Supporting compressor stator airfoils with diaphragms of inner and outer shrouds results in a ring-strut-ring assembly in which the multiple, nearly identical components (the airfoils) are connected by half-rings. When installed in an engine the half-rings behave as beams with asymmetrical boundary conditions. Such structures can exhibit variability in the vibration mode shapes of the various airfoils in the diaphragm, which is observed in finite element analysis, modal impact testing and holographic interferometry. For example, in one particular diaphragm assembly the second torsion modes are mostly consistent between airfoils, but the mode shapes of chord-wise and third torsion families can be combinations of these two modes. This mode-blending behavior is expected because the chord-wise and third torsion eigenfrequencies are relatively close together. The mode-blending results in significant changes in mode shape with small changes in frequency, which makes it necessary to use experimental techniques to characterize the vibration behavior. Since multiple system modes are excited simultaneously, and the airfoils in these system modes vibrate with different mode shapes, the relative response of strain gages can vary dramatically between vanes at the same driving frequency and on the same vane with small changes in the driving frequency. The diaphragm shroud also plays a significant role in determining the behavior of the system. This shroud participation implies that there can be significant cross talk between the airfoils and that the vanes near the ends of the half-rings will behave differently than those near the center.
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