I am doing experimental modal analysis on cantilever beam model . Natural frequency changes due to boundary conditions changes at every instant which affects the characteristic of structure.This is very important issue in the vibration based damage detection methods. Any one suggest me ''How to handle this issue?''
Hi, Prakash,
When we make modal analysis of free bodies, the rule we use for the suspension is that its natural frequency has to be at most 10% of the first natural frequency of the structure (usually obtained from a FEM model).
However, we know that it is practically impossible to represent the cantilever boundary condition perfectly, because when the structure vibrates, the efforts on the fixed end become really high.
My suggestion is that, on a FEM model, instead of modelling a fixed end, you should model an elastic end and vary its stiffness (always high values) in order to understand how it affects your natural frequencies. Furthemore, it will give you more parameters to compare your experiment and your model.
The only thing i suggest you is that, if you are using screws, be careful with the tightening torque you are using. Too much torque may create inner tensions in your beam, and then the model will never fit the experiment.
I hope I could help you a little bit.
Best wishes,
Vinícius
Thank you Vinicius for your reply....
off course, then how do I control the torque, because its varies every time. Is there standard way to solve this practical issue.
I think you should check how close you want to get to the cantilever condition. The main issue in this condition is to restrain the angle.
If the torque varies, I think that the best solution is to increase the number of screws. It is better than tightening the screws harder. Another thing you could do is to reduce the amplitude of the excitation. If your coherences are OK, there is no problem to have low amplitude of excitation. Besides, it would help to prevent loosing the screws.
Hi Prakash,
The answers you received from Vinicius are quite pertinent. Perhaps a picture or a link to the experimental setup would help in better understanding the issue. We had similar issues is comparing experiment and theory for cantilever beam, even if this case is simpler than the simply supported case. First of all make sure that your beam is slender, that is, in accordance with the theoretical model.
One aspect which was not discussed by Vinicius is the body (bench, wall, etc.) in which you are clamping the beam. Very important are the mass ratio body/beam which has to be large and the rigidity of the body (fixing to the ground). Then you can think how to tighten or weld the beam to this body and perform a correct (small amplitude) perturbation of the equilibrium state.
A model of elastic clamping, as suggested by Vinicius, involving a "best fit" of stiffness search algorithm could lead to accurate results.
Hope this helps you. Give us your feedback after the experiment.
When using screws to fix a system to a frame, it is common that not all the expected contacting surface is effectively in contact. In general the contact pressure vanishes over the bolt head radius and you end up with an interface with large free areas.
If you obtain experimental frequencies under your model frequencies, and a repeatability issue regarding mounting/unmounting, it is clear that you have an experimental clamping problem.
To control the screw tension, you can use a torque wrench. To optimize the contact surface control in your clamping joint, the use of washers is recommended as their softer mechanical properties will allow a better conformity of the contacting surfaces.
An alternative way of proceeding is to localize contact using pins for example, so that the effective surface is ensured with a higher contact pressure.
Regarding the model, trying to apply coupling to a more representative coupling area geometrically and using grounded spring of high stiffness (that can be calibrated) instead of direct fixed dof boundaries will certainly help.
Regards,
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