Hi Naji. I suggest you please use a Coupled Field Modal analysis system (rather than the Piezo & MEMS ACT) to do your modeling. Piezoelectric analyses are now natively exposed in Coupled Field analysis systems. The Piezo and MEMS ACT is no longer supported and the last version may not be fully functional in later versions of ANSYS. Also, we now have a library of piezoelectric material properties in Engineering Data that you can use in Coupled Field analysis systems.
I created a small piezoelectric test case using a Coupled Field Modal analysis system to help explain one way to get modal damping ratios from a modal analysis of a piezoelectric device. I cannot share it with you on the Forum - we're not allowed to do that. But I can capture images from the project that should help you get started.
By default, our modal analyses use an undamped eigensolver that makes no effort to include damping in the calculation of natual frequencies and mode shapes. The default undamped solver will not provide any information allowing you to deduce damping ratio associated with calculated natural frequencies.
So, as Dave said earlier in this thread, to get a damping ratio from a modal analysis, you must do two things:
Use the damped eigensolver (image below). Either the "full damped" or "unsymmetric" option will do, but in your case full damped might be better - in addition to giving you the stability of the mode, it will explicitly report the damping ratio (your objective) in a table.
The other thing you need to do is define some source of damping. In my test case, I have no structural damping, only an isotropic electric loss tangent that I added to our predefined Barium Titanate properties in Engineering Data:
I model a fixed-fixed beam, atop which resides a piezoelectric transducer with VOLT coupled (an open electrode/terminal) on the top surface and VOLT=0 (grounded) on the bottom surface:
With loss tangent = 0.002, the calculated damping ratio of the first mode is 1.836e-6:
With loss tangent = 0.2, the calculated modal damping ratio is 1.798e-4:
Also, many people who analyze piezoelectric devices want to include the effect of passive circuit elements connected across the terminals of the transducer (an electrical load of some kind). As this is not yet natively exposed, this requires a command object that creates CIRCU94 elements:
I hope this helps.