ABSTRACT
In this paper we propose a novel ultrasonic longitudinal wave resonance method for measuring the thickness of metal walls using a laser-electromagnetic acoustic transducer (Laser-EMAT). The method is based on the surface constraint mechanism (SCM) of the material and is expected to be capable of accurately detecting local thinning of metal walls in a non-contact manner and at high temperatures. Based on finite element analysis of laser-EMAT ultrasonic resonance measurement of aluminum alloy thickness, we investigated the effects of such key factors as SCM, irradiation parameters of laser source, and the size of EMAT receiving coil on the accuracy of thickness measurement (resonance frequency position) and on the amplitude of the resonance wave. Both numerical simulations and experiments are conducted to demonstrate that the measurement accuracy of the proposed method is not affected by SCM, irradiation laser source parameters, and EMAT receiving coil size, and that accurate detection of stepped aluminum plates with thickness thinning from 3.0 mm to 0.5 mm is achieved. Furthermore, we were able to perform rapid detection of aluminum thin plate thickness at 500 °C temperature with an EMAT lift-off of 5.0 mm and achieved a relative experimental error as small as 3.40 %. The results obtained in this study showed that the proposed method performed well in non-contact measurement of metal thinning in harsh environment of high temperature.
ABSTRACT
In order to solve the difficulty in localization and poor signal-to-noise ratio (SNR) of the angled shear vertical wave (SV wave) electromagnetic acoustic transducer (EMAT) in cracks detection of high-temperature carbon steel forgings, a finite element (FE) model of the angled SV wave EMAT detection process was established, and the influence of specimen temperature on the EMAT excitation, propagation, and reception processes was analyzed. A high-temperature resistant angled SV wave EMAT was designed to detect carbon steel from 20 °C to 500 °C, and the influence law of the angled SV wave at different temperatures was analyzed. Then a circuit-field coupled FE model of angled SV wave EMAT in the carbon steel detection process based on the Barker code pulse compression technique was established, and the effects of the Barker code element length, impedance matching method, and matching component parameters on the pulse compression effect were analyzed. In addition, the noise suppression effect and the SNR of the crack-reflected wave in the tone-burst excitation method and the Barker code pulse compression technique were compared. The results show that the amplitude of the block-corner reflected wave decreases from 556 mV to 195 mV, and the SNR decreases from 34.9 dB to 23.5 dB when the specimen temperature increases from 20 °C to 500 °C. When the temperature is 500 °C, the SNR of the crack-reflected wave obtained by the Barker code pulse compression technique can be improved by 9.2 dB compared to the tone-burst excitation method with 16 synchronous averages. The study can provide technical and theoretical guidance for online crack detection for high-temperature carbon steel forgings.
ABSTRACT
To address the problem of low signal-to-noise ratio (SNR) of ultrasonic echo generated by laser-electromagnetic acoustic transducer (EMAT) ultrasonic method in metal materials, a method to improve the energy conversion efficiency of laser-EMAT ultrasonic testing using the surface constraint mechanism is proposed. Based on numerical simulation and experiment, the excitation mechanism of a laser surface heat source with and without a surface constraint mechanism is investigated, and the effect of the water film surface constraint on the laser-EMAT ultrasonic testing echo of different metal materials is analyzed. The effects of laser spot radius, laser power density, laser pulse duration, EMAT design parameters, and water film parameters on the ultrasonic echo amplitude and multimode ultrasonic energy distribution ratio are also investigated, and the optimal combination of laser excitation and EMAT reception parameters is provided. The results show that the laser power density and spot radius significantly affect the multimode ultrasonic amplitudes. Under the water film surface constraint, the energy distributions of shear waves (SWs) and longitudinal waves (LWs) change significantly, and the energy of the SW change rules of different metal materials are different. After using the surface constraint mechanism, the LW amplitude improves significantly, and the SNR of the LW is increased by at least 13.0 dB, the main bang duration is reduced by at least 29.4%, and the main bang amplitude is reduced by more than 80.5%. The relevant information of the surface constraint mechanism provides an effective reference for designing the laser-EMAT testing system with LW detections and reducing the dead zone in ultrasonic testing.
ABSTRACT
We report for the first time that the imidate radical can be efficiently added to glycals to generate glycosyl radicals, based on which a general, toxic-reagent-free synthesis of C-glycosides of 2-deoxy-2-amino sugars has been developed. Complementary to previous strategies, the reaction is 1,2-trans-stereoselective and could use aryl alkenes as substrates. The late-stage functionalization and density functional theory calculations are reported.
