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I'm trying to determine a level of vibration testing I should perform on my product. I understand electrical stress/life testing and I'm looking for a mechanical equivalent.

I have a DC motor running in my application that generates a certain level of vibration. I'm planning to do some environmental testing to my electronics and one of the tests is vibration, so I plan to subject the electronics to the level of vibration above what the motor produces, my thinking being that a higher level may be equivalent to electrical life testing. My product needs to last ten years so I'd like to know that the vibration I see now won't cause problems in several years time.

For instance, I can put an accelerometer on my electronics and measure ~2 g. To simulate over the product's lifetime should I be applying a higher level of vibration, say 3-5 g?

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    $\begingroup$ More information about the product and the usage will help us answer this question. It needs to last 10 years, but is that 10 years of continuous usage? 40hrs/wk? Also, more information about the product itself will help to identify which parts the vibration may cause to fail. What you're asking about is a fatigue life calculation, and there are a lot of inputs required to get an accurate answer. Simply scaling up the load can very easily give you an incorrect life answer, as the relationship between stress and life is not linear. $\endgroup$ – Trevor Archibald Oct 26 '15 at 16:24
  • $\begingroup$ @Captain Correct me if I'm wrong, but I think the core of your question is the relationship between failure time and vibration strength. I.E. you want to know how much the failure time will decrease if you increase the vibration amplitude by a certain factor. I'm no expert, but I think that there are too many vibration failure modes for there to be a simple relationship like you are seeking. $\endgroup$ – Chris Mueller Oct 27 '15 at 0:55
  • $\begingroup$ Thanks for the replies so far. I think Chris is close to what I'm after. For electronics I have the arrhenius equations so I can run my electronics hotter and with a higher load for a short period to simulate them running for a long time. To answers Trevors questions it's a DC motor (high voltage) running approx 4-6hrs a day, 7 days a week, and the applciation is 10-15yrs lifetime. $\endgroup$ – Captain Barnacles Oct 27 '15 at 8:59
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It's not just about selecting a level of vibration. There's a lot more that goes into developing a vibration test program that does what you're talking about. There's probably not any one vibration test that will address everything you're looking to do.

First, you need to understand the vibration environment that your product will see. Are you only interested in the operating environment? What about the vibration during manufacturing, transport and installation. If it's portable, you should consider that as well.

Second, what kind of thing are you designing? A mass produced near-commodity product, say something along the lines of a cheap clock radio? Or is it a unique high cost item, like a satellite that can't be damaged in testing? Or something in the middle of that spectrum? How much are you willing to invest in testing? How much are your customers willing to pay for the resulting reliability? At a certain point, it's better to simply replace/repair a product than it is to design the thing that will last forever.

When developing a vibration test program, intensity is only one component of the input, frequency is another. The rotating frequency of your motor will likely dominate, but there will be other factors as well and if you're serious about this, it will pay off to develop a random vibration profile based on your device's vibration environment. Does the operating vibration of your device excite any of the resonances of your subcomponents/systems? I've seen transformers and large caps pull away from pcb's because the vibration the system experienced was at the component's resonant frequency.

How many degrees of freedom are significant? In other words, is the vibration mainly up/down, right/left, fore/aft? What about pitch, roll yaw?

To deliver a product out of the gate with high reliability, you're talking about developing a HALT/HASS (Highly Accelerated Life Test/ Highly Accelerated Stress Screening) test program for your product. You'll have to break a few test samples, but if that kind of reliability is what you're after, HALT/HASS will get you closer than just picking a few specs from the internet and testing to those.

Basically, the plan goes something like this:

  1. Identify frequencies of interest (Operating frequencies, component resonances, environmental inputs etc.) You can do this experimentally (sine sweep, resonance search), analytically or in some combination.
  2. Collect vibration data from the device in operation for each axis you want to test and for each, compute a PSD (Power Spectral Density) which tells you how much energy is distributed over any bandwidth. That level represents one standard deviation away from the mean acceleration of zero and the integral of the PSD plot is the overall RMS acceleration.
  3. Start shaking the DUT (device under test) while it's operating at increasing levels of vibration until something breaks. Find and fix that failure and repeat until something else fails and so on. Each time something fails, use a time varying stress model, typically some sort of cumulative damage model, to estimate the B1 life (the time at which reliability = 99% at 100% stress) Typically, this data analysis would be done with software like ALTA.
  4. If the B1 life isn't enough at that point, fix the issue and retest.

Typically, multiple units will be tested at once until they all fail or the B1 life prediction becomes adequate. Fix the failures that showed up during testing and re-test.

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Since you are planning on testing you are already ahead of a lot of companies. Trevor makes some good points in the comments. Failure from vibration is certainly non-linear, but it typically errors in favor of failure. A good initial test would be to take it to the extreme; put 10g or 20g accelerations on it at various frequencies. If it survives a week of that, it is a very high certainty that a 2g vibration will have an infinite fatigue limit. If it doesnt survive, you can dial it back until it does, you now have a metric for improving the vibration isolation or testing higher quality components.

There are published standards for this, but official standards have to be purchase. Its a corrupt system in my opinion, but you can find older versions with some creative web searching.

Examples:
LCD tested per IEC 68-2-6
Transformer tested per IEC 68-2-6

A list of a companies testing abilities (full list):
Sinusoidal Vibration CEI / IEC 68-2-6, Edition 6, 1995
Combined Cold/Vibration (Sinusoidal) CEI / IEC 68-2-50, Edition 1, 1983
Combined Dry Heat/Vibration (Sinusoidal) CEI / IEC 68-2-51, Edition 1, 1983
Combined Temperature (Cold and Dry Heat)/Vibration (Sinusoidal) CEI / IEC 68-2-53, Edition 1, 1984

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  • $\begingroup$ Just testing to an arbitrary standard is pretty meaningless in terms of predicting reliability. $\endgroup$ – DLS3141 Oct 27 '15 at 17:10
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Q- The aim of this research study is mainly based on the comparison of measured frequency response function results for the threshold with and without damage, and comparing them with the theoretical model to update the modal parameters. The study will be performed on isotropic beam, to study the vibration characteristics. First Step, the beam will be tested without damage, and the frequency response function will be measured, then the natural frequencies and mode shapes, can be evaluated. To ensure these values, analytical model based on modal analysis approach will be utilized to predict the structural dynamic response of the beam and comparing it with the experimentally FRF. Second Step, cut in the beam, will be induced and getting again the FRF experimentally which will be used as input data to the analytical model to get the modal parameters. Last Step, the results from the previous steps will be treated to update the modal parameters (mass and stiffness matrix).

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  • $\begingroup$ estimation mass and stiffness matrices for modal testing $\endgroup$ – Mohamed aljadi Jan 12 '16 at 9:03
  • $\begingroup$ Is this an answer to the question or an edit to the question? How does you statement "The aim of this research study ..." relate to the question? Are you collaborating with the OP or providing a general answer on how to conduct vibration testing? $\endgroup$ – Fred Jan 12 '16 at 10:18

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