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Introduction to FE Analysis

Tutorial CD

Introduction to FE Analysis

Tutorial CD

Frequency Response Analysis. Why?

applied a pressure loading which varied as a function of frequency in a frequency

response analysis. Finally I input an acceleration at the attachment point.

The Frequency response of the rod under the pressure loading showed a peak

response at the longitudinal natural frequency. The Frequency response of the rod

under input acceleration showed a different natural frequency. Is this due to damping

perhaps?

modes analysis and the pressure loading the structure it was unrestrained. It is what is

called a free-free model. The pressure does not apply any form of displacement

boundary condition and the structure will have a net motion given by F(t) = M*a(t).

In the case of the input acceleration this is an enforced motion and you are applying a

boundary condition to the structure which will change it's natural frequencies. Imagine

each node has to be constrained to move in your specified input direction. Any relative

movement of the fixed face will be eliminated, preventing the rod end from having lateral

displacements due to poissons ratio and any possible end 'belling' shapes. The flexural

modes will also be higher as you are stiffening the structure with the constraints.

If the base of the rod is actually the attachment point to the rest of the structure, then

pressure loading and use the same degrees of freedom at the base when applying the

base motion (acceleration). All these analyses should then have the same natural

frequencies.

There should not be a frequency shift due to damping at these levels. In fact most FE

solvers ignore the correction term between damped natural frequencies and undamped

natural frequencies. You won't shift the frequencies even if you put in really high

damping.