Direct-Write Piezoelectric Ultrasonic Transducers for Non-Destructive Testing of Metal Plates
Current real time structural health monitoring is implemented by assembling multiple discrete sensors on a structure with each sensor providing only point measurement. The installation of the multiple sensors results in high global cost and low reliability. This paper reports the design, direct-write fabrication, and testing of ultrasonic transducers on a plate structure and a non-destructive testing method for detecting defects using the sensor array comprising the direct-write ultrasonic transducers. The transducers are made of piezoelectric poly(vinylidenefluoride/trifluoroethylene) (P(VDF/TrFE)) polymer coatings that are aerosol-spray deposited and patterned directly on the plate structure to be monitored. The ultrasonic transducers bearing annular array comb electrodes are designed for generating and selectively detecting fundamental antisymmetric Lamb-mode ultrasonic waves in the plate structure. The ultrasonic transducers can serve as both the actuators to generate ultrasonic waves and the sensors to detect ultrasonic waves. The ultrasonic waves propagating through the plate structure contain the information about the structural integrity. With copper bars of different thicknesses introduced at the plate center as mock defects of different severity, the correlation between the transducer response and the defect thickness and hence, the severity is verified. It is also demonstrated that four ultrasonic transducers located at the square plate (100 mm $\times 100$  mm $\times 1.27$  mm) corners forming a transducer array, which can locate the defect on the plate. A short time Fourier transform algorithm and an imaging algorithm are developed for processing signal of the pitch-catch ultrasonic wave spectra to determine the location of the defects, which is verified by experimental results.