Ramathasan Thevamaran's blog
https://imechanica.org/blog/29917
enSubmit Abstract to the SES 2024 Symposium: 7.4. Controlling Mechanical Waves with Metamaterials
https://imechanica.org/node/27176
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/74">conference</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Dear Colleagues,</p>
<p>We would like to invite you to participate in the Metamaterials and Architected Materials (Track 7) Symposium titled “<a title="https://urldefense.com/v3/__https://www.2024ses.com/index/page.html?id=1018__;!!Mak6IKo!PNJ1Fz3KoXOuUE2jiJ_BIBjSaZdgPmXpnPNFdm0q0Qwv_pcnyI6C1xe6t2RyC3ONCq90OC-z_2zhvyXi9Q$" href="https://urldefense.com/v3/__https://www.2024ses.com/index/page.html?id=1018__;!!Mak6IKo!PNJ1Fz3KoXOuUE2jiJ_BIBjSaZdgPmXpnPNFdm0q0Qwv_pcnyI6C1xe6t2RyC3ONCq90OC-z_2zhvyXi9Q$" target="_BLANK" data-outlook-id="742b4a27-4c94-451b-a395-dad079a04f8d">Controlling Mechanical Waves with Metamaterials</a>”. The symposium will be organized at the Annual Technical Meeting of the Society of Engineering Science (SES 2024) to be held in Hangzhou, China from August 20 to 23, 2024.
</p><p>The focus of this symposium, in its 6th year at SES, is on the control of mechanical waves and vibrations using phononic crystals and metamaterials—advanced synthetic materials that are deliberately designed with specific geometry and constituent materials to provide a desired bulk functionality. The topics covered will include acoustic and ultrasonic bandgap materials, wave guides and filters, nonlinear dynamic metamaterials and structures, instability-induced switching behavior, non-reciprocal and directional wave transport, non-Hermitian and Parity-Time symmetric metamaterials, and topological metamaterials. Submissions focusing on the control of mechanical waves using multi-physics couplings such as electro- and magneto-mechanical couplings and chemically induced transformations are also encouraged. The symposium will bring together experimental, theoretical, and computational research in this emerging area to articulate new ideas and collaborations. We aim to provide a platform that enables the participants to advance current understanding of the control of mechanical waves and vibrations by manipulating the underlying structural features and their organization into a metamaterial in a well-informed and predictable manner.</p>
<p><strong>Please submit your abstracts on or before April 1, 2024 </strong>online at <a title="https://urldefense.com/v3/__https://www.2024ses.com/index/page.html?id=1073__;!!Mak6IKo!PNJ1Fz3KoXOuUE2jiJ_BIBjSaZdgPmXpnPNFdm0q0Qwv_pcnyI6C1xe6t2RyC3ONCq90OC-z_2w8bh71-Q$" href="https://urldefense.com/v3/__https://www.2024ses.com/index/page.html?id=1073__;!!Mak6IKo!PNJ1Fz3KoXOuUE2jiJ_BIBjSaZdgPmXpnPNFdm0q0Qwv_pcnyI6C1xe6t2RyC3ONCq90OC-z_2w8bh71-Q$" target="_BLANK" data-outlook-id="0c7c6ba5-2a3d-4c2f-bad1-b57229b9ac4f">https://www.2024ses.com/index/page.html?id=1073</a> We look forward to seeing you in Hangzhou, China. With best regards,Charlie Dorn (ETH Zurich), Katie Matlack (UIUC), Serife Tol (U of Michigan), and Ramathasan Thevamaran (UW-Madison)</p>
</div></div></div>Tue, 26 Mar 2024 01:39:28 +0000Ramathasan Thevamaran27176 at https://imechanica.orghttps://imechanica.org/node/27176#commentshttps://imechanica.org/crss/node/27176Mitigating Oblique Impacts by Unraveling of Buckled Carbon Nanotubes in Helmet Liners
https://imechanica.org/node/27017
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/13944">Compression-shear response</a></div><div class="field-item odd"><a href="/taxonomy/term/13945">Helmet liner</a></div><div class="field-item even"><a href="/taxonomy/term/13946">Oblique impact</a></div><div class="field-item odd"><a href="/taxonomy/term/13947">Collective buckling</a></div><div class="field-item even"><a href="/taxonomy/term/678">Energy dissipation</a></div><div class="field-item odd"><a href="/taxonomy/term/13948">Vertically aligned carbon nanotubes</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span><strong>Abstract</strong></span></p>
<p> </p>
<p><span>Background: Helmet systems most commonly experience oblique blunt impacts which cause simultaneous linear and rotational accelerations. The ability to attenuate both linear and rotational accelerations by absorbing the normal shock while accommodating large shear deformations with energy dissipation is critical to developing superior helmet liners that prevent traumatic brain injury (TBI).</span></p>
<p> </p>
<p><span>Objective: To investigate the quasistatic compression-shear response of vertically aligned carbon nanotube (VACNT) foams—which are known for their exceptional specific energy absorption in compression—and explore their potential of accommodating large shear strains at lower shear stress levels, under large compression-shear loadings.</span></p>
<p> </p>
<p><span>Methodology: We investigate the quasistatic compression shear response of freestanding vertically aligned carbon nanotube foams subjected to varied initial precompressions. We use in situ high speed microscopy to visualize the microscale deformations during shear.</span></p>
<p> </p>
<p><span>Results: Vertically aligned carbon nanotube foams exhibit a nonlinear hysteric shear stress-strain response that varies as a function of initial normal precompression. At a given precompression, initial linear response at very low shear strains leads to a behavior showing increasing compliance</span></p>
<p><span>leading to a plateau like regime at moderate shear strains and then transitions into a stiffening behavior at high shear strains. The shear stress-strain response softens with the increase in initial precompression demonstrating the vertically aligned carbon nanotube foam’s potential to accommodate large shear strains more effectively at severe compression-shear loads unlike other solids that typically jam. In situ high-speed microscopy reveals the unraveling of carbon nanotubes that collectively buckled during precompression, allowing them to accommodate large shear strains at low shear stress levels.</span></p>
<p> </p>
<p><span>Conclusion: We demonstrate the ability of vertically aligned carbon nanotube to accommodate large shear strains at lower shear stress levels under large compression-shear loadings. We propose a model to predict the compression-shear response at different precompressive strains and use this model to develop a deformation modality diagram that categorizes the dominant deformation mechanisms at different loads along different loading angles.</span></p>
<p><span><strong>Read the article here:</strong> <a href="https://rdcu.be/ds3OS">https://rdcu.be/ds3OS</a></span></p>
<p><span><img title="Head helmet impact kinetics and deformations induced on the protective liner" src="https://imechanica.org/files/Fig.1.pdf" alt="Head helmet impact kinetics and deformations induced on the protective liner" width="648" height="483" /></span></p>
<p> </p>
</div></div></div><div class="field field-name-upload field-type-file field-label-hidden"><div class="field-items"><div class="field-item even"><table class="sticky-enabled">
<thead><tr><th>Attachment</th><th>Size</th> </tr></thead>
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<tr class="odd"><td><span class="file"><img class="file-icon" alt="PDF icon" title="application/pdf" src="/modules/file/icons/application-pdf.png" /> <a href="https://imechanica.org/files/Fig.1.pdf" type="application/pdf; length=1361808" title="Fig.1.pdf">Head helmet impact kinetics and deformations induced on the protective liner</a></span></td><td>1.3 MB</td> </tr>
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</div></div></div>Fri, 08 Dec 2023 12:52:41 +0000Ramathasan Thevamaran27017 at https://imechanica.orghttps://imechanica.org/node/27017#commentshttps://imechanica.org/crss/node/27017Block copolymer additives for toughening 3D printable epoxy resin
https://imechanica.org/node/26984
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/9171">3D printing</a></div><div class="field-item odd"><a href="/taxonomy/term/699">polymer</a></div><div class="field-item even"><a href="/taxonomy/term/10841">toughening</a></div><div class="field-item odd"><a href="/taxonomy/term/9651">strengthening</a></div><div class="field-item even"><a href="/taxonomy/term/13935">phase separation</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Dear Colleagues,</p>
<p>Please see our recent article on toughening 3D printable resins with block copolymer additives:</p>
<p>Abstract<br />
</p><p id="spara014">We explore the potential for using a brush-coil triblock copolymer to enhance the mechanical properties of epoxy resin for 3D printing applications. Epoxy resins are widely used in structural material and adhesive and have great potential for 3D printing. However, the highly brittle nature of epoxy resins requires the use of large concentrations of toughening agents that pose significant challenges in meeting rheological requirements of 3D printing. We report a reactive brush-coil block copolymer with three distinct blocks that can phase separate and chemically crosslink with the base epoxy resin to form spherical aggregates. Detailed scanning electron microscopy imaging shows that these aggregates can arrest and deflect cracks during propagation and can synergistically strengthen (∼ 1.5×) and toughen (∼ 2×) the epoxy resin with even 1 wt% of the BCP additive to the base resin. Importantly, both the modulus and the glass transition temperatures are preserved. Direct ink writing (DIW) and digital light processing (DLP) 3D printing of the modified resins also shows the same strengthening and toughening effects seen in mold-cast samples, demonstrating its compatibility with 3D printing processes. These findings suggest that brush-coil triblock copolymers additives at very low concentrations can synergistically improve the mechanical properties of epoxy resin for 3D printed parts.</p>
<p>Read full article here: <a href="https://www.sciencedirect.com/science/article/pii/S2666542523000668?via%3Dihub">https://www.sciencedirect.com/science/article/pii/S2666542523000668?via%3Dihub</a></p>
<p> </p>
<p><img title="A brush-coil triblock copolymer to enhance the mechanical properties of epoxy resin for 3D printing applications" src="https://imechanica.org/files/1-s2.0-S2666542523000668-ga1_lrg.jpg" alt="" width="800" height="329" /></p>
</div></div></div><div class="field field-name-upload field-type-file field-label-hidden"><div class="field-items"><div class="field-item even"><table class="sticky-enabled">
<thead><tr><th>Attachment</th><th>Size</th> </tr></thead>
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<tr class="odd"><td><span class="file"><img class="file-icon" alt="Image icon" title="image/jpeg" src="/modules/file/icons/image-x-generic.png" /> <a href="https://imechanica.org/files/1-s2.0-S2666542523000668-ga1_lrg.jpg" type="image/jpeg; length=419232" title="1-s2.0-S2666542523000668-ga1_lrg.jpg">A brush-coil triblock copolymer to enhance the mechanical properties of epoxy resin for 3D printing applications</a></span></td><td>409.41 KB</td> </tr>
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</div></div></div>Tue, 14 Nov 2023 18:56:43 +0000Ramathasan Thevamaran26984 at https://imechanica.orghttps://imechanica.org/node/26984#commentshttps://imechanica.org/crss/node/26984Assistant Professor in Fluid Dynamics at University of Wisconsin-Madison
https://imechanica.org/node/26862
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/73">job</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Dear Colleagues,</p>
<p>The Department of Mechanical Engineering (ME) at the University of Wisconsin-Madison is seeking applicants for a tenure-track Assistant Professor in the area of experimental fluid dynamics with applications to aerodynamics, turbulence, wind energy, flow control, bio-inspired flows, or related fields. Please see the job description here: <a href="https://jobs.wisc.edu/jobs/assistant-professor-in-fluid-dynamics-madison-wisconsin-united-states">https://jobs.wisc.edu/jobs/assistant-professor-in-fluid-dynamics-madison-wisconsin-united-states</a></p>
</div></div></div>Fri, 15 Sep 2023 20:58:17 +0000Ramathasan Thevamaran26862 at https://imechanica.orghttps://imechanica.org/node/26862#commentshttps://imechanica.org/crss/node/26862Assistant Professor in Polymers and Composites at University of Wisconsin-Madison
https://imechanica.org/node/26861
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/73">job</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Dear Colleagues,</p>
<p>The Department of Mechanical Engineering at the University of Wisconsin, Madison seeks candidates in polymer and composite processing and engineering to join their interdisciplinary advanced manufacturing group as a tenure-track Assistant Professor. Please see the job description here: <a href="https://jobs.wisc.edu/jobs/assistant-professor-in-polymers-and-composites-madison-wisconsin-united-states">https://jobs.wisc.edu/jobs/assistant-professor-in-polymers-and-composites-madison-wisconsin-united-states</a></p>
</div></div></div>Fri, 15 Sep 2023 20:56:59 +0000Ramathasan Thevamaran26861 at https://imechanica.orghttps://imechanica.org/node/26861#commentshttps://imechanica.org/crss/node/26861Assistant Professor in Structural Dynamics at the University of Wisconsin-Madison
https://imechanica.org/node/26860
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/73">job</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Dear Colleagues,</p>
<p>The Department of Mechanical Engineering at the University of Wisconsin-Madison invites applications for a tenure-track Assistant Professor position in structural dynamics. Please see the job description here: <a href="https://jobs.wisc.edu/jobs/assistant-professor-in-structural-dynamics-madison-wisconsin-united-states">https://jobs.wisc.edu/jobs/assistant-professor-in-structural-dynamics-madison-wisconsin-united-states</a></p>
