Scattering of the fundamental torsional mode at an axial crack in a pipe

Abstract

A quantitative study of the interaction of the T(0,1) torsional mode with an axial defect in a pipe is presented. The results are obtained from finite element simulations and experiments. The influence of the crack axial extent, depth, excitation frequency, and pipe circumference on the scattering is examined. It is found that the reflection from a defect consists of a series of the wave pulses with gradually decaying amplitudes. Such behavior is caused by the shear waves diffracting from the crack and then repeatedly interacting with the crack due to circumferential propagation. Time-domain reflection coefficient analysis demonstrates that the trend of the reflection strength for different crack lengths, pipe diameters, and frequencies from a through-thickness crack satisfies a simple normalization. The results show that the reflection coefficient initially increases with the crack length at all frequencies but finally reaches an oscillating regime. Also, at a given frequency and crack length the reflection decreases with the increase in pipe circumference. An additional scattering study of the shear wave SH(0) mode at a part-thickness notch in a plate shows that the reflection coefficient, when plotted against depth of the notch, increases with both frequency and notch depth.

Reflection of torsional T(0,1) guided waves from defects in pipe bends

“…It is expected that a similar variation would be seen for small, roughly equiaxed corrosion defects, but the sensitivity to axial cracks would be very different as they do not affect the transmission of axial stresses. T(0,1) guided wave inspection is in general less sensitive to axial defects compared to defects which also exhibit some circumferential extent [34]. …”

High-frequency lowest torsional wave mode ultrasonic inspection using a necked pipe waveguide unit

“…Although high-frequency waves may be preferred for small-sized defect inspection, the co-existence of multiple modes including dispersive higher modes makes signal analysis difficult. For this reason, a torsional wave has been used typically at a low-frequency below the cutoff frequency of the T(0,2) mode or in a thinner pipe than the first cutoff thickness at a given excitation frequency [1][2][3]. The symbol T(0,m) denotes the mth torsional wave mode [4].…”

On the scattering of torsional elastic waves from axisymmetric defects in coated pipes

“…The advantage of limiting this study to the scattering of the T(0,1) mode from an axisymmetric defect is that one may then readily compare predictions against theoretical and experimental data reported in the literature for an uncoated pipe (see for example [26][27][28][29]). Furthermore, this also facilitates comparison with experimental measurements since it is relatively straightforward to generate and measure the T(0,1) mode and at the same time to minimise coherent noise.…”