Patent ID: 7886604

Claim:
An electromagnetic ultrasonic flaw detection method comprising: moving an electromagnetic ultrasonic transducer in a fin-implanted heat transfer tube of an air cooling heat exchanger axially of the tube; causing the electromagnetic ultrasonic transducer to generate an axially symmetric SH wave by utilizing an electromagnetic force to vibrate a tube body of the fin-implanted heat transfer tube to cause resonance; causing the electromagnetic ultrasonic transducer to detect a resonant frequency; and if the detected resonant frequency is different from a resonant frequency observed when the tube body has a normal wall thickness, judging that an interior surface of the tube body has a corroded portion, wherein the electromagnetic ultrasonic transducer includes a permanent magnet unit having a hollow or solid cylindrical shape, and a transmission coil and a reception coil which are wound around the permanent magnet unit, and wherein the permanent magnet unit includes a plurality of permanent magnets each having opposite poles respectively disposed at a greater radius portion and a smaller radius portion thereof and arranged circularly with their polarities alternately reversed circumferentially of the permanent magnet unit.