Hi all,

I am very confused with some of the results that I am getting using a static frequency analysis.

Frequency is calculated as the squareroot of stiffness devided by mass (as found here towards the bottom: http://en.wikipedia.org/wiki/Fundamental_frequency).

My question: when I run an analysis on the same beam but using a different material, say steel (ASTM 36) vs aluminum (6061 alloy) vs plastic (Nylon 101) I get almost the exact same results (difference of 27.61Hz vs 27.65Hz vs 27.48Hz). Can sombody explaine to me why I might get approximately the same result even though the masses are significantely different fro each of these 3 materials?

Thank you in advance!

Noah,

For steel and Aluminum, the answer is fairly easy. The stiffness of the bar is directly proportional to Young's modulus for the material and the weight of the bar is directly proportional to the density of the material. Steel is 3 times as stiff as Aluminum and also about 3 times as dense, so the natural frequency should be very close to the same. But for the Nylon, you should get a much lower natural frequency, about 1/5th of the other two. Something is wrong.

Jerry S.