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European Journal of Applied Sciences – Vol. 13, No. 1

Publication Date: February 25, 2025

DOI:10.14738/aivp.131.18366.

Murayama, R. (2025). Examination of Pole Structure for Static Magnetic Field Constituting Electromagnetic Acoustic Transducer

(EMAT) for Lamb Wave Using Electromagnetic Induction Coil with Ferromagnetic Core. European Journal of Applied Sciences, Vol

- 13(1). 453-464.

Services for Science and Education – United Kingdom

Examination of Pole Structure for Static Magnetic Field

Constituting Electromagnetic Acoustic Transducer (EMAT)

for Lamb Wave Using Electromagnetic Induction Coil with

Ferromagnetic Core

Riichi Murayama

Fukuoka Institute of Technology, Japan;

3-30-1 Wazirohigashi, Higashi, Fukuoka, 811-0295, Japan

ABSTRACT

To increase the mechanical and thermal strengths of the EMAT for Lamb waves, the

air-core electromagnetic induction coil that constitutes the EMAT was changed to a

structure in which a conductor is wound around a ferromagnetic material. Initially,

the coils for the static and dynamic magnetic fields were vertically wound around

low-carbon steel, a ferromagnetic material. As a result, a transmission signal was

detected. To further improve the received signal strength, a structure separating

the poles for the static and dynamic magnetic fields was evaluated. It was then

confirmed that an improvement in the received signal strength was obtained.

Conventionally, the magnetic poles for a static magnetic field and dynamic magnetic

field were alternately arranged. In the present paper, the structure was changed to

a two-pole structure for the static magnetic poles, and all the dynamic magnetic

poles were sandwiched between the two poles. As a result, a dramatic improvement

in the sensitivity was achieved.

Keywords: Lamb wave, EMAT, Nondestructive inspection, EM-coil, Ferromagnetic core.

INTRODUCTION

Ultrasonic waves are widely used as the most effective means in the nondestructive testing of

materials and structures. Lamb waves, a type of ultrasonic waves, are used as an effective

means for the large-scale inspection of cold-rolled steel plates, hot-rolled steel plates, steel

pipes, etc. [1-3]. An ultrasonic sensor for the Lamb wave used for the on-line inspection of cold- rolled and hot-rolled steel sheets is tire-shaped in appearance. This ultrasonic probe enables

the on-line full-length inspection by rotating the tire-shaped case in the rolling direction.

Longitudinal ultrasonic waves transmitted from a piezoelectric transducer mounted inside the

tire are transmitted at an angle to the steel plate, and Lamb waves are transmitted and received

inside the steel plate by mode conversion. It is also necessary to continuously apply oil or other

contact mediums to the contact points of the steel plate surface with the tire-shaped probe.

However, unevenness in the coating of the contact medium occurs, and the detection sensitivity

fluctuates dramatically [4]. In recent years, various kinds of inspections of pipes and outer walls

of structures have been investigated by taking advantage of the characteristics of the Lamb

waves. For example, it is known that the temperature of heat exchangers used in power plants

rises to about 500°C [5-8]. However, general piezoelectric transducer ultrasonic probes are

designed to be used at room temperature and cannot be used at temperatures above 50°C for

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Services for Science and Education – United Kingdom 454

European Journal of Applied Sciences (EJAS) Vol. 13, Issue 1, February-2025

long periods of time. Electromagnetic acoustic transducers (EMATs) were originally proposed

as ultrasonic transducers that do not require a contact medium and can be applied to structures

and materials that are essentially hot. However, several issues became apparent during the

development process. One of the problems is that the separation distance between the EMAT

and the specimen surface is extremely small, only a few millimeters. As a result, thermal

damage when applied to hot objects and mechanical damage due to collisions when applied to

moving objects became major problems. In other words, the electromagnetic induction coil that

constitutes the EMAT is structurally vulnerable to thermal and mechanical damages, which

prevents the EMAT from being applied to moving objects and high-temperature structures [9-

10]. There are some cases of EAMTs that were specifically developed for use in high- temperature structures [11], but they require cooling facilities and are not easy-to-use

ultrasonic sensors.

