Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session L49: Focus Session: Mechanics of Defects and Discontinuities |
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Sponsoring Units: GSOFT GSNP Chair: James Hanna, Virginia Polytechnic Institute and State University Room: 217D |
Wednesday, March 4, 2015 8:00AM - 8:12AM |
L49.00001: Fracture of brittle coatings on soft plastic substrates Joel Marthelot, Davy Dalmas, Jeremie Teisseire, Jose Bico, Benoit Roman Mechanics of stiff and electrically conductive films deposited on soft plastic substrates have recently gain interest due to the development of stretchable electronics applications. When submitted to tensile stress, such films tend to fail with the apparition of arrays of parallel channel cracks transverse to the direction of deformation, with fatal consequenses for electrical conductivity. We study the propagation of such fractures in oxyde monolayers coated on a polymer substrate under uniaxial stretching. We show how the crack density undergoes a transition from a statistic failure distribution of brittle material to a deterministic failure set by the elastic mismatch between the film and the substrate. A two-dimensional model of a film bonded to an elastic substrate fails to describe the saturation observed at high strain. We present experimental evidences of the localization of strain in the substrate by in-situ AFM imaging of the fracture process. We propose an increment of the model to account for the plasticity of the substrate. This description allows to pass continuously from the elastic to the plastic regime and to predict the saturation of the fragmentation as observed experimentally at large deformation. [Preview Abstract] |
Wednesday, March 4, 2015 8:12AM - 8:24AM |
L49.00002: ABSTRACT WITHDRAWN |
Wednesday, March 4, 2015 8:24AM - 8:36AM |
L49.00003: Effect of topological defects and curvature on anisotropic crystal growth Amir Azadi, Gregory M. Grason The equilibrium shapes and symmetries of crystals are vestiges of the physical principles underlying their formation. We perform particle-based simulations guided by analytical analysis to investigate the structure of crystalline domains on curved substrates, a focus on the impact of topological defects on domain morphology. We find at low area fraction, as has been argued previously, that isotropic crystal growth with relatively compact domains generates large curvature-induced strains accommodated by relative ductile interactions, while the formation of anisotropic ribbon-like structures with lower-curvature induced stresses, introduces a larger line tension cost, and is thus favored for brittle crystals. Our results show that for ductile crystals with large surface coverage, appearance of stable topological defects precludes the formation of anisotropic, ribbon domains. However branch-like structures with large interfacial area are stable for certain values of intermediate curvature and crystalline ductility. These processes are guided by the interplay between elastic shape instability, defects, and curvature, where pattern formations are not related to kinetic instabilities. [Preview Abstract] |
Wednesday, March 4, 2015 8:36AM - 8:48AM |
L49.00004: Energy Barriers for Defects in Disordered Solids Sven Wijtmans, Lisa Manning In solids, defects govern flow and failure. In crystals, defects are easily-identified dislocations, while in disordered solids, defects can be found by analyzing the vibrational modes of the system, which are eigenvectors of the matrix describing the linear response. The low frequency modes are typically quasi-localized hybrids of excitations localized at the defects and plane-wave like modes. Additional analysis can separate these components, giving the location of a defect and displacement of particles along that defect. To define an energy barrier for each defect, we displace particles along an isolated defect mode and calculate the energy at which the system transitions to a new energy basin. Different definitions of a new basin, such as a change in the particle contact network or particle displacements above a specific threshold, give different results. We identify several criteria that are consistent and provide a reasonable, robust definition of an energy barrier. Somewhat surprisingly, we find that energy barriers for isolated defects are generally higher than energy barriers for typical quasi-localized modes in the system. [Preview Abstract] |
Wednesday, March 4, 2015 8:48AM - 9:00AM |
