Study and comparison of different EMAT configurations for SH wave inspection

Abstract

Guided wave inspection has proven to be a very effective method for the rapid inspection of large structures. The fundamental shear horizontal (SH) wave mode in plates and the torsional mode in pipe-like structures are especially useful because of their non-dispersive character. Guided waves can be generated by either piezoelectric transducers or electro- magnetic acoustic transducers (EMATs), and EMATs can be based on either the Lorentz force or magnetostriction. Several EMAT configurations can be used to produce SH waves, the most common being Lorentz-force periodic permanent magnet and magnetostrictive EMATs, the latter being directly applied on the sample or with a bonded strip of highly magnetostrictive material on the plate. This paper compares the performance of these solutions on steel structures. To quantitatively assess the wave amplitude produced by different probes, a finite element model of the elementary transducers has been developed. The results of the model are experimentally validated and the simulations are further used to study the dependence of ultrasonic wave amplitude on key design parameters. The analysis shows that magnetostrictive EMATs directly applied on mild steel plates have comparatively poor performance that is dependent on the precise magneto-mechanical properties of the test object. Periodic permanent magnet EMATs generate intermediate wave amplitudes and are noncontact and insensitive to the variations in properties seen across typical steels. Large signal amplitudes can be achieved with magnetostrictive EMATs with a layer of highly magnetostrictive material attached between the transducer and the plate, but this compromises the noncontact nature of the transducer.

Circumferential guided wave EMAT system for pipeline screening using shear horizontal ultrasound

“…The spatially periodic magnetic field they produce then interacts with the eddy current induced in the sample by a racetrack coil below the magnet array. The Lorentz and magnetostrictive forces generated by this arrangement generate a shearing force at the surface of the sample, with the Lorentz force being the dominant mechanism in the generation of ultrasound within the sample [9,10].…”

Electromagnetic ultrasonic guided wave long-term monitoring and data difference adaptive extraction method for buried oil-gas pipelines

“…The coils winded on the surface of the nickel strap are excited by alternating currents and thus can provide the axial alternating magnetic field. In this configuration, circumferentially uniform torsional waves will be stimulated and the torsional waves will propagate along the axial direction of the pipe [27]. When the torsional waves encounter defects at a certain propagation distance, the waves will be reflected and then finally received by the EMAT.…”

Structural health monitoring using torsional guided wave electromagnetic acoustic transducers

“…While Electromagnetic Acoustic Transducers are usually referred to as contactless transducers, this is not true for the magnetostrictive types of EMATs which are usually used with a bonded highly magnetostricitve strip (23; 24) . The use of a magnetostrictive strip enhances the excited wave strength but sacrifices the contactless feature of the transducer (25).…”

“…Usually both the magnetostrictive and Lorentz forces are generated on ferromagnetic media. The dominant force generated by a transducer is dependent on the EMAT configuration and the material properties (25). This paper introduces a Lorentz force based transducer and neglects the magnetostrictive and magnetisation forces.…”