Holographic vibration analysis of turbocharger turbine wheel Instruction for use and benefit

摘要

Since the early 1990's, holography has been used worldwide to study the vibration of mechanical parts at the design stage. The so-called time average technique on holographic plates was able to give accurate information on the vibration modes of structures. TV-Holography has simplified the data capture, and also has the ability to easily produce amplitude and phase maps. This optical method is a powerful tool for vibration analysis but it needs to be used carefully to gain the full benefit of the data recorded. Then several types of analysis of these data may bring to the mechanical designer key information for the future life of the designed mechanical part. In this paper, we present a complete vibration analysis of a turbocharger turbine wheel, including the two main following points : the holographic recording method and the data post-processing that is done by the vibration experts. Concerning the data recording we will emphasize the experimental conditions that lead to data that are useful for the mechanical engineer : wheel preparation, wheel boundary conditions, method of excitation, geometrical conditions, tests complementary to the holographic recording. Experimental results are reported, showing the effect of the experimental conditions on the eigenfrequencies, eigenmodes and damping factor. Concerning High Cycle Fatigue (HCF) on turbine blades of turbochargers, Holography is of great help in two instances, predictive behaviour at design stage and field failures analysis . For the first task, Holography confirms/refines the 3 or 4 first modes predicted by FEA models , it gives the high order modes not predictable by models (especially coupled inducer/backdisc modes) and also the damping factors that are not accurately predicted .Those data are then fed into an "Harmonic Analysis" which allows the prediction of a forced response and, subsequently, an answer about robustness with respect to HCF. For the second task, Holography provides accurate nodal lines which can be easily correlated with location of field cracks or fractures. A Campbell diagram can be used to identify the order of the aero excitation responsible for the failure. Corrective actions to the product design or recommendations for speed limit can then be taken accordingly.