</div></div></div>Thu, 14 Sep 2023 21:41:06 +0000Ramathasan Thevamaran26860 at https://imechanica.orghttps://imechanica.org/node/26860#commentshttps://imechanica.org/crss/node/26860A Postdoctoral Research Associate Position at the University of Wisconsin-Madison
https://imechanica.org/node/26789
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/73">job</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/8826">postdoctoral research associate</a></div><div class="field-item odd"><a href="/taxonomy/term/6419">hierarchical materials</a></div><div class="field-item even"><a href="/taxonomy/term/13845">dimensionally stable space composites</a></div><div class="field-item odd"><a href="/taxonomy/term/13846">micro/nanomechanics</a></div><div class="field-item even"><a href="/taxonomy/term/13847">thermomechanical testing</a></div><div class="field-item odd"><a href="/taxonomy/term/13559">Composite material design</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span>A Postdoctoral Research Associate Position is available immediately in Professor R. Thevamaran’s laboratory (<a href="https://thevamaran.engr.wisc.edu/"><span>https://thevamaran.engr.wisc.edu</span></a>) at the Department of Mechanical Engineering of the University of Wisconsin-Madison. The research will focus on design, fabrication, micro/nanostructural characterization, and thermo-mechanical testing, and modeling of carbon nanotube-based hierarchical composites. The candidate will get opportunities to work in a dynamic team focusing on challenging interdisciplinary composite materials research to enable next generation space structures. Highly motivated scholars with a PhD in Mechanical Engineering, Aerospace Engineering, Engineering Mechanics, or Materials Science and Engineering with research expertise in mechanics of materials are encouraged to apply. Prior experience on fabrication and testing of composite materials and nanofibrous yarns, microstructural characterizations, and additive manufacturing is desirable.<br />Interested recent graduates, or students who will be graduating soon shall email <a href="mailto:thevamaran@wisc.edu">Prof. Thevamaran</a> with their CV, two significant publications, and a cover letter. The one-page cover letter should clearly describe your prior research experience, research interests, and your fit to the research focus of Thevamaran Laboratory. The research at Thevamaran Lab will offer prospects of working on challenging research problems in an energetic team with ample opportunities for collaboration across disciplines. The University of Wisconsin-Madison is located in the City of Madison, which is known for its diverse community, high quality of life, and its natural beauty with multiple lakes and pleasant seasonal variations—often ranked as the best college town in the nation.</span></p>
</div></div></div>Fri, 28 Jul 2023 20:56:53 +0000Ramathasan Thevamaran26789 at https://imechanica.orghttps://imechanica.org/node/26789#commentshttps://imechanica.org/crss/node/26789Requisites on material viscoelasticity for exceptional points in passive dynamical systems
https://imechanica.org/node/26756
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/12815">Exceptional points</a></div><div class="field-item odd"><a href="/taxonomy/term/12577">non-Hermitian materials</a></div><div class="field-item even"><a href="/taxonomy/term/13427">#viscoelasticity</a></div><div class="field-item odd"><a href="/taxonomy/term/744">elastodynamics</a></div><div class="field-item even"><a href="/taxonomy/term/2222">Actuators and Sensors</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Dear Colleagues,</p>
<p>I invite you to read our recent publication in the Journal of Physics: Materials: <a href="https://iopscience.iop.org/article/10.1088/2515-7639/ace381">https://iopscience.iop.org/article/10.1088/2515-7639/ace381</a></p>
<p>Here, we demonstrate the necessary characteristics of viscoelastic materials to form an exceptional point degeneracy in elastodynamic framework. We experimentally show a few materials that respect such characteristics that can be used for designing passive (without the use of gain/amplification mechanisms) non-Hermitian systems for enhanced sensitivity, enhanced emissivity, and mechanical wave control.</p>
<p><img title="Exceptional point formation or lack thereof in non-Hermitian systems with different viscoelastic materials" src="https://imechanica.org/files/Figure_3_7.pdf" alt="Exceptional point formation or lack thereof in non-Hermitian systems with different viscoelastic materials" width="600" height="347" /> <strong>Abstract</strong>
</p><p>The recent progress of non-Hermitian physics and the notion of exceptional point (EP) degeneracies in elastodynamics has led to the development of novel metamaterials for the control of elastic wave propagation, hypersensitive sensors, and actuators. The emergence of EPs in a Parity-Time symmetric system relies on judiciously engineered balanced gain and loss mechanisms. Creating gain requires complex circuits and amplification mechanisms, making engineering applications challenging. Here, we report strategies to achieve EPs in passive non-Hermitian elastodynamic systems with differential loss derived from viscoelastic materials. We compare different viscoelastic material models and show that the EP emerges only when the frequency-dependent loss-tangent of the viscoelastic material remains nearly constant in the frequency range of operation. Such type of loss tangent occurs in materials that undergo stress-relaxation over a broad spectrum of relaxation times, for example, materials that follow the Kelvin-Voigt fractional derivative (KVFD) model. Using dynamic mechanical analysis, we show that a few common viscoelastic elastomers such as Polydimethylsiloxane (PDMS) and polyurethane rubber follow the KVFD behavior such that the loss tangent becomes almost constant after a particular frequency. The material models we present and the demonstration of the potential of a widely available material system in creating EPs pave the way for developing non-Hermitian metamaterials with hypersensitivity to perturbations or enhanced emissivity.</p>
<p> </p>
</div></div></div>Sun, 09 Jul 2023 15:32:52 +0000Ramathasan Thevamaran26756 at https://imechanica.orghttps://imechanica.org/node/26756#commentshttps://imechanica.org/crss/node/26756Journal Club for July 2023: A Space-Time Odyssey - Taming Exceptional Points in Elastodynamics for Sensitivity and Emissivity Enhancement and Asymmetric Wave Steering
https://imechanica.org/node/26740
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/12815">Exceptional points</a></div><div class="field-item odd"><a href="/taxonomy/term/12818">non-Hermitian systems</a></div><div class="field-item even"><a href="/taxonomy/term/13822">Parity-Time symmetric systems</a></div><div class="field-item odd"><a href="/taxonomy/term/13823">Degeneracies</a></div><div class="field-item even"><a href="/taxonomy/term/744">elastodynamics</a></div><div class="field-item odd"><a href="/taxonomy/term/1459">sensors</a></div><div class="field-item even"><a href="/taxonomy/term/6393">actuators</a></div><div class="field-item odd"><a href="/taxonomy/term/13824">Viscoelastic materials</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p class="MsoNormal"><strong><em>By: Ramathasan Thevamaran (Theva)</em></strong></p>
<p class="MsoNormal"><em>Department of Mechanical Engineering, University of Wisconsin-Madison, Madison, WI 53706, USA.</em></p>
<p class="MsoNormal"><em>Research Lab: </em><a href="https://thevamaran.engr.wisc.edu/"><em>https://thevamaran.engr.wisc.edu</em></a></p>
<p class="MsoNormal">Geometric symmetries—such as translation, rotation, mirror, and fractal symmetries—are commonly found in nature and are utilized often in architecture and engineering as well as for realizing photonic crystals, phononic crystals, and metamaterials. The dynamical (hidden) symmetries—such as time-reversal symmetry and parity-time (PT) symmetry—however, are not apparent in the system geometry and instead require a careful analysis of the equations of motion describing the dynamic behavior of the system to recognize them. Incorporation of such dynamical symmetries and their violations at certain critical points of the parameter space that control the system behavior can lead to intriguing and unusual consequences that have utility in engineering. This perspective article aims to present some of the recent advancements in an emerging field in mechanics—non-Hermitian elastodynamics [1-10]—with their prospective utility in engineering applications that rely on mechanical wave-matter interactions. While an introduction to non-Hermitian systems and exceptional point degeneracies (EPDs) is provided, this article is not intended to be a comprehensive review. Such detailed reviews can be found elsewhere [11-16].</p>
<p class="MsoNormal"><strong>What is an exceptional point degeneracy (EPD)? And what is its uniqueness?</strong></p>
<p class="MsoNormal">The EPD is a spectral singularity where two or more eigenvalues and the corresponding eigenvectors of a non-Hermitian system coalesce (see Figure 1 for an order two EPD where two eigenmodes coalesce). The non-Hermitian systems can have complex eigenvalues and non-orthogonal eigenvectors—in contrast to the Hermitian Hamiltonians that have real eigenvalues and orthogonal eigenvectors. Because of this, the system does not necessarily have conservative properties, for example, observables such as energy and momentum are not necessarily conserved. </p>
<p class="MsoNormal">A special class of these non-Hermitian systems is the Parity-Time (PT)-symmetric system, where the system is symmetric under combined Parity and Time reversal operations. As an example, Figure 1a illustrates a coupled mechanical oscillator (resonator) with one oscillator having energy loss (dissipative mechanism) and the other having an equivalent energy gain (amplification mechanism) respecting the PT-symmetry. When this system is Hermitian, i.e., neither gain nor loss is present, it exhibits two eigenmodes corresponding to an in-phase (low frequency) and an out-of-phase (high frequency) motion. When the gain and loss intensity (non-Hermitian strength) is increased while keeping them balanced, these two modes approach each other to coalesce at a critical non-Hermitian strength (Figure 1b). This coalescence point—EPD—is often interpreted as a phase transition point between an exact PT-phase (where the eigenvalues are real) and a broken PT-phase (where the eigenvalues are complex) (Figure 1b). The eigenvectors that describe the mode shapes are initially orthogonal when the system is Hermitian, and become non-orthogonal when the gain/loss is introduced, and eventually become parallel at the EPD.</p>
<p class="MsoNormal"><img title="Figure 1. (a) A Parity-Time (PT) symmetric elastodynamic system depicted by a coupled oscillators with one (blue) oscillator having loss (damping mechanism) and the other oscillator (red) having equivalent gain (amplification mechanism). The system is neither symmetric under Parity operation alone nor Time operation alone but recovers the symmetry under the combined PT-symmetry operation. (b) The formation of an exceptional point degeneracy (EPD) as the gain/loss intensity is increased; left axis shows the real part of the eigenfrequency and the right axis shows the imaginary part of the eigenfrequency; the regime where the eigenvalues are real (i.e., imaginary part is zero) is referred to as the exact PT-phase and the other as the broken PT-phase. (c) Comparison of an EPD (red) that forms in a non-Hermitian system as a function of the coupling between the two resonant elements to the commonly known diabolic point that forms in a Hermitian system, showing the sublinear bifurcation which enables hypersensitivity." src="https://imechanica.org/files/Figure%201_2.png" alt="Figure 1. (a) A Parity-Time (PT) symmetric elastodynamic system depicted by a coupled oscillators with one (blue) oscillator having loss (damping mechanism) and the other oscillator (red) having equivalent gain (amplification mechanism). The system is neither symmetric under Parity operation alone nor Time operation alone but recovers the symmetry under the combined PT-symmetry operation. (b) The formation of an exceptional point degeneracy (EPD) as the gain/loss intensity is increased; left axis shows the real part of the eigenfrequency and the right axis shows the imaginary part of the eigenfrequency; the regime where the eigenvalues are real (i.e., imaginary part is zero) is referred to as the exact PT-phase and the other as the broken PT-phase. (c) Comparison of an EPD (red) that forms in a non-Hermitian system as a function of the coupling between the two resonant elements to the commonly known diabolic point that forms in a Hermitian system, showing the sublinear bifurcation which enables hypersensitivity." width="600" height="466" /></p>
