RESUMEN
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.
RESUMEN
Fatigue life of materials or structures can be classified into three stages: fatigue hardening or softening, crack initiation and crack propagation, which includes two stages. Current mature non-destructive testing (NDT) methods can only detect macro or visible cracks in stage II crack propagation. In order to detect and evaluate the fatigue damage occurring before stage II crack propagation quickly and effectively, magneto acoustic emission (MAE) measurement was carried out on laboratory specimens with different numbers of fatigue cycles. With the accumulation of fatigue damage, the RMS of MAE decrease steadily on the whole, making MAE a promising non-destructive method for evaluating fatigue damage. To make MAE applicable in noisy environments, square waveform voltage were selected to excite magnetic fields, and 'T' type MAE signals with higher amplitude were produced. The variation of MAE with number of fatigue cycles at different excitation intensity indicated that the defects associated with fatigue damage have greater effects on the creation and annihilation of domain walls. The point where the MAE amplitude begins to increase instead of decrease with fatigue can be an indicator for the onset of stage II crack propagation. This article is part of the theme issue 'Advanced electromagnetic non-destructive evaluation and smart monitoring'.
