Here are some questions from students that I recently received for shaker table testing of equipment.
- How are vibration standards developed and how do they relate to the operational environment?
Standards may have different purposes. Some of the common ones are:
- Envelope the actual field or service vibration environment
- Screen for manufacturing and workmanship defects
- Combined envelope & screen
- Identify natural frequencies and damping
The envelope levels are ideally based on measured accelerometer data from actual service environments. But the background data might not be readily available. The levels become a sort of tribal folklore.
Some standards specify generic envelope levels, but also have caveats that the users should make their own accelerometer measurements for their specific applications and develop levels accordingly.
NAVMAT P9492 is a seminal reference with a generic workmanship vibration level. But there is an emphasis on users developing their own levels through trial-and-error.
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Natural Frequency Identification
The natural frequency identification tests are flexible and can be performed either with random or sine sweep base input. Also the input levels can be varied for a linearity study.
Another option is modal testing with an applied force input.
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- What are there two options of dwell and random and when is it suitable or not to use either one?
Both sine dwell and random can be used for either enveloping an operational environment or for a workmanship screen. Ultimately any sort of testing comes down to experience and engineering judgment. Contractual requirements may also be a driver.
A sine dwell would be especially appropriate if the service environment was sinusoidal.
Sine dwell may also refer to exciting a test item at its natural frequency with a sinusoidal input, for a resonant response.
Likewise, a random test would be advisable if the service environment was random.
Some environments may also be sine-on-random, such as helicopter vibration.
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- Can we obtain transient time domain data from a PSD? Is there a method for doing this?
We can synthesis a time history to satisfy a PSD. The time history will typically be stationary and Gaussian, but other distributions are possible. This is essentially what a vibration control computer does for a closed-loop shaker test. We can also do this for analysis purposes.
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- When creating a PSD profile for a shaker table test, are there specific methods that can be followed for reducing the number of breakpoints and keeping the Grms in the test the same as in the measured data?
A PSD test specification usually consists of a few straight line segments in log-log format. The number of breakpoints can be reduced, but this will tend to increase the overall GRMS level. The GRMS increase is usually considered acceptable as part of the added uncertainty margin.
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- For torsional acceleration should we use a torsional accelerometer (expensive) or can we obtain the torsional acceleration from the translational accelerometers?
You should be able to calculate the torsional acceleration from two closely-spaced translational accelerometers, by subtracting one signal from the other. You would need to assume that the difference was due to pure rotation. This requires engineering judgment.
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- How do we determine the dependencies between the different PSDs-PSDx and PSDy?
You really need to measure the accelerometer data in all three axes. There is also a cross-PSD function that shows the correlation as a function of frequency between two PSDs. The cross-PSD functions are usually only a concern for simultaneous multi-axis testing.
In aerospace avionics testing, a common level which envelops all three axes is typically derived, for the environment enveloping. Then some statistical uncertainty margin is added. Then the component is tested to the same level in each of three orthogonal axes.
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- PSDs at a different axis: Say we obtain the PSD in a given axis. Is there a way we can obtain the PSD is some other axis? Are there transformation equations?
No methods or equations are available. You need to take measurements in all three axes.
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- We are hoping to gain a better understanding of testing methodologies (comparing sine sweeps and random vibration methods).
There are peak and fatigue damage spectra comparison methods for this very purpose. These functions relate the peak response or fatigue damage as a function of natural frequency.
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- How do the Normal distribution and Rayleigh distributions of random vibration differ?
The PSD test specifications have an implicit assumption that the base input random instantaneous values will have a normal distribution. Now consider a lightly-damped single-degree-of-freedom oscillator subjected to the base input. Reduce the response time history to a subset of its local positive and negative peaks. Use absolute values of each. Plot those as a histogram. The resulting response histogram will have a Rayleigh distribution. Rayleigh distributions are used for fatigue calculations and for the estimation of the absolute maximum peak response.
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- How can we compare different PSDs? For example we have a Standard PSD profile and can we compare it to our PSD from measured data? How can we determine if excursions from our PSD over the Standard PSD are significant?
Assume that the test item behaves as an SDOF system. The vibration response spectrum (VRS) may then be used, which gives the response as a function of natural frequency and the amplification factor Q.
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- What is the relationship between an accelerated life testing input level and the time duration such that input level and the time can be adjusted to get the equivalent accelerated life testing.
There are some approaches, each of which requires assumptions. The key assumption is the inverse slope of the S-N fatigue curve. Also, linearity must be assumed.
See Time-Level Equivalence in:
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– Tom Irvine