Electromagnetic Acoustic Transducer Array Research Articles

Phased Electromagnetic Acoustic Transducer Array for Rayleigh Wave Surface Defect Detection.

A phased electromagnetic acoustic transducer (EMAT) array system has been developed for the detection and characterization of surface-breaking defects. An array of four linear coils that are individually controlled is used to generate a Rayleigh wave. The high-current electronics combined with the coil designs enables the array to generate either narrowband or broadband signals, and controlling the phase delay between the channels makes it possible to change the ultrasound wavelength without requiring the physical separation of the coils to be changed. The experimental results show that the four-coil phased array can generate a wavelength range from 3.0 to 11.7 mm. Surface-breaking defects were characterized using a transmit-receive setup with a broadband EMAT detector being used to detect the Rayleigh wave. Machined surface slots with different depths were used for technique validation. The results show that the array is sensitive to surface defects and a wide depth sensitivity range for defect sizing can be easily achieved by applying phasing to tune the wavelength of operation. A large increase in detection flexibility is immediately shown.

Electromagnetic acoustic transducer optimisation for surface wave applications

A simplified model of surface wave generation from finite width sources is presented, and it is shown that the generated wave is given by the convolution of the spatial profile of the wave source with the temporal displacement profile of the driving function. The model is then used to analytically derive the optimum coil width for an electromagnetic acoustic transducer (EMAT) for the case of harmonic excitation. The analysis is extended to predict the optimal coil width for a pulsed coil excitation approximated by a Gaussian function, and it is shown that optimum generation is achieved for a coil width of a≈4.0δc, where δ is the width parameter of the Gaussian excitation and c is the Rayleigh wave speed. Finite element analysis and experimental data are then presented in support of this result. A phased Rayleigh wave EMAT array concept is demonstrated as a means to improve generated signal amplitudes beyond the limit for an optimised coil suggested by the analytical and finite element models.

Helical Mode Guided Waves in Pipe Observed by Electromagnetic Acoustic Transducer Array

Helical Mode Guided Waves in Pipe Observed by Electromagnetic Acoustic Transducer Array

Structural health monitoring using torsional guided wave electromagnetic acoustic transducers

Torsional guided wave inspection is widely used for pipeline inspection. Piezoelectric and magnetostrictive transducers are most commonly used to generate torsional guided waves. These types of transducers require bonding or mechanical contact to the pipe which can result in changes over time which are undesirable for structural health monitoring. This article presents a non-contact Lorentz force–based electromagnetic acoustic transducer for torsional guided wave monitoring of pipelines. First, the excitation mechanism of the transducer is simulated by analyzing the eddy current and the static magnetic field using the finite element method. An electromagnetic acoustic transducer transformer model is presented which describes the eddy current generation transfer function and the ultrasound excitation. Independently simulated eddy current and magnetic fields are used to calculate the Lorentz force that an electromagnetic acoustic transducer array induces on the surface of a 3-in schedule 40 pipe, and an explicit finite element solver is then used to simulate the elastic wave propagation in the pipe. Then, the reception mechanism and the expected received signal levels are discussed. The construction of an experimental transducer is described, and measurement results from the transducer setup are presented. The measured and modeled performance agree well. Finally, a monitoring example is presented where an artificial defect with 3% reflection coefficient is introduced and successfully detected with the designed sensor.

A wholly analytical method for the simulation of an electromagnetic acoustic transducer array

This paper presents a wholly analytical method which combines electromagnetic and ultrasonic simulations for the study of an Electromagnetic Acoustic Transducer Array (EMAT). Analytical solutions to the eddy current problem for the meander coil used in an EMAT are adapted from the classic Deeds and Dodd solution, which was originally intended for circular coils. The analytical solution resulting from this novel adaptation exploits the large radius extrapolation and shows several advantages over the finite element method (FEM), especially in the higher frequency regime. The calculated Lorentz force density from the EM solver is then coupled to the ultrasonic simulations, which exploit an analytical solution for generating the distribution of surface waves. Beam features are studied, which can produce performance parameters for an EMAT array, facilitating the optimum design of such sensors. This total analytical approach to an EMAT simulation has not been reported previously to our knowledge.

Simulation of an Electromagnetic Acoustic Transducer Array by Using Analytical Method and FDTD

Previously, we developed a method based on FEM and FDTD for the study of an Electromagnetic Acoustic Transducer Array (EMAT). This paper presents a new analytical solution to the eddy current problem for the meander coil used in an EMAT, which is adapted from the classic Deeds and Dodd solution originally intended for circular coils. The analytical solution resulting from this novel adaptation exploits the large radius extrapolation and shows several advantages over the finite element method (FEM), especially in the higher frequency regime. The calculated Lorentz force density from the analytical EM solver is then coupled to the ultrasonic simulations, which exploit the finite-difference time-domain (FDTD) method to describe the propagation of ultrasound waves, in particular for Rayleigh waves. Radiation pattern obtained with Hilbert transform on time-domain waveforms is proposed to characterise the sensor in terms of its beam directivity and field distribution along the steering angle, which can produce performance parameters for an EMAT array, facilitating the optimum design of such sensors.

