MIL-PRF-23653D
4.4.6.3 Testing. The equipment and procedure for determining the vibration damping characteristics of the test
specimens are detailed in 4.4.6.3.2 and 4.4.6.3.3, respectively. The bare bar shall be conditioned at 75±2 °F
(24±1.1 °C) for a minimum of 2 hours and tested at this temperature. The covered bars shall be conditioned and
tested at each of the temperatures specified in tables II, III, IV, and V, as appropriate. Covered bars shall be
conditioned for at least 16 hours at the temperature specified prior to testing, and the specified temperature
maintained within ±2 °F (±1.1 °C) during conditioning and testing.
4.4.6.3.1 Accelerated aging. After completion of the initial damping measurements, the covered bars shall be
placed in a forced-draft oven for 168±1 hours at 212±2 °F (100±1.1 °C). Vibration damping measurements shall
then be repeated. The covered bars shall be conditioned and tested at each of the temperatures specified in tables II,
III, IV, and V, as appropriate. Covered bars shall be conditioned for at least 16 hours at the temperature specified
prior to testing, and the specified temperature maintained within ±2 °F (±1.1 °C) during conditioning and testing.
4.4.6.3.2 Equipment. A spectrum analyzer shall be used to take vibration damping measurements. The specific
arrangement of equipment used must meet all requirements of 4.4.6.3.3. A laboratory calibration shall be made of
all vibration measuring instrument components, including accelerometers, within 12 months prior to each use, or
after exposure to mechanical shock or other unusual disturbance, or upon request by a Government inspector. All
calibration instrumentation shall have been calibrated within six months prior to each use traceable to the National
Bureau of Standards. The laboratory calibration of the components shall be accurate within the instrument
manufacturer's specifications.
4.4.6.3.3 Procedure.
4.4.6.3.3.1 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 a number 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 percent critical damping by the relation:
100
100
R
%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 (decibels(dB)/second)
Q = "quality factor" at the resonant mode
4.4.6.3.3.2 Excitation. The test specimens (bare steel bar and steel bars with damping material bonded to one
side) shall be excited using a vibration exciter. The location of the excitation point for the bar shall be at the
geometric center of the 3- by 40-inch side of the bar not covered by the damping material. A sinusoidal signal
source shall be used to drive the vibration exciter. The amplitude of excitation shall be adequate to provide an
accelerometer signal at least 40 dB greater than the combined effects of ambient vibration and measurement system
noise at each frequency. A 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.
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