Structural Health Monitoring with Piezoelectric Wafer Active Sensors for Space Applications

Ultrasonic guided waves inspection using Lamb waves is suitable for damage detection in metallic structures. This paper will present experimental results obtained using guided Lamb waves to detect flaws in aluminum specimens with design features applicable to space applications. Two aluminum panels were fabricated from a variable-thickness aluminum top plate, with two bolted I-beams edge stiffeners and four bonded angle stiffeners. Artificial damages were introduced in the two panels: cracks, corrosions, and disbonds. The proposed investigation methods used bonded piezoelectric wafer active sensors to excite and receive Lamb waves. Three wave propagation methods were used: pitch–catch, pulse–echo, and the embedded ultrasonic structural radar. In addition, we also used a standing-wave damage detection technique, the electromechanical impedance method. The paper will present in detail the salient results from using these methods for damage detection and structural health monitoring. Where appropriate, comparison between different methods in detecting the same damage will be performed. The results have demonstrated the ability of piezoelectric wafer active sensors working in conjunction with guided Lamb waves to detect various types of damages present in complex geometry structures typical of space applications. Nomenclature A0 = antisymmetric Lamb-wave mode C = zero-load capacitance of the piezoelectric wafer active sensor Dj = electrical displacement dkij = piezoelectric coupling effect Ek = electrical field Sij = mechanical strain S0 = symmetric Lamb-wave mode sEijkl = mechanical compliance Tkl = mechanical stress Z = electromechanical impedance ZPWAS = electrical impedance of the piezoelectric wafer active sensor Zstr = mechanical impedance of the structure "Tjk = dielectric permittivity at zero mechanical stress 31 = electromechanical cross-coupling coefficient I.