experimental modal anlysis

[anssaf]

Hi
I have a structure of cantilever beam amd (T-shape)with two joints which connect the two parts of the beam to be in T-form.when we mounted the beam by its free end to a table and excited it by a hammar to find the two first modes. the bending mode disappearedit is suspected that the dynamics of the table influenced the the total behavior.what I can do to know the reason.I need help
thank you

 

[GregLocock]

That's why we tend to test free-free. A proper test fixure to test a cantilever will weigh >200 times as much as the component, and be much stifffer, so thta all of its modes are outside the range of interest.

I suggest you do a modal test of the table itself, and do a free free modal of your component, and then use the theory of receptances to simulate bolting the two together.

Cheers

Greg Locock

 

[xydrex]

Regarding Greg's response, I have recently been doing modal testing on a car bonnet using shaker & laser vibrometer. We had to hang the bonnet vertically because the scanning head is to be ~ 5m away from the surface, the roof is too low if we had it horizontally (scanner above). Anyway, our table is a massive steel structure, weighing several tons which should not pose any problems. However to get the bonnet hanging vertical we had to make a frame on top of the table which isnt as massive (40x40x3mm square tube). Shaker is mounted on this frame and you can feel the vibrations through it (frame) when you put your hand on it. My boss says it shouldnt be a problem because the 'laser vibrometer's detection of bonnet response is so fast that response of frame will not have a chance to register'. Bonnet hangs from frame by flexible shock cable, so frame dynamics can only enter bonnet through the shaker itself. A force transducer at stinger tip will pick up what the shaker is putting in, even if influenced by the frame so theoretically it shouldnt be an issue. But it means that you will not get the nice constant input force amplitude over the excitation range. Would anyone know any downfalls that could reult from this?

 

[GregLocock]

Poor signal to noise ratio when the frame goes into resonance. Your boss is half right, you'll probably get usable data. If you can feel the frame vibrating then that suggests the isolation is not working very well - try doubling the length of the shock cords and see if the responses change.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MikeyP]

I think it would be fine. The effects of shaker force dropout will be just as severe as the influence of the frame. Keep an eye on the coherence when you measure the FRFs jsut to be on the safe side. Just make sure that the rigid body frequencies are AT LEAST 10 times smaller than the first flexible mode.

M

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Dr Michael F Platten