Using Phased Array Technology and Embedded Ultrasonic Structural Radar for Active Structural Health Monitoring and Nondestructive Evaluation

The embedded ultrasonic structural radar (EUSR) was developed based on phased array technology. It can interrogate large structural areas from a single location using ultrasonic guided Lamb and Rayleigh waves generated by tuned piezoelectric wafer active sensors (PWAS) that are permanently attached to the structure. This paper brings together several aspects of the implementation and application of EUSR to structural damage detection: (a) improving the near field damage detection; (b) designing optimized phased-array patterns; (c) designing a mini phased array for compact structures with complicated geometries and multiple boundaries. Firstly, we deduced a generic formulation for phased array directional beamforming using the exact traveling waves formulation without the limiting parallel-rays assumption used by other investigators. This algorithm has been implemented in the EUSR LabVIEW program and its performance has been verified through simulation and experimental tests. Secondly, we studied the beamforming and lobe steering characteristics of a 1-D linear array design. The influence of several geometry parameters was discussed in order to achieve the optimal directionality, including the number of sensors in the phased array, the spacing between adjacent sensors, and the steering direction angles. Extensive simulation studies have shown that the well-behaved directional beamforming can be achieved with judicious array design. Proof-of-concept experiments for testing these results have also been set up and the preliminary results are confirming the effectiveness of our approach. Thirdly, we investigated the possibility of applying the EUSR phased array method to compact specimens and proposed the design of a mini phased array. Laboratory experiments have been carried out to prove the successful implementation of this concept. Finally, the paper ends up with discussions and conclusions regarding the beamforming, optimization and implementation of the PWAS phased arrays, as well as suggestions for further work. INTRODUCTION Phased arrays are made of multiple piezoelectric elements excited by predetermined time-delayed signals to generate structural interference patterns. By properly adjusting the time delays, phased array can phase steer and focus the ultrasonic beams at certain direction. Some of the advantages of phased arrays over conventional ultrasonic transducers include high inspection speed, flexible data processing capability, improved resolution, and the capability of scanning without requiring mechanical movement, i.e., dynamic beam steering and focusing [1]. The backscattered ultrasonic signals can be analyzed and then mapped into an image. Current ultrasonic inspection of thin wall structures (e.g., aircraft shells, storage tanks, large pipes, etc.) is a time consuming operation that requires meticulous through-thethickness C-scans over large areas. One method to increase the efficiency of thin-wall structures inspection is to utilize guided waves (e.g., Lamb waves) instead of the conventional pressure waves [2, 3]. Guided waves propagate along the mid-surface of thin-wall plates and shallow shells [4, 5]. They can travel at relatively large distances with very little amplitude loss and offer the advantage of large-area coverage with a minimum of installed sensors. Guided Lamb waves have opened new opportunities for cost-effective detection of damage in aircraft structures, and a large number of papers have recently been published on this subject. The use of guided waves in conjunction with phase-array principles has proliferated widely in recent years due to its obvious benefits. However, an important roadblock on the way towards the utilization of these techniques in aircraft structural health monitoring is that the conventional ultrasonic transducers used for guided wave and phased-array applications are bulky and expensive, thus making their use in wide-area structural health monitoring expensive and problematic. Hence, a different type of sensors than the conventional ultrasonic transducers is required for the SHM systems. Piezoelectric wafer active sensors (PWAS) are small, non-intrusive, inexpensive piezoelectric wafers that are