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|  MIL-v-81995(AS)
4.5.6.2.2
Frequency scan. The equipment shall be vibrated
with essentially simple harmonic motion in each of three mutually perpendicular
directions to explore for resonant frequencies of component parts over
the ranges or frequencies required in the specification sheet. Frequency
scans shall be made very slowly and carefully. The frequency range
shall be broken into small convenient intervals and each interval scanned
at a constant applied force amplitude which produces approximately the
fixture displacement or acceleration amplitude defined in the specification
sheet. The fixture motion shall be observed closely during the frequency
scans to detect the frequencies of minimum fixture motion which defines
the frequencies at which major components of the unit are in resonance.
These frequencies are quite sharply tuned and do not necessarily coincide
with the frequencies at which maximum component amplitudes or noises
occur when scanning at constant applied force amplitude. When the
resonant components are small, the reduction of fixture motion at resonance
may not be-discernible, in which case the resonant frequencies may be
determined from observations of maximum component amplitudes or noises,
phase shift between fixture motion, and motion of a resonant part. In
all cases the resonant frequencies shall be verified, if possible, by
checking for minimum fixture motion. The test report must contain
complete descriptions of all resonances, including the resonant frequencies,
parts in resonance, and modes of vibration.
4.5.6.2.3
Resonance endurance tests. After completion of the
frequency scan, resonance endurance tests shall be conducted at all
resonant frequencies of the components, with the applied force adjusted
as necessary to obtain the applicable fixture displacements or accelerations
specified in the specification sheet." The vibrator and power supply
must produce the prescribed fixture motions at all frequencies. Endurance
tests shall not be conducted at any frequency at which the fixture
amplitude abruptly increases when scanning at a constant applied force
amplitude. Endurance tests shall be interrupted periodically to rescan
for resonance. If a change in resonant frequency occurs during endurance
testing at resonance, the frequency of vibration shall be adjusted to
follow the resonance. However, if large or abrupt resonant frequency
shifts occur, the item shall be examined for component failures. Resonance
endurance tests shall be conducted for one million cycles or 8 hours,
whichever occurs first, at each resonance. If the same resonance is
excited by vibration applied in more than one direction, the endurance
test for that resonance shall be accomplished with vibration applied in
the direction which produces the greatest response. If no resonances
are found, the unit shall be vibrated at 100 hertz and the corresponding
amplitude as specified in the specification sheet for one million cycles
in each of three mutually perpendicular directions.
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