Converging thermoelastic bulk waves using a ring-shaped laser ultrasonic technique and application to thickness detection

The laser ultrasonic contralateral detection method for material inspections mainly uses longitudinal waves with strong energy in the ablation mechanism. The effect is perfect, but it does not meet non-destructive testing (NDT) requirements due to the damage to the material surface. The thermoelastic mechanism conforms to the principle of NDT, but the excited ultrasonic energy is relatively weak. In this work, an ultrasonic converging method was studied to enhance the signal in the thermoelastic regime based on the phased array principle using a ring-shaped laser ultrasonic technique. An axicon and a convex lens were used to irradiate a ring-shaped laser on an aluminum sample to induce the bulk ultrasound. A two-wave mixing interferometer with a photorefractive crystal was used to measure the ultrasound. The signals were superim-posed at the detection point on the center axis of the laser ring, and the amplitude was enhanced. The ultrasonic signals were affected by the width and diameter of the laser ring. A novel thickness measurement method was proposed in a non-destructive and non-contact manner. The flight times of successive echoes were observed as features to reconstruct thickness.