Passive-only damage detection by reciprocity of Green's functions reconstructed from diffuse acoustic fields with application to wind turbine blades

The increasing demand for renewable and clean power generation has resulted in the increased size of rotor blades in wind turbine systems. The operational environments have introduced the need for structural health monitoring of the blades to help prevent unexpected failures. A new passive approach is here presented to detect damage in a wind turbine blade using the reciprocity of Green's function. This is achieved using the derivative of an ensemble average of cross-correlation functions to reconstruct the forward and backward time-domain Green's functions between any two detection points on the structure within a diffuse acoustic field. Damage is detected when the similarity between the forward and backward signals decreases due to the wave nonlinearities introduced by the discontinuity that break the field reciprocity condition. Proof-of-principle results are presented first for an aluminum plate and then for a 9-m-long composite wind turbine blade containing known, simulated damage.