Controlling the Dispersion Characteristics of Pipe-like Structures with Phononic Crystals

Guided Wave Ultrasonics and Acoustic Emission are two common Nondestructive Evaluation (NDE) methods, which are based on propagating elastic waves in solids. Current challenges in the NDE of monolithic structures include decay in wave energy with distance and insensitivity of wave modes to certain types of defects. In this research, Gradient-Index Phononic Crystal (GRIN-PC) is exploited to modify the dynamic properties of structures, allowing elastic waves to be transmitted longer distances and better sensitivity to damage modes. With this motivation, a GRIN-PC lens consisting of periodic arrangement of steel stubs is integrated with a pipe-like structure made of steel. The Bloch-Floquet theorem is employed to build unit cell models with periodic boundary conditions representing infinite waveguides. Dispersion characteristics are investigated to determine the influence of the GRIN-PC lens on controlling the wave propagation. Piezoelectric transducers are utilized for actuation and sensing. The excitation and focusing of plane waves are demonstrated via time-dependent numerical simulations using finite element analysis. Full-scale experiments are conducted to validate numerical findings. Signal processing techniques, such as the Continuous Wavelet Transform and two-dimensional Fast Fourier Transform, are applied to analyze complex time-series data. This research has three main outcomes: (i) a plane wave entering the GRIN-PC lens region focuses on a point, and a dynamic source originating from this point is converted to a plane wave. In both directions, the elastic wave results in increased amplitude. The GRIN-PC lens is reciprocally applicable for burst-type acoustic emissions representing crack-like activities in a pipe-like structure. (ii) The optimal number of actuators to excite the fundamental torsional wave mode of guided waves is determined. Wave focusing with torsional mode is achieved using both original and shortened versions of the GRIN-PC lens, resulting in amplification of wave energy. (iii) The torsional wave mode becomes more sensitive to overall and localized thickness changes, since the GRIN-PC lens modifies the dispersion characteristics of the structure. The improvement of dynamic NDE methods is achieved by reducing the decay in wave energy due to increased wave amplitude. The GRIN-PC lens is demonstrated to enhance defect detection in pipe-like structures, including uniform and localized corrosion.