What can we get from modal analysis beside natural frequencies and corresponding shapes ?

[ymn]

My question is : Can we get a hint about overall structural behaviour of a system from modal analysis ? Such as: even under a static case the system will try to deform like its mode shapes. Is this statement true ? I am trying to understand what can I get from modal analysis so I can use these informations in the early design phase of a system. (Where to add more parts/structural elements etc.)

 

[FEA way]

Modal analysis (eigenfrequency extraction) is often needed for other dynamic simulations that utilize modal superposition method. It's also possible to obtain participation factors from these analyses. They indicate the direction for which the mode will be most easily excited or will have the strongest response. Yet another output is modal effective mass - it tells you how much each mode contributes to total mass of the model for motion in a given direction. Modal analyses are often used to check connections in assemblies (if some part is unconnected, it will detach from the rest of the structure). Also, the first natural frequency can be used to estimate impact velocity for quasi-static simulations.

 

[rb1957]

"the first natural frequency can be used to estimate impact velocity for quasi-static simulations" … can you explain more please ? links ??

another day in paradise, or is paradise one day closer ?

 

[FEA way]

That's something Abaqus/Explicit users are taught. The procedure is as follows:
- determine the lowest natural frequency
- calculate the corresponding time period (t) from that frequency
- estimate the deflection in the direction of impact (d)
- calculate impact velocity: v=d/t
- make sure that v is less than 1% of the wave speed of particular material

 

[IceBreakerSours]

That is a general trick/recipe/hack users of other codes follow as well. Here is one such example. In the link I shared they also show how to go about choosing damping parameters based off of an eigenvalue analysis.

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[GregLocock]

"even under a static case the system will try to deform like its mode shapes. Is this statement true ? "

Not generally true, no. The response at 0 Hz is governed by the low frequency residuals of the modes, is the closest I'd get to that. Often the lowest frequency mode dominates, but that is very dependent on the point of application of the load and its direction.

Once you have a correlated set of modes then you can be fairly sure your model isn't fundamentally wrong, and you can go ahead and use it for other linear range simulations.

Cheers

Greg Locock


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[karachun]

Structure will buckle with first mode shape. If the structure is preloaded with critical buckling load then its first frequency in prestressed modal analysis will be zero.

 

[GregLocock]

Not generally true. A car does not buckle at the first resonance, which is bouncing on the springs.

"even under a static case the system will try to deform like its mode shapes. Is this statement true ? "

Still no, but if you just hit a system, much of the response will usually be at the first mode shape.

Cheers

Greg Locock


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