Detection of cracks on a plate by piezoelectric interdigital transducers

Much work has been reported on the use of piezoceramic patch sensors in structural health monitoring due to their simple structure and moderate performance. However, the patch uses the inherent electromechanical properties of a piezoceramic element itself. Once the piezoceramic element is given, not much controllability is available on its operation frequency, directionality, and sensitivity. This paper proposes and verifies the feasibility of a new ultrasonic sensor to estimate the quantitative configuration of cracks on a plate: an inter-digital transducer (IDT) type Lamb wave sensor. This IDT sensor is more readily controllable than conventional patch sensors in terms of its operation frequency and directionality by altering the IDT pattern on a given piezoceramic element. In this work, two different types of IDT sensors are designed and fabricated: annular IDT and rectangular IDT sensors. The IDT sensors are implemented in the experiments to investigate the length, the number and the inclination angle of the cracks artificially imposed on an aluminum plate. The variation of amplitude and time-of-flight (TOF) of Lamb waves are measured and analyzed to estimate the crack configurations. The results show that the annular IDT sensor is similar to the conventional patch sensors in terms of its omni-directional beam pattern. The rectangular IDT sensor is highly directional, thus the sensor has superior sensitivity to a particular direction, which means more robust to environmental noise coming from arbitrary directions. The measured configuration shows excellent correlation with real configuration of the cracks, which confirms the efficacy of the IDT sensors.