MIL-DTL-24487B (SH)
4.5.8.2 Testing. The equipment and procedure for determining the vibration damping characteristics of
the test specimens are given in 4.5.8.2.2 and 4.5.8.2.3 respectively. Both bare bars and covered bars shall be
conditioned at 50 ± 2 °F (10 ± 1.1 °C) for a minimum of 8 hours and tested at this temperature.
4.5.8.2.1 Accelerated aging. After completion of the initial damping measurements, the test bars shall be
placed in a forced-draft oven for 168 ± 1 hour at 212 °F (100 ± 1.1 °C). Vibration damping measurements shall
then be repeated as specified (see 4.5.8.2).
4.5.8.2.2 Equipment. A spectrum analyzer shall be used to take vibration damping measurements. The
specific arrangement of equipment used must meet all requirements of 4.5.8.2.3. All equipment, including the
spectrum analyzer and accelerometers, shall be in current calibration.
4.5.8.2.3 Procedure.
4.5.8.2.3.1 Method. Vibration damping tests shall be conducted by the method of the decay rate of free
vibrations. This method requires that the test specimen be excited at several of its free-free flexural resonant
frequencies in turn. Resonant frequencies shall be determined by applying a random noise source to the bar. A
force at the resonant frequencies is then applied and the vibration of the specimen allowed to stabilize. The force is
then removed and the vibration measured while decaying freely. The rate of decay of the test specimen's vibration
is measured and related to the percent of critical damping by the relation:
R
100
100
%D =
=
= 1.83
(27.3 f )
2Q
f
2
R
where:
%D = percent of critical damping
f = frequency of the vibrational mode excited (Hz)
R = decay rate of vibration (dB/second)
Q = "quality factor" at the resonant mode.
4.5.8.2.3.2 Excitation. The test specimens (steel bars and steel bars with damping treatment bonded to one
surface) shall be excited using a vibration exciter. The location of the bar excitation point shall be at the geometric
center of the 3-inch by 26-inch (7.6 by 66 cm) or 3-inch by 29 inch side (7.6 by 73.7 cm), as appropriate, that is not
covered with damping treatment. A sinusoidal signal source shall be used to drive the vibration exciter. The
excitation shall be adequate to provide an accelerometer signal at least 40dB greater than the combined ambient
vibration and measurement system noise at each frequency. An spectrum analyzer or oscilloscope shall be used to
monitor the source excitation and measurement system response waveforms to verify that signal levels remain
within the linear limits of the equipment used.
4.5.8.2.3.3 Accelerometer. An accelerometer weighing not more than 1 ounce (28.3 grams) and having a
mounted resonant frequency of 20 kilohertz (kHz) or greater shall be used to sense the vibration decay. The
accelerometer shall have a minimum sensitivity of 9 mV/g. The accelerometer shall be attached to the bar opposite
to the excitation point with a stud, epoxy adhesive, hard acrylic adhesive or dental cement. It is permissible to
remove sufficient damping treatment as required to permit attachment.
4.5.8.2.3.4 Instrumentation. The instrumentation system is used to determine the frequencies of the
resonant modes and to determine the decay rate at each required modal frequency. The instrumentation used must
be capable of measuring a decay rate corresponding to at least 15% of critical damping. This value shall be verified
by electrical impulse applied to the input of the measuring system. The spectrum analyzer shall be configured to
obtain a time record. The decay rate shall be determined by capturing the log magnitude of the accelerometer signal
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