Title: Electro-Mechanical Impedance Method for Crack Detection in Thin Wall Structures.

The Electro-Mechanical (E/M) impedance method has achieved acceptance among NDE methods due to its relative simplicity, applicability to complex structures, and low cost of active piezoelectric elements. Thus, allow one to identify the structural dynamics directly by obtaining its E/M impedance or admittance signatures. Previous work reported on this method contained little theoretical work but described many practical applications. The scope of presented research was to extend the positive results obtained for 1-D structure onto 2-D thin-wall structures with circular symmetry. Theoretical analysis was analytically performed on simple geometries (circular plates) subjected to particular set of boundary conditions. The experimental results shown that E/M impedance (or admittance) spectrum accurately identify the natural frequency spectrum of the specimens manufactured from the aircraft-grade thin-gage material. Changes in the E/M impedance spectrum due to presence of crack were investigated. The dependence of the frequency spectrum on crack location was studied. The results show changing in E/M impedance spectrum due to damage presence. Additional recommendations are given to improve applications of the E/M impedance method for aging aircraft-type structures to detect incipient cracks and corrosion damage. 1. INTRODUCTION 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 high-frequency impedance spectrum is directly affected by the presence of damage or defects in the monitored structure (Figure 1). The pioneering work on utilization of E/M impedance method for structural health monitoring was presented by Liang et al. (1994) who performed the coupled