Health monitoring of aging aerospace structures using the electromechanical impedance method

This paper describes the use of the electro-mechanical (E/M) impedance method for health monitoring of aging aerospace structures. As a nondestructive evaluation technology, the E/M impedance method allows us to identify the structural dynamics directly by obtaining the E/M impedance signatures of attached piezoelectric wafer active sensors (PWAS). The theoretical model for 2-D structures, which predicts the E/M impedance response at PWAS terminals, was developed and validated. The model accounts for axial and flexural vibrations of a host structure and considers both, structural dynamics and dynamics of the sensor. The study of sensor's sensitivity to structural damage is presented. The presence of damage modifies the E/M impedance spectrum causing frequency shifts, peak splitting and appearance of new harmonics. The overall-statistics damage metrics and probabilistic neural network (PNN) were used to classify data according to damage severity. When installed on the aging aircraft panel, the sensors response features: (a) in the near field, spectral baseline change; (b) in the medium field, changes in harmonics distribution. These effects were successfully captured with overall-statistics damage metrics (correlation coefficient deviation) and PNN respectively. The health monitoring of aging aerospace specimens shows that unobtrusive permanently attached PWAS in conjunction with E/M impedance method can be successfully used to assess the presence of incipient damage through the examination and classification of the E/M impedance spectra.