<p class="MsoNormal"><strong>Figure 1.</strong> <strong>(a)</strong> A Parity-Time (PT) symmetric elastodynamic system depicted by a coupled oscillators with one (blue) oscillator having loss (damping mechanism) and the other oscillator (red) having equivalent gain (amplification mechanism). The system is neither symmetric under Parity operation alone nor Time operation alone but recovers the symmetry under the combined PT-symmetry operation. <strong>(b)</strong> The formation of an exceptional point degeneracy (EPD) as the gain/loss intensity (non-Hermitian strength) is increased; left axis shows the real part of the eigenfrequency and the right axis shows the imaginary part of the eigenfrequency; the regime where the eigenvalues are real (i.e., the imaginary part is zero) is referred to as the exact PT-phase and the other as the broken PT-phase. <strong>(c)</strong> An EPD (red) that forms in a non-Hermitian system as a function of the coupling between the two resonant elements compared to the commonly known diabolic point (black) that forms in a Hermitian system, showing the sublinear bifurcation associated with EPD which enables hypersensitivity.</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">One of the unique features of these EPDs is the sublinear approach of the modes towards the EPD. It can be seen from the Figure 1c that in contrast to a Hermitian degeneracy such as a diabolic point which exhibits a linear bifurcation, the order-two EPD exhibits a square-root bifurcation. Besides this novel nonlinear behavior emerging from a linear system (note that there is no nonlinearity introduced to the system so far), this nature of the coalescence has immediate utility. Compared to diabolic points, the EPD exhibits hyper-sensitivity to small perturbations in the system parameters. Hence, it has been exploited to create hypersensitive electronic, optical, microwave, acoustic, electromechanical, and elastodynamic sensors depending on the framework in which they are implemented [1,16,17].</p>
<p class="MsoNormal">With this background, let me turn to a more interesting new viewpoint on the non-Hermitian systems where we are just starting to scratch the surface of.</p>
<p class="MsoNormal"><strong>Can we enhance signals using damping?</strong></p>
<p class="MsoNormal">Damping is commonly used as a design element to attenuate signals, i.e., mechanical waves and vibrations, in engineering systems. But can we use it to enhance signals, particularly without deteriorating the signal quality? The EPD allows us to do so as we have demonstrated experimentally in a recent study [5]. Besides what I have presented above about EPDs in PT-symmetric systems, EPDs can be realized even in the absence of any gain and purely with damping. However, it requires differential damping between the two elements—for example, if we consider the coupled oscillator system in Figure 2, introducing some damping to one and no damping to the other oscillator or introducing higher damping to one and lower damping to the other oscillator can potentially create an EPD. More generally, non-proportional damping where the damping matrix [<em>C</em>]does not commute with mass matrix [<em>M</em>] and stiffness matrix [<em>K</em>]—unlike in proportional damping case where [<em>K</em>]=<em>α</em>[<em>M</em>]+<em>β</em>[<em>K</em>]—can lead to EPDs [10]. This is quite interesting, because certain avoided crossings that we find in the band diagrams commonly studied in phononics and metamaterials could be engineered to form EPDs with the introduction of such non-proportional damping.</p>
<p class="MsoNormal"><img title="Figure 2. Comparison of EPDs in (a) a PT-symmetric system with balanced gain and loss and (b) a passive non-Hermitian system with differential damping (zero gain)." src="https://imechanica.org/files/Figure%202_7.jpg" alt="Figure 2. Comparison of EPDs in (a) a PT-symmetric system with balanced gain and loss and (b) a passive non-Hermitian system with differential damping (zero gain)." width="600" height="386" /></p>
<p><strong>Figure 2.</strong> Comparison of EPDs in<strong> (a)</strong> a PT-symmetric system with balanced gain and loss and <strong>(b)</strong> a passive non-Hermitian system with differential damping (zero gain). [6]</p>
<p class="MsoNormal">The consequences are quite remarkable. It has been argued theoretically [18-24] that the eigenvector degeneracy (EVD) associated with the EPDs can lead to an anomalous emissivity enhancement of a source when it is brought at the proximity of an EPD. As we demonstrated experimentally in a non-Hermitian elastodynamic system recently [5], the emitted power from an actuator is enhanced by a universal factor of four (exerted force is enhanced by two-fold) when the system is reconfigured near the EPD compared to its expected resonance-induced enhancement when the system operates far away from the EPD (Figure 3). More importantly, this enhancement is achieved while keeping the quality factor of the signal constant—an intriguing consequence of exact phase of non-Hermitian systems where the imaginary part of the signal remains constant during such reconfiguration (Figure 3d). In other words, incorporation of EPDs into mechanical systems such as MEMS resonators, nano-/AFM-indenters, and robotic actuators provides a new pathway to further boost the actuation power by four-fold while maintaining signal quality—importantly by entirely passive means with no gain/amplification elements. Beyond the technological implications, this finding also provides new insights at a fundamental level to the Purcell Physics [24] and Fermi’s Golden Rule of quantum mechanics [20] which links emission rates to local density of states (LDoS) of a system. Purcell showed that the spontaneous emission rate of a quantum emitter is enhanced by appropriately engineering its surrounding environment, and therefore, the LDoS [24]. These analogies can be understood clearly when the imaginary part of the Green’s function that describes our elastodynamic system is interpreted as the LDoS of the system [5].</p>
<p class="MsoNormal"><img title="Figure 3. Enhanced emissivity near an EPD. (a) A non-Hermitian elastodynamic system made of coupled resonant elements (a Hookean spring connected to a mass and a KVFD viscoelastic material connected to a mass) that supports the formation of an EPD as a function of the coupling spring stiffness. (b) Experimentally measured response showing the formation of an EPD with actuation force amplification. The (c) real and (d) imaginary parts of the modes showing the exact and broken phases and the EPD. (e) The force amplification factor in the exact phase increasing by a factor of 2 as the system approaches the EPD from far away (right-side of the curve); here, the y-axis shows the force amplification—i.e., output force divided by input force—which is further normalized by the force amplification corresponding to the system operating far away from the EPD (i.e., at higher coupling strength)." src="https://imechanica.org/files/Figure%203_1.png" alt="Figure 3. Enhanced emissivity near an EPD. (a) A non-Hermitian elastodynamic system made of coupled resonant elements (a Hookean spring connected to a mass and a KVFD viscoelastic material connected to a mass) that supports the formation of an EPD as a function of the coupling spring stiffness. (b) Experimentally measured response showing the formation of an EPD with actuation force amplification. The (c) real and (d) imaginary parts of the modes showing the exact and broken phases and the EPD. (e) The force amplification factor in the exact phase increasing by a factor of 2 as the system approaches the EPD from far away (right-side of the curve); here, the y-axis shows the force amplification—i.e., output force divided by input force—which is further normalized by the force amplification corresponding to the system operating far away from the EPD (i.e., at higher coupling strength)." width="600" height="508" /></p>
<p class="MsoNormal"><strong>Figure 3.</strong> Enhanced emissivity near an EPD in an entirely passive non-Hermitian system. <strong>(a)</strong> A non-Hermitian elastodynamic system made of coupled resonant elements (a Hookean spring connected to a mass and a KVFD viscoelastic material connected to a mass) that supports the formation of an EPD as a function of the coupling spring stiffness. <strong>(b)</strong> Experimentally measured response showing the formation of an EPD with actuation force amplification. The <strong>(c)</strong> real and <strong>(d) </strong>imaginary parts of the modes showing the exact and broken phases and the EPD. <strong>(e)</strong> The force amplification factor in the exact phase increasing by a (universal) factor of 2 as the system approaches the EPD from far away (right-side of the curve); here, the y-axis shows the force amplification—i.e., output force divided by input force--that is normalized by the force amplification corresponding to the system operating far away from the EPD (i.e., at higher coupling strength). [5]</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal"><strong>A materials challenge: How do we experimentally realize EPDs in non-Hermitian elastodynamics?</strong></p>
<p class="MsoNormal">While EPDs can be realized when balanced gain and loss are introduced, realization of EPDs in entirely passive systems with no gain seemed non-trivial. The simplest way to introduce damping in a load bearing elastodynamic system is by incorporating viscoelastic materials as the non-Hermitian elements. However, when we examined various viscoelastic materials that respect different models such as Kelvin-Voigt (KV), Standard Linear Solid (SLS), and Kelvin-Voigt Fractional Derivative (KVFD) viscoelastic models, it turns out only the KVFD viscoelastic material—described by a spring and a springpot in parallel—supports the formation of an EPD (Figure 4) [6]. The KV solids support only an approximate transition with no degeneracy. Even though an SLS material with short relaxation time can still support EPD in theory, such relaxation times are unrealistic. We trace the reasons for this to the loss tangent (<em>tan(δ)=Ed/Es</em>) of the material—given by the ratio of the loss modulus (imaginary part of the dynamic modulus) to the storage modulus (real part of the dynamic modulus)—remaining nearly constant in the frequency regime where the system operates [6]. Only a KVFD viscoelastic material such as polyurethane rubber and PDMS support EPD formation because of their broad spectrum of relaxation times while the natural rubber does not [6].</p>
<p class="MsoNormal"><img title="Figure 4. The viscoelastic material characteristics for the realization of an EPD." src="https://imechanica.org/files/Figure%204_1.png" alt="Figure 4. The viscoelastic material characteristics for the realization of an EPD." width="600" height="364" /></p>
<p><strong>Figure 4.</strong> The viscoelastic material characteristics for the realization of an EPD. [6]</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal"><strong>What are the challenges and potential opportunities?</strong></p>
<p class="MsoNormal">Last couple of decades have seen the design and realization of phononic crystals and metamaterials that utilize impedance contrast induced wave scatting to control mechanical waves and vibrations as well as to demonstrate exotic wave phenomena that were thought to be impossible, e.g., cloaking, and nonreciprocal transport. In this respect, non-Hermitian symmetries add further elegance to these phononic crystals and metamaterials where their impedance profiles are engineered to be complex rather than just real—i.e., incorporating damping (with/without gain) contrast instead of stiffness/density contrast—to unleash their potential further in unprecedented ways [2, 25-29].</p>
<p class="MsoNormal">Our prior studies [4] examining PT-symmetric quasiperiodic, aperiodic, and fractal metastructures, which exhibit scale-free emergence of EPDs in their unfolding (fractal) spectra, suggest that there are underlying universal rules that govern the emergence of EPDs in such non-Hermitian elastodynamic systems which could facilitate their design and experimental realization (Figure 5). For example, the non-Hermitian strength (gain/loss intensity) required to create an EPD turns out to be directly proportional and of the same order as the initial split between the two coalescing modes in the corresponding Hermitian spectrum (Figure 5e) [4]. The density of EPDs that emerge in the spectra could also be predicted by the fractal dimension of the initial Hermitian spectra (Figure 5f) [4]. Beyond these generalized scaling laws, such systems with fractal spectra themselves could potentially be exploited in interesting ways. For example, <em>could the scale-free nature of EPDs in systems exhibiting fractal spectra enable multi-point and multi-scale sensing? Can the scale-free nature of EPDs in fractal systems be exploited for multi-scale emissivity enhancement?</em> </p>
<p class="MsoNormal"><em><img title="Figure 5. The universal rules governing the formation of EPDs. Finite Element Models of (a) a PT-symmetric Aubrey-Harper quasi-periodic elastodynamic metastructure, (b) a PT-symmetric H-tree geometric fractal elastodynamic metastructure, (c) a PT-symmetric aperiodic (following Fibonacci sequence) elastodynamic metastructure. (d) A coupled-mode theory model that follows a Fibonacci substitution rule. All these systems exhibit an unfolding (fractal) spectrum as shown in the center and when the gain/loss intensity (non-Hermitian strength) is increased, numerous EPDs form at different regions of the spectra in a scale-free fashion enabling us to investigate the underling relations between initial modes in the Hermitian spectrum and the EPDs that form in the non-Hermitian spectra once the gain/loss is introduced. (e) The relationship between the initial split between the coalescing modes in the corresponding Hermitian spectrum (Δ_0) and the critical non-Hermitian strength (γ_EP) required to coalesce those modes. (f) The relationship between the density of EPDs in the spectrum and the fractal dimension of the spectrum." src="https://imechanica.org/files/Figure%205_0.png" alt="Figure 5. The universal rules governing the formation of EPDs. Finite Element Models of (a) a PT-symmetric Aubrey-Harper quasi-periodic elastodynamic metastructure, (b) a PT-symmetric H-tree geometric fractal elastodynamic metastructure, (c) a PT-symmetric aperiodic (following Fibonacci sequence) elastodynamic metastructure. (d) A coupled-mode theory model that follows a Fibonacci substitution rule. All these systems exhibit an unfolding (fractal) spectrum as shown in the center and when the gain/loss intensity (non-Hermitian strength) is increased, numerous EPDs form at different regions of the spectra in a scale-free fashion enabling us to investigate the underling relations between initial modes in the Hermitian spectrum and the EPDs that form in the non-Hermitian spectra once the gain/loss is introduced. (e) The relationship between the initial split between the coalescing modes in the corresponding Hermitian spectrum (Δ_0) and the critical non-Hermitian strength (γ_EP) required to coalesce those modes. (f) The relationship between the density of EPDs in the spectrum and the fractal dimension of the spectrum." width="600" height="274" /></em></p>
