<p>Guided wave ultrasonics is one of the widely used nondestructive evaluation methods for structural health monitoring of civil infrastructures, such as, pipeline networks, bridges and so on. It is cost-effective, easy to implement and sensitive to most defects but suffers from the attenuation of wave energy, which limits the sensor spacing, increases the cost of structural integrity monitoring and decreases reliable inspectability. Especially, ultrasonic inspection of polymer structures is challenging because of the high material damping and low wave speeds. Elastic wave focusing can improve the inspectability of ultrasonic inspection by amplifying guided wave signal carrying defect information at sensor locations. In this work, we present a structurally embedded gradient index phononic crystal (GRIN-PC) lens for elastic wave focusing in polymer plates. The GRIN-PC lens is composed of equally spaced cylindrical brass inserts embedded into the plate wall. All the inserts are of same diameter and the insert heights are varied in the direction transverse to wave propagation to realize hyperbolic secant distribution of refractive index across the width of the plate. A parametric study is run to determine the insert material which results in minimum reflection at the lens interface but at the same time produces sufficient refractive index gradient required for wave focusing. Hence, the GRIN lens we present can focus the wave energy effectively at multiple focal points of the lens dictated by the hyperbolic secant profile. To estimate the material damping of PVC plate, the decay in the amplitude of out-of-plane velocity signal on the plate surface is measured experimentally using scanning laser vibrometer. An exponential curve fit to the measured velocity data gives decay exponent which is utilized to estimate the viscous damping loss factor used in numerical simulations. The decay exponent is larger at higher frequencies with values ranging from 5.5 to 7.5 m<sup>-1</sup> for the frequency range of 20 to 50 kHz. With the proposed lens, the ultrasonic signal can be transmitted long distances by overcoming the attenuation due to material damping. We successfully verify focusing of flexural A0 plate modes at the first two focal points of the lens through time domain numerical simulations in COMSOL Multiphysics. Due to attenuation, the signal amplitude decays almost 5 times over the focal length of 0.25 m in the PVC plate (baseline) at the design frequency of 40 kHz. Whereas, with the GRIN lens the signal amplitude at first focal point of the lens is 1.6 times higher than the baseline case. The focal points estimated from ray trajectory predictions are also confirmed by RMS velocity fields measured along the centerline of the lens. The GRIN-PC lens design is further validated through experiments. The experimental setup is designed by drilling holes over a PVC plate and sticking the brass insert inside the holes using epoxy. A scanning laser vibrometer is used to capture out-of-plane velocity over the plate surface for the baseline case and embedded GRIN-PC lens case. The velocity amplitude is compared for the two cases demonstrating the decay of velocity amplitude along the length of the plate and the amplification of velocity signal in the focal region of GRIN-PC lens as compared to the baseline. The amplified signal carrying the defect information cab be detected by sensors overcoming the limitations due to strong material damping.</p>

Experimental Demonstration of the Structurally Embedded Gradient Index Lens for Guided Wave Amplification in Polymer Plates

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2021/volume/85499

ASME 2021 Smart Materials, Adaptive Structures and Intelligent Systems (SMASIS2021)

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