Fatigue crack detection using dual laser induced nonlinear ultrasonic modulation

In this study, a nonlinear ultrasonic modulation technique based on dual laser excitation is proposed for fatigue crack detection. Two pulse lasers are shot on the target specimen for ultrasonic generation. The corresponding ultrasonic responses are measured by a laser Doppler vibrometer (LDV) and analyzed to extract the crack induced nonlinear ultrasonic modulation. First, the effect of the pulse laser beam size on the frequency content of the generated ultrasonic waves is numerically and experimentally investigated. Then, this finding of the laser beam size effect is utilized to generate wideband (WB) and narrowband (NB) ultrasonic waves by adjusting the laser beam sizes of the two pulsed excitation lasers. Nonlinear ultrasonic modulation results from the interaction of WB and NB ultrasonic waves when a fatigue crack exists in the target specimen. The fatigue crack is then detected by comparing the spectral responses obtained under a single WB input and both WB and NB inputs. In the end, a fully noncontact dual laser ultrasonic system is developed and used to detect micro fatigue cracks in aluminum and steel plate specimens.