Influence of guided ultrasonic wave scattering directionality on the detection sensitivity for SHM of fatigue cracks

Localized and distributed guided ultrasonic waves array systems offer an efficient way for the long-term monitoring of the structural integrity for large structures. The use of permanently attached sensor arrays has been shown to be applicable to detect simulated corrosion damage. However, the detection sensitivity for fatigue cracks depends on the location and orientation of the crack relative to the transducer elements, and for some transducer locations no change in the signal even for a significant defect will be detected. Crack-like defects have a directionality pattern of the scattered field depending on the angle of the incident wave relative to the defect, the defect depth, and the ratio of the characteristic defect size to wavelength. The directionality pattern of the scattered field for the A(0) Lamb wave mode is predicted from 3D Finite Element simulations and verified from experimental measurements at machined part-through and through-thickness notches using a laser interferometer. Good agreement is found and the directionality pattern can be predicted accurately. These results provide the basis for the quantification of the detection sensitivity for defects in plate structures using guided wave sensors.