R. Dhayalan

Bio: R. Dhayalan is an academic researcher from Indira Gandhi Centre for Atomic Research. The author has contributed to research in topics: Ultrasonic sensor & Electromagnetic acoustic transducer. The author has an hindex of 6, co-authored 14 publications receiving 160 citations. Previous affiliations of R. Dhayalan include Indian Institute of Technology Madras.

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

Abstract: 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 time–frequency 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.

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

Abstract: 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.

An EMAT-based shear horizontal (SH) wave technique for adhesive bond inspection

Abstract: The evaluation of adhesively bonded structures has been a challenge over the several decades that these structures have been used. Applications within the aerospace industry often call for particularly high performance adhesive bonds. Several techniques have been proposed for the detection of disbonds and cohesive weakness but a reliable NDE method for detecting interfacial weakness (also sometimes called a kissing bond) has been elusive. Different techniques, including ultrasonic, thermal imaging and shearographic methods, have been proposed; all have had some degree of success. In particular, ultrasonic methods, including those based upon shear and guided waves, have been explored for the assessment of interfacial bond quality. Since 3-D guided shear horizontal (SH) waves in plates have predominantly shear displacement at the plate surfaces, we conjectured that SH guided waves should be influenced by interfacial conditions when they propagate between adhesively bonded plates of comparable thickness. This paper describes a new technique based on SH guided waves that propagate within and through a lap joint. Through mechanisms we have yet to fully understand, the propagation of an SH wave through a lap joint gives rise to a reverberation signal that is due to one or more reflections of an SH guided wave mode within that lap joint. Based upon a combination of numerical simulations and measurements, this method shows promise for detecting and classifying interfacial bonds. It is also apparent from our measurements that the SH wave modes can discriminate between adhesive and cohesive bond weakness in both Aluminum-Epoxy-Aluminum and Composite-Epoxy-Composite lap joints. All measurements reported here used periodic permanent magnet (PPM) Electro-Magnetic Acoustic Transducers (EMATs) to generate either or both of the two lowest order SH modes in the plates that comprise the lap joint. This exact configuration has been simulated using finite element (FE) models to describe the SH mode generation, propagation and reception. Of particular interest is that one SH guided wave mode (probably SH0) reverberates within the lap joint. Moreover, in both simulations and measurements, features of this so-called reverberation signal appear to be related to interfacial weakness between the plate (substrate) and the epoxy bond. The results of a hybrid numerical (FE) approach based on using COMSOL to calculate the driving forces within an elastic solid and ABAQUS to propagate the resulting elastic disturbances (waves) within the plates and lap joint are compared with measurements of SH wave generation and reception in lap joint specimens having different interfacial and cohesive bonding conditions.

Numerical and experimental analysis of unidirectional meander-line coil electromagnetic acoustic transducers

Abstract: The elastic waves generated by traditional meander- line coil electromagnetic acoustic transducers (EMATs) propagate in two directions, overlapping the echo signals from defects with the same distances, and the defect echo signal is hard to distinguish from the edge-reflected signal when the EMATs are near the edge of a specimen. In this paper, a unidirectional EMAT with two meander-line coils is proposed. A finite element model is used to simulate the directivity of the Rayleigh and shear vertical waves generated by these EMATs. Six transducers are fabricated using the printed circuit technique. The unidirectional Rayleigh wave and shear vertical wave are tested, and the results agree well with the simulation.

Optimization of the Bias Magnetic Field of Shear Wave EMATs

Abstract: The main advantage of electromagnetic acoustic transducers (EMATs) over piezoelectric transducers is that no direct contact with the specimen under test is required. Therefore, EMATs can be used to test through coating layers. However, they produce weaker signals, and hence, their design has to be optimized. This paper focuses on the design of a Lorentz force shear wave EMAT and its application in thickness gaging; special emphasis is placed on the optimization of the design elements that correspond to the bias magnetic field of the EMAT. A configuration that consists of several magnets axisymmetrically arranged around a ferromagnetic core with like poles facing the core was found to give the best results. By using this configuration, magnetic flux densities in excess of 3 T were obtained in the surface of a specimen; the maximum value achieved by a single magnet under similar conditions is roughly 1.2 T. If the diameter of an EMAT ultrasonic aperture is 10 mm, the proposed configuration produces signals roughly 20 dB greater than a single magnet, while for a given overall EMAT volume, signals were greater than 3–6 dB. Linear and radial shear wave polarizations were also compared; a higher mode purity and signal intensity were obtained with the linear polarization.