Yunxin Wu

Comparison of three formulations for eddy-current and skin and proximity problems in a meander coil electromagnetic acoustic transducer

Three differential equations based on different definitions of current density are compared. Formulation I is based on an incomplete equation for total current density (TCD). Formulations II and III are based on incomplete and complete equations for source current density (SCD), respectively. Using the weak form of Finite Element Method (FEM), the three formulations were applied in a meander coil Electromagnetic Acoustic Transducer (EMAT) example to solve magnetic vector potential (MVP). The FEM results from frequency domain and time domain models are in excellent agreement with previously published works. Results show that the errors for Formulations I and II vary with coil dimensions, coil spacing, lift-off distance and external excitation frequency, for the existence of eddy-current and skin and proximity effects. And the current distribution across the coil conductors also follows the same trend. It is better to choose Formulation I instead of Formulation III to solve MVP when the coil height or width are less than twice the skin depth, due to the low cost and high efficiency of Formulation I.

Comparisons of Different Models on Dynamic Recrystallization of Plate during Asymmetrical Shear Rolling

Asymmetrical shear rolling with velocity asymmetry and geometry asymmetry is beneficial to enlarge deformation and refine grain size at the center of the thick plate compared to conventional symmetrical rolling. Dynamic recrystallization (DRX) plays a vital role in grain refinement during hot deformation. Finite element models (FEM) coupled with microstructure evolution models and cellular automata models (CA) are established to study the microstructure evolution of plate during asymmetrical shear rolling. The results show that a larger DRX fraction and a smaller average grain size can be obtained at the lower layer of the plate. The DRX fraction at the lower part increases with the ascending speed ratio, while that at upper part decreases. With the increase of the offset distance, the DRX fraction slightly decreases for the whole thickness of the plate. The differences in the DRX fraction and average grain size between the upper and lower surfaces increase with the ascending speed ratio; however, it varies little with the change of the speed ratio. Experiments are conducted and the CA models have a higher accuracy than FEM models as the grain morphology, DRX nuclei, and grain growth are taken into consideration in CA models, which are more similar to the actual DRX process during hot deformation.

Optimal design of spiral coil electromagnetic acoustic transducers considering lift-off sensitivity operating on non-ferromagnetic media

Abstract The generation efficiency of electromagnetic acoustic transducers (EMATs) dramatically reduces with increasing lift-off, thus restricting their applications. This paper aims to provide methods to decrease lift-off sensitivity. The process of generating a shear wave in a spiral coil EMAT is established based on a 2-D axisymmetric model using the finite element method. An equivalent excitation circuit, including an impedance-matching network and a limited power source, is presented for calculating the excitation current. Such an equivalent circuit has not been given adequate attention in previous publications in this field, but it has proven to be of great importance in determining the efficiency of ultrasonic generation. The predictions of static magnetic field, eddy current density and excitation current are validated by experimental results. The conversion efficiency and lift-off sensitivity of the EMAT are analysed with respect to impedance matching parameters, magnet dimensions, the presence of a copper backplate between the magnet and the coil, and backplate-to-coil distance. The results indicate that impedance matching parameters, magnet diameter and plate-to-coil distance have obvious effects on lift-off sensitivity. The backplate can be used to decrease lift-off sensitivity, and it can rearrange the source current density across the cross-sectional area of the coil conductor. Furthermore, compared with the original EMAT, the lift-off sensitivity of the optimised EMAT can be decreased by 7.9 dB at 2 mm lift-off.

A modified model for simulating the effect of temperature on ultrasonic attenuation in 7050 aluminum alloy

Knowing propagating properties of an ultrasonic wave can enhance the non-destructive testing techniques in alloy materials field, such as the electromagnetic acoustic transducer techniques, and the piezoelectric ultrasonic transducer techniques. When temperature is taken into consideration, the ultrasonic propagating attenuation become very complex process. In this paper, a loss factor coefficient function with change in temperatures is established and the loss factor damping model with temperature term is coupled into the equations of elastic wave motion. A modified frequency domain model for calculating the ultrasonic attenuation due to temperature changes in 7050 Aluminum alloy is then developed. The model is validated experimentally using a high power pulse transmitter/receiver RPR-4000, a resistant high temperature electromagnetic acoustic transducer set-up and a 7050 Aluminum alloy sample. The simulation and the experimental results are determined to be in good agreement. The numerical model is used to calculate the ultrasonic-waves field, the ultrasonic attenuation, and the ultrasonic propagation directivity considering the temperature effect. The modeling results indicate that the ultrasonic energy attenuation is significantly affected by temperature. When the temperature increases from 20°C up to 480°C, the ultrasonic energy attenuates by 32.31%. It is also found that the length of near acoustic field increases with the increase in temperature. There is a common basic mode for the attenuation of ultrasonic waves, in which the attenuated mode cannot be affected by other factors. Increasing the temperature or the frequency, the ultrasonic propagation can obtain an excellent directivity. Results obtained from the present model will provide a comprehensive understanding of design parameter effects and consequently improve the design/performance in the non-destructive testing techniques.Knowing propagating properties of an ultrasonic wave can enhance the non-destructive testing techniques in alloy materials field, such as the electromagnetic acoustic transducer techniques, and the piezoelectric ultrasonic transducer techniques. When temperature is taken into consideration, the ultrasonic propagating attenuation become very complex process. In this paper, a loss factor coefficient function with change in temperatures is established and the loss factor damping model with temperature term is coupled into the equations of elastic wave motion. A modified frequency domain model for calculating the ultrasonic attenuation due to temperature changes in 7050 Aluminum alloy is then developed. The model is validated experimentally using a high power pulse transmitter/receiver RPR-4000, a resistant high temperature electromagnetic acoustic transducer set-up and a 7050 Aluminum alloy sample. The simulation and the experimental results are determined to be in good agreement. The numerical model is used...