Interaction Between SH₀ Guided Waves and Tilted Surface-Breaking Cracks in Plates

The interaction between SH₀ guided waves and simple defects is well understood and documented, and the SH₀ and related torsional guided waves are commonly used in inspection. However, tilted and branching cracks, for which vertical notches are a poor approximation, are found in some environments, particularly when pipes are buried in alkaline soils. This paper studies the interaction between SH₀ guided waves and tilted, surface-breaking cracks, investigating the effect of the tilt and depth of the defect. The incident wave interacts with the tilted crack to generate a transmitted wave, a reflected wave, and a wave trapped below the crack. It is shown that the direction of the tilt of the crack relative to the incident wave direction does not affect the scattering behavior. In addition, the axial extent of the crack plays a major role in the reflectivity of the crack, leading to transmission nulls in some configurations. These transmission nulls appear for all crack depths, the frequency range over which the transmission is significantly reduced increasing with crack depth. This behavior is shown to be analogous to the acoustic energy flow in a duct when a Helmholtz resonator is introduced. The null is not seen above the SH₁ cutoff as the propagating signals are no longer monomodal. The existence of a transmission null and corresponding reflection maximum is promising for the detection of small defects and measurement of the frequency at which the null occurs will assist with defect characterization. Experimental validations of the key results are presented.