<p class="MsoNormal"><strong>Figure 5.</strong> The universal rules governing the formation of EPDs. Finite element models (FEM) of <strong>(a)</strong> a PT-symmetric Aubrey-Harper quasi-periodic metastructure, <strong>(b)</strong> a PT-symmetric H-tree geometric fractal metastructure, <strong>(c)</strong> a PT-symmetric aperiodic (following Fibonacci sequence) metastructure. <strong>(d)</strong> A coupled-mode theory model that follows a Fibonacci substitution rule. All these systems exhibit an unfolding (fractal) spectrum as shown in the center and when the gain/loss intensity (non-Hermitian strength) is increased, numerous EPDs form at different regions of the spectra in a scale-free fashion enabling us to investigate the underlying relations between the modes in the initial Hermitian spectrum and the EPDs that form in the non-Hermitian spectra once the gain/loss is introduced. <strong>(e)</strong> The relationship between the initial split between the coalescing modes in the corresponding Hermitian spectrum (Δ0) and the critical non-Hermitian strength (γEP) required to coalesce those modes. <strong>(f)</strong> The relationship between the density of EPDs in the spectrum and the fractal dimension <em>D</em> of the spectrum. [4]</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal"><em>Can a PT-symmetric defect in a periodic system yield anything useful?</em> We recently showed that a PT-symmetric defect embedded in a periodic metastructure itself enable the formation of multiple EPDs with their formation tailored by the defect position—i.e., the distance between the Parity-symmetric components of the defect—in addition to the non-Hermitian strength [7]. Besides giving additional experimental controls for tailoring the underlying relations between the initial spectra and the critical non-Hermitian strength required for the EPD formation, the ability to use merely a PT-symmetric defect embedded in a periodic media to create multiple EPDs is certainly beneficial for experimental realization compared to a more complex PT-symmetric system requiring gain and loss control in every unit cell [7]. <em>What are the other consequences of such PT-symmetric defects to the band topology?</em></p>
<p class="MsoNormal"><em>What happens to noise at the proximity to EPDs?</em> It has been pointed out that the hypersensitivity emerging from EPDs may not be beneficial if the noise is also amplified near the EPDs [30,31]. The noise can be of fundamental nature owing to the eigenbasis collapse at EPDs or of technical nature associated with the amplification mechanisms utilized for the realization of EPDs. In a recent study in electromechanical framework [17], we have shown that the signal-to-noise of an accelerometer can be significantly enhanced by overcoming the fundamental noise enhancement if the EPD sensor is weakly coupled to a transmission line where a transmission peak degeneracy (TPD) is realized, and its detuning is utilized as the measure of sensitivity. We have demonstrated a three-fold enhancement in signal-to-noise ratio near the TPD with high dynamic range where this TPD based accelerometer outperforms conventional linear sensors [17]. <em>While this study steers the utilization of EPDs towards beneficial applications, it also raises further questions and opens research opportunities to address the effects of framework/platform specific (technical) noise and strategies for realizing sensors with selectivity, high signal-to-noise ratio, and high dynamic range.</em></p>
<p class="MsoNormal">As we have shown in a prior study in acoustic framework [26], incorporation of dissipative (imaginary) nonlinearities—in contrast to stiffness (real) nonlinearities—can lead to novel asymmetric wave steering. For example, we showed that an amplitude-dependent asymmetric wave rectification with high frequency purity—i.e., rectification performed without higher harmonic generation—can be achieved. <em>How could such dissipative nonlinearities be engineered in material systems without the presence of stiffness nonlinearities? </em>(Note that this question has a fundamental limitation in the linear regime; e.g., the Kramer-Kronig relation dictates that the real and imaginary parts of the dynamic modulus of a viscoelastic material are intimately connected in linear viscoelasticity).</p>
<p class="MsoNormal"><em>Another promising direction is the exploitation of non-resonant EPDs, i.e., the formation of singularities in spectra of operators other than the effective Hamiltonian that describes the dynamics of the resonant system.</em> For example, in the framework of photonics, it is known for a while that EPDs in Bloch modes of a periodic structure can lead to slow light phenomena [32]. A similar approach can be taken in the framework of elastodynamic media for creating slow waves. Such systems do not involve gain or loss elements and instead exploit the non-Hermitian nature of the transfer matrix that describe the wave propagation across the unit cell of the periodic media. These types of EPDs have also been proposed for hypersensitive sensing [33] where the observable now is a sublinear (Wigner) cusp anomaly in the differential cross-section of scattering processes. To some extent, this way of utilizing EPDs can be thought of as an alternative to topological protection since the scattering characteristics remain robust against local perturbations [34] and respond only to global perturbations, e.g. frequency variations of the incident wave.</p>
<p class="MsoNormal">Last but not least, <em>what are the consequences of incorporating non-Hermitian elements in topological systems to topological phases [35] and boundary effects [36,37]?</em> This direction also has plethora of fundamental questions that could lead to interesting wave physics.</p>
<p class="MsoNormal">In summary, non-Hermitian symmetries engineered into metamaterials and phononic crystals can lead to unusual properties and functionalities that could benefit mechanical wave and vibration control, highly responsive robotic systems, and precision instrumentation.</p>
<p class="MsoNormal"><strong>Acknowledgements</strong></p>
<p class="MsoNormal">I would like to thank my present and past research <a href="https://thevamaran.engr.wisc.edu/research-team/">team members</a> Abhishek Gupta, Yanghao Fang, Jizhe Cai, Jiayan Zhang, Kyle Seledic, and Melissa Schmidt-Landin—whose contributions over the years on the non-Hermitian metamaterials research thrust of our lab educated me enough to draft the above article—for their enthusiastic commitment to science and our growth as a team. A special thanks to <a href="http://cqdmp.research.wesleyan.edu/">Prof. Tsampikos Kottos</a>, our theoretical physics collaborator from the Wesleyan University for his unwavering appreciation and commitment towards experimentation and engineering that led to our long-term collaboration resulting in the breadth and the depth of the work presented here. Finally, we acknowledge the financial support for this research thrust from the National Science Foundation, the Army Research Office, and the Wisconsin Alumni Research Foundation.</p>
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<p>23. A. Hashemi, K. Busch, D. Christodoulides, S. Ozdemir, R. El-Ganainy, Linear response theory of open systems with exceptional points, <a href="https://www.nature.com/articles/s41467-022-30715-8">Nature Commun</a>. 13 (1) (2022).</p>
<p>24. E.M. Purcell, H.C. Torrey, R.V. Pound, Resonance absorption by nuclear magnetic moments in a solid, <a href="https://journals.aps.org/pr/abstract/10.1103/PhysRev.69.37">Phys. Rev</a>. 69 (1–2) (1946).</p>
<p>25. J. Christensen, M. Willatzen, V. R. Velasco, M.-H. H. Lu, Parity-Time Synthetic Phononic Media. <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.207601">Phys. Rev. Lett</a>. 116, 1–5 (2016).</p>
<p>26. R. Thevamaran, R. M. Branscomb, E. Makri, P. Anzel, D. Christodoulides, T. Kottos, E. L. Thomas; Asymmetric acoustic energy transport in non-Hermitian metamaterials. <a href="https://pubs.aip.org/asa/jasa/article/146/1/863/995626/Asymmetric-acoustic-energy-transport-in-non">J Acoust Soc Am</a> 146 (1): 863–872 (2019).</p>
<p>27. C. Shi et al., Accessing the exceptional points of parity-time symmetric acoustics. <a href="https://www.nature.com/articles/ncomms11110">Nat. Commun</a>. 7, 1–5 (2016).</p>
<p>28. D. Psiachos, M. M. Sigalas, Acoustic response in a one-dimensional layered pseudo-Hermitian metamaterial containing defects. <a href="https://pubs.aip.org/aip/jap/article-abstract/123/24/245109/155430/Acoustic-response-in-a-one-dimensional-layered?redirectedFrom=fulltext">J. Appl. Phys</a>. 123, 1–8 (2018).</p>
<p>29. V. Achilleos, G. Theocharis, O. Richoux, V. Pagneux, Non-Hermitian acoustic metamaterials: Role of exceptional points in sound absorption. <a href="https://journals.aps.org/prb/abstract/10.1103/PhysRevB.95.144303"><em>Phys. Rev. B</em></a>. <strong>95</strong>, 144303 (2017).</p>
<p>30. Y.-H. Lai, Y.-K. Lu, M.-G. Suh, Z. Yuan, K. Vahala, Observation of the exceptional-point-enhanced Sagnac effect. <a href="https://www.nature.com/articles/s41586-019-1777-z"><em>Nature</em></a> <strong>576</strong>, 65–69 (2019).</p>
<p>31. H. Wang, Y.-H. Lai, Z. Yuan, M.-G. Suh, K. Vahala, Petermann-factor sensitivity limit near an exceptional point in a Brillouin ring laser gyroscope. <a href="https://www.nature.com/articles/s41467-020-15341-6"><em>Nat. Commun</em></a><em>.</em> <strong>11</strong>, 1610 (2020).</p>
<p>32. Figotin, A., Vitebskiy, I. Slow light in photonic crystals. <a href="https://www.tandfonline.com/doi/abs/10.1080/17455030600836507">Waves Random Complex Media</a> 16, 293 (2006).</p>
<p>33. W. Tuxbury, R. Kononchuk, T. Kottos, Non-resonant exceptional points as enablers of noise-resilient sensors, <a href="https://www.nature.com/articles/s42005-022-00973-5">Communications Physics</a> 5, 210 (2022)</p>
<p>34. H. Li, I. Vitebskiy, T. Kottos, Frozen mode regime in finite periodic structures, <a href="https://journals.aps.org/prb/abstract/10.1103/PhysRevB.96.180301">Phys. Rev. B</a> 96, 180301(R) (2017)</p>
<p>35. Z. Gong <em>et al.</em>, Topological Phases of Non-Hermitian Systems. <a href="https://journals.aps.org/prx/abstract/10.1103/PhysRevX.8.031079"><em>Phys. Rev. X</em>.</a> <strong>8</strong>, 031079 (2018).</p>
<p>36. N. Okuma, M. Sato, Non-Hermitian Topological Phenomena: A Review. <a href="https://www.annualreviews.org/doi/abs/10.1146/annurev-conmatphys-040521-033133">Annu. Rev. Condens. Matter Phys</a>. 14, 83–107 (2023).</p>
<p>37. Ding, K., Fang, C. & Ma, G. Non-Hermitian topology and exceptional-point geometries. <a href="https://www.nature.com/articles/s42254-022-00516-5"><em>Nat Rev Phys</em></a> <strong>4</strong>, 745–760 (2022).</p>
</div></div></div>Fri, 30 Jun 2023 22:58:25 +0000Ramathasan Thevamaran26740 at https://imechanica.orghttps://imechanica.org/node/26740#commentshttps://imechanica.org/crss/node/26740Orientation-dependent plasticity mechanisms control synergistic property improvement in dynamically deformed metals
https://imechanica.org/node/26719
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Dear Colleagues,</p>
<p>I invite you to read our recent paper on the orientation dependent plasticity mechanisms that control the dynamic creation of nanostructures and their behavior under subsequent quasistatic microcompression published in the <em>International Journal of Plasticity</em>: <a href="https://doi.org/10.1016/j.ijplas.2023.103657">https://doi.org/10.1016/j.ijplas.2023.103657</a></p>
<p>Abstract<br /></p><p id="spara011">We demonstrate the ability to synergistically enhance strength and toughness in metals by tailoring the dominant plasticity mechanisms from impact-induced heterogeneous nanostructures. Using a laser-induced projectile impact testing apparatus, we impact initially dislocation-free single crystal microcubes at high velocities to induce grain size and dislocation density gradients. These gradient nanostructures exhibit heterogeneous deformation under subsequent quasi-static loading leading to enhanced strength and toughness. Transmission Kikuchi diffraction (TKD) analyses show that the synergistic property improvement arises from the complimentary intragranular and intergranular plasticity mechanisms: the pronounced intragranular dislocation plasticity within coarse grained regions provides increased strain hardening and toughness while the nanograined regions with high dislocation densities provide high strength and supplemental toughness enhancement through cooperative grain rotation and grain boundary migration. These plasticity mechanisms are activated to different extents depending on the specific impact-induced nanostructures, which depend on the orientation of the single crystal upon impact. Controlled [100]-face, [110]-edge, and [111]-corner impact of single crystal microcubes, subsequent quasi-static mechanical testing, and pre- and post-compression nanostructural characterization provide a fundamental understanding of the comprehensive process-structure-property relations in heterogeneous nanostructured metals.</p>
