Maximum load from vibration data 1

[fgutenburg]

Hi,

I have vibration data from an automotive product which plots accleration in G's vs time. I am attempting to estimate a force value from this data to use in pull testing and linear FEA calculations (which will be done before a final vibration test). For example, there are impulse values as high as 10 G's, but only last a few miliseconds. The values are generally close to 3 G's. I know 10 G's of sustained acceleration would induce failure, based on basic calculations, yet no failure was seen. How do I determine what the highest force value I should use for my calculations, and eliminate the values that don't induce a sustained force?

Thanks,

Frank

 

[CESSNA1]

FGUTENBERG: The first thing I would do is find out some basic information about the data.

How was it obtained?
What is the highest frequency of interest?
What anti-aliasing filter was used?
Is there a possibility that noise is present in the data?
What is the sampling frequency (should be at least 10 times the highest frequency of interest,preferably more)?
Were the accelerometers calibrated in G peak or G rms?
What accelerometers were used and what is their resonant frequency?

Regards
Dave

 

[BobM3]

Was the accelerometer attached directly to the part? If not, anything springy between the accelerometer and the part could mean the part acceleration isn't the same as the accelerometer data. Are there any other ways that loads could be applied to the part other than acceleration? Were there items attached to the part when the data was taken?

 

[fgutenburg]

The accelerometer was mounted directly to the part. The sampling frequency was 1 millisecond. The accelerometer measured G rms. I don't currently have the rest of the information at hand. Regarding the highest frequency of interest, I guess that is part of my question. How do you determine at what point a value is of interest?

 

[GregLocock]

Your accelerometer measured g, it may have been calibrated in g RMS.

Anyway, do not be dismayed by the test data, it is perfectly normal to get very high g values in short term peaks that would destroy the part if they were applied at low frequency. For instance on an engine you'll see spikes to 30-100g, eg if the block weighs 200 kg then applying a 20 ton force to the block would hurt it. In reality it will withstand millions of cycles.

Turning vibration data into load data is a black art... could you consider applying a load spectrum to the model and adjusting it until you get the same acceleration response? This is actually remarkably difficult.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[IRstuff]

Case in point, our company's cigarette boat routinely sees 40 g to 50 g spikes, but they only last a couple of milliseconds. While most things do survive prolonged exposure to that environment, we do have a single component that routinely fails in the that environment.

TTFN

FAQ731-376

 

[Twoballcane]

Just a question...are you doing shock or sin wave vib? When I think of shock I think of it in the time domain (Gs vs time). The way I understand it is that you take your mill sec multiply it by two and then take the inverse and that will tell you what frequency the 50g will happen. In a shock event, if you have somting vibrating at that calcuated frequecy, that part will see the 50g at full response.

Fore example: If you get a shock event and the info come in G vs time say 50g at .050 sec. The frequency this would happen would be at 1/(2*.050)= 10Hz. Thru FEM you find out that the part you are conserned with has a first mode at 10Hz then the 50g is the load on that part.

Tobalcane
"If you avoid failure, you also avoid success."