A crack ultrasonic detection method considering the beam deflection at the crack tips

In the ultrasonic penetration method for detecting internal cracks in steel structures, the accuracy of detection is directly correlated with the precision of crack tip location determination. When the ultrasonic beam passes through the crack tip, its axis undergoes deflection. This study systematically investigates the deflection of ultrasonic beams at crack tips and applies this phenomenon to achieve internal crack detection in steel structures. Based on the ultrasonic wave propagation theory, the transmission coefficient and transmittance when the crack tip is aligned with the acoustic beam axis were derived. Through experiments on cylindrical specimens and finite element simulations, the variation patterns of two key indicators of beam deflection – sound pressure and deflection angle – with respect to testing parameters and crack characteristics were examined. Plate crack experiments employed the sound pressure ratio (ωK) and deflection angle (αd) as key parameters for crack characterisation. The findings indicate that sound pressure redistribution constitutes the primary factor contributing to the deflection of the beam axis. ωK and αd exhibit a correlation with excitation frequency and crack parameters. Using ωK and αd, two data processing methods for detecting hidden cracks in plates are proposed, which provide significant insights for fatigue crack testing in steel bridge structures.