| Popis: |
A field-circuit coupled finite element (FE) model of an angled vertical shear vertical wave (SV wave) electromagnetic acoustic transducer (EMAT) was established to investigate influencing factors of angled SV wave EMAT in high-temperature aluminum alloy detection. The FE model also considered equivalent circuits of the EMAT excitation and reception. Based on the FE model, we investigated the effect of temperature on the excitation current from the EMAT excitation equivalent circuit, EMAT generation efficiency, ultrasonic propagation and acoustic radiation field, EMAT reception efficiency, and output gain of the EMAT reception equivalent circuit. Subsequently, the influence of temperature on the detected ultrasonic wave signal was analyzed with a constant frequency method (CFM) of 2 MHz and variable frequency method (VFM) and verified by the experimental measurement with the aluminum alloy with temperatures ranging from 20°C to 440°C. The results show that for non-ferromagnetic metal materials such as aluminum alloy, the main reason for the declining ultrasonic amplitude at high temperatures is that the ultrasonic attenuation coefficient increases with the increasing temperature, followed by the power changing characteristics of the EMAT excitation and reception circuit with the rising temperature. When the temperature rises, the equivalent resistance of the excitation coil increases, the current of the equivalent excitation circuit and the input voltage of the preamplifier from the reception equivalent circuit decline, and the output gain decreases. With the increase in temperature, the experimental amplitude of the angled SV wave shows a downward trend. When the temperature is 440 ℃, the amplitude of the angled SV wave with VFM is 42.85% higher than that with CFM, and the SNR is enhanced by 4.9 dB. The application of VFM to detect high-temperature aluminum alloy can significantly increase the ultrasonic signal amplitude and improve the SNR of ultrasonic waves. |
