404 P.M. Daborn et al. Fig. 37.12 Impedance Matched Multi-Axis Test for dummy missile. (a) IMMAT—view from above test setup. (b) IMMAT—view from tail of the missile. (c) IMMAT—view from below test setup frequency bandwidth. Of more concern was the observed undertest at some uncontrolled locations. In addition, the evidence of this may not be discovered during typical qualification programs as often only the control locations are considered. Furthermore, there was evidence of cross-axis overtesting which means that any subsequent testing in orthogonal axes would subject the structure to unrealistically high stresses at the relevant frequencies. Other limitations of the twin-shaker test include the need to carry out three orthogonal tests sequentially to adequately excite all directions. In addition the fixture arrangement is likely to significantly alter the dynamics of the structure in terms of natural frequencies, modeshapes and damping. The twin-shaker test is a significant improvement compared to traditional large shaker, single-axis tests which are still commonplace today for many qualification programs on a variety of structures. The limitations described above could be considerably exaggerated for vibration tests that involve these large shaker systems. The new Impedance Matched Multi-Axis Testing (IMMAT) technique potentially offers considerable improvements. IMMAT includes enhancements such as matching the local impedance of the parent structure and controlling the response of the structure at many locations using Multi-Input-Multi-Output control and has been successfully demonstrated in this paper. A noteworthy improvement of IMMAT over current methods includes the excitation of all axes simultaneously with a considerable reduction in test time and the elimination of problems such as cross-axis excitation. Another major benefit of the IMMAT approach is the simulation of the original aerodynamic environment, including the power spectral densities, the cross spectral densities and the relative phase between locations. This ensures that the operating deflection shape of the structure in the IMMAT is very similar to the aerodynamic environment, leading to similar stress patterns. Finally, test houses could make substantial long-term cost savings by replacing some of their large shaker systems with a few smaller shaker systems for random vibration tests. A.1 Appendix 1: Twin-Shaker Vibration Test Results See Figs. 37.13 and 37.14. A.2 Appendix 2: IMMAT Results See Figs. 37.15, 37.16, and 37.17.
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