Programmable elastic metamaterials for wave and vibration control

Abstract

[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Emerging from electromagnetic waves and fast extending to acoustics and elastics, metamaterials as artificial structures exhibit extraordinary wave control abilities attracting more and more researchers in this field. Over the past two decades, elastic metamaterials/metasurfaces with engineered microstructures provided an appealing solution in wave and vibration control. By tailoring the microstructure on a subwavelength scale, elastic metamaterial holds the ability to distinct itself from traditional materials or phononic crystals by its striking functions in wave trajectory manipulation, cloaking, nonreciprocal and topological wave control, and low-frequency wave/vibration mitigation and absorption in particular. Unfortunately, metamaterials made with passive materials alone limit their performances, functionalities, tunabilities etc. One of the greatest challenges in the development of elastic metamaterials is the ability to change their properties without physical modifications in order to broaden and control their performance in real-time without microstructural alterations. On the other hand, with active elements incorporated into metamaterial/metasurface designs, tunable and/or programmable devices with functionalities controlled by external stimuli become possible, opening a new platform to dynamically manipulate elastic waves. Introducing the piezoelectric shunting technique into the design of passive metasurfaces/metamaterials is a promising way to overcome those limitations. This technique is convenient because the transducer can be not only functioned as both sensor and actuator, but also with a reduced weight. In this dissertation, we introduce digitally programmable metamaterials, meta-boundaries and meta-layers for broadband, real time, reprogrammable, elastic wave and vibration control. Through theoretical modeling, numerical validation and experimental realization of programmable metamaterials, meta-boundaries and meta-layers, extraordinary wave steering abilities in broad bandwidth with on demand tunabilities are demonstrated, which have potential applications in active noise and vibration control. Specifically, a self-adaptive metamaterial for strong broadband low-frequency wave attenuation with adaptive mechanical local resonators, by leveraging the concept of the frequency-dependent stiffness, is experimentally demonstrated. To further explore tunabilities and functionalities of digitally controlled metamaterials, we introduce a strategy to design elastic metamaterials with coupling stiffness modulated in space and in time by programmably controlling pumping ac currents into coils and demonstrate, experimentally and theoretically, tunable nonreciprocal flexural wave propagation. Next, we introduce a programmable meta-boundary with deep subwavelength thickness that is composed of an array of piezoelectric sensing-and-actuating units controlled by electrical circuits such that pressure to shear wave conversions are able to be electrically reconfigured. Lastly, an active meta-layer for flexural wave absorption is proposed. Meta-layers are numerically and experimentally demonstrated for broadband wave absorption, vibration control and skin cloaking of large voids. This design approach proposed in this dissertation might shed light in dynamically programmable wave and vibration control devices and could enable alternative solutions to complicated ultrasonic sensing and evaluation of engineering structures.