Linear buckling - strange behaviour

[Anonymous]

Just did a simple linear buckling analysis with 2 MPa external pressure and both ends fixed. When I display the results, it shows some spurious results for first mode shape. When I disable display results and click on contour, a more reasonable result for mode shape appears. The Eigen value remains same thankfully. Then, if I run the analysis again, I encounter an error. 

What's happening here? 

When I run a nonlinear buckling analysis with ARC length enabled (I didn't have to include any small eccentric load for the convergence here), I find a similar shape as that seen in "spurious" result above for the load increment of 0.9999. 

However the final result for mode 1 is correct for nonlinear buckling:

[John_Holtz · Accepted solution]

Hi @Anonymous 

Based on your video, there is a discrepancy between the result shown in the window and the "result" indicated by the drop-down selection in the ribbon.

• At the beginning of the video, the result shown in the window is the static stress result due to the external pressure. This is shown by the text in the lower left-hand corner of the window. The text does not say that the result is the EIGV result (meaning it is not the buckling results; therefore it is the stress result). Meanwhile, the ribbon incorrectly indicates that the buckling result is shown (EIGV=8.5).
• Later on (at 0:35 into the video), the contour plot is showing the buckling shape as indicated by the text "EIGV=8.5" in the left-hand corner.
• I happen to have a buckling model that I have been working on for a different issue. I will check to see if it shows the discrepancy between the ribbon and what's shown in the window.

Then, the same thing is shown in the images for the nonlinear buckling analysis. The first four results with load factors between 0.1 and 1.0 are the static stress results due to the external pressure. (I assume the text in the lower left corner indicates the results for for INCR 4 Load = 1.0, but that text is cut-off in your image.) Uniform compression of the tube is what you would expect. The fifth result for "Mode 1 EIGV=8.55" shows the buckling result.

[Anonymous]

John, I see what you mean. I have never bothered to look at the drop down window for linear buckling and assumed that linear buckling will show only the buckling results and not the linear static results for the present load. For nonlinear analysis, the buckling results are shown as a default and same should happen for linear case after the mismatch of dropdown and results is removed.