Structural Health Monitoring (SHM) and Nondestructive Evaluation (NDE) systems generally adopt piezoelectric transducers which emit omnidirectional wave fields. The achievement of directionality of guided wave generation will benefit the structural sensing purpose, which allows better detection and localization of the damage sites. Active metamaterials are drawing increasing attention for its superb capability in flexible wave manipulation and control. The peculiar stiffness change of nitinol, a type of shape memory alloy (SMA), during the heating process, provide the feasibility to actively steer guided waves for emitting a unidirectional wave field from a single piezoelectric transducer via the metasurface design.

The current study proposes a type of metamaterial ultrasonic radar for the steerable unidirectional wave manipulation. It contains 180 stubbed unit cells sitting on an aluminum plate which are delicately arranged in 5 circles. Each unit cell is composed of a shape memory alloy substrate serving as a spring and a lead stub working as a mass to form a resonator. The controllable bandgap of such metamaterial system can be achieved due to the stiffness change of nitinol between its martensite phase and austenite phase under a thermal load. First, a Finite Element Model (FEM) of the unit cell is constructed to compute its frequency-wavenumber domain dispersion characteristics; the adjustable feature of this proposed metamaterial system is shown by the demonstration of a bandgap shift between the martensite and austenite states. Then, numerical modeling of the metamaterial radar is performed by shifting the bandgap of one sector of the metasurface away from the excitation frequency to form a wave emission tunnel. For this model, harmonic analysis is conducted and guided waves are generated by a circular piezoelectric wafer active sensor (PWAS) bonded at the center of the metasurface area. The modeling results demonstrate that the martensite phase metasurface area forms a bandgap region where guided wave energy cannot penetrate. On the other hand, the bandgap of the austenite sector under thermal load shifts away from the excitation frequency, opening up a transmission path for the ultrasonic waves. By rotating the austenite sector, the corresponding wave emission direction can be steered. In this manner, the metamaterial structure can work like a wave emission radar which demonstrate the realization of the steerable unidirectional wave emission with a single transducer. Such an active metasurface possesses great application potential in future SHM and NDE systems. The paper finishes with summary, concluding remarks, and suggestions for future work.

Keywords: steerable unidirectional wave emission, metamaterial, shape memory alloy, ultrasonic radar, structural health monitoring


Steerable Unidirectional Wave Emission From a Single Piezoelectric Transducer Using a Shape Memory Alloy Composite Metasurface

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

IMECE2020 - Virtual Conference

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.