Laser induced narrowband ultrasound for fatigue crack detection

Abstract

Laser ultrasonic technique is often adopted in non-destructive evaluation (NDE) and structural health monitoring (SHM) for broadband generation of ultrasonic waves. However, in some cases, it is advantageous to manipulate the laser input for the ultrasonic generation with a concentrated frequency of interest. In this study, a spatial mask is used to shape the input pulse laser shot on the target structure, and to excite a narrowband ultrasonic wave with specified central frequencies. Numerical simulations and experimental tests are performed, and the obtained ultrasonic waves are compared with various spatial masks. Then, a dual laser excitation approach is developed to simultaneously introduce two distinct narrowband frequency inputs into the target structure. Nonlinear ultrasonic modulation will occur between the two inputs when the structure is damaged. Finally, the feasibility of the proposed technique is validated by detecting fatigue crack on aluminum plates.