MIL-PRF-23653D
4.4.6.3.3.3 Accelerometer. An accelerometer weighing no more than 1 ounce and having a mounted resonance
of 20 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 (see 6.8). Accelerometers shall be mounted in accordance with the
mounting guidance provided in ANSI S2.61, except that no oil or grease shall be used. It is permissible to remove
sufficient damping tile as required to permit attachment.
4.4.6.3.3.4 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 percent 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 and
measuring the slope of the decaying free vibration. Signal conditioning is desirable in this case and may be
accomplished by translation ("zoom") analysis if the limiting decay criteria is met.
4.4.6.3.3.5 Testing. Each test bar shall be suspended from two lightweight nylon or cotton cords at least 24
inches long. The bar may be suspended edgewise or flat, as appropriate for the type of vibration exciter being used.
The suspension cords are to be attached to the bar at the appropriate nodal points of the first flexural mode of
vibration (approximately 9 inches from each end of the bar). Alternatively, each test bar may be suspended
vertically from one end of the bar using a single cord. For vertically suspended bars, a hole approximately inch in
diameter and located nominally 1 inch from the end of the bar may be drilled through the bar and the damping
material. The free bar shall be excited at each of the odd-numbered lengthwise flexural modes of the bar as required
in tables II, III, IV, and V, as appropriate. Care shall be exercised to identify the modes excited and to avoid
torsional, longitudinal, and combined modes. Care shall also be taken to prevent overloading of any part of the
exciting and sensing circuits to eliminate spurious results. It is very important that the alignment of the vibration
exciter be such that the direction of excitation is normal to the face of the test bar to which the vibration exciter is
attached. Of primary concern is the need to eliminate the effect of the exciter on the test specimen during the time
of the decaying free vibration. After connecting the instrumentation, but prior to measuring the damping properties,
the resonant frequency response of the test specimen shall be measured. In addition to determining the modal
frequencies, this process is used in determining misalignment and overload problems in the exciter and measurement
systems.
4.4.6.3.3.6 Measurement system damping. The percent critical damping at each mode of the bare bar shall be
measured at 75 °F (24 °C). This test shall be conducted to demonstrate that the data acquisition system and physical
test set-up do not impose extraneous damping on the measurement. The percent critical damping of the bare bar
shall not exceed 0.5 percent at any mode.
4.4.6.4 Calculation. The percent critical damping at each mode found for the three coated bars at each of the
test temperatures shall be averaged and compared with the requirements in tables II, III, IV, and V.
4.4.7 Hardness. Hardness of 10 sample tiles shall be measured at 73.5±2.0 °F (23.0±1.1 °C) on whole tiles
which have been conditioned at this temperature for not less than 16 hours. This test shall be performed on Class 1
and Class 2 tiles with a durometer conforming to ASTM D2240, Type A. The hardness of Class 2.5 and Class 3
tiles shall be measured with a durometer conforming to ASTM D2240, Type D. The durometer shall be read 15
seconds after the presser foot makes firm contact with the tile. Hardness measurements shall be made at five points
in accordance with ASTM D2240 except that each test point shall be spaced not less than 2½ inches from any other
test point. The average of the five determinations shall be reported.
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