Strategies for small-footprint devices in structural health monitoring

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Structural Health Monitoring (SHM) has been proposed to allow the aerospace industry transitioning from scheduled-based maintenance, conducted using Non-Destructive Testing (NDT) techniques, to condition-based maintenance. The most promising approach uses in situ piezoceramic (PZT) transducers mounted on metallic or composite structures to generate and receive ultrasonic guided waves propagating and interacting with defects. Although a number of SHM technologies have been proposed, and despite the potential reduction of aircraft maintenance costs and increased aircraft availability, the civil aerospace industry has not implemented the approach yet. This program intends to achieve a major breakthrough on the requirements still to be addressed: damage quantification (resolution below 1 mm), smaller footprint, and demonstrated durability of SHM technologies, by developing strategies for small-footprint arrays of PZT transducers (in the order of 1 cm2) in damage quantification at high frequency. Damage quantification will come from smaller wavelengths at high frequency, and better resolved generation and measurement locations with a compact array. This research program will be conducted along three distinct research thrusts. 1) The discrimination of the guided wave mode propagating is critical in damage quantification. Thus, high-frequency mode-selective transducers will be designed to generate target stress profile at the interface between the PZT and the host structure, obtained from systematic optimization of modal power flow. Combined shear and normal stresses will be exploited through innovative 3D configurations to overcome the reduced wave power generated in the structure because of the transducer size reduction, potentially leading to reduced sensitivity to damage. 2) As high resolution damage imaging is required to transition from detection and localization to quantification, super-resolution tools will be integrated into the correlation-based damage imaging algorithms already validated for larger arrays. The dynamic model of the PZTs, as well as their mechanical and electrical interactions within the array will be integrated in the analysis atoms used in the correlation for higher resolution. 3) 3D microfabrication of the PZT arrays will combine ablation from a bulk piece of PZT for high power density, using laser micro-machining techniques, sol-gel deposition with photolithography techniques for through-the-thickness and multi-layer features, and assembly of components such as inertial masses. Velocity field measured with a 3D Laser Doppler Vibrometer and damage imaging will be used to validate the fabricated PZT arrays. Strategies for small-footprint arrays of PZT will allow damage quantification through higher resolution and such systems are expected to achieve the required durability by being less intrusive and less sensitive to environment.