Electromagnetic Acoustic Transducers for Robotic Nondestructive Inspection in Harsh Environments

Abstract

Elevated temperature, gamma radiation, and geometric constraints inside dry storage casks for spent nuclear fuel represent a harsh environment for nondestructive inspection of the cask and require that the inspection be conducted with a robotic system. Electromagnetic acoustic transducers (EMATs) using non-contact ultrasonic transduction based on the Lorentz force to excite/receive ultrasonic waves are suited for use in the robotic inspection. Periodic permanent magnet EMATs that actuate/receive shear horizontal guided waves are developed for application to robotic nondestructive inspection of stress corrosion cracks in the heat affected zone of welds in stainless steel dry storage canisters. The EMAT’s components are carefully selected in consideration of the inspection environment, and tested under elevated temperature and gamma radiation doses up to 177 °C and 5920 krad, respectively, to evaluate the performance of the EMATs under realistic environmental conditions. The effect of gamma radiation is minimal, but the EMAT’s performance is affected by temperatures above 121 °C due to the low Curie temperature of the magnets. Different magnets are needed to operate at 177 °C. The EMAT’s capability to detect notches is also evaluated from B-scan measurements on 304 stainless steel welded plate containing surface-breaking notches.

Application of ultrasonic guided waves to robotic occupancy grid mapping

“…SH waves oscillate perpendicular to the propagation path, and have particle motion in the transverse plane, while Lamb waves have particle vibrations in the sagittal plane [16]. The transverse polarization of SH wave modes may enhance the sensitivity of this type of guided wave to certain types of structural features such as welded regions, as well as the sensitivity to defects oriented parallel to the wave vector [32,33].…”

“…EMATS use a combination of a pulsed current and a magnetic field to generate or detect a disturbance in the metal lattice, which propagates as an elastic wave [34]. Unlike traditional piezoelectric transducers, they do not require couplant, which simplifies deployment on a robotic platform [33]. However, the magnetic field can cause difficulties when scanning magnetic materials.…”

“…In this work, we used miniaturised periodic permanent magnet (PPM) EMATS which consist of a racetrack-shaped coil and an array of permanent magnets spaced to match the SH wavelength -for more discussion of the design see Ref [35]. PPM EMATS/SH waves are being used increasingly for NDE applications as SH waves scatter from both perpendicular and parallel defects relative to the wavefront [33], [36,37] . It has also been reported that SH1 wave modes are sensitive to wall thinning damage due to the dispersive behaviour of this wave mode [38][39][40].…”

Visually Encoded Contact Inspection System for EMATs

“…A relevant type of NDE inspection probe for scans are electromagnetic acoustic transducer (EMAT) [16], [17]. EMAT probes have the advantage that they do not require direct contact with the inspected component, and thus there is no need for surface preparation.…”

Enhancement of Lamb-EMAT Signal Using a Modified One-Side Pitch-Catch Design

“…Compared with conventional UT technique using piezoelectric transducer, EMAT can generate and receive various wave modes in conductive and ferromagnetic materials without physical contact or liquid couplant with specimen [2], [3]. Such contact&couplant-free feature makes EMAT especially well-suited for industrial applications where needs high-temperature inspection, or the surface condition of the specimen is not ideal [4]- [7]. Lamb wave is one of special guide structure mode which is widely used for accessing the damages and characterization of metal materials due to its low attenuation and long distance propagation [8], [9].…”