This paper reports the experimental demonstration of deep subwavelength ultrasonic imaging of defects in metallic samples with a feature size of λ/25 using holey-structured metamaterial lenses. Optimal dimensions of the metamaterial’s geometric parameters are determined using numerical simulation and the physics of wave propagation through holey lenses. The paper also shows how the extraordinary transmission capacity of holey structured metamaterials comes about by the coupling of higher frequencies in the incident ultrasonic wave field to resonant modes of the lens. © 2017, The Author(s).

Krishnan Balasubramaniam

Department of Mechanical Engineering

Prabhu Rajagopal

Kiran Kumar Amireddy

article

Computer simulation

Physics

Ultrasound

Fulltext

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IIT Madras

Scientific Reports

The focusing of ultrasound using topographic lenses, typically made of plates with step changes that cause an interaction between forward- and backward-propagating guided waves, has been widely studied in recent years. However, such ‘step-change’ lenses require precise machining and moreover, the thick-thin structure can be unstable during deployment in practical inspection applications. The work reported here follows from the insight that perhaps any approach to induce a mismatch in acoustical impedance as achieved by the step-change can also lead to focusing of ultrasonic guided waves. By carefully choosing the impedance pairing, a novel material contrast lens stacking Aluminium and Molybdenum plates in series is shown to achieve focusing of ultrasound through negative refraction. The interface between the two metals causes the interaction of the forward-propagating second symmetric Lamb mode S 2 into the backward- propagating first symmetric S 2b . The focusing of Lamb waves is demonstrated using numerical simulations validated by experiments. Comparison with a simple Aluminium-Aluminium plate combination brings out the underlying physics of focusing using the proposed material contrast lens. Simulation results showing super-resolution imaging using the proposed material contrast lens are also presented, demonstrating the power of the proposed approach. This report opens up the possibilities of developing new lensing devices for use in medical imaging and nondestructive evaluation, among other possible applications. © 2019, The Author(s).

Lensing in the Ultrasonic Domain using Negative Refraction Induced by Material Contrast

Image resolution in classical wave applications is limited by the diffraction limit which corresponds to half the operating wavelength (λ). To achieve higher resolution, ways of overcoming the natural diffraction limits are of interest. In this paper, an experimental demonstration of deep sub-wavelength resolution in the ultrasonic regime using a metamaterial lens is presented. Metamaterial lenses effectively transfer the evanescent waves to the imaging plane, which carry the details of the sub-wavelength features. The successful transmission of the decaying evanescent waves which contain much larger wave vectors than the propagating waves enables to overcome the diffraction limit set by the operating wavelength. We report an imaging technique using optimized holey-structured metamaterial lens to characterize a horizontal crack of size λ/25 in a layered aluminium sample. © 2019, The Indian Institute of Metals - IIM.

Transactions of the Indian Institute of Metals

Characterization of Deep Sub-wavelength Sized Horizontal Cracks Using Holey-Structured Metamaterials

This paper reports the application of a porous medium as an aperiodic metamaterial lens for ultrasonic imaging in the context of nondestructive evaluation and non-invasive diagnostics. Experimental results are presented, demonstrating a deep sub-wavelength imaging down to 1/36th of the operating wavelength, which is the highest resolution demonstrated worldwide using bulk ultrasound. The improvement in the resolution is shown to be linked to aperiodicity overcoming the Wood anomaly, which sets limits on wave transmission by holey structured lenses. © 2018 Author(s).

Applied Physics Letters

Porous metamaterials for deep sub-wavelength ultrasonic imaging

This paper studies the feasibility of using ultrasonic guided waves for fast inspection of conformal deltoid radius filler or "Noodle" regions of joints in stringer composite structures. Semi-analytical finite element simulations, supported by experiments and three-dimensional finite element models, are used to demonstrate the existence of a longitudinal guided ultrasonic mode confined or trapped in the Noodle regions. Studies reveal that this mode has attractive properties for rapid screening of Noodle joints, including strong energy concentration, low dispersion, and attenuation. Discussing the physics of mode confinement in light of material differences and geometry, the phenomenon is shown to be related to feature-guiding effects noted in literature recently. © 2016 Acoustical Society of America.

Journal of the Acoustical Society of America

Longitudinal guided waves confined in radius filler regions of composite joints

This paper presents the implementation of holey structured metamaterial lens for ultrasonic characterization of subwavelength subsurface defects in metallic components. Experimental results are presented, demonstrating ultrasound-based resolution of side drilled through-holes spaced (λ/5) in an aluminum block. Numerical simulation is then used to investigate the parameters that can help improve the resolution performance of the metamaterial lens, particularly, the addition of end-conditions. This work has important implications for higher resolution ultrasonic imaging in the context of practical non-destructive imaging and non-invasive material diagnostics. © 2016 Author(s).

Holey-structured metamaterial lens for subwavelength resolution in ultrasonic characterization of metallic components

Additive manufacturing methods are gaining increasing popularity for rapidly and efficiently manufacturing parts and components in the industrial context, as well as for domestic applications. However, except when used for prototyping or rapid visualization of components, industries are concerned with the load carrying capacity and strength achievable by additive manufactured parts. In this paper, the wire-arc additive manufacturing (AM) process based on gas tungsten arc welding (GTAW) has been examined for the internal structure and constitution of components generated by the process. High-resolution 3D X-ray tomography is used to gain cut-views through wedge-shaped parts created using this GTAW additive manufacturing process with titanium alloy materials. In this work, two different control conditions for the GTAW process are considered. The studies reveal clusters of porosities, located in periodic spatial intervals along the sample cross-section. Such internal defects can have a detrimental effect on the strength of the resulting AM components, as shown in destructive testing studies. Closer examination of this phenomenon shows that defect clusters are preferentially located at GTAW traversal path intervals. These results highlight the strong need for enhanced control of process parameters in ensuring components with minimal defects and higher strength. © 2015 AIP Publishing LLC.

Journal	Scientific Reports
Publisher	Nature Publishing Group
ISSN	20452322
Open Access	Yes