“…Even if the transducer could reach these components, the ultrasonic beams could not cover the target inspection area because of the limited scanning space. Compact ultrasonic probes have emerged as a solution to this problem (Yoon et al, 2014;Erhard et al, 2001;Song et al, 2002;Kim et al, 2016b;Abdallah and Namgung 2018). Pajnić et al (Pajnić et al, 2009) developed a slim sword-like trinary probe with a pair of time-offlight diffraction (TOFD) transducers for the detection and sizing of circumferential and axial cracks in the 3 mm gap between the inner surface of the penetration nozzle and the thermal sleeve.…”