Static vs Dynamic Forces - Trouble Interpreting Vibrations in Data

[toothless48]

Hello all,

the is x-posted from Vibrations Engineering, hope that is OK.

I am working on an analysis of a large chassis frame for an autonomous vehicle. We are validating the performance capabilities of the system using vehicle-mounted accelerometers. The raw data that we are collecting is filled with intermittent high-amplitude noise, most likely due to vibrations when the vehicle drives over rough patches in the concrete floor. This system follows a motion command array, but the system's true performance capabilities are not currently known.

The intention is to take the collected data, and feed it into static load cases for an FEA study. The current plan is to filter the data to remove "non-commanded" motion, and then use the peak filtered results for the static analysis. My question is this: how do I interpret the high-frequency vibrations in terms of structural analysis? In another way, at what frequency can dynamic loading be modeled as static loading? I am of course not looking for a numerical or concrete answer, but maybe for some guidance from anyone who had similar experience. It seems fair to say that 100Hz frequency components on a structure similar to a truck are not static. The vehicle design and hardware are vibration-tolerant, so I am really just looking at this from a structural standpoint.

Thanks very much

 

[WARose]

In another way, at what frequency can dynamic loading be modeled as static loading?

I've never input any load that varies with time as anything but dynamic. It's not the same thing as mass.

 

[MotorCity]

So you are collecting dynamic load data and you want to dumb it down into a static load and then put the static load into an FEA program? Why not input the dynamic loading into the FEA program? You are taking a rather round about method that will undoubtedly lose degrees of precision along the way.

Generally, structural folks tend to avoid dynamics as much as possible. For example, we tend to apply an impact factor to a wheel load to account for the dynamic load produced by the wheel hitting a bump. This approach results from the fact that we have a hard time getting our hands on dynamic data because it is costly to obtain in our applications.

Automotive folks embrace dynamics because obtaining dynamic data is part of their everyday routine, using their big and fancy testing equipment and labs. Since you have the dynamic data available to you, I would use it as such and not try to convert it to some "equivalent" static load that will most likely lead to misleading results.

The quick and dirty, structural guy solution: dynamic load = 2.0 * static load........I will send you the bill

 

[WARose]

The quick and dirty, structural guy solution: dynamic load = 2.0 * static load...

That's true for a load applied once (the maximum possible response).....but if we are talking a (equivalent) sinusoidal force (i.e. applied over and over).....that's a different animal. Even at a 100 Hz, its a risk to not completely model the situation and input accurate loads to get an idea as to an accurate response.