A study of the transduction mechanisms of electromagnetic acoustic transducers (EMATs) on pipe steel materials

doi:10.1016/j.sna.2015.03.034

Sensors and Actuators A: Physical

Volume 229, 15 June 2015, Pages 154-165

https://doi.org/10.1016/j.sna.2015.03.034 Get rights and content

Highlights

•
The transduction mechanisms in operation when MS and NB EMATs are used on steels are the MF, SLF and DLF.
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For NB-EMATs, the conductivity of the material is proportional to the critical excitation current.
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The research established the minimum excitation current required to produce acoustic wave by DLF.
•
NB-EMATs require less energy to attain the critical excitation current than the MS-EMATs.
•
A novel algorithm was developed to decouple and quantify the transduction mechanisms inherent in EMATs.

Abstract

This research paper presents in detail a novel modeling strategy to decouple and quantify the various transduction forces in operation when EMAT (Normally Biased and magnetostrictive configurations) are used on various grades of pipe steel materials. The strategy established the value of the critical excitation current (CEC) of the EMAT configurations studied. The CEC, when magnetostrictive EMAT configurations (MS-EMAT) are used to generate acoustic waves, was found to be: 260A, 268A, 270A, 274A, 279A, 280A, 286A, 289A, 294A and 305A for EN16, CS70, EN24, L80SS, L80A, TN80Cr3, EN8, EN36, EN3 and J55, respectively, while for Normally Biased EMAT configurations (NB EMAT), the critical excitation currents was found to be 181A, 190A, 197A, 205A, 240A, 122A, 203A, 160A, 231A and 200A for EN16, CS70, EN24, L80SS, L80A, TN80Cr3, EN8, EN36, EN3 and J55 respectively. This implies that beyond the CEC, where the dynamic Lorentz force mechanism dominates, it is possible to develop a coil only EMAT, thereby eliminating the undesirable effect of using a permanent magnet or electromagnet and also reducing the overall size of the EMAT. Furthermore, the research gave an insight into the relationship between the CEC and the electrical and magnetic properties of the steel materials used in this study and also compared the CEC for both the NB and MS-EMAT configurations. A conclusion drawn is that NB-EMATs are more efficient in the generation of acoustic waves as they have a lower CEC than their MS EMAT counterpart.

Introduction

An electromagnetic acoustic transducer is a noncontact ultrasonic transducer used for the non-destructive testing of electrically conducting materials. EMATs are grouped, based on the orientation of their bias magnetic field, into: Normally Biased EMATs (NB-EMATs) and Magnetostrictive EMATs (MS-EMATs). An NB-EMAT is an EMAT configuration where the bias field is normal to the surface of the test material as seen in Fig. 1a, while an MS-EMAT is an EMAT configuration in which the bias field is parallel to the surface of the test material as shown in Fig. 1b. There exist other types of ultrasonic transducers. Prominent amongst them are conventional piezoelectric transducers (PZT). Some of the main advantages of EMATS over PZT include the fact that no couplant and surface preparation is required during measurement; thus, the cost of surface preparation during measurement and irregularities that arise from the use of a couplant are eliminated. An EMAT, as a transducer, has some inherent deficiencies that affect and limit its operation on ferromagnetic materials. These deficiencies are its relatively low signal to noise ratio when compared with the conventional PZT, the size of the transducer, and the undesirable effect of using a permanent or magnet [1], [2], [3], [4].

For improved performance and better understanding of EMAT operation, researchers have developed several numerical models [5], [6], [7], [8]. These models cannot account for all of the transduction mechanisms that take place when EMATs are employed on ferromagnetic material. These researchers have merely created a simple model that can calculate the magnetostrictive mechanism and Lorentz mechanism, implying that the effects of static and dynamic fields are lumped together as the Lorentz mechanism. For a better understanding of the operation of EMATs on ferromagnetic material it is important to decouple and quantify the transduction forces inherent in EMAT.

Accounting for the three major transduction mechanisms operating in EMATs has been a challenge for researchers for years; Thompson [9], [10], in his work, separated analytically the magnetostrictive force (MF) and Lorentz force (LF) in an MS-EMAT operating on ferromagnetic material. He concluded that the magnetostriction is the dominant transduction mechanism. Ribichini [11] further evaluated, both numerically and experimentally, the Magnetostrictive and Lorentz force density in an NB-EMAT operating on various grades of structural steel and concluded that the Lorentz force mechanism is the dominant transduction mechanism on ferromagnetic materials. Ludwig [12], [13] pioneered research that established theoretically the three main transduction mechanisms that an EMAT exploits when operating on a ferromagnetic material. He achieved this fit by applying the momentum law to describe the EMAT operation. More recent research by Wang et al. [7], on an NB-EMAT operating on an aluminum specimen, separated and numerically compared the displacement due to the dynamic and static Lorentz force at various excitation currents.