<p><img src="https://imechanica.org/files/1-s2.0-S0749641923001432-gr1.jpg" alt="" width="678" height="401" /></p>
</div></div></div>Tue, 13 Jun 2023 04:16:23 +0000Ramathasan Thevamaran26719 at https://imechanica.orghttps://imechanica.org/node/26719#commentshttps://imechanica.org/crss/node/26719Synergistic strength and toughness through impact-induced nanostructural evolutions in metals
https://imechanica.org/node/26718
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Dear Colleagues,</p>
<p>I invite you to read our recent paper on the process-structure-property relations in heterogeneous nanostructured metals created through a laser induced projectile impact of single crystal microcubes published in the <em>Extreme Mechanics Letters</em>: <a href="https://doi.org/10.1016/j.eml.2023.102037">https://doi.org/10.1016/j.eml.2023.102037</a></p>
<p><strong>Abstract</strong><br /></p><p id="d1e163">Synergistically improving strength and toughness is imperative to designing next generation materials for critical engineering applications. Incorporating structural heterogeneities at the nanoscale can synergistically improve the strength and toughness of metals through eliciting complementary deformation mechanisms. Here, we present a synergistic enhancement in strength and toughness in microscale samples with a gradient-nano-grained structure produced through impact-induced recrystallization of initially single-crystal defect-free silver microcubes. The impacted samples exhibit ultra-high strain hardening rates and up to 4.7x the yield strength and 5x the toughness as the single-crystal silver micropillars, and up to 7.4x the yield strength and 4x the toughness as bulk coarse-grained silver. Synergistic improvements in mechanical properties are achieved through the activation of intragranular dislocation slip and intergranular grain boundary rotation and migration enabled through inversely correlated gradients in grain size and dislocation density.</p>
<p><img src="https://imechanica.org/files/1-s2.0-S2352431623000834-gr1_0.jpg" alt="" width="320" height="469" /></p>
</div></div></div>Tue, 13 Jun 2023 04:08:28 +0000Ramathasan Thevamaran26718 at https://imechanica.orghttps://imechanica.org/node/26718#commentshttps://imechanica.org/crss/node/26718Disrupting Density-Dependent Property Scaling in Hierarchically Architected Foams
https://imechanica.org/node/26682
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/13800">stiffness−density scaling</a></div><div class="field-item odd"><a href="/taxonomy/term/13801">architected foams</a></div><div class="field-item even"><a href="/taxonomy/term/13802">vertically aligned carbon nanotube foams</a></div><div class="field-item odd"><a href="/taxonomy/term/13803">structural hierarchy</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Dear Colleagues,</p>
<p>I invite you to read our latest paper published in ACS Nano: <a href="https://pubs.acs.org/doi/10.1021/acsnano.3c01223">https://pubs.acs.org/doi/10.1021/acsnano.3c01223</a></p>
<p>Creating lightweight architected foams as strong and stiff as their bulk constituent material has been a long-standing effort. Typically, the strength, stiffness, and energy dissipation capabilities of materials severely degrade with increasing porosity. We report nearly constant stiffness-to-density and energy dissipation-to-density ratios─a linear scaling with density─in hierarchical vertically aligned carbon nanotube (VACNT) foams with a mesoscale architecture of hexagonally close-packed thin concentric cylinders. We observe a transformation from an inefficient higher-order density-dependent scaling of the average modulus and energy dissipated to a desirable linear scaling as a function of the increasing internal gap between the concentric cylinders. From the scanning electron microscopy of the compressed samples, we observe an alteration in the deformation modality from local shell buckling at a smaller gap to column buckling at a larger gap, governed by an enhancement in the number density of CNTs with the increasing internal gap, leading to better structural stiffness at low densities. This transformation simultaneously improves the foams’ damping capacity and energy absorption efficiency as well and allows us to access the ultra-lightweight regime in the property space. Such synergistic scaling of material properties is desirable for protective applications in extreme environments.</p>
<p><img src="https://imechanica.org/files/nn3c01223_0008.jpg" alt="" width="500" height="265" /></p>
</div></div></div>Fri, 26 May 2023 20:33:18 +0000Ramathasan Thevamaran26682 at https://imechanica.orghttps://imechanica.org/node/26682#commentshttps://imechanica.org/crss/node/26682Dynamic Hardness Evolution in Metals from Impact Induced Gradient Dislocation Density
https://imechanica.org/node/26547
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Dear Colleagues,</p>
<p>I invite you to read our recent publication on Acta Materialia on the dynamic hardness evolution in metals.</p>
<p>Abstract<br /></p><p id="spara007">A clear understanding of the dynamic behavior of metals is critical for developing superior structural materials as well as for improving material processing techniques such as cold spray and shot peening. Using a high-velocity (from ∼120 m/s to 700 m/s; strain rates >107 1/s) micro-projectile impact testing and quasistatic (strain rates: 10−21/s) nanoindentation, we investigate the strain-rate-dependent mechanical behavior of single-crystal aluminum substrates with (001), (011), and (111) crystal orientations. For all three crystal orientations, the dynamic hardness initially increases with increasing impact velocity and reaches a plateau regime at hardness 5 times higher than that of at quasistatic indentations. Based on coefficient of restitution and post-mortem transmission Kikuchi diffraction analyses, we show that distinct plastic deformation mechanisms with a gradient dislocation density evolution govern the dynamic behavior. We also discover a distinct deformation regime—stable plastic regime—that emerges beyond the deeply plastic regime with unique strain rate insensitive microstructure evolution and dynamic hardness. Our work additionally demonstrates an effective approach to introduce strong spatial gradients in dislocation density in metals by high-velocity projectile impacts to enhance surface mechanical properties, as it can be employed in material processing techniques such as shot peening and surface mechanical attrition treatment.</p>
<p>Read full paper at: <a href="https://authors.elsevier.com/a/1ggZ84r9SUTLyg" target="_blank" rel="noopener noreferrer">https://authors.elsevier.com/a/1ggZ84r9SUTLyg</a></p>
<p><img src="https://imechanica.org/files/1-s2.0-S1359645423001386-ga1_lrg.jpg" width="600" height="315" /></p>
</div></div></div>Wed, 01 Mar 2023 22:34:51 +0000Ramathasan Thevamaran26547 at https://imechanica.orghttps://imechanica.org/node/26547#commentshttps://imechanica.org/crss/node/26547Reconfigurable enhancement of actuation forces by engineered losses in non-Hermitian metamaterials
https://imechanica.org/node/26535
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Dear Colleagues,
</p><p>I invite you to read our recent Extreme Mechanics Letters paper presenting a novel viewpoint on enhancing emissivity near exceptional point singularities in a non-Hermitian metamaterial. We present the experiments and numerical modeling in an elastodynamic framework--where the actuation forces from an actuator are enhanced by coupling to a passive dissipative non-Hermitian metamaterial--and present the theory further generalized and applicable to other physical frameworks from acoustics to optics and microwaves.</p>
<p>Abstract<br /></p><p id="d1e513">While boosting signals with amplification mechanisms is a well-established approach, attenuation mechanisms are typically considered an anathema because they degrade the efficiency of the structures employed to perform useful operations on these signals. An emerging alternate viewpoint promotes losses as a novel design element by utilizing the notion of exceptional point degeneracies (EPDs)—points in parameter space where the eigenvalues of the underlying system and the associated eigenvectors simultaneously coalesce. Here, we demonstrate a direct consequence of such eigenbasis collapse in elastodynamics—an unusual enhancement of actuation force by a judiciously designed non-Hermitian metamaterial supporting an EPD that is coupled to an actuation source. Intriguingly, the EPD enables this enhancement while maintaining a constant signal quality. Our work constitutes a proof-of-principle design which can promote a new class of reconfigurable nano-indenters and robotic-actuators. Importantly, it reveals the ramifications of non-Hermiticity in boosting the Purcell emissivity enhancement factor beyond its expected value, which can guide the design of metamaterials with enhanced emission that does not deteriorate signal quality for mechanical, acoustic, optical, and photonic applications.</p>
<p>Read the full article here: <a href="https://doi.org/10.1016/j.eml.2023.101979">https://doi.org/10.1016/j.eml.2023.101979</a></p>
<p><img src="https://imechanica.org/files/1-s2.0-S2352431623000251-gr1.sml_.gif" alt="" /></p>
</div></div></div>Sun, 19 Feb 2023 17:17:44 +0000Ramathasan Thevamaran26535 at https://imechanica.orghttps://imechanica.org/node/26535#commentshttps://imechanica.org/crss/node/26535Superior mechanical properties by exploiting size-effects and multiscale interactions in hierarchically architected foams
https://imechanica.org/node/26297
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/6419">hierarchical materials</a></div><div class="field-item odd"><a href="/taxonomy/term/468">mechanical properties</a></div><div class="field-item even"><a href="/taxonomy/term/10891">Energy absorption</a></div><div class="field-item odd"><a href="/taxonomy/term/3653">thin walled structures</a></div><div class="field-item even"><a href="/taxonomy/term/218">buckling</a></div><div class="field-item odd"><a href="/taxonomy/term/264">carbon nanotubes</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Dear Colleagues,</p>
<p>I invite you to read our recent paper on hierarchically architected foams published in <em>Extreme Mechanics Letters</em>: <a href="https://authors.elsevier.com/a/1fy4l8MuOhC1HQ"><strong>Read full article here</strong></a>.</p>
<p><strong>Abstract</strong></p>
<p>Protective applications in extreme environments demand materials with superior modulus, strength, and specific energy absorption (SEA) at lightweight. They must also have the ability to attenuate intense stress waves and absorb kinetic energy from impact while providing thermally stable functionality. However, these properties typically have a trade-off. Hierarchically architected materials—such as the architected vertically aligned carbon nanotube (VACNT) foams—offer the potential to overcome these trade-offs and achieve synergistic enhancement in mechanical properties because of their multiscale origins of bulk properties derived from structural features that span nano to millimeter scales. Such architected materials with complex hierarchical structures require careful investigation of the effects of multitier design parameters and their interactions on the resultant bulk mechanical properties. Here, we adopt a full-factorial design of experiments (DOE) approach to identify an optimal set of design parameters to achieve synergistic enhancement in SEA, compressive strength, and modulus at lightweight in VACNT foams with mesoscale cylindrical architecture. We exploit size effects from geometrically-confined synthesis and highly interactive morphology of the CNTs to enable higher-order design parameter interactions that intriguingly disrupt the diameter-to-thickness (D/t)-dependent scaling laws found in common architected materials having steel and composite tubular structures. We show that exploiting complementary hierarchical mechanisms in architected material design can lead to superior and synergistic enhancement of mechanical properties and performance desirable for extreme protective applications. </p>
<p><img src="https://imechanica.org/files/1-s2.0-S2352431622001754-ga1_lrg.jpg" width="600" height="226" /></p>
</div></div></div>Thu, 20 Oct 2022 15:37:20 +0000Ramathasan Thevamaran26297 at https://imechanica.orghttps://imechanica.org/node/26297#commentshttps://imechanica.org/crss/node/26297Emergence of Exceptional Points in Periodic Metastructures with Hidden PT-symmetric Defects
https://imechanica.org/node/26263
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/12816">Parity-Time symmetry</a></div><div class="field-item odd"><a href="/taxonomy/term/744">elastodynamics</a></div><div class="field-item even"><a href="/taxonomy/term/3280">defects</a></div><div class="field-item odd"><a href="/taxonomy/term/13599">#metamaterials</a></div><div class="field-item even"><a href="/taxonomy/term/12454">Elastic Wave</a></div><div class="field-item odd"><a href="/taxonomy/term/12815">Exceptional points</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Dear Colleagues, Our recent publication on the Journal of Applied Mechanics:</p>
