MIL-STD 810 Vibration Characterization
One of the key components of MIL-STD 810 vibration testing is vibration characterization. To assess vibration compliance, an understanding of the characterization is necessary. This is because the majority of vibration experienced by equipment is broadband in spectral content. That is, the vibration is present at all frequencies over a relatively wide frequency range at varying intensities.
Vibration amplitudes may sometimes vary randomly, or as a combination of mixed random and periodic. Keystone Compliance is a vibration test lab with significant mil-std-810 vibration test experience. Our vibration laboratory and test engineers understand the nuances of the vibration profiles and vibe testing.
The following information is extremely technical in nature. It was derived from version H of Mil-810 vibration section. Even though the language is from mil-810h vibration, it applies to previous versions of the standard. This includes MIL-810G vibration.
810H Random vibration
Random vibration is expressed as auto spectral density (a.k.a. power spectral density, or PSD). To find the auto spectral density (ASD), the engineer must take the square of the root mean square (rms) value of the acceleration. The next step is to divide the rms by the bandwidth of the measurement.
To find the accuracy of spectral values multiple the measurement bandwidth and the time over which the spectral value is computed. The normalized random error for a spectral estimate given by 1/√BT. B is the analysis bandwidth in Hz, and T is the average time in seconds. In general, use the smallest practical bandwidth or minimum frequency resolution bandwidth.
Most commercially available vibration control systems assume that the acceleration amplitude has a normal distribution. Other amplitude distributions may be appropriate in specific cases. Ensure that test and analysis hardware and software are appropriate when non-Gaussian distributions are encountered.
This procedure is directly from MIL-STD 810H, but is accurate for previous versions including MIL-STD 810G random vibration. Random vibration is one of the most common procedures used in shaker testing, especially to 514.8 vibration.
Sinusoidal (Sine) vibration
Sine vibration is expressed as acceleration and frequency. An environment dominated by sine vibration is characterized by a fundamental frequency and harmonics (multiples) of that fundamental. Often there will be more than one fundamental frequency. Each fundamental will generate harmonics.
The service vibration environment sometimes contains excitation that is basically sinusoidal in nature, and has a very low broadband background. Examples of such environments include low performance propeller aircrafts and helicopters. The excitation derives from engine rotational speeds, propeller and turbine blade passage frequencies, rotor blade passage, and their harmonics.
Environments such as this may be best simulated by a sine test. Ensure the frequency range of the sinusoidal exposure is representative of the platform environment. Often, the broadband random may be of sufficient amplitude. Thus, simply omitting the broadband energy and conducting a pure sine test is either questionable or not acceptable.
Mixed broadband and narrowband vibration
Sometimes, the vibration environment is characterized by somewhat periodic excitation. This comes from rotating structures and mechanisms like rotor blades, propellers and pistons. When this form of excitation predominates, source dwell vibration is appropriate.
Source dwell is characterized by broadband random vibration, with higher level narrowband random, or sine vibration superimposed. Data reduction techniques affect the apparent amplitudes of these different types of signals. Exercise care when determining levels of random and sinusoidal vibration from measured data.
Narrowband random over broadband random
Ensure that the amplitudes and frequencies of the total spectrum envelope the environment. The narrowband bandwidth(s) should encompass or be cycled through frequencies that represent variations of the environment. These frequencies should represent variations of the resonant frequency of material(s).
Sinusoid(s) over broadband random background
Ensure the random spectrum is continuous over the frequency range. It should envelop all of the environment, except for the amplitude(s) to be represented by the sinusoid(s). The sinusoid(s) amplitude(s) should envelope the sinusoid(s) in the environment. Cycle the sinusoid(s) frequency(s) through bands representative of frequency variation in the environment and resonant frequency variation in material.
Transient vibration varies in time and is a “windowed” portion of a random vibration. It has a relatively short duration (0.5-7.5 seconds). Currently, such a measured environment is replicated in the laboratory on a vibration exciter under waveform control.
Verification of the laboratory test is provided by three things. There must be a display of the laboratory measured amplitude time history. There must be an optimally smooth estimate of the amplitude time history time-varying root-mean-square. There must be either an energy spectral density estimate, or a Shock Response Spectrum (SRS) estimate.
These estimates are for short environments. Such environments have transient vibration durations that are less than the period of the first natural mode of the test item. Or for a time-varying auto spectral density estimate of longer duration environments (2.5-7.5 seconds).
The environment of the material is replicated in the vibration lab under waveform control. If the impulse response function of the system is determined and applied correctly, the replication should closely resemble the measured environment. The transient vibration environment is an important environment for stores resident in platform weapon bays.
This procedure is directly from MIL-STD 810H, but is accurate for previous versions including MIL-STD 810G vibration.
Random versus sinusoidal vibration equivalence
In the past, most vibration was characterized in terms of sinusoids. Currently, most vibration is correctly understood to be random in nature and is characterized as such. This results in a demand to determine equivalence between random and sine vibration. This demand is generated by the need to use material that was developed to sine requirements.
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There have been several versions of vibration testing procedures in MIL-STD-810 vibration characterization testing. Below is a list of each version and the appropriate method number: