Learning Forum

[hajidjamal]

I read a research/report on how to determine rayleigh coefficient from modal analysis. how can we determine damping from modal analysis as it doesnt include any type of load? I need the rayleigh damping(alpha and beta) for me to conduct a transient analysis. Thank You.

[peteroznewman]

Modal analysis can't tell you the Rayleigh damping constants alpha and beta, those are inputs to a transient analysis.
An experimental modal survey can acquire data from instrumentation such as an accelerometer on a physical structure that is excited with a hammer strike or some harmonic excitation. Analysis of that data can calculate a damping ratio for each mode.
If you don't have that, then you can make an assumption on damping ratio based on the type of materials and joints in the structure.
There are equations to compute alpha and beta such that the damping ratio is fairly constant between two frequencies.
Those equations are explained in the free course on Damping on this site.

[hajidjamal]

Thank you for the answer, if we mention type of material when conducting transient/harmonic analysis, does damping will occur in the result or we still need to input damping ratio(alpha/beta) in the analysis setting? thank you

[peteroznewman]

Either add damping to the material in Engineering Data or add it in the Analysis settings. If you do both you will double the damping.

[hajidjamal]

Back to the first answer, so without doing the physical impulse hammer test, there are really no other way to determine the damping ratio for the damping ratio in the transient/harmonic analysis setting other than make an assumption on the dampin ratio? thank you

[peteroznewman]

In general, you need experimental data to provide inputs to simulation.

[Hassan Elshokrofy]

if i have different structures made from the same material, can i do an experimenatal estimation of damping ratio for only one sample and use the result as input for FEA model for other structures?