MIL-DTL-19379C(SH)
4.4.2.1.1 Test apparatus for measuring dynamic stiffness. The test apparatus shall be capable of superimposing
cyclical vibration while statically loading the mount. Refer to the forced non-resonant section of ASTM D5992 for
discussion and schematic of test systems capable of conducting low-frequency dynamic stiffness measurements. A
commonly used type of equipment for non-resonant testing is a closed-loop servo-hydraulic machine or a vibration
shaker with load frame (see figure 1). Transducers shall be arranged to measure force at the blocked output or
foundation side of the mount and displacement at the equipment or input side of the mount. The stiffness of the test
system (machine, load-frame, force transducer, etc.) in series with the mount under test shall be determined
experimentally or via calculation. A test system which is at least 100 times stiffer than the dynamic stiffness of the
mount being tested shall not require measurement adjustment to account for test system flexibility. If the test system
fails to meet this relative stiffness requirement, the test system compliance shall be subtracted from the measurement
to obtain the true dynamic stiffness of the mount. The elastic stiffness of the mount (Kmount) can be calculated using
the classic equation governing the summation of springs in series:
(1/Kmeasured) = (1/Kmount) + (1/Ksystem)
To avoid over-correction, the stiffness of the test system shall be greater than 30 times the stiffness of the
mount. Refer to ASTM D5992 for guidelines and requirements of the mechanical apparatus and instrumentation
used in the test system.
Dynamic calibration of the complete system shall be verified using steel springs of known spring constants,
having negligible damping, prior to the start of testing. Refer to the "calibration for measurement of phase" section
of ASTM D5992 for measurement discussion and selection of springs. In addition, verification springs shall be
selected on the basis of their linear force-deflection properties and ability to produce a spring constant comparable to
the stiffness of the mount to be tested. System verification shall be performed using springs of known spring
constants at the excitation amplitudes and frequencies at which the mount is to be tested. System verification shall
be performed at loads which produce spring constant measurements that bracket the expected dynamic stiffness
range of the mount to be tested. This shall be accomplished by adding or removing springs within the stiffness
range of intended mount measurements. Care shall be taken to assure system verification measurements are
conducted within the linear load range of the springs. All spring dynamic stiffness measurements (corrected for test
machine stiffness) should be within 3 percent of the known spring constants. If this is not the case, the error shall be
investigated and corrected. This procedure is not intended to replace the periodic calibration of transducers, signal
conditioners, and other measuring instruments as required by ASTM D5992.
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