Magnetostriction-Based Omni-Directional Guided Wave Transducer for High-Accuracy Tomography of Steel Plate Defects

An omni-directional (OD) magnetostriction-based electromagnetic acoustic transducer (EMAT) is suitable for use in the guided wave (GW) tomography of steel plate defects. Because the GW is characterized by multiple modes and frequency dispersion, the waveforms detected by the traditional magnetostriction-based OD GW EMATs are complex, which substantially reduces the accuracy of projection data extraction and then decreases the quality of GW tomography and the accuracy of defect quantification. This paper proposes a new magnetostriction-based OD EMAT to generate shear-horizontal (SH) GWs of a special mode to reduce the interference of multiple modes and the frequency dispersion of GWs. The new EMAT consists of a contra-flexure coil and a pre-magnetized thin circular nickel foil. The theoretical background and working principle of this EMAT are presented. The results of pitch–catch experiments on a healthy steel plate demonstrate that the new EMATs can transmit and receive purer single SH1-mode GWs and have improved the accuracy of projection data extraction, compared with the traditional EMATs. The performance of SH1 wave tomography of the traditional and new EMAT arrays are compared via experiments implemented on a 3-mm-thick steel plate with an artificial corrosion defect. The experimental results indicate that higher tomographic reconstruction quality, clearer normalized slowness curves with less noise and higher accuracy of tomographic defect quantification can be achieved by the new EMAT array, which has improved the projection data extraction accuracy, compared with the traditional EMAT array.

Mode control of guided wave in magnetic hollow cylinder using electromagnetic acoustic transducer array

The aim of this work is to demonstrate a method for exciting and receiving torsional and longitudinal mode guided waves with an electromagnetic acoustic transducer (EMAT) ring array. First of all, a three-dimensional guided wave simulator is developed in order to numerically analyze the propagation of the guided wave. The finite difference time domain method is used for the simulator. Second, two guided wave testing systems using an EMAT ring array are provided: one is for torsional mode (T-mode) guided wave and the other is for longitudinal mode (L-mode). The EMATs used in the both systems are the same in design. A method to generate and receive the T- and L-mode guided waves with the same EMAT is proposed. Finally, experimental and numerical results are compared and discussed. The results of experiments and simulation agree well, showing the potential of the EMAT ring array as a mode controllable guided wave transmitter and receiver.

A New Omni-Directional EMAT for Ultrasonic Lamb Wave Tomography Imaging of Metallic Plate Defects

This paper proposes a new omni-directional electromagnetic acoustic transducer (EMAT) for the ultrasonic Lamb wave (ULW) tomography imaging (TI) of defects in metallic plates. The proposed EMAT is composed of a permanent magnet and a coil with a contra-flexure structure. This new EMAT coil structure is used for omni-directional ULW transmission and reception and ULW TI for the first time. The theoretical background and the working principles of this EMAT are presented and analyzed. The experimental results of its use on a 3 mm thick aluminum plate indicate that the EMAT with a contra-flexure coil (CFC) can transmit and receive a pure single A0 mode ULW with a high signal-to-noise ratio (SNR). Thus, the extraction of the projection data used for ULW TI may be performed accurately. The circumferential consistency of the projection data is only slightly influenced by the distortion of the eddy current field that is induced by the new CFC with an irregular shape. When the new EMAT array is used for ULW TI using the cross-hole method and SIRT arithmetic, a desirable imaging quality can be achieved, and the estimated size of an artificial corrosion defect agreed well with its actual value. The relation between the reconstruction resolution and the number of the new EMATs used is analyzed. More TI experiments are carried out when the aluminum plate defect is in two different locations relative to the EMAT array, for the further investigation of the performances of the new EMATs.

Anisotropy measurements in metal alloys using a laser/electromagnetic acoustic transducer array system

Rayleigh waves have been used to measure anisotropy in metal alloys using a transient Rayleigh pulse and an eight-element electromagnetic acoustic transducer array. The array spacing determined sensor separation, so that velocity measurements were made independent of the source-to-detector distance. Elimination of this distance, which would normally lead to systematic errors, has resulted in a measurement system capable of measuring velocities in metal to an uncertainty of 0.1%. As an example, variations in the Rayleigh velocity have characterized the anisotropy in rolled bars of aluminum.

Omnidirectional guided wave inspection of large metallic plate structures using an EMAT array

The design of an electromagnetic acoustic transducer (EMAT) array device for the inspection of large areas of metallic plate-like structures using the S0 guided wave mode is described. The reasons for using the S0 mode are discussed and it is shown how the choice of mode determines the nature of the EMAT array elements. A novel array construction technique is shown to be necessary whereby the EMAT coils for adjacent elements are overlapped in order to achieve the required element density. Results are presented that illustrate the operation of the device on steel and aluminum plate specimens in the thickness range from 5 to 10 mm. An area of at least 10 m2 can be inspected from a single location. Spurious signals in the results are caused both by the unwanted A0 mode and by S0 sidelobes, the latter occurring at the same radial distance from the array as the genuine S0 signal from a reflector, but in the wrong direction. The signal-to-coherent noise performance of the complete system is determined by the amplitude ratio of the largest genuine S0 signal to the largest spurious signal. This is typically around 30 dB. The sensitivity of the device to artificial defects and genuine corrosion patches is demonstrated and the limitations of its operation are discussed. The feasibility of using the device with the S1 guided wave mode to inspect a 20 mm thick plate is also demonstrated.