Resonance Frequency-modal Analysis

In summary, the conversation discusses the use of MSC Nastran 103 to calculate the resonance frequency of a car's sheet metal platform using the finite element method. The question is raised whether constraints need to be applied, to which the response is that a free modal analysis means no constraints, but the final outcome will depend on where forces are applied. It is advised to at least constrain the panel in its installed state, as this will affect the results. The importance of constraining the model as it will be in real-life is emphasized, as unconstrained models will produce meaningless results. Different types of boundary conditions and their impact on the results are also discussed.
  • #1
ataras
2
0
Using MSC Nastran 103 to calculate BIW (car's sheet metal platform only)resonance frequency by FE method, do we need to apply any constrains?
I think, a free modal analysis means no constrains, however, final outcome (Hz)will depends on where forces are applied to the model ? Cross car torsional stiffness frequency will be diferent from front to back. Please, advise.
 
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  • #2
I would think, at the bare minimum, you would constrain the panel as it would be in the installation. A panel confined around its edges is going to act differently than a free floating one. Just thinking of the solving of the differential equations, you need to specify the BCs to get a particular solution.
 
  • #3
To get results that are worth something, you need to constrain the model as it will be constrained in real-life. While it might be possible that the solver will solve without constraints (depends on the software) the results are meaningless. You'll end up with an infinite number of vibrational modes that CANNOT happen in real life because there should be a constraint there.

Think of the possible differences just on a simple structure like a standard rectangular beam. Simply supported, single cantilever, or double cantilever will all give you far different modes of vibration, yet the structure looks exactly the same in each case, only the boundary conditions have changed.
 
  • #4
Thanks, location of excitations points is crucial. Every time you change them you do get different results. Point I was trying to make is that you induce vibration through excitation points on particular modes without having to constrain the object.
 
  • #5
ataras said:
Thanks, location of excitations points is crucial. Every time you change them you do get different results. Point I was trying to make is that you induce vibration through excitation points on particular modes without having to constrain the object.

If you're doing a modal analysis, you shouldn't need to have any excitation points defined. The program should solve for the natural modes of vibration without them; but boundary conditions are crucial. This solution should show you the nature of the mode, and it's modal ferquency.

If you're doing a frequency-response analysis, you will need to define an excitation force, a range of frequencies, AND constrain the model fully. This solution can show you the model's reaction to an input vibration. As before, if the model is unconstrained you will not get any results that are useful (rigid-body motion, modes that don't exist in the constrained model, etc).

Basically, no matter what, if you want to believe the results you're getting, you need to constrain the model as it will be constrained in real-life. There is a VERY fine line between getting numbers, and getting numbers that make sense in FEA.
 
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1. What is resonance frequency-modal analysis?

Resonance frequency-modal analysis is a technique used to determine the natural frequencies and mode shapes of a structure. It involves exciting the structure at different frequencies and measuring the corresponding responses to identify the resonant frequencies and their corresponding mode shapes.

2. Why is resonance frequency-modal analysis important?

Resonance frequency-modal analysis is important because it allows engineers and scientists to understand the dynamic behavior of a structure, which is crucial for designing and optimizing structures to withstand external forces and vibrations. It also helps in identifying potential failure modes and improving the overall performance and reliability of the structure.

3. How is resonance frequency-modal analysis performed?

Resonance frequency-modal analysis is typically performed using specialized equipment such as vibration shakers, accelerometers, and data acquisition systems. The structure is excited at different frequencies using the shaker, and the responses are measured using accelerometers. The data is then processed to determine the natural frequencies and mode shapes.

4. What types of structures can be analyzed using resonance frequency-modal analysis?

Resonance frequency-modal analysis can be applied to a wide range of structures, including bridges, buildings, aircraft, and machines. It is particularly useful for large and complex structures that are difficult to analyze using traditional methods or when the behavior of the structure is highly dependent on its dynamic characteristics.

5. What are the limitations of resonance frequency-modal analysis?

Resonance frequency-modal analysis has some limitations, including the assumption of linear behavior and the neglect of damping effects. It also requires specialized equipment and expertise, which can be costly and time-consuming. Additionally, the results may vary depending on the excitation and measurement techniques used. Therefore, it is essential to validate the results and consider other factors when interpreting the data.

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