Why do we need to introduce geometric imperfection in ABAQUS?

[ShadowWarrior]

So this is a bit of theoretical question, kindly bear with me.

When doing column loading analysis, which tend to be dominated by buckling, why do we introduce geometric imperfection? Please list all possible scenarios.

 

[StefCon]

Hi,
I found this in the manual (11.3.1 Introducing a geometric imperfection into a model)

"In Abaqus/Standard the Riks method (“Unstable collapse and postbuckling analysis,” Section 6.2.4) can be used to solve postbuckling problems, both with stable and unstable postbuckling behavior. However, the exact postbuckling problem often cannot be analyzed directly due to the discontinuous response (bifurcation) at the point of buckling. To analyze a postbuckling problem, you must turn it into a problem with continuous response instead of bifurcation, which can be accomplished by introducing a geometric imperfection pattern in the “perfect” geometry so that there is some response in the buckling mode before the critical load is reached."

I've also found that the magnitude of the imperfection has an impact on the buckling load.

 

[BenStewart]

Why: A "perfect" column won't buckle.

An imperfection, to some degree needs to be introduced. This imperfection could in fact be from rounding in the calculations but best not to rely and it is recommended to manually seed an imperfection i.e. perturb your model so that it can buckle.

One method would be to run an eigenvalue analysis and seed the appropriate mode shape scaled by a fabrication tolerance. For column loading the straightness manufacturing tolerance (i.e. mm out of straight per m of length of column) may be appropriate.