This research will develop a novel modeling strategy to decouple, quantify and numerically compare the three main transduction forces (Dynamic Lorentz force (DLF), Static Lorentz force (SLF) and Magnetostrictive force (MF)), operating simultaneously when MS and NB EMATS are used on pipe steel materials. This will lead to the establishment of the critical excitation current (CEC); that is the minimum excitation current required by each EMAT configuration to generate an acoustic wave more efficiently using the dynamic Lorentz force mechanism. The research will further investigate the relationship between the CEC and the magnetic and electrical properties of the steel materials. The composition, magnetic and electrical properties of the steel materials used in this study are shown in Table 1.

Section snippets

Physical principles of modeling

Faraday's and Ampere's equations, which forms part of Maxwell's equation within the quasistatic field limit is of the form [12]: $\nabla \times E = μ_{0} \frac{\partial H}{\partial t} - J_{m}$ $\nabla \times H = J_{f}$ $\nabla \cdot H = - \nabla \cdot M$ where the magnetic source density J_m can be expressed in terms of magnetization and mechanical particle velocity as; $J_{m} = μ_{0} \frac{\partial M}{\partial t} + μ_{0} \nabla \times (M \times \frac{\partial u}{\partial t})$ where μ₀ and u are absolute permeability and particle displacement respectively.

The total free conducting current density J_f is given as: $J_{f} = σ E+ μ_{0} σ \frac{\partial u}{\partial t} \times (H + M) + J_{s}$

with σ and J_s denoting material conductivity

Finite element modeling

A numerical modeling technique using finite element commercial software (Comsol multi-physics) is proposed. The technique involves creating a Whole EMAT model (WEM) that accounts for the DLF and SLF, a magnetostrictive model (MEM) that accounts for the MF, DLF and SLF, and a dynamic model that accounts for only the DLF.

The model solves simultaneously the electrodynamic problems that account for the Eddy current induction and elastic phenomenon that give rise to wave generation. The

Distribution of the transduction forces in NB-EMAT

A typical plot of the particle displacement due to the transduction mechanisms versus variation of excitation current from 20–400 A for five grades of commonly used pipe steel is shown in Fig. 8a–e. In the entire steel sample studied, prior to attaining the critical excitation current, the SLF appears to be dominant, followed by the DLF, while magnetostriction appears to be the least dominant. This trend continues up until a certain excitation current, and it was observed that the DLF equals and

Experimental validation of model

The experimental validation of the model was carried out with the setup shown in Fig. 13; the setup includes a Laser Doppler Vibrometer (LDV) and a Teletest Mark III pulser/receiver unit. The EMAT generate the acoustic wave, while the Laser Vibrometer detects the velocity of the sound wave. The amplitude of the signal detected by the LVD is proportional to the velocity of the generated signal. An NB-EMAT was employed in the experiment. It consisted of a spiral copper coil (with internal

Conclusion

This paper was able to decouple the transduction forces in operation when NB and MS EMATs respectively are used on pipe steel material. The transduction forces obtained and quantified were the MF, SLF and DLF. In all of the pipe steel materials investigated, the SLF was found to be dominant prior to attaining the critical excitation current (CEC), while the DLF was the dominant mechanism afterwards, as seen in Fig. 8, Fig. 9. Furthermore, the dominant transduction force in the MS-EMAT prior to

Acknowledgment

This work is supported by the Petroleum Technology Development Fund (PTDF) Nigeria, under the OSS/PhD program 2011.

Evans Ashigwuike received his B.Eng. (Hons) and M.Eng. degree from the department of Electrical/Electronic Engineering, Nnamdi Azikiwe University, Awka, Nigeria in 1999 and 2005, respectively. He worked as a lecturer in the department Electrical/Electronic Engineering at the University of Abuja Nigeria, between 2005 and 2011. He is currently a PhD student with the Centre for Electronic System Research (CESR) Brunel University, London, UK. Evans's main research interests are in the areas of

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View full text

A study of the transduction mechanisms of electromagnetic acoustic transducers (EMATs) on pipe steel materials

Highlights

Abstract

Introduction

Section snippets

Physical principles of modeling

Finite element modeling

Distribution of the transduction forces in NB-EMAT

Experimental validation of model

Conclusion

Acknowledgment

Mechatronics

NDT E Int.

EMATs for Science and Industry: Non-contacting Ultrasonic Measurements

Electromagnetic acoustic wave transducers, EMATS – their operation and mode patterns

Field dependence of coupling efficiency between electromagnetic field and ultrasonic bulk waves

J. Appl. Phys.

A model for the electromagnetic generation and detection of Rayleigh and Lamb waves

Sonics Ultrason. IEEE Trans.

Recent advancements in the application of EMATs to NDE

Improved finite element method for EMAT analysis and design

Magn. IEEE Trans.