<p>We study the elastodynamics of a periodic metastructure incorporating a defect pair that enforces a parity-time (PT) symmetry due to a judiciously engineered imaginary impedance elements– one having energy amplification (gain) and the other having equivalent attenuation (loss) mechanism. We show that their presence affects the initial band structure of the periodic Hermitian metastructure and leads to the formation of numerous exceptional points (EPs) which are mainly located at the band edges where the local density of modes is higher. The spatial location of the PT-symmetric defect serves as an additional control over the number of emerging EPs in the corresponding spectra as well as the critical non-Hermitian (gain/loss) strength required to create the first EP–a specific defect location minimizes the critical non-Hermitian strength. We use both finite element and coupled-mode-theory-based models to investigate these metastructures, and use a time-independent second-order perturbation theory to further demonstrate the influence of the size of the metastructure and the PT-symmetric defect location on the minimum non-Hermitian strength required to create the first EP in a band. Our findings motivate feasible designs for experimental realization of EPs in elastodynamic metastructures.</p>
<p>Read the full article here: <a class="ww-doi-link" href="https://doi.org/10.1115/1.4055618" target="_blank" rel="noopener noreferrer">https://doi.org/10.1115/1.4055618</a></p>
</div></div></div>Thu, 06 Oct 2022 15:35:17 +0000Ramathasan Thevamaran26263 at https://imechanica.orghttps://imechanica.org/node/26263#commentshttps://imechanica.org/crss/node/26263Exceptional-point-based accelerometers with enhanced signal-to-noise ratio
https://imechanica.org/node/26132
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/12815">Exceptional points</a></div><div class="field-item odd"><a href="/taxonomy/term/12818">non-Hermitian systems</a></div><div class="field-item even"><a href="/taxonomy/term/12816">Parity-Time symmetry</a></div><div class="field-item odd"><a href="/taxonomy/term/12872">#Sensors</a></div><div class="field-item even"><a href="/taxonomy/term/5522">accelerometer</a></div><div class="field-item odd"><a href="/taxonomy/term/12817">Hypersensitivity</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Exceptional points (EP) are non-Hermitian degeneracies where eigenvalues and their corresponding eigenvectors coalesce. Recently, EPs have attracted attention as a means to enhance the responsivity of sensors, through the abrupt resonant detuning occurring in their proximity. In many cases, however, the EP implementation is accompanied by noise enhancement, leading to the degradation of the sensor’s performance. The excess noise can be of fundamental nature (owing to the eigenbasis collapse) or of technical nature associated with the amplification mechanisms utilized for the realization of EPs. Here we show, using an EP-based parity–time symmetric electromechanical accelerometer, that the enhanced technical noise can be surpassed by the enhanced responsivity to applied accelerations. The noise owing to eigenbasis collapse is mitigated by exploiting the detuning from a transmission peak degeneracy (TPD) — which forms when the sensor is weakly coupled to transmission lines — as a measure of the sensitivity. These TPDs occur at a frequency and control parameters for which the biorthogonal eigenbasis is still complete and are distinct from the EPs of the parity–time symmetric sensor. Our device shows a threefold signal-to-noise-ratio enhancement compared with configurations for which the system operates away from the TPD.</p>
<p>Read the full article here: <a href="https://www.nature.com/articles/s41586-022-04904-w">https://www.nature.com/articles/s41586-022-04904-w</a></p>
<p> </p>
</div></div></div>Wed, 27 Jul 2022 22:02:04 +0000Ramathasan Thevamaran26132 at https://imechanica.orghttps://imechanica.org/node/26132#commentshttps://imechanica.org/crss/node/26132Origins of mechanical preconditioning in hierarchical nanofibrous materials
https://imechanica.org/node/25638
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Dear Colleagues,</p>
<p>I invite you to read our recent paper on Origins of mechanical preconditioning in hierarchical nanofibrous materials, published in Extreme Mechanics Letters.</p>
<p><span><strong>Abstract</strong></span></p>
<p id="d1e365"><span>Many structured materials exhibit softening in constitutive response for the first few loading-unloading cycles–a phenomenon known as the preconditioning effect. However, the micro and nanostructural mechanisms responsible for the preconditioning effect are not well understood. We investigate the multiscale origins of the preconditioning effect in vertically aligned carbon nanotube (VACNT) foams using synchrotron X-ray scattering and mass attenuation measurements. Because of their self-organized nanofibrous structure that spans a broad lengthscale from a few angstroms to several millimeters, the VACNT foams serve as an excellent model system for soft hierarchical materials. Their hierarchical structure also leads to superior mechanical properties that are critical for extreme engineering applications. They exhibit a softening hysteretic response in the initial few compression cycles, which then remains constant over thousands of subsequent cycles. When compressed, they deform through a sequentially progressive collective buckling of the CNTs, and have the ability to recover from large strains upon unloading. We observe that the preconditioning effects arise not only from mesoscale reorganization of nanofibers as hypothesized previously, but also permanent strain in the nanoscale structure. Our findings provide insights for engineering hierarchical fibrous materials to achieve superior sustained hysteretic energy absorption and strain recovery as well as guidance for the development of physically-motivated models.</span></p>
<p><span>Read full article here: <a href="https://www.sciencedirect.com/science/article/abs/pii/S2352431621002352?via%3Dihub">https://www.sciencedirect.com/science/article/abs/pii/S2352431621002352?...</a></span></p>
<p> </p>
</div></div></div>Fri, 17 Dec 2021 03:18:50 +0000Ramathasan Thevamaran25638 at https://imechanica.orghttps://imechanica.org/node/25638#commentshttps://imechanica.org/crss/node/25638Extreme Dynamic Performance of Nanofiber Mats under Supersonic Impacts Mediated by Interfacial Hydrogen Bonds
https://imechanica.org/node/25637
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/6094">Nanofibers</a></div><div class="field-item odd"><a href="/taxonomy/term/264">carbon nanotubes</a></div><div class="field-item even"><a href="/taxonomy/term/8939">kevlar</a></div><div class="field-item odd"><a href="/taxonomy/term/13119">Ballistic Impact Response</a></div><div class="field-item even"><a href="/taxonomy/term/13333">interfacial interactions</a></div><div class="field-item odd"><a href="/taxonomy/term/9858">dynamic performance</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>Dear Colleagues,</p>
<p>I invite you to read our recent paper on the Extreme Dynamic Performance of Nanofiber Mats under Supersonic Impacts Mediated by Interfacial Hydrogen Bonds published on <em>ACS Nano</em>.</p>
<p><strong>Abstract</strong></p>
<p><span><span>Achieving extreme dynamic performance in nanofibrous materials requires synergistic exploitation of intrinsic nanofiber properties and inter-fiber interactions. Regardless of the superior intrinsic stiffness and strength of carbon nanotubes (CNTs), the weak nature of van der Waals interactions limits the CNT mats from achieving greater performance. We present an efficient approach to augment the inter-fiber interactions by introducing aramid nanofiber (ANF) links between CNTs, which forms stronger and reconfigurable interfacial hydrogen bonds and π–π stacking interactions, leading to synergistic performance improvement with failure retardation. Under supersonic impacts, strengthened interactions in CNT mats enhance their specific energy absorption up to 3.6 MJ/kg, which outperforms widely used bulk Kevlar-fiber-based protective materials. The distinct response time scales of hydrogen bond breaking and reformation at ultrahigh-strain-rate (∼10</span><span>7</span><span>–10</span><span>8</span><span> s</span><span>–1</span><span>) deformations additionally manifest a strain-rate-dependent dynamic performance enhancement. Our findings show the potential of nanofiber mats augmented with interfacial dynamic bonds─such as the hydrogen bonds─as low-density structural materials with superior specific properties and high-temperature stability for extreme engineering applications.</span></span></p>
<p><span><span>Read full article at: <a href="https://pubs.acs.org/doi/10.1021/acsnano.1c07465">https://pubs.acs.org/doi/10.1021/acsnano.1c07465</a></span></span></p>
</div></div></div>Fri, 17 Dec 2021 03:12:41 +0000Ramathasan Thevamaran25637 at https://imechanica.orghttps://imechanica.org/node/25637#commentshttps://imechanica.org/crss/node/25637Origins of size effects in initially dislocation-free single-crystal silver micro- and nanocubes
https://imechanica.org/node/25305
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/1641">Size effects</a></div><div class="field-item odd"><a href="/taxonomy/term/13199">single crystals</a></div><div class="field-item even"><a href="/taxonomy/term/8273">silver</a></div><div class="field-item odd"><a href="/taxonomy/term/169">Plasticity</a></div><div class="field-item even"><a href="/taxonomy/term/13200">crystal defects</a></div><div class="field-item odd"><a href="/taxonomy/term/13201">MD simulations</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span>We report phenomenal yield strengths—up to one-fourth of the theoretical strength of silver—recorded in microcompression testing of initially dislocation-free silver micro- and nanocubes synthesized from a multistep seed-growth process. These high strengths and the massive strain bursts that occur upon yield are results of the initially dislocation-free single-crystal structure of the pristine samples that yield through spontaneous nucleation of dislocations. When the pristine samples are exposed to a focused ion-beam to fabricate pillars and then compressed, the dramatic strain burst does not occur, and they yield at a quarter of the strength compared to the pristine counterparts. Regardless of the defect-state of the samples prior to testing, a size effect is apparent—where the yield strength increases as the sample size decreases. Since dislocation starvation and the single-arm-source mechanisms cannot explain a size effect on yield strength in dislocation-free samples, we investigate the dislocation nucleation mechanisms controlling the size effect through careful experimental observations and molecular statics simulations. We find that intrinsic or extrinsic symmetry breakers such as surface defects, e</span><span>dge roundness, external sample shape, or a high vacancy concentration can influence dislocation nucleation, and thus contribute to the size effect on yield strength in initially dislocation-free samples.</span></p>
<p> </p>
<p>Read the full article here: <a href="https://www.sciencedirect.com/science/article/pii/S1359645421004006?dgcid=author">https://www.sciencedirect.com/science/article/pii/S1359645421004006?dgci...</a></p>
<p> </p>
</div></div></div>Sat, 03 Jul 2021 17:35:30 +0000Ramathasan Thevamaran25305 at https://imechanica.orghttps://imechanica.org/node/25305#commentshttps://imechanica.org/crss/node/25305Universal route for the emergence of exceptional points in PT-symmetric metamaterials with unfolding spectral symmetries
https://imechanica.org/node/25304
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/632">metamaterials</a></div><div class="field-item odd"><a href="/taxonomy/term/12816">Parity-Time symmetry</a></div><div class="field-item even"><a href="/taxonomy/term/12577">non-Hermitian materials</a></div><div class="field-item odd"><a href="/taxonomy/term/12815">Exceptional points</a></div><div class="field-item even"><a href="/taxonomy/term/8659">fractal materials</a></div><div class="field-item odd"><a href="/taxonomy/term/744">elastodynamics</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span>We introduce a class of parity-time symmetric elastodynamic metamaterials (Ed-MetaMater) whose Hermitian counterpart exhibits unfolding (fractal) spectral symmetries. Our study reveals a scale-free formation of exceptional points in those Ed-MetaMaters whose density is dictated by the fractal dimension of their Hermitian spectra. We demonstrate this scale-free EP-formation in a quasi-periodic Aubry-Harper Ed-MetaMater, a geometric H-tree-fractal Ed-MetaMater, and an aperiodic Fibonacci Ed-MetaMater—each having a specific fractal spectrum—using finite element models and establish a universal route for EP-formation via a coupled-mode theory model with controllable fractal spectrum. This universality may enable the rational design of novel Ed-MetaMater for hypersensitive sensing and elastic wave control.</span></p>
<p><span>Read the full article here: </span><a href="https://iopscience.iop.org/article/10.1088/1367-2630/ac09c9">https://iopscience.iop.org/article/10.1088/1367-2630/ac09c9</a></p>
<p> </p>
</div></div></div>Sat, 03 Jul 2021 17:30:27 +0000Ramathasan Thevamaran25304 at https://imechanica.orghttps://imechanica.org/node/25304#commentshttps://imechanica.org/crss/node/25304A Postdoctoral Research Associate Position at the University of Wisconsin-Madison
https://imechanica.org/node/24485
