This paper reports on a new means of generating higher-order mode clusters of guided waves (HOMC-GW) using a meander-coil (MC) electromagnetic acoustic transducer (EMAT) in plates at frequencies significantly higher than the lower-order plate modes. These wave modes are considerably less dispersive and they occur at much higher frequency¿thickness (f x d) products. Our studies cover the f x d range of 13 to 20 MHzmm. Experimental measurements were carried out on Al plate samples of different thicknesses using three different EMAT coil periods. To understand the generation and propagation characteristics of HOMC-GW with EMATs, several simulations were carried out using 2-D finite element models at different f x d products. These simulations captured all features observed in the experiments. The time¿frequency smoothed pseudo Wigner-Ville distribution (SPWVD) was used to analyze the HOMC-GW modes. Defect detection measurements using HOMC-GW generated using EMATs were made on Al plates with machined defects. © 2012 IEEE.

Krishnan Balasubramaniam

Department of Mechanical Engineering

Bruce W. Maxfield

DEFECT DETECTION

ELECTROMAGNETIC ACOUSTIC TRANSDUCERS

Experimental measurements

Finite element models

Higher frequencies

Higher-order modes

PROPAGATION CHARACTERISTICS

WAVE MODES

WIGNER-VILLE DISTRIBUTION

Guided electromagnetic wave propagation

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IIT Madras

Generation and detection of higher-order mode clusters of guided waves (HOMC-GW) using meander-coil EMATs

IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control

Guided waves inspection is a well-established method for the long-range ultrasonic inspection of pipes. Guided waves, used in a pulse-echo arrangement, can inspect a large range of the pipe from a single point as the pipe structure carries the waves over a large distance due to the relatively low attenuation of the wave modes. However, the complexity of the dispersion characteristics of these pipe guided wave modes are well known, and can lead to diffculty interpreting the obtained results. The torsional family of guided wave modes are generally considered to have much simpler dispersion characteristics; especially the fundamental T(0,1) mode, which is nominally non-dispersive, making it particularly useful for guided wave inspection. Torsional waves have been generated by a circumferential ring of transducers to approximate an axi-symmetric load to excite this T(0, 1) mode. Presented here is a new design of Electromagnetic Acoustic Transducer (EMAT) that can generate a T(0, 1) as a single transducer, rather than a circumferential array of transducers that all need to be excited in order to generate an axisymmetric force. The EMAT consists of a periodic permanent magnet array and a single meander coil, meaning that the excitation of the torsional mode is greatly simplified. The design parameters of this new EMAT are explored, and the ability to detect notch defects on a pipe is demonstrated. © 2017 Author(s).

Fulltext

AIP Conference Proceedings

A new electromagnetic acoustic transducer design for generating torsional guided wave modes for pipe inspections

Guided ultrasonic waves are widely used for long range inspection. Higher Order Modes Cluster (HOMC), discovered at the author's research group [1-3] consist of multiple higher order guided wave modes that travel together as a single wave-packet and without appreciable dispersion for distances in the range of meters. These waves not only propagate along the length of the structure but also cover the entire thickness, and in view of the higher frequencies, they can offer improved resolution over conventional low-frequency guided waves. This paper studies the sensitivity of axial plate HOMC to notch-like defects, evaluated by calculating wave reflection co-efficient. The studies are carried out using finite element models validated by experiments. Analysis is presented for better understanding of wave-defect interaction. Advantages and limitations for practical realization of the above approach are also discussed. © 2017 Author(s).

Interaction of Higher Order Modes Cluster (HOMC) guided waves with notch-like defects in plates

Interaction of fundamental torsional ultrasonic pipe guided mode T(0, 1) from defects caused by induction pressure welding (IPW) process is studied using three-dimensional (3D) finite element (FE) analysis validated by experiments. Defects are assumed as cross-sectional notches along the weld bond-line, and both surface-breaking and embedded features are considered. Results show that T(0, 1) mode reflection from weld defects is strongly influenced by features of the weld itself. However, with supplementary results such as the mode-converted flexural F(1, 3) and F(1, 2) modes and circumferential variation of T(0, 1) reflection, there is potential for an effective screening solution. © 2017 by ASME.

Journal of Pressure Vessel Technology, Transactions of the ASME

Reflection and Mode Conversion of Fundamental Torsional Guided Wave Mode from Lack of Bonding in Induction Pressure Welding

Finite Element Analysis (FEA) is an important tool that has been extensively used in wave propagation models for NDE applications. With the increase and diversification of Non Destructive Evaluation (NDE) applications, the complexity and size of FE models increase considerably. Also for modeling wave propagation models, the size of the FE problem depends on the frequency that is used and velocity of sound in the material. High ultrasonic frequency applications and materials with lower ultrasonic velocity contribute immensely to the increase in the FE problem size. Additionally, the requirements for 3D models are also increasing. This calls for higher computation resources to be available at the hands of the researcher at significant costs. We have developed a new approach to model such extremely large FE problems that involves splitting the entire model to be analyzed into smaller sub-models and by solving each sub-models sequentially, while transferring the outputs at the boundaries between the sub-models. Thus, the entire "large" model now consists of a few "smaller" sub-models. The size of these smaller models can be decided upon by the researcher depending on the computing capacity available. While this approach may not provide significant improvements in the time of computation, it allows for the computation of large models possible with relatively less computational resources. The model was tested on a few case studies and validated with the results obtained from a full "large" model. © 2014 AIP Publishing LLC.

