Manipulating the torsional mode in pipe-like structures with mechanical lens for extended propagation distance

Abstract

Guided wave ultrasonic testing (GWUT) has been commercially used in pipelines since the last two decades. T(0,1), the axisymmetric torsional mode, is one of the preferred wave modes in GWUT because it is nondispersive. In this study, the T(0,1) plane wave within a steel pipe is generated using an array of d35 piezoelectric actuators attached around the pipe circumference. A previously designed mechanical lens built of gradient-index phononic crystal is implemented to extend the propagation distance of torsional wave mode by reducing attenuation. The decrease in attenuation is achieved by manipulating the refractive properties of the propagating elastic waves such that the wave energy is focused at a specific point along the pipe. Time-dependent finite element simulations show that the amplitude of the signal received at the focal point is amplified by approximately 80%. Experiments validate the excitation of T(0,1) mode but not the wave focusing as the adhesive layer creates a band gap by changing the dispersion characteristics. Moreover, findings from the numerical studies involving unit cell and full-sized pipe models and are presented. The two-dimensional Fast Fourier Transform on full-pipe models confirms that the pipe with GRIN lens is more sensitive to overall thickness change within the pipe wall, in agreement with the unit cell simulations. The sensitivity of the GRIN lens depends on the location around the pipe circumference in the case of localized defects. Experiments with T(0,1) focusing need to be explored further considering the adhesive.