Wavelet Analysis of Experimental Blade Vibrations During Interaction With an Abradable Coating

Minimizing the clearance between turbofan blades and the surrounding casing is a key factor to achieving compressor efficiency. The deposition of an abradable coating on casings is one of the technologies used to reduce this blade-casing clearance and ensure blade integrity in the event of blade-casing contact. Aircraft in-service conditions may lead to interactions between the blade tip and the coated casing, during which wear of the abradable coating, blade dynamics, and interacting force are critical yet little-understood issues. In order to study blade/abradable-coating interactions of a few tens of milliseconds, experiments were conducted on a dedicated test rig. The experimental data were analyzed with the aim of determining the friction-induced vibrational modes of the blade. This involved a time-frequency analysis of the experimental blade strain using continuous wavelet transform (CWT) combined with a modal analysis of the blade. The latter was carried out with two kinds of kinematic boundary conditions at the blade tip: free and modified, by imposing contact with the abradable coating. The interaction data show that the blade vibration modes identified during interactions correspond to the free boundary condition due to the transitional nature of the phenomena and the very short duration of contacts. The properties of the continuous wavelet transform were then used to identify the occurrence of blade-coating contact. Two kinds of blade/abradable-coating interactions were identified: bouncing of the blade over short time periods associated with loss of abradable material and isolated contacts capable of amplifying the blade vibrations without causing significant wear of the abradable coating. The results obtained were corroborated by high-speed imaging of the interactions.