Twice reflected ultrasonic bulk wave for surface defect monitoring.

This work offers an ultrasonic structural health monitoring (SHM) approach for assessing the defects located on the same surface and at one side of piezoelectric ultrasonic transducer array. It is based on the analysis of ultrasonic bulk wave travelling in the thickness direction obtained from an enhanced full-skip configuration of the time-of-flight diffraction (TOFD) technique. In contrast to existing TOFD setup only considering the direct paths between the ultrasonic transducer and defect, our ultrasound monitoring configuration involves twice reflected ultrasonic bulk wave (TRBW). The TRBW travels following the propagation route from an ultrasonic transmitter located at the same side of the defect initiated, the backwall, the defect tip, the backwall again and finally to the same or another ultrasonic transducer. Both theoretical analyses and experimental validations have been conducted in our study. A simplified algorithm for efficient detection and mapping the growth of a surface defect in an aluminum alloy block has been demonstrated with an incremental surface defect growth starting from 2.80 mm in depth, in which conformable direct-write ultrasonic transducers (DWT) made of in-situ piezoelectric coating are implemented. Our approach provides an ultrasonic method for effective monitoring the near surface defects with the ultrasonic transducers conveniently implemented on the same surface and at the same side of the defects.