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Abstract

In order to reduce weight and build more economic rotating parts, the use of bladed integral disks (Blisks) is getting more widespread. These components are milled from one single piece instead of being assembled like conventional parts.
Blisks reduce weight, but bring technical challenges. As they are made from one single piece, they show extremely low damping, leading to very sharp, pronounced vibration resonances. An ideal Blisk is perfectly symmetric. Very small imperfections in the manufacturing process lead to mistuning of the resonances. The vibration energy is then concentrated on one or a few blades, leading to higher stress in these blades and ultimately to an earlier failure of the component.
To predict the real stresses, a detailed Finite Element (FE) modelling is needed. As a first step, a FE model for a symmetric part needs to be validated by test. As a second step, it will then be adapted to account for the observed mistuning.
An experimental modal test is performed on a compressor Blisk of 240mm diameter, provided by the ITSM from the University of Stuttgart. Broadband excitation up to > 20kHz is performed by an automated modal hammer. A 3D Laser Doppler Vibrometer is used to measure the complete 3D vibration deflection shapes in a non-contact manner, leading to a data set that can easily be used for FE correlation. No mass loading or increased damping as for e.g. accelerometers appears.
Transfer functions for all scanpoints and the corresponding deflection shapes are recorded. For further analysis and comparison with FE, modal extraction is performed. These modes are then compared to the FE modes for a purely symmetrical structure. The frequencies are in excellent agreement, indicating only small mistuning. Nevertheless, the slight split in the frequencies of the symmetric mode pairs shows its presence. This is consolidated by comparing the mode shapes with a MAC analysis. While the lower order modes show excellent correlation, some of the higher order modes show lower MAC values. This indicates some localization of the vibration amplitudes to a few blades more than to others.
The used 3D Scanning Vibrometer is an excellent tool for acquiring hi-fidelity data on challenging objects like Blisks. Modal extraction of the data enables a detailed correlation with FE modes, showing slight mistuning effects in the real part. An updated FE model, which takes into account mistuning, can be set up, which is mandatory for predicting high cycle fatigue in real operation.