Numerical study of length and angle gauging for subsurface-inclined cracks based on reflected surface waves with laser ultrasonic technique

To achieve quantitative detection of subsurface-inclined cracks, the reflected surface waves from subsurface cracks with different lengths and angles were obtained by the finite element method (FEM). The relationship between the time difference of each feature wave in the reflected waves and the length and angle of the subsurface inclined crack was analysed. The arrival time difference between the lowest trough and the first trough of the reflected wave is independent of the angle of the subsurface crack and approximately a linear function of the crack length. Furthermore, when the crack lengths remain unchanged, the arrival time difference between the highest peak and the second trough of the reflected wave is a quadratic function with the angle of the subsurface crack. Thus, a quantitative measurement method for gauging the length and angle of subsurface inclined cracks is proposed based on the above phenomena. The relationships between the length and angle of the subsurface inclined crack and the arrival time difference of the reflected surface waves were obtained by curve fitting. The proposed method was verified by simulation data. The maximum relative error of the crack length obtained by the method was 7.76%, and the maximum relative error of the obtained inclination angle was 11.61% when the crack angle was large. The proposed approach will open the way for simultaneous measurement of the length and angle of subsurface inclined cracks and structures.