Hi – we look forward to questions about vibration and shock testing – everything is fair game:

Test equipment

Test specs

Test parameters (selection OR measurement)

Fixture design

Data analysis

Or just let us know that you have an interest.

Steve Brenner, Consultant and V&S Blogmeister

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5 Responses to Welcome to the Vibration & Shock Testing Blog

  • Shailender Reddy K. says:

    Hi All,

    Good Morning!

    The vibration section in HALT test plans generally instructs the operator to set the vibration level to certain Grms (20 or 35 etc…), but do not say anything about the frequency of vibration to be set in HALT test plan. why is it so and what is the appropriate frequency of vibration that should be used in HALT testing.

    Please let me know how frequency effects vibration.

    Thank you,
    Regards,
    Shailender

  • Hi Samir,

    “G’s” are usually G’s zero to peak (Go-p), G’s peak to peak (Gp-p), Grms, or g^2/Hz.

    If there is no annotation to the “G” it is usually Go-p.

    Your local accelerometer sales person will be useful in helping select an accelerometer with the correct range BUT be aware of the following:

    An accelerometer used for HALT must be accurate over the entire temperature range AND be accurate during RAPID TRANSITIONS. Not many accelerometers meet that requirement. And don’t forget, the accelerometer CABLES also need to be properly rated for those conditions also.

  • Samir Choksi says:

    Hi,

    I was referring to some of papers available on internet for HALT test guidelines. For Vibration tests, the paper says that chamber requirements should be -> Vibration output without load from 1 Grms to a minimum of 35 Grms.

    But for test equipment, accelerometer range is +/- 500g.

    I am confused what is the difference between g and Grms. How to select proper accelerometer?

    Please help.

    Thanks for your help in advance.

    Regards,
    Samir

  • Steve Brenner says:

    Hi – Thanks for your questions – They are very far reaching, so I’ll supply some brief answers here and you can reach me at Steveb@opsalacarte.com if you want more detailed information.

    Since you numbered your questions, I’ll use your numbers.

    1 – g^2/Hz, dB/Oct, and Grms are all ways to describe random vibration AFTER they have been converted into frequency domain – that is after a PSD (Power Spectral Density) graph has been created from the original time domain (amplitude vs time) data was recorded. The original time domain data looks pretty much like noise (which it is) and doesn’t give us much information. If we look at a PSD, the x-axis units are in Hz, and the y-axis units are in g^2/Hz. If we describe a PSD graph by it’s shape, we often define the shape by the slope of the lines – in dB/Oct. RMS is the area under the entire curve, which corresponds to the overall energy. Since all of this is an analysis, in the frequency domain, the only relationship to the physical world we can readily see is the Grms values in that the more Grms, the more vibration (and energy) is being input to the system.

    2 – Actually we CAN use Random Vibration to determine Resonant Frequencies but we generally prefer a nice slow Sine sweep. There are two reasons. First, if you have two resonances close together, Random Vibration input may cause them to damp each other out and you may not see them or see them as a single resonance. Secondly, a nice slow sine sweep (1 Oct/min or slower) will allow each resonance to build up to its maximum Q (transmissibility) so you can find out things like the Q itself, damping and half power points.

    3 – Well, the obvious thing is to look for peaks, but remember, by definition, the resonant frequency is not necessarily the peak, but the frequency at which the output (response) is 90 degrees out of phase with the input. Comparing the input vs, response on an oscilloscope (which every good vibration lab has) is an excellent way to determine resonant frequencies.

    4 – Most vibration damage occurs due to resonances. One option is to isolate the potentially damaged part from the input vibration by the use of isolators. Another is to increase damping by changing materials – but remember, increasing damping reduces the Q but does NOT change the resonant frequency. To do that see 5 below.

    5 – If you want to shift the resonant frequency you can change the mass, or change the stiffness. Stiffness changes, such as adding a “stiffener” to a PWB will raise the resonant frequency BUT the added mass of the stiffener will lower the resonant frequency – so there is always a tradeoff. Sometimes the answer is simply to drill some holes in the structure, reducing stiffness and thusly shifting the resonant frequency down. Another caveat is that if you are testing in one axis at a time, fixes which help you in one axis may give you problems in another.

    I hope this helps – and if you would like more detail, please contact me directly.

    Steve Brenner

  • Vincent says:

    Hi all,

    Good morning,

    I have the following questions on Mechanical vibration and shock testing,

    1. What are g^2/Hz, db/Oct and Grms and how are they related ? In physical sense, what do they really means ?

    2. why only Sine Sweep vibration can be used to determine Reasonance Frequency of UUT, while Random Vibration cannot ?

    3. What are the things to look out for during Sine Sweep vibration, in order to determine Reasonance Frequency of UUT ?

    4. In general, what are the possible options when a UUT get damaged when vibrate at Reasonance Frequency ?

    5. How can one change the UUT mechanical structure to shift the Reasonance Frequency out of the Vibration Test spec ?

    Hope to hear from you soon.
    Thank you very much

    Vincent

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