For the modal testing of free boundary condition structure, such as a steel beam put on the bungee cords, is it possible to apply force using the actuator, like modal shaker?
I guess it is impossible since the attachment of actuator introduces not only stiffness and damping but a restriction in the attached directional DOF.
Then, is the excitation by impact hammer an only option for the modal testing in such cases?
Looking forwards to any help.
Thanks.
Often, experimental measurements of a nonlinear system convolute the effects due to the experimental setup and the effects due to the nonlinearity of interest. For instance, a clamped boundary condition is often prohibitively difficult to achieve; in most cases, some amount of torsional stiffness, friction, and even intermittent contact can be introduced by the boundary condition. At the opposite extreme, the free boundary condition is often replicated via bungee cords that act as weak springs.
Some researches have been performed to identify a set of best-practices for conducting experiments of nonlinear systems, specifically those with jointed interfaces. The effects of the boundary conditions, excitation techniques, and measurement techniques on the measurements of the system’s stiffness and damping properties should be carefully studied which the following specific recommendations should be applied:
- For impact hammer measurements, the tip and hammer mass do not significantly affect the measurements of damping and stiffness. Thus, the impact hammer configuration should be chosen based on the frequency range of interest with harder tips without added mass being more appropriate to study higher frequencies.
- The responses of systems with free boundary conditions created via the use of bungee cords are insensitive to the bungee length, stiffness and position.
- The use of accelerometers, even when they are a small fraction of the mass of the system, can significantly affect the measurements of damping and stiffness. If used, accelerometers should be located away from node points, and the number should be kept to a minimum.
- For shaker excitation, care should be used in selecting an appropriate stinger. Of those tested, the thinner stingers are found to exhibit bending, which results in the appearance of nonlinearities in the system’s frequency responses.
- The excitation sweep rate, magnitude, and direction are not found to have significant effects for the existing systems.
- The conditions of the interface have significant effects on the responses of the system, and future work should aim to thoroughly quantify the interface conditions.
Good luck in your test.
Kabir Sadeghi Thank you very much for your answer.
It is interesting that I also have confirmed the fifth one, on the sweep methodology, in my previous test.
I will keep this in mind, especially the six consideration points. Thanks again.
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