Monitoring surface breaking defects with piezoelectric active systems

This paper is concerned with an experimental study and a numerical study of the application of two active techniques to monitor the growth of fatigue cracks and simulated defects in beams using piezoelectric sensors and actuators. The first technique is a transfer function method that consists of exciting the structure with piezoelectric actuators, while recording the electromechanical response of sensors placed close to the defect. Results have shown that the choice of adequate parameters, such as sensor size and its distance to the defect edge, allows the detection of small changes in defect depth. Finite-element simulations were also performed to determine a correlation between sensor response and defect depth. The second method is an impedance technique where a single piezoelectric element is used to excite and to monitor the growth of the defect. This technique was evaluated through experiments where a machined slot of increasing depth was monitored by measuring the electrical admittance of the piezoelectric sensor.