Dear Bharatesh, I hope you are not joking!  By definition of aspect ratio of a beam is the length versus breadth, But this doesn't enter into the formula for oscillation frequencies of elastic cantilever beam, which is given by for the nth  mode:

Omegan= alphan  SQR( E I / rho A L4)   E  Young Modulus, I Moment of Inertia, L  Length,  A  cross section,  rho  density.  I= b d3/ 12   rectangular cross section.  b is breadth, d  width.

Omegan= alphan SQR( E d2 / 12 rho  L4)     rectangular cross section.

Only 

Dear Bharatesh, I agree with Tarik.

From experimental point of view you could test cantilever for evaluate its damping properties on the basis of the so-called Oberst-bar method, as described in ASTM Standard E 756-98. I do hope it helps.

Dear Bharatesh, 

I'm not sure you are looking for Rayleigh damping coefficients?

If so, you should first define the mass and stiffness matrices.   

Dear All,

Thank you for the inputs