Directivity analysis of meander-line-coil EMATs with a wholly analytical method

Abstract

This paper presents the simulation and experimental study of the radiation pattern of a meander-line-coil EMAT. A wholly analytical method, which involves the coupling of two models: an analytical EM model and an analytical UT model, has been developed to build EMAT models and analyse the Rayleigh waves' beam directivity. For a specific sensor configuration, Lorentz forces are calculated using the EM analytical method, which is adapted from the classic Deeds and Dodd solution. The calculated Lorentz force density are imported to an analytical ultrasonic model as driven point sources, which produce the Rayleigh waves within a layered medium. The effect of the length of the meander-line-coil on the Rayleigh waves' beam directivity is analysed quantitatively and verified experimentally.

Phased Electromagnetic Acoustic Transducer Array for Rayleigh Wave Surface Defect Detection

“…where d meander is the coil turn spacing, m is an integer, and λ is the ultrasound wavelength, and can be used when designing a meander coil for a chosen frequency of operation. The behaviour of a meander EMAT has been studied by several authors [14], [26], [27]. The use of a single meander EMAT limits inspection to the chosen wavelength defined by equation 1, which can limit the sensitivity to defects.…”

An Improved Design of the Spiral-Coil EMAT for Enhancing the Signal Amplitude

“…Electromagnetic acoustic transducer (EMAT) is a type of non-contact ultrasonic transducer, which is the core component of ultrasonic excitation and acceptance in UT [1,2,3,4,5]. Due to a plenty of outstanding advantages such as non-contact operation, requiring no coupling fluid, the flexibility to generate and detect multiple wave modes, and the capability of operating in high temperature, isolation layer and other harsh environment, EMATs are particularly useful and active in nondestructive evaluation (NDE) and internal structure evaluation (ISE) [6,7,8,9,10,11,12]. Besides, EMATs have be distinctively attractive in thickness measurement for their good penetration ability.…”

“…Obviously, it can be obtained from Equation (1) that the square of the magnetic flux density is proportional to the SNR of EMATs with the mechanism of Lorentz force [7,8,9,10,11,12,13,14,15]. The SNR can be given as $\frac{V_{\mathrm{EMAT}}}{V_{\mathrm{noise}}}=\frac{\sqrt{P_0}{N}^2{B}^{2}A}{2Z_S\sqrt{KT\beta }{R}_{\mathrm{EMAT}}}\exp \left(-\frac{\alpha h}{D}\right)$ where $V_{\mathrm{EMAT}}$ is the received voltage across the terminals of the coil (V), $V_{\mathrm{noise}}$ is the received noise level (V), $P_0$ is the power of the EMAT (W), $N$ is the number of turns per unit length, $B$ is the magnetic flux density (T), $A$ is the active area of EMAT (${\mathrm{m}}^2$), $Z_S$ is the acoustic impedance of the part ($\mathsf{\Omega }$), either longitudinal or shear wave, $K$ is Boltzmann’s constant times electronic charge (Meter-Kilogram-Second units), $T$ is temperature in degrees Kelvin (K), $\beta$ is the bandwidth of the amplifier (Hz), $R_{\mathrm{EMAT}}$ is the resistance of EMAT coil ($\mathsf{\Omega }$), $\alpha$ is a geometry dependent constant, $h$ is the lift-off distance between EMAT coil and the surface of the part (m), and $D$ is the diameter of the coil (m).…”

“…Kang et al improved the signal amplitude of surface wave EMAT based on 3-D simulation analysis and orthogonal test method [9]. Xie et al proposed a wholly analytical modelling method to study the Rayleigh waves’ beam directivity on the surface of the material [10]. Seung et al presented a new EMAT for generation and measurement of omnidirectional shear-horizontal (SH) guided waves in metallic plates [11].…”

“…Seung et al presented a new EMAT for generation and measurement of omnidirectional shear-horizontal (SH) guided waves in metallic plates [11]. There have been previous studies of SH wave EMAT, but they mainly focused on the meander-line-coil (MLC) EMAT for surface wave generation and detection in the metallic plate or pipe [7,8,9,10]. …”