Directivity of guided ultrasonic wave scattering at notches and 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. Crack-like defects have a directivity pattern of the scattered wave field depending on the angle of the incident wave relative to the defect orientation and on the ratio of the characteristic defect size to wavelength. From FE simulations it has been shown that for cracks and notches almost no energy is scattered in certain directions from the defect, i.e., the data processing algorithm must take into account that for some transducer combinations no change in the signal even for a significant defect will be detected. The directivity pattern of the scattered field for the A0 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 was found and the directivity pattern can be predicted accurately. The amplitude of the scattered wave is quantified for a variation of the angle of the incident wave relative to the defect orientation, the defect depth, and the ratio of the characteristic defect size to wavelength. These results provide the basis for the quantification of the detection sensitivity for defects in plate structures using guided wave array sensors. A hybrid model has been developed, taking the different propagation distances and scattering characteristics into account, in order to predict the relative amplitudes of received pulses for given sensor locations. From a comparison with the signal to noise ratio of the array system, detection capabilities can be predicted for given defect size and orientation.

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