into 20-node hexahedral elements before the analysis. Since I am using a single element across the wall thickness for model A, I actually expect the results to be identical for both models.

Unfortunately, I donâ€™t see that. I get the following displacements for a pressure of 1 MPa and boundary conditions for infinitely long pipes:

I also see large deviations in the stresses:

The buckling pressures also differ:

I did the analyses under Windows 11. The two inp files can be found here: ell_inf.zip - Google Drive

I believe that the results of model A are correct because I confirmed them with NX Nastran. They also agree quite well with the publication /1/ from 1950.

It is also interesting that I get no differences in the displacements and stresses for perfectly round cylinders. Only the buckling pressures are unchanged compared to the non-round models. The models for the perfectly round cylinders can be found here: cyl_inf.zip - Google Drive

I would like your support and advice. I donâ€™t want to rule out that the error is mine. On the contrary, it could be likely;-)

Greetings, thanks in advance

muppets

/1/ S. Schwaigerer, A. Konejung, Die Festigkeitsberechnung von Flammrohren. In: Konstruktion, 2 (1950), No. 1, pp. 17-23