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Experimental Methods

Fluctuating blade stress measurements have been studied using spectral analysis (64). Laser speckle photographs have been used to measure rotating blade stresses 165). Kiraly (66) applied the digital system to almost 400.000 data points to investigate dynamie blade displacements. Advantages of the non-interferometry technique over the strain gage measurement method have been predicted (67). Test results of dynamie stresses are available [68-71].

Glober and Mały (72) examined both the stress distribution and the frequency distribution with holographic interferometry, strain gauges, and piezo-electric 9auges. Strain gauges and the telemetry technique have been used to measure dynamie stresses and natural frequencies (73). Wachter and 8eckmann [74) presented test results for the effects of exciting forces due to flow distortion and damping forces due to lacing wires on the blade stresses. Fu [75) explained the merits of applying holographic interference technique in combination with FEM. One advantage was the ability to estimate the correct-ness of results even at higher modęs by using an infinite number of measuring points.

Finbow and Watson (76J have summarized different blade vibration measurement techniques. Shalabanov [77) revealed the limitations of the holographic interferometry method as opposed to a new tech-nique for obtaining blade frequencies and nodes. lnvestigations of blade damage have been carried out with optical and scanning electron micro9Copes [781. Egorov [79] has provided a spectral analysis of blade ring vibrations. Various kinds of measurement tech-niques have been reviewed and the merits and faults investigated [80, 81). Blade vibrations due to rotating stall have been studied (82), and reduction in vibration amplitudę due to lacing wire has been studied experimentally (83). Leon [84] predicted the exact location of excessive blade vibration from the Doppler technique.

Shroud Stresses

The effects of shroud on blade vibrations has been studied experimentally and the results compared with FEM [85J. Shroud flange contact stresses have been analyzed, and severe conditions at which maxi-mum stress occurs have been pointed out (86). Stresses around shroud holes have also been analyzed [87). It is of interest that a decrease in inclination of the shroud hole axis altered the maximum stress points and inereased the maximum stress value. The strain freezing method has been used to study exper i-mentally the behavior of stresses around the shroud hole; the results were compared with analytical results (88). The vibration behavior of an assembly of shrouded blades has been investigated by studying the vibrational characteristics of the components (89). Butt welding of long arc shrouds has been found to reduce dynamie stresses (90).

Effects of Damping

The effects of dry friction damping on blade vibra-tion have been investigated in terms of anisotropic dry friction theory (911. The system was treated as a lumped mass system, and a Computer package was developed to study the blade to disc damping (92). An experimental method for obtaining the friction coefficient was also outlined. An optimized system for the combined effects of dry friction damping and hysteresis damping at resonant condition has been suggested (93). Non-aerodynamic damping effects have been discussed analytically as well as experi-mentally (94). Griffin (95) has presented an effi-cient dry friction damping model and compared it with experimental results. Predictions of various damping parameters have been discussed [96. 97).

Griffin [98] studied the sensitivity of mistuning and its effect on slip force using modal analysis. Klompas [99] examined blade vibration due to elliptical whirling in viscous dampers and presented a method for obtaining whirl harmonics. An expression for blade motion that includes the effects of slip damping and hysteresis damping has been formulated i 1001. A study of shrouds with corrugated bands has re-vealed the phenomenon of damping due to the bands (1011. The wire stiffness coefficient. influence of centrifugal force. position of wire. and magnitude of dynamie stresses have been determined and used to calculate the damping effect of blade wires (1021. Kapralov and Skvortsov [1031 have discussed the vibrational effects of dry friction damping due to box-type dampers. Coulomb friction between the shroud ring elements has been studied to determine its effect on the vibrational characteristics of blades; anisotropic dry friction theory and FEM were used [104].

Forced vibration analysis for a compressor blade as a function of rotor speed and frequency of flow exci-