Emat Pipe Inspection with Guided Waves

Abstract

Electro Magnetic Acoustic Transducers (EMATs) are a well-known type of ultrasonic probes used for nondestructive evaluation (NDE) of electrically conductive materials. Conventional Ultrasonic Testing (UT) is performed by longitudinal waves and mode converted shear waves generated by piezoelectric probes. EMATs are broadening the range of usable wave modes by the direct conversion of polarized shear waves with normal and angle beams and the selective generation and detection of nearly all types of guided waves. Despite their limitations (low efficiency, lift-off sensitivity, limited frequency range etc.) they have the big advantage to perform UT without couplants. The dry coupling allows UT at elevated temperatures, in media which do not tolerate liquids (e.g. natural-gas pipelines) or on sensitive and coated surfaces of blanks used for car bodies in the automotive industry etc. Ultrasonic Testing (UT) is still concentrated on the ‘classical’ wave modes generated by piezoelectric probes. Other wave modes as the Shear Horizontal (SH) waves and the wide class of guided waves offer new solutions for UT not yet widely used due to the lack of availability of appropriate probes and equipment. EMATs are the most far developed ultrasonic probes for UT using SH waves and guided waves. This contribution will present new solutions for UT of pipes using guided waves such as Shear Horizontal waves: for weld inspection, for long range inspection of pipes, for screening UT for hidden corrosion, for crack inspection in gas pipelines. The latest developments of the probe design are briefly reported. The equipment and their integration in production lines for in-service application are shown together with examples of inspection results during their practical applications.