An improved vibration testing technique has been recently developed and applied to eliminate overtesting caused by the infinite mechanical impedance of the shaker in conventional vibration tests. With the new technique, the acceleration input is notched at the resonance frequencies of the test item in order to limit the shaker force to that predicted for night. Since few night vibratory force data are available, the force limits must be calculated from measurements or analyses of the night mounting structure and test-item mechanical impedances. An improved method of calculating these force limits is derived from evaluating the test-item apparent mass at the coupled-system resonance frequencies. Application of the method to a simple and to a complex coupled oscillator system yields nondimensional results which may be used to calculate limits for future force-limited vibration tests. For example, using the simple system results with Q = 50 and equal impedances of the night mounting structure and test item, the input acceleration amplitude will be notched by a factor of 31.25 relative to a conventional test. References: 17
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