Mode selection for corrosion detection in pipes and vessels via guided wave tomography.

Guided wave tomography offers a method to accurately quantify wall thickness losses in pipes and vessels caused by corrosion, using ultrasonic waves transmitted over distances of approximately 1 to 2 m, and measured by an array of transducers. These measurements are then used to reconstruct a map of wall thickness throughout the inspected region. To achieve accurate estimations of remnant wall thickness, it is vital that a suitable Lamb mode is chosen. This paper presents a detailed evaluation of the two most suitable modes, S0 and A0, to compare their performance using both numerical and experimental data. The sensitivity of A0 to thickness variations was shown to be superior to S0; however, the attenuation from A0 when a liquid loading was present was much higher than S0. A0 was less sensitive to the presence of coatings on the surface than was S0. Finally, it was shown that both modes could achieve a similar level of resolution in the plane of the plate surface.

Study and comparison of different EMAT configurations for SH wave inspection.

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.

Quantitative modeling of the transduction of electromagnetic acoustic transducers operating on ferromagnetic media.

The noncontact nature of electromagnetic acoustic transducers (EMATs) offers a series of advantages over traditional piezoelectric transducers, but these features are counter-balanced by their relatively low signal-to-noise ratio and their strong dependence on material properties such as electric conductivity, magnetic permeability, and magnetostriction. The implication is that full exploitation of EMATs needs detailed modeling of their operation. A finite element model, accounting for the main transduction mechanisms, has been developed to allow the optimization of the transducers. Magnetostriction is included and described through an analogy with piezoelectricity. The model is used to predict the performance of a simple EMAT: a single current-carrying wire, parallel to a bias magnetic field generating shear horizontal waves in a nickel plate close to it. The results are validated against experiments. The model is able to successfully predict the wave amplitude dependence on significant parameters: the static bias field, the driving current amplitude, and the excitation frequency. The comparison does not employ any arbitrary adjustable parameter; for the first time an absolute validation of a magnetostrictive EMAT model has been achieved. The results are satisfactory: the discrepancy between the numerical predictions and the measured values of wave amplitude per unit current is less than 20% over a 200 kHz frequency range. The study has also shown that magnetostrictive EMAT sensitivity is not only a function of the magnetostrictive properties, because the magnetic permeability also plays a significant role in the transduction mechanism, partly counterbalancing the magnetostrictive effects.