Downloaded from https://www.everyspec.com MIL-STD-81OD 19 July 1983 a c. lest s et~. The secured cargo transportation test phase shall ,be accomplished as indicated in paragraph 1-3.2.1. The additional vibratiori’ ~ exposure shall be accomplished with the test item secured to the fixture”/ exciter. Items which are mounted on vibration isolation equipment should be tested with the isolation removed. The items shall be tested in each ,ofthree orthogonal axes. I-4 SPECIAL CONSIDERATIONS . . .. I-4.1 Combined temDerature. vibration t~.
To expose materiel to realistic servioe stresses, a combined temperature-vibration test,may be necessary.”.The high and low temperatures that materiel is expeoted to endure while being transported are usually specified in the requirements dooumente ‘interme of the climatic categories described in MIL-STD-21O or AR 70-38. These same criteria are presented for use in methods 501.2 (High,Temperature) and 502.2 . . (LOW Temperature). The temperature values selected for those testa can be used for combined temperature-vibrationaltesting. ,. I-4.2 Gommon test techn- 1-4.2.1 ~.
The “majorityof vibration experienced by””equipment in operational service has been determined by analysis to be broadband in speotral content. That is, all frequencies are present at all times in varioua combinations of intensity. Controlled experiments have demonstrated that random vibration effective y simulates”broadband in a test situation. Therefore most of the tests in this method use random vibration. 0 Random vibration spectra are defined in terme of acceleration spectral density (also referred to as power spectral density, or PSD) profiles, which relate energy density levels to epecific frequency bands. The vibration is defined. over a relevant frequency range.
The use of g-rms values alone to describe vibration tests is not valid, since a a g-rms value does not characterize a specific vibration profile. An infinite combination of frequency bandwidths and spectral shapes can satisfy one g-rms. value. Therefore, vibration measurements and test spectra should alwaye relate energy content to specific frequency bands. Aa traditionally applied in the test laboratory, random vibration is charac- terized by a gaussian energy distribution,.but other forms of broadband excitation (mechanical or electro-hydraulic) may be employed if the desired spectral control can be achieved. I-4.2.2 Source dwel la.
In some cases, the vibration environment is charac- terized by periodic excitation from reciprocatin~ or rotating structure and mechanisms (e.g., rotor blades, propellers, pistons, gunfire). This excitation may be transmitted through fluids (air or liquid) or structures. When this form of excitation predominates in a critical frequency band, source dwell vibration is appropriate. A source dwell excitation is characterized by broadband random, narrowband random, or one or more sine waves. I METHOD 514.3 19 July 1983 514.3-58