Wide and narrow-spectrum laser ultrasonics for fatigue crack detection using nonlinear modulation analysis

Laser ultrasonic technique is widely applied in non-destructive evaluation (NDE) and structural health monitoring (SHM) for the broadband generation of ultrasonic waves. However, controlling the spectral content of the laser-generated waves can enhance the sensitivity of damage detection. In this study, a finite element model is developed to investigate how laser beam diameter influences ultrasonic wave generation. A small beam diameter is used to produce a wide-spectrum (WS) response, while a larger beam generates a narrow- spectrum (NS) response. The two independent ultrasonic responses are then combined to form a dual-laser excitation approach. When a structure contains fatigue damage, nonlinear ultrasonic modulation occurs between the WS and NS responses, leading to the generation of sidebands. A spectral energy-based index is employed to quantify these sidebands and identify fatigue-induced nonlinearity. The results confirm that the proposed dual-laser ultrasonics method improves sensitivity to fatigue cracks compared to conventional single laser excitation.