Classification of flaw severity using pattern recognition for guided wave-based structural health monitoring

Miller, Corey; Hinders, Mark K.

doi:10.1016/j.ultras.2013.04.020

Cited by 32 publications

(16 citation statements)

References 43 publications

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“…In equation (2), J e represents the externally applied current density. From the magnetic vector potential, the magnetic flux density (B = ∇ × A) and electric field E = −Ȧ may be computed.…”

Section: Electromagnetic Acoustic Couplingmentioning

confidence: 99%

“…The induced current density can be given as J = −σȦ using Ohm's law. In addition to equation (2), for the computation of electromagnetic fields, a gauging condition is introduced, such as the Coulomb gauge (∇ · A = 0). The solution to Maxwell's equation in any representation requires a set of constitutive relationships interrelating the various electromagnetic field variables.…”

Section: Electromagnetic Acoustic Couplingmentioning

confidence: 99%

“…A challenging aspect of guided wave tomography is that multiple Lamb wave modes can exist at any one frequency, even at low frequencies. 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.…”

Section: Introductionmentioning

confidence: 99%

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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: Electromagnetic Acoustic Couplingmentioning

confidence: 99%

Section: Electromagnetic Acoustic Couplingmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

et al. 2015

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“…Since at every frequency multiple Lamb wave modes may exist, there are many approaches to guided wave tomography, such as sending and receiving multiple modes [1]- [3]. By comparison, attempts to excite and measure a single Lamb wave mode by careful transduction have shown superior results, using the S0 mode in [4]- [7] and the A0 mode in [8]- [14] allowing for accurate thickness maps of complex defects to be produced.…”

Section: Introductionmentioning

confidence: 99%

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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“…The other group is the Fourier-based methods like filtered back projection (FBP) [7,8,12] and interpolated filtered back projection (IFBP) [13]. The geometries of the transducer arrays include the circular array configuration with parallel or fan beam projection [14], the linear array configuration such as cross-hole [10] and double cross-hole [15] and so on. In recent studies, a subset of the reconstruction algorithms of ULW TI, namely reconstruction algorithm for probabilistic inspection of damage (RAPID), has received great attention [16–19].…”

Section: Introductionmentioning

confidence: 99%

A New Omni-Directional EMAT for Ultrasonic Lamb Wave Tomography Imaging of Metallic Plate Defects

Huang

Wei

Zhao

et al. 2014

Sensors

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This paper proposes a new omni-directional electromagnetic acoustic transducer (EMAT) for the ultrasonic Lamb wave (ULW) tomography imaging (TI) of defects in metallic plates. The proposed EMAT is composed of a permanent magnet and a coil with a contra-flexure structure. This new EMAT coil structure is used for omni-directional ULW transmission and reception and ULW TI for the first time. The theoretical background and the working principles of this EMAT are presented and analyzed. The experimental results of its use on a 3 mm thick aluminum plate indicate that the EMAT with a contra-flexure coil (CFC) can transmit and receive a pure single A0 mode ULW with a high signal-to-noise ratio (SNR). Thus, the extraction of the projection data used for ULW TI may be performed accurately. The circumferential consistency of the projection data is only slightly influenced by the distortion of the eddy current field that is induced by the new CFC with an irregular shape. When the new EMAT array is used for ULW TI using the cross-hole method and SIRT arithmetic, a desirable imaging quality can be achieved, and the estimated size of an artificial corrosion defect agreed well with its actual value. The relation between the reconstruction resolution and the number of the new EMATs used is analyzed. More TI experiments are carried out when the aluminum plate defect is in two different locations relative to the EMAT array, for the further investigation of the performances of the new EMATs.

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Classification of flaw severity using pattern recognition for guided wave-based structural health monitoring

Cited by 32 publications

References 43 publications

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

A New Omni-Directional EMAT for Ultrasonic Lamb Wave Tomography Imaging of Metallic Plate Defects

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