Metallic plates are widely used in a variety of industrial applications, from which conductivity is a key parameter related to the quality of metals. In order to accurately measure the conductivity, this article proposes a novel eddy current method based on sweep-frequency strategy. First, a simplified theoretical model for conductivity measurement is derived from the classical Dodd and Deeds analytical solution. Based on the analytical solution, the real and imaginary parts of the mutual inductance signal have an intersection in the frequency domain. According to mathematical derivation, the corresponding frequency (crossover frequency) of the intersection is inversely proportional to the conductivity of the sample. Moreover, the liftoff compensation method is proposed to reduce the effect of liftoff variation. Experiments have been carried out for samples with different conductivities, ranging from 0.5 to 58 MS/m, to verify the proposed method. The results show that the average error of the conductivity estimation with the proposed approach can be controlled within 1.1%, and the liftoff compensation method can significantly reduce the conductivity estimation error.

Eddy current (EC) sensors have been applied to detect defects for many years. Since both defects and the lift-off variation will lead to measurement signal fluctuation, it is necessary to distinguish the defect response and lift-off variation response. In this work, a novel EC method is proposed for defect detection immune to lift-off via a triple-coil structure. Firstly, a simplified theoretical model is built to investigate the relationship between the lift-off and the inductance variation. It is found the lift-off is proportional to the logarithm of inductance variation in real part under high frequency. In terms of sensor design, the triple-coil sensor consists of two pairs of transmitting–receiving coils, from which one of the receiving coils is larger than the other one. With such sensor structure, the small-size coil aims to detect the defect, while the large-size coil aims to calibrate lift-off. The proposed method is verified by experiments with the sensor scanning over a Titanium specimen with defects of different sizes. The results indicate the proposed method can achieve efficient and precise defect detection immune to lift-off.

Titanium and stainless steel are widely used in industries due to their advantages of high strength and good corrosion resistance. Such materials have common electromagnetic features – with a relatively low conductivity ～10⁵ − 10⁶ S/m and a relative permeability of 1. In terms of structure health monitoring, conductivity is an important physical parameter of metal materials, which is closely related to its purity, heat treatment state, internal stress state, hardness and temperature. Therefore, accurate and efficient conductivity estimation of such metals is extremely important to ensure product quality and normal operation of equipment. This paper presents an efficient conductivity estimation method of metals based on eddy current testing. The sensors used are with two coaxially cylindrical coils acting as transmitting coil and receiving coil respectively. According to the characteristics of low-conductivity and sensor configuration, a simplified theoretical model derived from the classic Dodd–Deeds theory is proposed. Moreover, the finite element method (FEM) simulation and experiments are carried out to verify the proposed model. The experiments are carried out on stainless steel 316 and different Titanium alloys with a commercial impedance analyzer, and the results indicate that the average relative measurement error is less than 2.8%.

The thickness and conductivity of metal materials are important parameters because they can determine the purity, hardness, and heat treatment state of the metal. In order to better monitor the state of the metal, this paper proposes a method to simultaneously measure the thickness and conductivity of the metal sample based on a single-frequency eddy current testing scheme. First of all, it is found that the phase of the coil mutual impedance is approximately proportional to the thickness of the sample based on the Dodd and Deeds analytical solution when the conductivity is relatively low. However, as the conductivity of the sample increases, the linear fitting cannot be strictly satisfied, which means a quadratic polynomial fitting is more suitable to illustrate the relationship between the thickness and measured signal. In fact, the fitted coefficients of quadratic polynomial are approximately linearly related to conductivity, and hence a second fitting can be applied to the quadratic polynomial fitting. The correlation coefficients of the above two fitting can reach 0.998. Based on the above analysis, the relationship between the sensor phase, the thickness and conductivity of the sample can be obtained. The sensor structure consists of one excitation coil and two receiving coils(1T-2R sensor) are employed. According to the phase information of two pairs of coils, two equations related to thickness and conductivity can be obtained. Therefore, thickness and conductivity of the sample can be deduced by solving these two equations. Finally, the proposed method is verified by finite element method(FEM) numerical solutions and experiments. The mean relative error of thickness and conductivity are less than 5.4% and 5.2% for titanium alloy and stainless steel. The experimental results show the method is capable of on-line monitoring the conductivity and thickness simultaneously. More importantly, the method only needs the mode of single frequency excitation and has low requirements for the design of hardware circuit.

Conductivity is an essential parameter for solid metal rods, which can be applied to assess quality. In this paper, an eddy current testing(ECT) method is investigate to measure the conductivity of solid metal rods. Above all, the simplified theoretical model is derived for a coil encircling a cylindrical sample. According to the proposed model, the real part of the coil impedance variation at high frequency (~ MHz) is closely related with the reciprocal of root of the rod conductivity. Therefore, a simple and effective conductivity measurement method for solid rod conductivity measurement is investigated based on the relationship between the impedance variation and the solid rod conductivity. Moreover, the effect of metal rods and coils parameters on the proposed method are also analyzed. Finally, the proposed method is verified with measurements of conductive materials. The results demonstrate the investigated simplified theoretical model is accurate and reasonable, and the relative error of the solid rod conductivity measurement is 2.2%.

In this paper, we propose a method using T(0,1) guided waves combined with coil coding technique to detect defects in buried liquid-filled pipes implemented by an electromagnetic acoustic transducer (EMAT). Due to its non-dispersive properties and the fact that there is no energy loss in non-viscoelastic fluids, the T(0,1) mode is selected for pipe defects detection. The electromagnetic device that generates the circumferential magnetic field is optimized to excite the pure T(0,1) mode. To realize energy enhancement and defect location identification, the electromagnetic acoustic coil is spatially encoded by 11-bit Barker code and the receiver coil is multiplexed consisting of a spatial coded coil and a unit coil. The defect detection is accomplished through time-of-flight (TOF) time-frequency analysis, and the defect location identification is achieved by digital signal processing methods (cross-correlation and convolution). The feasibility of this method is verified by the finite element model and experimental analysis, indicating the defect locating error in a liquid-filled pipes is less than 1%. Overall, the proposed method achieves a high-precision flaw detection and location identification.