Laser-induced surface acoustic waves for measurement of elastic constants based on mode conversion

The measurement of elastic constants is very important to the study of materials. Laser ultrasonic technology has been widely concerned for its unique advantages and used to measure elastic constants, but the current measurement methods have their limitations. In general, traditional measurement methods require the use of a fatigue tester or measurement of all modal waves during the experiment. It is found that cracks can transform the mode of ultrasonic wave, in other words, the surface acoustic waves can be transformed into shear waves by mode conversion. So, we proposed a new method of measuring elastic constants of materials by laser ultrasound based on mode conversion. Surface acoustic waves are induced by laser ultrasound system, the reflection and mode conversion of surface acoustic waves can be realized by cracks. Therefore, direct waves, reflected waves and mode converted waves are included in the collected surface wave signals, then the wave velocity of surface waves and shear waves are calculated by the surface acoustic waves, and the elastic constants of materials are obtained by the acoustic elasticity equation. Since the experiment and processing are all based on surface acoustic waves, multiple waveform acquisition and processing are avoided. After calculation, the error is kept below 3% compared with the standard measurement value. In order to further confirm the application range of the method, cracks of different depths are used in the experiment, it is further proved that this method has nothing to do with the crack size. The research results of this paper can extend the application range of laser ultrasound. It also provides a new approach to the characterization of materials.