Collaborative Research: Conformal Gradient-Index Lenses for Ultrasonic Wave Amplification and Improved Diagnostics

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Current structural systems are made of homogenous materials such as cast iron, wrought iron, bare steel, and brittle plastic, which are highly susceptible to failure and therefore require accelerated inspection and repair. Ultrasonics is a nondestructive evaluation method based on propagating elastic waves in structures, which are affected by defects in the structure and can therefore be used for damage diagnostics. However, in conventional structures, the amplitude of elastic waves decays with distance due to spreading/scattering, which limits the detectability of critical defects. In this research, new structural systems will be designed with an embedded or externally added lens so that ultrasonic signals can be focused and amplified as they propagate in the structure. In this way, ultrasonic wave energy can be transmitted and preserved over long distances. The research will allow detecting defects at their earliest stage and preventing unexpected failures. The target application will be pipeline systems due to their high susceptibility to failure. Therefore, results from this research will benefit the U.S. economy and society. This multi-disciplinary research encompasses metamaterials, sensors, additive manufacturing, structural monitoring, and design. The project features a synergistic educational component that integrates the strengths of two institutions in recruiting students who are underrepresented in engineering. In particular, female undergraduate and graduate students at both institutions will be connected via mutual workshops and the Society of Women Engineers (SWE).This research will introduce a conformal gradient-index (GRIN) metamaterial lens as part of a structural system such that elastic waves will be amplified as they propagate through the non-planar structure. With the GRIN lens, different ultrasonic wave modes (i.e., longitudinal, flexural, or torsional) will be focused and transmitted such that higher frequencies (50?200 kHz) will be able to propagate with the increased sensitivity to structural damage. The GRIN lens will be designed by varying the refractive index of unit cells, and the existing model of the GRIN lens for flat surfaces will be modified for conformal surfaces. The metamaterial lens layer will be created with 3D printing, which will allow more practical and light weight structures to be easily integrated into the host structure. Additionally, a novel composite pipe structure, produced with multi-material additive manufacturing technologies, will be designed with the embedded metamaterial lens to address the highest risk of major incidents from the conventional materials used in pipelines.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.