Focused bulk ultrasonic waves generated by ring-shaped laser illumination and application to flaw detection

Focused bulk ultrasonic waves have been generated in aluminum plates by surface irradiation with ring-shaped laser light. The waves are detected by a piezoelectric transducer. Compression and shear peak amplitudes drop quickly when the detector is moved away from the epicenter. This shows that strong focusing exists at the epicenter as the result of constructive interference of the waves generated by different parts of the ring. The focusing persists when the radius of the laser light is scanned over a large range, indicating that the elastic disturbance concentrates in depth along the ring's central axis. Numerical simulations are presented for comparison. The "pencil-like" acoustic wave structure is used to observe a sample plate with an artificial flaw. Strong new features including compress-shear mode conversion at the site of the flaw are observed. These features are used to locate the flaw within the sample.