L49.00005: Tuning a material's properties through the excitation of localized defect modes Marc Serra Garcia, Joseph Lydon, Chiara Daraio Technological applications such as acoustic super-lenses and vibration mitigation devices require materials with extreme mechanical properties (Very high, zero, or negative stiffness). These properties can be achieved through buckling instabilities, local resonances and phase transitions, mechanisms that are limited to particular frequencies, strains or temperatures. In this talk I will present an alternative mechanism to tune the stiffness of a lattice. The mechanism is based on the excitation of a nonlinear localized defect mode. The oscillation of the defect mode affects the bulk properties of the lattice. This is due to the thermal expansion of the defect mode and the nonlinear coupling between the mode amplitude and the strain of the lattice. Due to the singular properties of nonlinear systems near bifurcation points, the lattice can achieve an arbitrarily large stiffness. It is possible to select point of the force-displacement relation that is being tuned by selecting the defect's excitation frequency and amplitude. Depending on the nonlinear interaction potential at the defect site, the stiffness can be tuned to extremely positive or extremely negative values. While our theoretical and experimental results have been obtained in a granular crystal, the analysis suggests that an equivalent effect should be present in other lattices with localized modes and nonlinearity. [Preview Abstract] |
Wednesday, March 4, 2015 9:00AM - 9:12AM |
L49.00006: Scars and the stability of crystalline shells under external pressure Duanduan Wan, Mark Bowick, Rastko Sknepnek While continuum elastic theory predicts the mechanical properties of ideal spherical shells under external pressure, on microscopic scale the response of shells to pressure may be affected by their crystalline order and defect structure. Here we compare the stability, under external pressure, of shells with a minimal set of topologically-required defects to shells with extended defect arrays (grain boundary ``scars"). In particular, we perform Monte Carlo simulations to compare how shells with and without scars deform quasi-statically under external hydrostatic pressure. We find that the critical pressure at which shells collapse is lowered when the scar distribution breaks icosahedral symmetry and raised when symmetry is preserved. The particular shapes resulting from collapses which break icosahedral symmetry depend crucially on the F{\" o}ppl-von K{\'a}rm{\'a}n number. [Preview Abstract] |
Wednesday, March 4, 2015 9:12AM - 9:48AM |
L49.00007: Dynamics and geometry of interacting fractures in torn elastic sheets: convergent, divergent, and multiple swirling cracks Invited Speaker: Eugenio Hamm I will present some recent results on the dynamics of multiple interacting cracks in torn elastic sheets. Specifically, I will consider a peeling - like configuration, in which two cracks converge in a robust fashion, and a ``concertina'' configuration in which two cracks systematically diverge. Based on experiments, I will discuss the non-trivial aspects of both problems, namely the way in which elasticity and fracture mechanics are concomitant when it comes to predict crack paths. Besides, I will show cases in which the trajectory of a crack is dictated by the path followed by another crack. This delayed interaction of cracks allows the construction of multiple crack configurations in which each crack recursively interacts with a nearby crack, giving rise to divergent self-similar spiral trajectories. Finally, I will discuss the effects of material anisotropy on the propagation of cracks. If time permits, I will also present a concise review of a second example of defect dynamics: the motion of conical singularities in thin elastic sheets, subjected to external forcing, and their mutual interaction. Specifically I will consider the gliding, climbing, annihilation and rotation of such structures. [Preview Abstract] |
Wednesday, March 4, 2015 9:48AM - 10:00AM |
L49.00008: Untangling Superfluid Vortices Dustin Kleckner, Martin W. Scheeler, Davide Proment, William T. M. Irvine What is the role of topology, or knottedness, in superfluid phase defects (quantum vortices)? In ideal classical fluids, vortex knots may never untie, and so there is an associated conserved quantity -- helicity -- which measures how tangled a flow is. One might expect a similar robustness for superfluid defects, however, simulations of the Gross-Pitevskii equation demonstrate that vortex knots and links spontaneously untie and unlink. Nonetheless, the topology dramatically affects the vortex evolution, and a component of the initial helicity is transferred to helical coils as the knots unravel. These effects are remarkably similar to the behavior of tangled vortices in viscous fluids, suggesting they are universal features of non-ideal fluids. [Preview Abstract] |
