Damage identification by NSMS blade resonance tracking in mistuned rotors

Damage in rotor blades, including cracks, tend to shift the blades' resonant frequencies. Blade resonances have thus been envisioned as a damage indicator. This paper studies issues that arise in blade resonance identification using Non-contacting Stress Monitoring Systems (NSMS) when blade resonances have slight variation (mistuning) and are dynamically coupled. The study compares simulation results to experimental data obtained during seeded fault tests. The main findings are as follows: 1) The forced resonant response of a blade is seldom dominated by the blade's uncoupled dynamics. Changes due to damage in one blade are thus often reflected in the responses of many blades. 2) To reliably identify damage by looking at one individual response at a time, the shift in resonant frequency resulting from damage must be larger than the frequency redistribution due to blade-to-blade coupling (e.g. 1%-2%). Only for uncoupled blades can smaller shifts in resonance frequency be reliably detected. 3) For finer damage identification (e.g. less than 1%), a global identification method utilizing all blade responses at once must be use to determine blade dynamics and coupling. This can be attempted with an NSMS system, which senses every blade response.