Improving laser-EMAT ultrasonic energy conversion efficiency using surface constraint mechanism

•Surface constraint mechanism can enhance the LW (longitudinal wave) amplitude and reduce main bang duration and its amplitude in the laser-EMAT ultrasonic detection.•Analyse the correlation between the laser spot radius, laser energy, laser duration, EMAT design parameters, water film parameters, and the ultrasonic echo amplitude and multimode ultrasonic energy distribution generated by the laser-EMAT.•Determine the optimal combination of pulse laser parameters and spiral coil EMAT parameters. 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.