I am analyzing the squeal of simple brake using Abaqus and I tried to find the unstable frequencies using complex frequency which caused the squealing noise; however, all of the modes have nearly zero dampings (lie on the imaginary axis) and the pairs of modes which become coupled and formed a stable/unstable pair couldn't be found. I attach the brake model in the comment. I did the following steps: Step 1 --> non-linear static analysis for applying brake-line pressure Step 2 --> non-linear static analysis to impose rotational speed on the disc Step 3 --> normal mode analysis to extract the natural frequency of the undamped system Step 4 --> complex eigenvalue analysis that incorporates the effect of friction coupling. How can I found some unstable frequencies using Abaqus?