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Unformatted text preview: interface stiffness should be a factor of ten greater than the local stiffness of the system under test. While either of these practical guidelines can be achieved for small test objects, a large class of flight vehicle systems can not be acceptably tested in either configuration. Arguments have been made that the impedance of a support system can be defined (via test and /or analysis) and the effects of such a support system can be eliminated from the measured data. This technique is theoretically sound but, due to significant dynamics in the support system and limited measurement dynamics, this approach has not been uniformly applicable. In response to this problem, many alternate structural testing concepts have been proposed and are under current evaluation. Active, or combinations of active and passive, suspension systems are being evaluated, particularly for application to very flexible space structures. Active inert gas suspension systems have been used in the past for the testing of smaller commercial and military aircraft and, in general, such approaches are formulated to better match the requirements of a free-free boundary condition. Another alternate test procedure is to define a series of relatively conventional tests with various boundary conditions. The various boundary conditions are chosen in such a way that each perturbed boundary condition can be accurately modeled (for example, the addition of a large mass at interface boundaries). Therefore, as the experimental model is acquired for each configuration and used to validate and correct the associated analytical model, the underlying model will be validated and corrected accordingly. This procedure has the added benefit of adding the influence of modes of vibration that would normally occur above the maximum frequency of the test into the validation of the model. For example, the inertial effect of the (5-75) +UC-SDRL-RJA CN-20-263-663/664 Revision: June 12, 2001 + addition of a mass at an interface will cause a downward shift in frequency of any mode that is active at the interface (modes that are not affected by the interface dynamics will not be shifted). Since this shift is measured and the analytical model can accurately define the dynamics of the added mass, any inaccuracy in the analytical prediction of the frequency shifts as well as the corresponding effects on the modal vectors will be due to the lack of fidelity of the underlying analytical model. Recently, other researchers have proposed multiple configurations of test conditions as a methodology of utilizing practical test configurations in the testing of flight vehicle systems. In a related research area, work is progressing on using constrained testing together with direct parameter estimation methods to define the characteristics of the unconstrained structure. In this test procedure, the excitation forces and the constraint forces are measured together with appropriate response information. The direct pa...
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This note was uploaded on 09/29/2013 for the course MECHANICAL ME taught by Professor Regalla during the Fall '11 term at Birla Institute of Technology & Science, Pilani - Hyderabad.

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