Application of an Optical Fiber Sensor for Nonlinear Ultrasonic Evaluation of Fatigue Crack

Nonlinear ultrasonic technique that evaluates the growth of the barely visible fatigue crack in metal is difficult to be achieved by conventional optical fiber sensor because of the critical requirements for sensor sensitivity and bandwidth. In this paper, a high-performance optical fiber sensor using phase-shifted fiber Bragg grating and balanced demodulation system was applied to detect the fundamental and second harmonic Lamb waves propagating in an aluminum plate. After the nonlinearity was calculated from the modes of the Lamb waveform detected by the optical fiber sensor, its change due to the second harmonic generation shows an exponential trend in a fatigue test, which can be used to evaluate the crack growth with sufficient reliability. The experimental results from the optical fiber sensor were validated by comparing with results from the PZT sensor. This paper facilitates the development of optics-based nonlinear ultrasonic structural health monitoring for fatigue crack evaluation.