<p>The fundamental torsional wave mode, namely T(0,1), is favorable in the guided wave ultrasonic testing of pipelines as it is not dispersive. In this study, the excitation of T(0,1) plane wave in a steel pipe is achieved by utilizing an array of d35 piezoelectric transducers. The required number of d35 sensors in the array is determined by multiphysics numerical simulations with the constraint of uniform torsional wave generation at the pipe circumference. The propagation distance of T(0,1) is extended by introducing a gradient-index phononic crystal (GRIN-PC) lens made up of steel stubs. The GRIN-PC lens modifies the refractive characteristics of propagating elastic waves along the axial direction such that the wave energy is localized at a focal point with improved amplitude, reducing the overall attenuation. The plane wave focusing at a point carries information along its path, making it possible to utilize an individual piezoelectric transducer for sensing, instead of an array. Pure T(0,1) wave mode excitation is verified by two-dimensional Fourier transform as well as the tangential displacement histories around the pipe circumference at the sensor location. Numerical simulations for wave propagation demonstrate that the amplitude of torsional wave mode increases at the focal point with the presence of GRIN-PC lens. Experiments validate the numerical findings when the adhesive layer between the pipe and the GRIN-PC lens is considered. Furthermore, uniform and localized thickness changes in the pipe wall are investigated in order to assess the performance of the GRIN-PC lens in the detection of corrosion defects using ultrasonic guided waves. Unit cell models with the stubs indicate that the dispersion of T(0,1) mode changes when the thickness of the pipe varies, signifying that the pipe with the GRIN-PC lens is more sensitive to the uniform thickness change in the pipe wall. Findings from the full-size pipe models simulating wave propagation are in good agreement with the unit cell models. The sensitivity of the GRIN-PC lens depends on the location around the pipe circumference in the case of localized defects. The extended propagation distance and focused plane wave result in increased transducer spacing and less number of sensors, decreasing the cost of guided wave ultrasonic testing for the studied pipe.</p>

Manipulating the Torsional Mode With the Gradient-Index Phononic Crystal Lens

Paper Proceedings: https://asmedigitalcollection.asme.org/qnde/QNDE2021/volume/85529

ASME 2021 48th Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE2021)

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