Annular plate inspection using guided wave tomography

Volker, Arno; Vos, Rik

doi:10.1063/1.4789120

Cited by 2 publications

(1 citation statement)

References 1 publication

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“…One strategy is to simply send and receive multiple modes [1,2], however, the thickness reconstruction may suffer due to uncertainties in the signal processing. By comparison, attempts to excite and measure a single Lamb wave mode by careful transduction have shown superior results, using the S0 mode in [3][4][5] and the A0 mode in [6,7], allowing for highly accurate thickness maps of complex defects to be produced. The relative merits of the A0 and S0 mode are discussed in [8] and based on this, here the focus is on exploiting A0 waves, which have been shown to be more sensitive than S0 to the majority of corrosion defects.…”

Section: Introductionmentioning

confidence: 99%

Model-Based Design of Low Frequency Lamb Wave EMATs for Mode Selectivity

et al. 2015

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A low-frequency, omni-directional A0 Lamb wave ElectroMagnetic Acoustic Transducer (EMAT) is developed for applications in guided wave tomography, operating at 50 kHz on a 10 mm thick steel plate. The key objective is to excite an acceptably pure A0 wave mode in relation to the S0 mode, which can also be present at this operating point and is desired to be suppressed by approximately 30 dB. For that, a parametric Finite Element (FE) model of the design concept is implemented in a commercially available FE software, where the bias magnetic field is calculated initially, then combined with the eddy current caused by the induction coil to produce a force. A numerical optimization process employing a genetic algorithm is set up and the EMAT design is optimized to yield an improved A0 mode selectivity. The parameters subjected to optimization are the magnet diameter and the magnet lift-off, which control the direction of the exciting force in the skin depth layer and therefore the mode selectivity. Although there are three possible electromagnetic acoustic interaction mechanisms, the optimisation considers only the Lorentz force, as its performance surface contains a clear optimum and from the optimised design a physical prototype is built. The FE model is validated against measurements on an aluminium plate for the Lorentz force excitation mechanism and on a steel plate for both the Lorentz and magnetisation force. For the steel plate, it is found that only considering the Lorentz force leads to a significant overestimation of the mode selectivity, as the S0 amplitude is underestimated by the Lorentz force, but the A0 amplitude remains mainly uninfluenced. Further, it has been found that additionally including the magnetisation force into the optimisation leads to a better mode selectivity, however, the optimisation drives the optimum to a minimum magnet diameter and therefore reduces the EMAT sensitivity. In a numerical study robustness is shown for fairly large variations of the magnet lift-off and the magnetic permeability. Based on the findings, a two-step model-based design approach is proposed whereby only the Lorentz force is used in the first step for the optimisation and then in a second step, a realistic estimate of the mode selectivity of the optimised design can be obtained by additionally considering the magnetisation force.

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Section: Introductionmentioning

confidence: 99%

Model-Based Design of Low Frequency Lamb Wave EMATs for Mode Selectivity

et al. 2015

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Experimental Studies of the Inspection of Areas With Restricted Access Using A0 Lamb Wave Tomography

Seher

Huthwaite

Lowe

2016

IEEE Trans. Ultrason., Ferroelect., Freq. Contr.

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Corrosion damage in inaccessible regions presents a significant challenge to the petrochemical industry, and determining the remaining wall thickness is important to establish the remaining service life. Guided wave tomography is one solution to this and involves transmitting Lamb waves through the area of interest and subsequently using the received signals to reconstruct a thickness map of the remaining wall thickness. This avoids the need to access all points on the surface, making the technique well suited to inspection for areas with restricted access. The influence of these areas onto the ability to detect and size surface conditions, such as corrosion damage, using guided wave tomography is assessed. For that, a guided wave tomography system is employed, which is based on low frequency A0 Lamb waves that are excited and detected with two arrays of electromagnetic acoustic transducers (EMATs). Two different defect depths are considered with different contrasts relative to the nominal wall thickness, both of which are smoothly varying and welldefined. The influence of areas with restricted surface access, support locations, pipe clamps and STOPAQ(R) coatings, are experimentally tested, and their influence assessed through comparison to a baseline reconstruction without the respective restriction in place, demonstrating only a small influence on the detected value of the remaining wall thickness.

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Annular plate inspection using guided wave tomography

Cited by 2 publications

References 1 publication

Model-Based Design of Low Frequency Lamb Wave EMATs for Mode Selectivity

Model-Based Design of Low Frequency Lamb Wave EMATs for Mode Selectivity

Experimental Studies of the Inspection of Areas With Restricted Access Using A0 Lamb Wave Tomography

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