I am performing nonlinear buckling analysis using COSMOS/M of long thin-walled cylinders under external lateral pressure, using the eigenvalue buckling mode shape as an the initial geometric imperfection.

I (perhaps naively) was expecting the resulting load-deflection curve to have a nice sharp knee indicating the buckling load. Instead, I get a nice asymptotic curve that has no discernable slope break point. Does anyone have a recommendation on how I can determine the actual buckling pressure from these results? Or, is there a better way for performing the non-linear analysis?

Many thanks in advance.