Fatigue crack visualization using noncontact laser ultrasonics and state space geometrical changes

Fatigue crack and its precursor often serves as a nonlinear source, and the nonlinear ultrasonic features created by a fatigue crack have a much higher sensitivity compared with linear features. This paper presents a fatigue crack visualization technique based on noncontact laser ultrasonics and state space techniques. Under a broadband laser pulse excitation, defect nonlinearity exhibits modulation at multiple frequency peaks in a spectral plot due to interactions among various input frequency components of the broadband input. These modulations are weak and hardly discernable in both the frequency and time domains. In order to detect the nonlinear changes caused by fatigue cracks in the time domain, a state space attractor is reconstructed using a single laser pulse response and its geometrical deviations from the baseline data obtained from the pristine condition of a target structure are computed. Through scanning tests using a Q-switched Nd:YAG laser and laser Doppler vibrometer (LDV), the proposed method can be used for visualizing fatigue cracks in metallic plates.