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>A Postdoctoral Research Associate Position is available (Fall 2020) in <a href="https://thevamaran.engr.wisc.edu">Professor R. Thevamaran’s laboratory</a> at the Department of Engineering Physics of the University of Wisconsin-Madison to study the dynamic behavior of hierarchical materials. The research will focus on design, synthesis, structural characterization, and quasistatic and dynamic testing, and modeling of carbon nanotube-based hierarchical materials and composites. The candidate will get opportunities to work in an interdisciplinary multi-university-industrial partnership team (<a href="https://www.panther.engr.wisc.edu">PANTHER</a>) that is working to understand traumatic brain injury and to develop new materials that address the challenges in preventing traumatic brain injury. Highly motivated scholars with a PhD in Mechanical Engineering, Aerospace Engineering, Engineering Mechanics, or Materials Science and Engineering with research expertise in impact mechanics are encouraged to apply. Prior experience in working with Kolsky bar/ Split-Hopkinson pressure bar testing, Direct impact testing, or Plate impact testing is desirable.</p>
<p>Interested recent graduates, or students who will be graduating soon shall email (<a href="mailto:thevamaran@wisc.edu">thevamaran@wisc.edu</a>) Prof. Thevamaran with their CV, two significant publications, and a cover letter. The one-page cover letter should clearly describe your prior research experiences, research interests, and your fit to the research focus of Thevamaran Laboratory. The research at Thevamaran laboratory will offer prospects of working on challenging research problems in an energetic team with ample opportunities for collaboration across disciplines. The University of Wisconsin-Madison is located in the City of Madison, which is known for its diverse community, high quality of life, and its natural beauty with multiple lakes and pleasant seasonal variations.</p>
</div></div></div>Sat, 01 Aug 2020 16:37:10 +0000Ramathasan Thevamaran24485 at https://imechanica.orghttps://imechanica.org/node/24485#commentshttps://imechanica.org/crss/node/24485Environmentally induced exceptional points in elastodynamics
https://imechanica.org/node/24189
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/12815">Exceptional points</a></div><div class="field-item odd"><a href="/taxonomy/term/12816">Parity-Time symmetry</a></div><div class="field-item even"><a href="/taxonomy/term/744">elastodynamics</a></div><div class="field-item odd"><a href="/taxonomy/term/12817">Hypersensitivity</a></div><div class="field-item even"><a href="/taxonomy/term/12818">non-Hermitian systems</a></div><div class="field-item odd"><a href="/taxonomy/term/11135">Wave dynamics</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span>We study the nature of an environment-induced exceptional point in a non-Hermitian pair of coupled mechanical oscillators. The mechanical oscillators are a pair of pillars carved out of a single isotropic elastodynamic medium made of aluminum and consist of carefully controlled differential losses. The interoscillator coupling originates exclusively from background modes associated with the “environment,” that portion of the structure which, if perfectly rigid, would support the oscillators without coupling. We describe the effective interaction in terms of a coupled-mode framework in which only one nearby environmental mode can qualitatively reproduce changes to the exceptional-point characteristics. Our experimental and numerical demonstrations illustrate strategic directions utilizing environmental mode control for the implementation of exceptional-point degeneracies. Potential applications include a new type of noninvasive differential atomic force microscopy and hypersensitive sensors for the structural integrity of surfaces.</span></p>
<p><span>Read the paper here: </span><a href="https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.13.014060">https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.13.01...</a></p>
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</div></div></div>Mon, 11 May 2020 17:10:37 +0000Ramathasan Thevamaran24189 at https://imechanica.orghttps://imechanica.org/node/24189#commentshttps://imechanica.org/crss/node/24189Independent control of dynamic material properties by exploiting structural hierarchy and intrinsic structural gradients
https://imechanica.org/node/24188
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/2269">Damping</a></div><div class="field-item odd"><a href="/taxonomy/term/2272">stiffness</a></div><div class="field-item even"><a href="/taxonomy/term/6419">hierarchical materials</a></div><div class="field-item odd"><a href="/taxonomy/term/9239">carbon nanotube array</a></div><div class="field-item even"><a href="/taxonomy/term/3099">Applied mechanics; Dynamics and Vibrations</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span><span>Achieving high damping and stiffness is challenging in common materials because of their inter-dependent scaling. Controlling extreme mechanical waves requires synergistically enhanced damping and stiffness. We demonstrate superior damping and stiffness in vertically aligned carbon nanotube (VACNT) foams that are also independently controllable by exploiting their synthesis-tailored structural hierarchy and structural gradients. They exhibit frequency- and amplitude-dependent responses with dramatically tunable dynamic stiffness while maintaining constant damping. Developing independent control over critical dynamic properties via engineered hierarchical and gradient materials will enable the creation of non-Hermitian </span><span>metamaterials</span><span> and active control of mechanical waves and vibrations.</span></span></p>
<p><span><span>Read the paper here: </span></span><a class="doi" title="Persistent link using digital object identifier" href="https://doi.org/10.1016/j.mtcomm.2019.100865" target="_blank" rel="noopener noreferrer">https://doi.org/10.1016/j.mtcomm.2019.100865</a></p>
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</div></div></div>Mon, 11 May 2020 17:06:02 +0000Ramathasan Thevamaran24188 at https://imechanica.orghttps://imechanica.org/node/24188#commentshttps://imechanica.org/crss/node/24188Superior Energy Dissipation by Ultrathin Semicrystalline Polymer Films Under Supersonic Microprojectile Impacts
https://imechanica.org/node/24186
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/190">impact</a></div><div class="field-item odd"><a href="/taxonomy/term/699">polymer</a></div><div class="field-item even"><a href="/taxonomy/term/8265">thinfilms</a></div><div class="field-item odd"><a href="/taxonomy/term/10891">Energy absorption</a></div><div class="field-item even"><a href="/taxonomy/term/2807">dynamic behavior</a></div><div class="field-item odd"><a href="/taxonomy/term/9452">ballistics</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span>Distinct deformation mechanisms that emerge in nanoscale enable the nanostructured materials to exhibit outstanding specific mechanical properties. Here, we present superior microstructure- and strain-rate-dependent specific penetration energy (up to ∼3.8 MJ/kg</span><span>) in semicrystalline poly(vinylidene fluoride-</span><em>co</em><span>-trifluoroethylene) (P(VDF-TrFE)) thin films subjected to high-velocity (100 m/s</span><span> to 1 km/s</span><span>) microprojectile (diameter: 9.2 μm) impacts. The geometric-confinement-induced nanostructural evolutions enable the sub-100 nm thick P(VDF-TrFE) films to achieve high specific penetration energy with high strain delocalization across the broad impact velocity range, superior to both bulk protective materials and previously reported nanomaterials. This high specific penetration energy arises from the substantial stretching of the two-dimensionally oriented highly mobile polymer chains that engage abundant viscoelastic and viscoplastic deformation mechanisms that are further enhanced by the intermolecular dipole–dipole interactions. These key findings provide insights for using nanostructured semicrystalline polymers in the development of lightweight, high-performance soft armors for extreme engineering applications.</span></p>
<p><span>Read the paper here: <a href="https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.0c00066">https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.0c00066</a></span></p>
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</div></div></div>Mon, 11 May 2020 16:56:10 +0000Ramathasan Thevamaran24186 at https://imechanica.orghttps://imechanica.org/node/24186#commentshttps://imechanica.org/crss/node/24186Dynamic Martensitic Phase Transformation in Single-crystal Silver Microcubes
https://imechanica.org/node/23740
<div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/1082">phase transformation</a></div><div class="field-item odd"><a href="/taxonomy/term/11609">#Plasticity</a></div><div class="field-item even"><a href="/taxonomy/term/7781">high velocity impact</a></div><div class="field-item odd"><a href="/taxonomy/term/1248">Single Crystal</a></div><div class="field-item even"><a href="/taxonomy/term/1161">FCC metals</a></div><div class="field-item odd"><a href="/taxonomy/term/12316">gradient nano-grained materials</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span>The ability to transform the crystal structure of metals in the solid-state enables tailoring their physical, mechanical, electrical, thermal, and optical properties in unprecedented ways. We demonstrate a martensitic phase transformation from a face-centered-cubic (fcc) structure to a hexagonal-close-packed (hcp) structure that occurs in nanosecond timescale in initially near-defect-free single-crystal silver (Ag) microcubes impacted at supersonic velocities. Impact-induced high pressure and high strain rates in Ag microcubes cause impact orientation-dependent extreme micro- and nano-structural transformations. When a microcube is impacted along the [100] crystal symmetry direction, the initial fcc structure transforms into an hcp crystal structure, while impact along the [110] direction does not produce phase transformations, suggesting the predominant role played by the stacking faults generated in the [100] impact. Molecular dynamics simulations at comparable high strain rates reveal the emergence of such stacking faults that coalesce, forming large hcp domains. The formation of hcp phase through the martensitic transformation of fcc Ag shows new potential to dramatically improve material properties of low-stacking-fault energy materials.</span></p>
<p><span>Download the paper here: </span><a href="https://authors.elsevier.com/a/1a0T34r9SUFFwe" target="_blank" rel="noopener noreferrer">https://authors.elsevier.com/a/1a0T34r9SUFFwe</a></p>
<p><img title="Dynamic Phase Transformation in Silver" src="https://ars.els-cdn.com/content/image/1-s2.0-S1359645419306664-fx1_lrg.jpg" alt="Dynamic Phase Transformation in Silver" width="1333" height="701" /></p>
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</div></div></div>Thu, 07 Nov 2019 12:17:38 +0000Ramathasan Thevamaran23740 at https://imechanica.orghttps://imechanica.org/node/23740#commentshttps://imechanica.org/crss/node/23740Asymmetric acoustic energy transport in non-Hermitian metamaterials
https://imechanica.org/node/23503
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/6765">acoustic waves</a></div><div class="field-item odd"><a href="/taxonomy/term/12577">non-Hermitian materials</a></div><div class="field-item even"><a href="/taxonomy/term/11453">left–right asymmetry</a></div><div class="field-item odd"><a href="/taxonomy/term/2269">Damping</a></div><div class="field-item even"><a href="/taxonomy/term/6924">acoustic metamaterials</a></div><div class="field-item odd"><a href="/taxonomy/term/5284">acoustic-structural interaction</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span>One of our studies on linear and nonlinear non-Hermitian metamaterials has been published on the recent special issue of the Journal of the Acoustical Society of America: Non-Reciprocal and Topological Wave Phenomena in Acoustics.</span></p>
<p class="MsoNormal"><strong><span>Abstract</span></strong></p>
<p><span>The ability to control and direct acoustic energy is essential for many engineering applications such as vibration and noise control, invisibility cloaking, acoustic sensing, energy harvesting, and phononic switching and rectification. The realization of acoustic regulators requires overcoming fundamental challenges inherent to the time-reversal nature of wave equations. Typically, this is achieved by utilizing either a parameter that is odd-symmetric under time-reversal or by introducing passive nonlinearities. The former approach is power consuming while the latter has two major deficiencies: it has high insertion losses and the outgoing signal is harvested in a different frequency than that of the incident wave due to harmonic generation. Here, we adopt a unique approach that exploits spatially distributed linear and nonlinear losses in a fork-shaped resonant metamaterials. Our compact design demonstrates asymmetric acoustic reflectance and transmittance, and acoustic switching. In contrast to previous studies, our non-Hermitian metamaterials exhibit asymmetric transport with high frequency purity of the outgoing signal.</span></p>
<p><span>Full article: </span><a href="https://asa.scitation.org/doi/abs/10.1121/1.5114919">https://asa.scitation.org/doi/abs/10.1121/1.5114919</a></p>
</div></div></div>Thu, 08 Aug 2019 18:30:39 +0000Ramathasan Thevamaran23503 at https://imechanica.orghttps://imechanica.org/node/23503#commentshttps://imechanica.org/crss/node/23503Postdoctoral Research Associate Position at the University of Wisconsin-Madison, Madison, WI.
