Abstract
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.
Highlights
W ITH over 1 500 000 km of pipelines around the world carrying oil, gas, or petroleum [1], monitoring their state and assessing their remaining life are a major concern
As would be expected from reciprocity, it was found that the direction of the tilt of the crack relative to the incident wave direction does not affect the reflection and transmission behavior; at tilt angles below about 20◦, the response is similar to that from a crack normal to the surface
As in the case of a rectangular notch, it was shown that the reflection and transmission coefficients are a function of the axial extent of the crack expressed as a fraction of the wavelength
Summary
W ITH over 1 500 000 km of pipelines around the world carrying oil, gas, or petroleum [1], monitoring their state and assessing their remaining life are a major concern. Regarding the simpler case of SH waves below the SH1 cutoff frequency interacting with defects, Demma et al [13] studied the reflection behavior of the fundamental SH0 mode from rectangular notches, investigating the effect of the axial extent of the defect, its depth, and the frequency-thickness product. They observed reflection nulls caused by destructive interference.
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