Electro-Mechanical Impedance Method for Crack Detection in Thin Plates

In recent years, the damage detection with E/M impedance method has gained increased attention. The method uses small-size piezoelectric active sensors intimately bonded to an existing structure, or embedded into a new composite construction. Experimental demonstrations have shown that the real part of the highfrequency impedance spectrum is directly affected by the presence of damage or defects in the monitored structure (Figure 1). Pioneering work on utilization of E/M impedance method was presented by Liang et al. (1994) who performed the coupled E/M analysis of adaptive systems driven by a surface-attached piezoelectric wafer. However, no modeling of the structural substrate was included, and no prediction of structural impedance for a multi-DOF structure was presented. This work was continued and extended by Sun et al. (1994, 1995) who used the halfpower bandwidth method to accurately determine the natural frequency values. While the structural dynamics was always accounted for in the solution, the majority of authors assumed that the stiffness of the piezoelectric sensor is static and no sensor dynamics was considered. Giurgiutiu and Zagrai (2001a) derived an expression where the dynamics of a sensor was incorporated and the E/M impedance spectrum was modeled to simulate the direct measurements at the sensor’s terminals. The authors limited themselves to the one-dimension structure. Thus, the modeling was verified with simple beam specimen. In the same reference, the method for sensor’s self-diagnostics was suggested.