Guided wave ultrasonics is one of the widely used nondestructive evaluation methods for regular inspection of pipeline networks. However, the wave propagation-based damage detection methods suffer from the attenuation of wave energy, which limits the sensor spacing, increases the cost of pipeline integrity monitoring and decreases reliable inspectability. Especially, ultrasonic inspection of polymer pipes is challenging because of the high material damping. In this paper, we propose a novel conformal metamaterial lens to amplify elastic wave energy at sensor locations allowing the guided wave signal containing damage information to travel over longer distances. To this end, we investigate multi-mode focusing of guided ultrasonic waves in a polyvinyl chloride (PVC) pipe by designing and integrating an embedded gradient index (GRIN) lens with the PVC pipe. The metamaterial lens is composed of equally spaced cylindrical brass inserts embedded into the pipe wall. All the inserts are of same height which is equal to the half-thickness of the pipe. Insert diameters are varied in circumferential direction to realize hyperbolic secant distribution of refractive index around the circumference of pipe. For tailoring cylindrical inserts, we first obtained the relationship between refractive index (or phase speed) and insert diameters by performing dispersion analysis of cylinder embedded unit cell for a range of diameter values. Dispersion curves are acquired through eigenfrequency analysis of unit cell in COMSOL Multiphysics by applying Floquet Periodicity Boundary conditions at the sides and sweeping the wave number along the edges of first Irreducible Brillouin Zone of the lattice. From the variations in the dispersion curves of the longitudinal and torsional wave modes, we obtained the optimal diameter profile for the inserts corresponding to hyperbolic secant distribution of refractive index for different modes and we designed a single lens to focus L(0,1), L(0,2) and T(0,1) modes. Hence, the GRIN lens we present can be tuned to different wave modes in order to understand the most sensitive characteristics to be transmitted long distances with the increased reliability and the reduced measurement variance. We successfully verify focusing of two longitudinal (L(0,1) and L(0,2)) and one torsional (T(0,1)) pipe modes through time domain numerical simulations in COMSOL Multiphysics. The focal points estimated from ray trajectory predictions are also confirmed by RMS velocity fields measured along the centerline of the lens. In addition to wave focusing, we also verify the structural integrity of the pipe embedded with GRIN lens and understand the stress distribution between steel inserts and polymer by performing an axisymmetric model and considering a unit internal pressure inside pipe.


Structurally Embedded Gradient Index Lens for Simultaneous Amplification of Guided Waves in Polymer Pipes

Division / Sponsors: 
Aerospace Division

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2020/volume/84027

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