The purpose of this study is to develop an EMAT that is resistant to thermal and mechanical

damages by changing the structure of the electromagnetic induction coil to one in which the

conductors are wound around a ferromagnetic core [12]. The resulting improved EMAT should

be measurable while in contact with hot objects. First, we decided to examine a Lamb wave

EMAT for thin plate inspection. As a first step, we examined a type of EMAT that is based on the

principle that bias magnetic field generation is also simultaneously performed using magnetic

poles for AC magnetic field generation [13]. The results of the evaluation test showed that

EMAT was noticeably less sensitive in this structure. Therefore, a structure was made to

separate the magnetic poles for DC and AC magnetization; the magnetic pole for the AC

magnetization was made of the PC Perm-alloy. As a result, the sensitivity was dramatically

improved [14]. However, the magnetic permeability of the Perm-alloy rapidly drops at

temperatures above 100°C, making it difficult to apply it to high-temperature structures, which

is one of the features of the EMAT. Therefore, in this paper, we reexamined the structure of the

magnetic pole material as a steel whose properties can be maintained up to nearly 800°C. The

results are now reported.

BASIC STRUCTURE OF CONVENTIONAL EMAT FOR LAMB WAVES [9,10]

Figure 1 shows the vibration pattern of a Lamb wave, which propagates along the thin plate

surface in the right direction on the paper surface, while the vibration pattern oscillates in the

vertical direction on the front and the horizontal direction of the thin plate at regular intervals.

Fig.1: Vibration pattern of Lamb wave

Lamb wave

Traveling direction

Thin plate

2/λ

d

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455

Murayama, R. (2025). Examination of Pole Structure for Static Magnetic Field Constituting Electromagnetic Acoustic Transducer (EMAT) for Lamb

Wave Using Electromagnetic Induction Coil with Ferromagnetic Core. European Journal of Applied Sciences, Vol - 13(1). 453-464.

URL: http://dx.doi.org/10.14738/aivp.131.18366

The ratio of the vertical to horizontal components of the Lamb wave calculate using equation

(1). Using the values described in the later section on experimental conditions, a plate thickness

of 0.8 mm and a drive frequency of 200 kHz, the result is approximately 0.12. The horizontal

displacement component is overwhelmingly dominant. This means that the Lamb wave EMAT

under these conditions should provide vibration displacement in the horizontal traveling

direction.

η

ξ

=

P

2−1

2P

∙ tan(kpd) (1)

η: Vertical vibration component, ξ: Horizontal vibration component

k: Wavenumber (2π/λ), d: Plate thickness, Vt: Velocity of transverse wave, Vp: Phase velocity of

Lamb wave

P= ((Vp/ Vt)

2-1)1/2

Two types of EMATs for Lamb waves exist: the first type uses the Lorentz force, which is the

most well-known source of driving EMATs. The other type uses the magnetostrictive effect. The

former is applicable to any metallic material, while the latter is applicable only to ferromagnetic

materials that can generate magnetostriction. Figure 2(a) shows a type that uses Lorentz force

and consists of a comb-shaped electromagnetic induction coil that reverses the direction of the

current at regular intervals and a magnet that provides a magnetic field in the vertical direction

of the thin specimen. The Lorentz force is generated in the vertical direction of the electrodes

of the comb-shaped induction coils by the interaction between the induced current and the

static magnetic field generated on the surface of the metal specimen directly under the comb- shaped induction coils. Since the direction of the current flowing between adjacent comb- shaped electromagnetic induction coil electrodes is reversed, the direction of the Lorentz force

is also 180 degrees in the opposite direction. If the distance between these electrodes matches

half of the Lamb wave wavelength to be generated, then the Lamb wave is the driving source.

Figure 2(b) shows a type that uses the magnetostrictive effect. Magnets are placed on both sides

of the comb electromagnetic induction coil to achieve a horizontal magnetic field on the comb

electromagnetic induction coil. In this case, an induced magnetic field is generated on the

surface of the metal material in a direction parallel to the metal surface and perpendicular to

the electrodes due to the current flowing through the electrodes of the comb-type

electromagnetic induction coil. Magnetostriction is generated in the direction parallel to this

induced magnetic field. The direction of the magnetostriction between adjacent electrodes is

reversed 180 degrees, and when the distance between these adjacent magnetostrictions is half

the Lamb wave wavelength, the Lamb wave is generated. In the case of the magnetostriction

effect type (Figure 1(b)), Lorentz force in the vertical direction is generated in principle, but it

is considered to be smaller than the magnetostriction effect. The reason is that in the case of

ordinary steel, when a static magnetic field of about 100mT, which is the most likely to generate

magnetostriction, is applied, the magnitude of magnetostriction due to the dynamic magnetic

field is expected to be about 5nm. In contrast, the Lorentz force is the product of the magnetic

flux density, current, and electrode length, as in equation (2). Assuming that the length of the

electrode is 10 mm used in this experiment and that the magnitude of 1(A) directly energizing

the comb-type electromagnetic induction coil flows directly to the opposing metal surface, the

calculated Lorentz force is about 0.001(N). If the longitudinal modulus of elasticity of ordinary