Investigations on Maximum Amplitude Amplification Factor of Real Mistuned Bladed Structures

Abstract

The production of bladed structures, e.g. turbine and compressor wheels, is a subject of statistical scatter. The blades are designed to be identical but differ due to small manufacturing tolerances. This can be local variance of material properties and geometrical deviations, which is indicated as mistuning of the structure. This article deals with the amplitude amplification factor of bladed structures caused by mistuning of real geometries. Theoretical investigations and also experimental results show, that mistuning of the structure leads to an amplification of blade vibration amplitudes and hence to increased stresses in blades. The existing theoretical considerations by Whitehead from 1966 and Kenyon & Griffin from 2001 are compared with results of blade vibration measurements. For that purpose, measurement results of turbine and compressor wheels from publication in recent years were analyzed. These experimental results stem from measurements with strain gauges as well as tip timing measurements to determine blade vibration amplitudes. In addition to this extended literature survey, the authors also examined results from in-house tip timing measurements of a radial turbine wheel from a vehicular turbocharger. The maximum amplification factor (MAF) and the degree of localization were evaluated. The comparison with the theory by Kenyon & Griffith revealed that the estimated maximum amplification factor was always higher than the values from the analyzed data. The measured MAF were besides one exception of a highly localized vibration form, which does not meet the theory’s assumptions, between 54% and 99.5% of the expected value. In this sense the theory has been proven.