https://imechanica.org/node/22402
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/73">job</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/9932">1 postdoctoral position</a></div><div class="field-item odd"><a href="/taxonomy/term/185">experimental mechanics</a></div><div class="field-item even"><a href="/taxonomy/term/659">nanostructured materials</a></div><div class="field-item odd"><a href="/taxonomy/term/6419">hierarchical materials</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>A Postdoctoral Research Associate Position is available in <a href="https://thevamaran.engr.wisc.edu">Professor R. Thevamaran's laboratory</a> at the <a href="https://www.engr.wisc.edu/department/engineering-physics/">Department of Engineering Physics</a> to study the dynamic behavior and properties of nanostructured metals and hierarchical materials. A strong background in experimental solid mechanics and materials science is required for this research. Highly motivated and talented scholars who have a PhD in Mechanical Engineering, Materials Science and Engineering, or Engineering Mechanics are encouraged to apply. Having prior experience in electron microscopy (SEM & TEM) and X-ray diffraction techniques, and in situ nanoindentation is desirable.</p>
<p>Interested recent graduates, or students who will be graduating soon shall email Prof. Thevamaran with their CV, two significant publications, and a cover letter. The cover letter should clearly describe your prior research experiences, research interests, and your fit to the research focus of Thevamaran Laboratory in one page.</p>
<p>The research at the Thevamaran laboratory will offer prospects of working on challenging research problems in a dynamic team with ample of opportunities for collaboration across disciplines. The University of Wisconsin-Madison is located in the City of Madison, which is known for its divers community, high quality of life, and its natural beauty with multiple lakes and pleasant seasonal variations.</p>
</div></div></div>Wed, 30 May 2018 18:17:53 +0000Ramathasan Thevamaran22402 at https://imechanica.orghttps://imechanica.org/node/22402#commentshttps://imechanica.org/crss/node/22402Dynamic creation and evolution of gradient-nano-grained structures in single-crystal metallic microcubes
https://imechanica.org/node/20495
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/11387">gradient-nano-grained metals</a></div><div class="field-item odd"><a href="/taxonomy/term/7781">high velocity impact</a></div><div class="field-item even"><a href="/taxonomy/term/10745">recrystallization</a></div><div class="field-item odd"><a href="/taxonomy/term/9773">extreme events</a></div><div class="field-item even"><a href="/taxonomy/term/7359">Severe Plastic Deformation</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p>In an article published today in <em><strong>Science</strong>,</em> we have demonstrated the creation of <strong>an extreme gradient-nano-grained (GNG) structure</strong> in single-crystal microcubes through high-velocity impact. We use the defect-free single-crystal silver microcubes, synthesized using a seed-growth process, as the model system, and fire them at supersonic velocities onto a rigid target to create the GNG structure. We also study the created GNG structure over the course of several weeks and reveal that this nanostructure evolves through recrystallization at room temperature without any external thermal annealing. We believe that the GNG-structured metals with intermediate states of recrystallization holds promise for creating ultra-strong and tough metals, which have the potential to alleviate the brittle failure commonly found in nanocrystalline metals.</p>
<p>More details can be found in the <strong>paper</strong>: <a href="http://science.sciencemag.org/content/354/6310/312?panels_ajax_tab_tab=jnl_sci_top_topics&panels_ajax_tab_trigger=">http://science.sciencemag.org/content/354/6310/312?panels_ajax_tab_tab=j...</a></p>
<p><strong>Rice University press release</strong>: <a href="http://news.rice.edu/2016/10/20/smashing-metallic-cubes-toughens-them-up-2/">http://news.rice.edu/2016/10/20/smashing-metallic-cubes-toughens-them-up-2/</a></p>
<p><strong>An explanatory animation</strong>: <a href="https://www.youtube.com/watch?v=yD8LEcc4hyA&feature=youtu.be">https://www.youtube.com/watch?v=yD8LEcc4hyA&feature=youtu.be</a></p>
<p><strong>Rice University press release video</strong>: <a href="https://www.youtube.com/watch?v=arVIs6xQaxE&feature=youtu.be">https://www.youtube.com/watch?v=arVIs6xQaxE&feature=youtu.be</a></p>
</div></div></div><div class="field field-name-upload field-type-file field-label-hidden"><div class="field-items"><div class="field-item even"><table class="sticky-enabled">
<thead><tr><th>Attachment</th><th>Size</th> </tr></thead>
<tbody>
<tr class="odd"><td><span class="file"><img class="file-icon" alt="Image icon" title="image/jpeg" src="/modules/file/icons/image-x-generic.png" /> <a href="https://imechanica.org/files/GNG%20structure.jpg" type="image/jpeg; length=4875949" title="GNG structure.jpg">GNG-structured metal.</a></span></td><td>4.65 MB</td> </tr>
</tbody>
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</div></div></div>Thu, 20 Oct 2016 21:21:26 +0000Ramathasan Thevamaran20495 at https://imechanica.orghttps://imechanica.org/node/20495#commentshttps://imechanica.org/crss/node/20495A few experimental studies on the dynamic behavior of foam-like aligned carbon nanotubes
https://imechanica.org/node/19468
<div class="field field-name-taxonomy-vocabulary-6 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/76">research</a></div></div></div><div class="field field-name-taxonomy-vocabulary-8 field-type-taxonomy-term-reference field-label-hidden"><div class="field-items"><div class="field-item even"><a href="/taxonomy/term/6419">hierarchical materials</a></div><div class="field-item odd"><a href="/taxonomy/term/264">carbon nanotubes</a></div><div class="field-item even"><a href="/taxonomy/term/10972">multiscale behavior</a></div><div class="field-item odd"><a href="/taxonomy/term/10891">Energy absorption</a></div><div class="field-item even"><a href="/taxonomy/term/190">impact</a></div><div class="field-item odd"><a href="/taxonomy/term/2807">dynamic behavior</a></div></div></div><div class="field field-name-body field-type-text-with-summary field-label-hidden"><div class="field-items"><div class="field-item even"><p><span>I'm posting a few experimental studies we have conducted on the dynamic behavior of hierarchical fibrous materials, using vertically aligned carbon nanotubes as a model material. I hope these will be useful to those who are interested in buckle-instabilities, multiscale behavior, and energy absorption mechanisms.</span></p>
<p><strong><span>An overview:</span></strong></p>
<p><span>Soft hierarchical materials often present unique functional properties that are sensitive to geometry and the organization of their micro- and nano-structural features across different lengthscales. Carbon Nanotube (CNT) foams are hierarchical materials with fibrous morphology that are known for their remarkable physical, chemical and electrical properties. Their complex microstructure has led them to exhibit intriguing mechanical responses at different length-scales and in different loading regimes. Even though these materials have been studied for mechanical behavior over the past few years, their response at high-rate finite deformations and the influence of their microstructure on bulk mechanical behavior and energy dissipative characteristics remain elusive. </span></p>
<p><span>In these articles, we present the response of aligned CNT foams at the high strain-rate regime of 10^</span>2<span> - 10^</span>4<span> s^</span>-1<span>. We investigate their bulk dynamic response and the fundamental deformation mechanisms at different lengthscales, and correlate them to the microstructural characteristics of the foams. We developed an experimental platform, with which to study the mechanics of CNT foams in high-rate deformations, that includes direct measurements of the strain and transmitted forces, and allows for a full field visualization of the sample’s deformation through high-speed microscopy.</span></p>
<p><span>We synthesize various CNT foams (e.g., vertically aligned CNT (VACNT) foams, helical CNT (HCNT) foams, micro-architectured VACNT foams and VACNT foams with microscale heterogeneities) and show that the bulk functional properties of these materials are highly tunable either by tailoring their microstructure during synthesis or by designing micro-architectures that exploit the principles of structural mechanics. We also develop numerical models to describe the bulk dynamic response using multiscale mass-spring models and identify the mechanical properties at length scales that are smaller than the sample height.</span></p>
<p><span>The ability to control the geometry of microstructural features, and their local interactions, allows the creation of novel hierarchical materials with desired functional properties. The fundamental understanding provided by this work on the key structure-function relations that govern the bulk response of CNT foams can be extended to other fibrous, soft and hierarchical materials. The findings can be used to design materials with tailored properties for different engineering applications, like vibration damping, impact mitigation and packaging.</span></p>
<p><strong><span>Articles:</span></strong></p>
<p><span>1. </span><span><span>Thevamaran R., Daraio C., An experimental technique for dynamic characterization of soft complex materials, </span><em>Experimental Mechanics</em><span>, 54 (8), 2014, pp.1319-1328, doi: <a href="http://link.springer.com/article/10.1007%2Fs11340-014-9896-9" target="_blank">10.1007/s11340-014-9896-9</a>.</span></span></p>
<p><span><span>2. </span><span>Thevamaran R., Meshot ER., Daraio C., Shock formation and rate effects in impacted carbon nanotube foams, </span><em>Carbon</em><span>, 84, 2015, pp.390-398,</span><span> </span><span><a href="http://www.sciencedirect.com/science/article/pii/S0008622314011567" target="_blank">doi: 10.1016/j.carbon.2014.12.006</a>.</span></span></p>
<p><span><span>3. </span><span>Thevamaran R., Karakaya M., Meshot ER., Fisher A., Podila M., Rao A., Daraio C., Anomalous impact and strain responses in helical carbon nanotube foams, </span><em>RSC Advances</em><span>, 5, 2015, pp.29306-29311, doi: <a href="http://pubs.rsc.org/en/content/articlelanding/2015/ra/c5ra03561a#!divAbstract" target="_blank">10.1039/C5RA03561A</a>.</span></span></p>
<p><span><span>4. </span><span>Lattanzi L., Thevamaran R., Daraio C., Dynamic behavior of vertically aligned carbon nanotubes with patterned microstructures, </span><em>Advanced Engineering Materials</em><span>, 17 (10) 2015, pp.1470-1479, doi:<a href="http://onlinelibrary.wiley.com/doi/10.1002/adem.201400571/abstract" target="_blank">10.1002/adem.201400571</a>.</span></span></p>
<p><span><span>5. </span><span>Thevamaran R., Raney JR., Daraio C., Rate-sensitive strain localization and impact response of carbon nanotube foams with microscale heterogeneous bands, </span><em>Carbon</em><span>, 101, 2016, pp.184-190. doi: <a href="http://www.sciencedirect.com/science/article/pii/S0008622315305352" target="_blank">10.1016/j.carbon.2015.12.069</a>.</span></span></p>
</div></div></div>Sun, 14 Feb 2016 16:00:41 +0000Ramathasan Thevamaran19468 at https://imechanica.orghttps://imechanica.org/node/19468#commentshttps://imechanica.org/crss/node/19468Error | iMechanica