Downloaded from https://www.everyspec.com MIL-STD-81OD. 19 July 1983” c. Test se tUQ. Testing shall be accomplished in three mutually perpen- dicular axes with the mounting lugs in the up position. The test item should be attached to the fixture by its normal mounting means (e.g., suspension lugs for 2.75 inch FFAR launchers). The vibration fixture shall utilize, if feasible, actual aircraft components for accomplishing this test attachment. Low level O .05g, 5 to 500 Hz sweeps to identify the store’s resonant fre- quencies end transmissibilitiesshall be conducted before and after exposure to the test vibration environment.
Significant changes in resonant properties shall be considered as failures even though the item gtill functions to specification levels: 1-3.2.10 kteuorv 8 Ground The ground mobile environment consists largely of broadband random vibration resulting from the interaction of vehicle suspension and structures with road and surface discontinuities. The nature of the terrain, vehicle gpeed, vehicle dynamic characteristics, and suspension loading all affect vibration responses. . -. There is presently no analytical model of this environment suitable for generalized test application.
In general, the vibration spectrum of wheeled vehicles and trailers is predominantly random, with pesks and notches, considerably higher and lower than the mean level, at various discrete frequency bands. This environment can be simulated by a wide-band random vibration test similar to the minimum integrity spectrum for aircraft as given in I-3.2.12. The use cf a smooth spectrum similar to figure 51!i.3-36, generally will produce an’overtest at .iomeparts of the frequency spectrum. The spectrums shown in I-3:2.1 and figure 514.3-2 thru 514.3-10 are composites of the cargo beds of typical wheeled vehicles and trailers and again could produce unrealistic test condition for installed equipment.
When these curves are used, consideration must be given to the structurets response at the location where the equipment is installed as it relates”to the tiajorstructural members supporting the cargo bed. The track-laying vehicle environment i’scharacterized by the strong influence of the track-laying pattern. The ,movement.of the’vehicle, its suspension’ system, and road d,isccntinu:itieeproduce a brcadband random excitation which is further extended or excited at frequencies associated with the track ‘pattern. This environment is best simulated by superimposing narrowband raridomover a broadband”random base. Test leveJa.
As discussed above, generalized test levels have not been developed which would be applicable to a specific case. The information, levels and curve presented in 1-3.2.1 and I-3.2.12 must be adapted for a specific test.item. Whenever possible and justified by the program requirements, the actual vibration environments should be measured. before testing the equipment and the results used to formulate a more accurate spectrum shape and,level. .> b. ~. The test duration must be related to the test item’s service scenario. Appropriate test durations are given in I-3.2.1 and ‘I-3.2.2; however, their application for this category must be related again ,tothe specific equipment1s service scenario.
In development of the test duration from measured data, the procedure outlined in appendix A should be reviewed. 19 July 1983 514.3-53 METHOD 514.3