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摘要
To solve the uncertainty problem of the effect of temperature on the amplitude of ultrasonic echoes from an Electromagnetic Acoustic Transducer (EMAT) testing with aluminum alloys, and to overcome the difficulty in compensation for defect sizing and location evaluation at an elevated temperature, a field-circuit coupling finite element model is built for the detection process of a spiral coil EMAT operated in the aluminum alloy. Based on this model, the effects of temperature on the excitation and detection efficiency of the EMAT, power allocation characteristics of the equivalent transmitting and receiving circuits for the EMAT, and the diffusion/medium attenuation, echo amplitude, and ultrasonic wave velocity are analyzed. Subsequently, a high-temperature EMAT probe is designed and applied to an aluminum block with temperature ranging from 25℃ to 500℃. The medium attenuation coefficient and velocity of ultrasonic waves at elevated temperature are also measured. Based on the simulation and experimental results, affecting factors and characteristics of ultrasonic echoes in high temperature testing are examined. Results indicate that, for those non-ferromagnetic metal materials such as aluminum alloys, the main reason leading to the decrease in the amplitude of ultrasonic echoes is attenuation of the ultrasonic medium which increases with temperature rise. The second reason is the variation of power allocation characteristics for the EMAT excitation and detection circuit at elevated temperature. When the Lorentz force on the surface of high temperature sample produced by the transmitting EMAT is constant, the amplitude of excited ultrasonic waves increases with temperature rise. The increasing amplitude of the shear wave can compensate for the downward trend of ultrasonic echo amplitude.
