MIL-STD-810G covers Vibro-Acoustic/Temperature in Method 523.3. Method 523.3 is performed to determine the synergistic effects of vibration, acoustic noise, and temperature on externally carried aircraft stores such as bombs, missiles and sensor pods during captive carry flight. It is 16 pages plus one 9-page Annex.
This method would apply to rugged embedded computer systems as installed in externally mounted sensor pods.
For captively-carried stores, this method is intended primarily to test electronics and other electro-mechanical assemblies within the store for functionality in a vibro-acoustic/temperature environment.
This method is not intended to provide for:
- An environmental design qualification test of a store or any of its individual components for functionality. (For such testing see Method 500.5, Altitude; Method 501.5, High Temperature; Method 502.5, Low Temperature; Method 503.5, Temperature Shock; Method 507.5, Humidity; Method 513.6, Acceleration; Method 514.6, Vibration; Method 515.6, Acoustic Noise; Method 516.6, Shock; Method 517.1, Pyroshock; and Method 520.3, Temperature, Humidity, Vibration, Altitude).
- An environmental design qualification test of a store air frame or other structural components for structural integrity.
- Any test to satisfy the requirements of the Life Cycle Profile except that for the combined vibration, acoustic, and temperature environments related to reliability testing.
Possible effects of a combination of vibration, acoustic noise, and temperature include all effects that these factors can cause separately (see Methods 514.6, 515.6, and 520.3). In addition, increased stress as a result of moisture from thermal change may produce possible effects seen in Methods 501.5, 502.5, 503.5, and 507.5. The combined vibration, acoustic noise, and temperature environments may interact to produce effects that would not occur in any single environment or a lesser combination of environments.
Not all environmental stresses contribute equally to materiel deterioration or failure. Analysis of service-use failures caused by aircraft environmental stress on the store (paragraph 6.1, reference a) has identified the following four most important causes of failure:
- loading of the store through captive carriage,
- vibration, and
Operating any materiel item produces stress that can cause failure. In the case of external aircraft stores, operation generally means providing full electrical power, that produces thermal, electromagnetic, and electrochemical stress.
The most severe temperature shock to internal components may come from powering the materiel when it is cold. In order to induce all the stresses related to temperature in their proper proportion, use a thermal model of the store to predict the temperatures and rates of change at several internal points under service mission profiles. Temperature is constrained by the following:
- Ambient Temperature
The greatest variations in ambient temperature occur near the surface of the Earth. The low temperature extreme experience by a store is, in many cases, due to low ambient temperatures immediately preceding flight.
- Aerodynamic Heating
During captive flight, the high convective heat transfer rate will cause the surface temperature of an external store to be near that of the boundary layer.
- Power Dissipation
Although the high heat transfer rate will tend to keep the surface of a store at the boundary layer recovery temperature, internal temperatures may be considerably higher due to power dissipation of electronic equipment.
- Temperature Gradients
The strongest temperature gradients will usually be those associated with powering the materiel when it is cold. Temperature gradients will also occur due to changes in flight speed and altitude that change the surface temperature more rapidly than internal temperatures.
Vibration may cause mechanical fatigue failure of parts, abrasion due to relative motion, dislodging of loose particles that can cause electrical shorts, and degradation of electronic functions through microphonics and triboelectric noise.
Moisture, in conjunction with soluble contaminants, can result in corrosion. In conjunction with electrical power it may result in shorts. Freezing of water in confined spaces may produce mechanical stress. The test cycle should provide for diffusion of water vapor and condensation.
Shock can cause failure through mechanical stress similar to that induced by vibration. Shocks that are more nearly transient vibrations (many zero crossings), such as aircraft catapult and arrested landing shock, may be included in this test. Short duration shocks such as pyrotechnic shocks associated with store or sub-munition launch, flight surface deployment, etc., are generally too difficult to reproduce at the store level. Ensure that these events that are potentially destructive to electronics are accounted for in other analyses and tests.
Barometric pressure is generally not a stress for external stores. However, variation in pressure may enhance the penetration by moisture. Reduced pressure may increase the temperature due to reduced power dissipation and there may be increased electrical arcing.
Testing for Method 523.3 is very difficult and requires highly specialized test facilities. The test item is subjected, simultaneously, to high energy acoustic noise up to 165dB over the range of 150 to 2500 Hz, vibration by electrodynamic or electrohydraulic exciters and temperature in the range of -40 deg C to +85 deg C with a rate of change as high as 4 deg C. The test item is power cycled to induce electrical stress. The test item is instrumented for acceleration, acoustic pressure, and temperature plus other measurements as required by the particular store.