Is "performing Impact Hammer test" an ART?

[onemilimeter]

I've been doing Impact Hammer test on a motor end cap (just the end cap alone, i.e. the end cap is not attached to the motor frame) for several days. However, I'm still not able to get consistent results, let alone to determine the natural frequency of the end cap. Is performing Impact Hammer test an ART which requires 'skill'?

I followed the procedure stated in "Section 11: Modal Testing Using a Hammer and Accelerometer" in the Supplemental Operator's Guide for the Agilent 35670A Dynamic Signal Analyzer (

http://www.home.agilent.com/upload/cmc_upload/All/EPSG090090.pdf).

I recorded the frequency response function (FRF) plot as well as the coherence plot (an indicator of measurement quality) of the hammer testing. I repeated the hammer testing many many times. I found out the coherence results were not consistent, and most of the coherence results showed that "my measurement" were not "good". How can I improve my measurement? How can I obtain consistent measurement? Please kindly share your experience on performing Impact Hammer test.

Thank you very much

 

[jeyaselvan]

You should carry out your design of experiments based on what you are looking for from the test.

Your remarks indicate that you are not able to make consistent results idicate that your system might not be linear. Is the time data and autospectrum of your excitation okay? any multiple hits? how about your time response? Look whether you have any sort of signal clippings, gain,etc.

This should depend on your structure, excitation force, contact stiffness, contact time, mass of the hammer,mass loading of the sensor, windows, etc & many more.

Atleast you should be able to see first few modes repeatedly to look anything further on the data.

Regards
Jeyaselvan

 

[onemilimeter]

Thanks jeyaselvan.

The end cap was placed on a sponge. An accelerometer is glued to the end cap by using wax. Hammer test was done at a point near to the accelerometer.

The time data of the excitation is OK. There is no multiple hits. The autospectrum of the excitation is OK too, e.g. the gain is rather flat over the frequency of interest. The level input signals are within the input range settings of the DSA. "Force Exponential" window is used and the setting is done according to the experimental procedure stated in Agilent application note.

I find out that the "shape" of Frequency Response Function (FRF) results for different hammer tests are quite similar. But the difference in coherence results are not consistent. I really wish to know how to perform Impact Hammer test to obtain consistent coherence test results.

Thank you very much

 

[ScottVanD]

onemillimeter,

What I would suggest is that you get your hands on this book:

http://www.amazon.com/Vibration-Analysis-Electronic-Equipment-Steinberg/dp/047137685X

beginning on page 290 (of my version) there is a very good section on hammer shock.

It includes many practical sample problems along with very clear analysis.

 

[onemilimeter]

Hi ScottVanD,

Thanks for your suggestion. I will get the book for reference.

Thank you very much

 

[jeyaselvan]

Hi

I do have another paper, which might be of interest to you title " My coherence is better in some measurements than others when impact testing. Am I doing something wrong?"

Good luck

Jeyaselvan

 

[onemilimeter]

Hi,

Jeyaselvan, the article is great. One of my problem might be the one explained in the article.

Thank you very much

 

[GregLocock]

I have't used a 35670a, but one thing that is very useful is to look at the coherence plot after each hit.

Obviously you'll have a coherence of 1 after the first hit, then it will be awful, and then after a few more hits it'll settle down. If your technique is good it'll slowly improve.

On some structures one good clean hit is more useful than several hits with varying forces, angles and locations (ie poor technique). So often I will take 3 independent FRFs at the same location, overlay them, and then actually use the one that looks like the best.

On the other hand if I just want to know if a panel is causing a particular peak, I'll deliberately get the average response by hitting all over the panel.

The article is good, but I'd often accept even the worst of his examples as being good enough!





Cheers

Greg Locock

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

 

[onemilimeter]

Hi GregLocok,

Thanks for your reply. I have some experimental results here:

http://www.eng-tips.com/viewthread.cfm?qid=253310&page=1

I will be glad if you could comment and provide some advices to improve my coherence plots.

Thank you very much

 

[GregLocock]

I'd say they were pretty good, but there is some evidence that you are hitting at an angle or in a slightly different place in each ensemble, and I think you need more resolution.

If you are desperate to improve the coherence round the antiresonances then you need a more sensitive accelerometer and to get rid of noise in that channel.





Cheers

Greg Locock

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

 

[GregLocock]

Oh, I don't think it will help but switch your force window on.

Cheers

Greg Locock

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

 

[GregLocock]

Also try suspending your test item from elastic cords rather than lying on the foam. Foam suspension is a good quick and dirty approach.



Cheers

Greg Locock

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

 

[onemilimeter]

Hi GregLocock,

Thanks for your replies and suggestions, which are practical knowledge and very useful.

 

[Tmoose]

I still struggle to see the value of impact testing a freely suspended component.

1 - When a V8 crankshaft is placed in the block with 5 main bearings, the lateral response changes immensely.
2 - Press a guitar string against a fret, and the note changes many octaves.

 

[GregLocock]

Main reason to do it is to validate an FEA model - at least you know what the boundary conditions are (n't). You are right, if you are troubleshooting an assembly you really need to test the assembly - in this case the gasket and bolts will damp many modes and suppress/modify others.

Oh, another practical point, take the hammer extension off and fit a softer tip, I think the hammer may be too heavy.





Cheers

Greg Locock

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

 

[jeyaselvan]

Just to reinstate, the modal frequencies and shapes depend on the boundary condition as well. Hence with the crankshaft on bearings, due to added stiffness, the system's dynamic behaviour will be totally different..( like the modal frequencies and mode shape of an academic cantilever and simmply supported beams are totally different).

As Greg says, it is primarily for validating FE model for a known boundary condition and then utilize the modal model for predicting whatsoever boundary conditions, loads, etc.

Jeyaselvan