Wednesday, March 4, 2015 10:00AM - 10:12AM |
L49.00009: Dynamics of vacancies in two-dimensional Lennard-Jones crystals Zhenwei Yao, Monica Olvera de la Cruz Vacancies represent an important class of crystallographic defects, and their behaviors can be strongly coupled with relevant material properties. We report the rich dynamics of vacancies in two-dimensional Lennard-Jones crystals in several thermodynamic states. Specifically, we numerically observe significantly faster diffusion of the 2-point vacancy with two missing particles in comparison with other types of vacancies; it opens the possibility of doping 2-point vacancies into atomic materials to enhance atomic migration. In addition, the resulting dislocations in the healing of a long vacancy suggest the intimate connection between vacancies and topological defects that may provide an extra dimension in the engineering of defects in extensive crystalline materials for desired properties. [Preview Abstract] |
Wednesday, March 4, 2015 10:12AM - 10:24AM |
L49.00010: Jump conditions for thin bodies from an action principle James Hanna Thin, flexible bodies such as strings, sheets, and rods often sustain kinky geometric features, or experience discontinuous contact forces in their interactions with obstacles. The physics of dynamic and static versions of these phenomena differ. Kink/shock propagation, impact, peeling, unwrapping, tearing and cracking all occur at geometric locations in a body that do not correspond to material points. I will discuss how the jump conditions for momentum and energy across such moving discontinuities may be derived from an action principle for an extended body with time-dependent, non-material boundaries. [Preview Abstract] |
Wednesday, March 4, 2015 10:24AM - 10:36AM |
L49.00011: ABSTRACT WITHDRAWN |
Wednesday, March 4, 2015 10:36AM - 10:48AM |
L49.00012: Nonlinear optical probing of electric field induced oxygen migrations in Fe doped SrTiO$_{3}$ Haochen Yuan, David Ascienzo, Onur Kurt, Zehra Cevher, Steve Greenbaum, Russell Maier, Clive Randall, Yuhang Ren We report on our recent study of the electric field induced oxygen migration dynamics and defect states near the interface in Fe-doped SrTiO$_{3}$ single crystals by optical second harmonic generation (SHG) using a femtosecond Ti:sapphire laser at 800 nm wavelength. By varying both the incidence and the output angles, we identified a strong correlation between the measured SHG signals and the microscopic defect textures of the samples. Significant changes in SHG intensities and phases are explained by the formation and extension of oxygen vacancies and crystalline distortions near Fe defect centers. Our results show that the SHG technique is a powerful tool for detecting local environment near interfaces and oxygen migrations in ferroelectric structures. [Preview Abstract] |
Wednesday, March 4, 2015 10:48AM - 11:00AM |
L49.00013: Positron beam spectroscopy of defect kinetics in highly oriented pyrolytic graphite Varghese Anto Chirayath, Amarendra G We report here slow positron beam spectroscopy of thermally activated defect annealing mechanisms in highly oriented pyrolytic graphite (HOPG) which has been implanted with 200 keV carbon ions. The HOPG samples were irradiated to a dose of 10$^{14}$ and 10$^{15}$ ions/cm$^{2}$ which are just below the dose required for amorphization. The open volume defect-sensitive positron studies have clearly shown a defect annealing mechanism at temperatures close to the Wigner energy release peak for both the lower and higher dose irradiated samples. The sample irradiated to higher dose has also shown a second defect annealing step at 723K from near the end of range of the implanted ions. This step however was not visible in the lower dose sample and has not been previously reported. Positron beam spectroscopy could also detect the presence of interstitial defects trapped at the inter-planar regions after the open volume defect recovery by 973 K. These results will be compared to the present understanding of the open volume defect structures and their migration in graphite as well as in other sp$^{2}$ hybridized nanostructures like graphene. [Preview Abstract] |
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