Investigation of the damage and modal behavior of fast rotating geometrically complex structures using in-situ measurement systems

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Composite materials offer excellent properties for use in modern high performance rotors due to their high specific stiffness and strength and their adjustable successive damage behavior. The heterogeneity and anisotropy of the material requires novel numerical models for the design and description of the structural behavior of damaged rotors and, in particular, the damage-induced shift in their natural frequencies. Validation of such models requires in-situ measurement techniques to determine the damage state and modal behavior as a function of the complex load conditions.During the first funding period, the Diffraction Grating Method (DGM), with which the optical far field of diffraction gratings - applied to elementary rotor structures – was measured, was systematically investigated and used to validate the numerical model. Damage propagations and vibrations could be measured as a function of the rotational speed and spatially resolved at velocities >260 m/s.The possibilities of optical coherence tomography (OCT) for the investigation of statically and dynamically loaded composite materials were demonstrated and the entire 3D structure of a rotor could be visualized. Polarization-sensitive (PS) OCT was used to simultaneously detect stress changes, inter-fiber fractures, and the deformation field of the specimen.Measurement and simulation results show high agreement for undamaged, but not for damaged and heated structures. Furthermore, it became apparent that minor damages are difficult to detect by determining the shift of the natural frequency.Consequently, in the second funding period, the damping behavior will be investigated as an indicator for small damages. In addition, the numerical models will be extended to include the influence of temperature fields and generalized from multilayer disk rotors to more complex 3D structures. Therefore, systematic deviations of the DGM will be reduced to <100 με by Direct Laser Interference Patterning of the gratings, measurement range extension, and in-situ calibration using adaptive optics. OCT will be used for the first time on rapidly rotating, complex specimens for 3D detection of damage evolution by tracking the measurement beam via rotating optical elements. On static specimens, PS-OCT will be used for the first time to quantitatively measure spatially resolved strain and stress to test and improve models of damage evolution.The result will be a numerical model that can predict the relationship of the structural dynamic behavior of geometrically complex composite structures as a function of rotational velocity, temperature field, and damage state. In addition, novel and robust methods as well as sensors will be available for an in-situ validation of the numerical model.