Split approach for FEA simulations of very large wave propagation models

This paper presents a two-stage numerical model of a meander coil electromagnetic acoustic transducer transmitter operating on the Lorentz force principle. This model was used to predict the ultrasonic A-scan signals for bulk (longitudinal and shear) and guided (Rayleigh) waves in isotropic homogeneous materials. The simulation results agree with the experimental results. The grating effects of the meandering coil, for different coil spacings, are also discussed in this paper. © 2011 Taylor & Francis.

Nondestructive Testing and Evaluation

A two-stage finite element model of a meander coil electromagnetic acoustic transducer transmitter

This paper reports simulations of the generation of Lamb wave modes in thin plates using a meander coil (meander line) EMAT, which works under the principle of Lorentz force mechanism in non-magnetic materials. The numerical simulations have been compared with measurements. The numerical work presented in this paper is divided into two parts. First, a 2D electromagnetic model is developed for calculating the Lorentz force density, which is the driving force for elastic wave generation within the plate. Second, the calculated force at each point in the metal is used as the driving force for generating elastic wave modes in the plate. These calculated Lamb wave modes are compared with those generated experimentally in a thin plate. Additionally, the measured wave modes are analyzed with the help of dispersion curves and a timefrequency analysis. Our numerical work is also extended to analyze the interaction of Lamb wave modes with defects. The simulations compare favorably with the measurements presented here. © 2010 Elsevier Ltd. All rights reserved.

NDT and E International

A hybrid finite element model for simulation of electromagnetic acoustic transducer (EMAT) based plate waves

Corrosion detection in the annular plate region of above- ground storage tanks is a critical needfor tank farm operators in the oil and gas industry. The critical zone between the tank shell wall and the first few inches of the annular plate is difficult to inspect with conventional floor scanning methods due to the presence of coatings, uneven surfaces and a lack of access due to the presence of the weld toe. This region, however, is prone to accelerated corrosion due to the additional stresses caused by the weight of the tank wall and the increased possibility of water entrapment under the annular plate. Repairs in this region demand replacement of the entire annular plate and this leads to long shutdown of the tank, and often failures occur without any warning. A new method using the Higher Order Modes Cluster Guided Waves (HOMC*) to detect and screen defects in the critical inaccessible zones in the annular plate is discussed. This testing can be carried out online while the tank is in operation using HOMC guided waves generated on the outer exposed region of the annular plate. Robotic scanners allow for the inspection to be carried out in relatively short periods of time. Data collected in the laboratory as well as from the field trials are presented. The HOMC technique has potential for non-destructive inspection of inaccessible regions in structures for medium-range applications (2 to 3 m).

Insight: Non-Destructive Testing and Condition Monitoring

Higher order modes cluster (HOMC) guided waves for online defect detection in annular plate region of above-ground storage tanks

A higher order cylindrically guided ultrasonic wave was used for the detection and sizing of hidden pitting-type corrosion in the hidden crevice regions (between the pipe and the pipe supports) without lifting or disturbing the structural layout arrangement of the pipelines. The higher order circumferential guided waves were generated using a piezoelectric crystal based transducer, located at the accessible top region of the pipes, in a pulse-echo mode. By studying the experimental parameters such as dispersion, particle displacement, and wavelength of the ultrasonic guided wave modes, an appropriate higher order mode was selected for excitation using an appropriately designed acrylic angle wedge that conforms to the pipe's outer curvature. A manual pipe crawler was designed with a provision for holding the wedge, and the essential hardware such as data acquisition card, encoder, etc., was integrated with the system so that the corrosion was mapped in real time during the scanning of the pipes. The system was validated on pipes ranging from 6 in. to 24 in. outer diameters of wall thicknesses up to 12 mm, by mapping defects as small as 1.5 mm diameter and 25% penetration wall thickness. A 2D finite element model using ABAQUS® was used to understand the wave propagation in pipe wall and its interaction with pinhole-type defects. Copyright © 2008 by ASME.

Ultrasonic circumferential guided wave for pitting-type corrosion imaging at inaccessible pipe-support locations

This paper reports the use of dispersive Higher Order Modes Guided Waves (HOM-GW) and non-dispersive Higher Order Modes Cluster Guided Waves (HOMC-GW) for the detection and sizing of corrosion defects in pipes and tank floors. Pin-hole and wall thinning defects of different sizes were artificially generated on the pipe and the tank floor specimens in order to simulate corrosion pits under pipe supports and tank floor annular plate regions respectively. Since, the nature of loss of material differs in each type of corrosion; the same mode excited cannot be effective in characterizing all the defect types, although they all may indicate the presence of defect. The effective modes are identified for each type of corrosion and their defect characterizing sensitivity was studied. The validity of this technique was tested with field samples. © 2008 American Institute of Physics.

Journal	IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control
ISSN	08853010
Open Access	No