Damage Detection in Simulated Aging-aircraft Panels Using the Electro-mechanical Impedance Technique

The aging of aerospace structures is a major current concern of civilian and military aircraft operators. PZT active sensors offer special opportunities for developing sensor arrays for in-situ health monitoring of aging aircraft structures. In this paper, we examine the structural health monitoring of aging aircraft structures with the electro-mechanical (E/M) impedance method. Local impedance methodology as well as damage detection strategy were developed. The detection of damage due to corrosion, structural cracks, and active sensor self-diagnostics with electro-mechanical (E/M) impedance method are discussed. The paper emphasis is sensor fabrication and installation procedures. Consistency and repeatability of sensor fabrication and installation were perfected and confirmed by statistical tests on a number of nominally identical specimens. Experiments on simple metallic beams were then performed and the results were compared with readily available theoretical predictions. Experiments on square plates manufactured from aircraft grade thin-gage stock followed. Statistical analysis of plate data confirmed that consistent and repeatability results could be achieved with our methodology. Finally, experiments on realistic aircraft panels with simulated crack and corrosion damage were performed. An array of four sensors were installed along a line emanating at a right angle from the crack, and E/M impedance readings were collected from all sensors. Consistency of sensor readings was verified. Also, the tendency of frequency to shift in the vicinity of the crack was noticed. However, further data processing, and detailed FEM modeling are still needed before full understanding of the correlation between crack presence and sensors readings is achieved. *) Associate Professor, Member ASME **) Graduate Research Assistant, Student Member ASME ELECTRO-MECHANICAL (E/M) IMPEDANCE METHOD The impedance method is a damage detection technique complementary to the wave propagation techniques. Ultrasonic equipment manufacturers offer, as options, mechanical impedance analysis (MIA) probes and equipment (Staveley NDT Technologies, 1998). The mechanical impedance method consists of exciting vibrations of bonded plates using a specialized transducer that simultaneously measures the applied normal force and the induced velocity. Cawley (1984) extended Lange’s (1978) work on the mechanical impedance method and studied the identification of local disbonds in bonded plates using a small shaker. Though phase information was not used in Cawley’s analysis, present day MIA methodology uses both magnitude and phase information to detect damage. v t V t ( ) sin( ) = ω PZT wafer transducer