Guided wave focusing electromagnetic acoustic transducer for flaw detection of laser welding in thin metallic sheets

Guided wave testing(GWT) has lower accuracy than bulk wave testing; however, it has been adopted for thin plate and cylindrical structure owing to limitations of conventional ultrasonic testing(UT) for these application. Even though contact-probe-based GWT can inspect the full thickness of plate-like structures, it can introduce uncertainties in manual ultrasonic testing because of accessibility and contact issues between the probe and the specimen. The electromagnetic acoustic transducer (EMAT) GWT, which is a non-contact technique, can overcome such problems; however, its application for inspection can be difficult owing to its low transduction efficiency compared to contact probes such as those using piezoelectric elements. Therefore, this paper proposes guided wave focusing using EMAT for local damage inspection of welds in thin metallic sheets. The focusing effect of the coil shape and pattern was identified by numerical analysis results, and verified by detecting the out-of-plane displacement of the guided waves propagated in a thin steel plate. To verify the validity of proposed technique, a guided wave focusing EMAT was designed and fabricated, and the distribution of acoustic fields with high energy density in the focusing area was identified. Consequently, the performance of the proposed guided wave focusing EMAT was verified by detecting the machining defects on the laser welding area