On the existence of local defect resonance in ultrasonic guided waves interaction with horizontal defects in plates

The interaction of ultrasonic guided waves with defected structures gives rise to local defect resonance (LDR), which manifests in large displacements in the vicinity of the defect and affects the reflection and transmission spectra. This paper investigates the fundamental mechanism at the origin of the LDR phenomenon in isotropic elastic plates, using a hybrid computational method (Global-Local). The analyses show that the coupling of ultrasonic guided waves with the vibrational resonance modes of the sub-structure, geometrically defined by the defect, causes LDR, and boundary conditions affect it secondarily. The coupling mechanism is captured by the Global-Local method and is investigated in relation to the characteristics of the defect and the relationship to the host-structure. The coupling occurs at defect lengths that are odd multiples of the modes' quarter wavelengths. Comparisons with analytical, finite element and methods in literature for the computation of the natural and LDR frequencies are provided. The presence of LDR and its effect on broadband reflection and transmission ultrasonic spectra away from the defected region are also verified experimentally and can be used for remote defect characterization in NDE applications. These studies clarify the fundamental understanding of LDR and provide an effective approach to capture and predict LDR in ultrasonic guided wave propagation in plate-like structures.