Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H48: Thermal versus Athermal PlasticityFocus Session
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Sponsoring Units: GSNP GSOFT Chair: Robert Hoy, University of South Florida Room: LACC 510 |
Tuesday, March 6, 2018 2:30PM - 3:06PM |
H48.00001: Strain Bursts and Dislocation Avalanches in Obstacle-Hardened Materials Invited Speaker: Nasr Ghoniem We develop a hybrid continuum-discrete model for the collective dynamics of dislocations in dense obstacle fields. The density of obstacles is described by continuum conservation equations, and solved numerically. With this hybrid model, we unravel the mystery of how and why irradiation-induced defects enhance or inhibit strain bursts in submicron single crystals. It is shown that smaller strain burst amplitudes in irradiated nano- and micro-pillars are obtained under stress control conditions. However, under strain control conditions, bursts are found not to be sensitive to irradiation, despite the arresting effect of radiation defects. This feature is a result of rapid stress relaxation truncating the strain burst, compared with the influence of irradiation-induced defects. The influence of dislocation-defect interaction mechanisms, cross slip, irradiation dose, as well as loading mode on strain bursts is systematically investigated, and the results compared with experimental observations. We also present results showing the relationship to spatial localization of plastic flow. |
Tuesday, March 6, 2018 3:06PM - 3:18PM |
H48.00002: Precursor Dislocation Avalanches in Small Crystals: The Irreversibility Transition Xiaoyue Ni, Haolu Zhang, Danilo Liarte, Louis McFaul, Karin Dahmen, James Sethna, Julia Greer The transition from elastic to plastic deformation in crystalline metals shares both history dependence and scale-invariant avalanche behaviors with other non-equilibrium systems under external loading. Many of these other systems, however, typically exhibit purely elastic behavior only after training through repeated cyclic loading; recent studies in these other systems show power laws and scaling of the hysteresis magnitude and training time as the peak load approaches a reversible—irreversible transition (RIT). We discover here that deformation of small crystals shares these key features. Yielding and hysteresis in uniaxial compression experiments of single-crystal Cu nano- and micropillars decay under repeated cyclic loading; the amplitude and decay time diverge as the peak stress approaches the failure stress, with power laws and scaling as seen in RITs in other nonequilibrium systems. We observe that these effects become smaller as the pillars become larger, perhaps explaining why scale-invariant training effects have not been observed in macroscopic samples. |
Tuesday, March 6, 2018 3:18PM - 3:30PM |
H48.00003: Critical scaling of temporal and spatial correlations with strain rate in athermal, disordered solids Joel Clemmer, K. Michael Salerno, Mark Robbins Athermal, disordered solids deform plastically via avalanches in the quasistatic limit of shear. These avalanches display critical behavior including power-law distributions. With increasing strain rate, the system moves away from the critical point causing a decrease in the correlation length of the system. We explore 2D and 3D simulations of overdamped binary Lennard-Jones solids in the limit of finite strain rate and characterize the temporal and spatial correlations that govern avalanche plasticity. With increasing strain rate, we identify a decreasing length scale in the power spectrum of the non-affine displacement of particles and a decreasing time scale in the power spectrum of temporal fluctuations in the average kinetic energy and stress. Using finite-size scaling, we find the critical exponents governing the scaling of correlations with rate. We will also relate these results to the scaling of other system properties with strain rate such as a size-dependent diffusion constant in 2D and the average shear stress. |
Tuesday, March 6, 2018 3:30PM - 3:42PM |
H48.00004: Dynamics of point dislocations along glide lines in two-dimensional colloidal crystal An Pham, Benjamin Yellen We have developed an experimental model to study the growth and healing of crystalline domains in a monolayer of paramagnetic particles exposed to a high-frequency rotating magnetic field. Here, our goal is to measure the stress/strain relationships of point dislocations that move along glide lines during the crystal healing process. These measurements are based on a combination of image processing to determine the local strain and force calculations to determine the local stress based on our prior calibration of the pairwise interaction potential. We will present measurements on both the elastic modulus and the yield stress as the point dislocation moves from the interior of the crystal towards a domain boundary. These measurements are conducted in different applied magnetic fields strengths in order to model the crystal healing properties at different temperatures. We also quantify the net torque and rotation of the domain as the point dislocation moves along a glide line, which allows the domain boundaries between neighboring crystals to heal. |
Tuesday, March 6, 2018 3:42PM - 3:54PM |
H48.00005: Ability of Local Structure to Predict Particle Rearrangements in Varying Spatial Dimension Sean Ridout, François Landes, Eric Corwin, Andrea Liu In glassy systems, it is difficult to predict which particles will rearrange under thermal fluctuations or applied load using only local structural information. The most successful and least computationally intensive of the many approaches that have been developed to address this problem uses machine learning to construct a scalar function of local structure, “softness.” This particle-based quantity has been shown to be highly predictive of rearrangements in several model disordered systems in two and three dimensions. One might expect that the ability of local structure to predict rearrangements in a given model will decrease with increasing spatial dimension, e.g. due to greater homogeneity in particles' local environments. We use soft-sphere systems under athermal shear to study systematically the ability of softness to predict rearrangements in different spatial dimensions and test this hypothesis. |
Tuesday, March 6, 2018 3:54PM - 4:06PM |
H48.00006: Thermal energy as structural indicator in glasses: From universal anomalous statistics to predicting plastic rearrangements Jacques Zylberg, Edan Lerner, Yohai Bar-Sinai, Eran Bouchbinder Identifying heterogeneous structures in glasses - such as localized soft spots - and understanding structure-dynamics relations in these systems remain major scientific challenges. Here we derive an exact expression for the local thermal energy of interacting particles in glassy systems by a systematic low-temperature expansion. We show that the local thermal energy can attain anomalously large values, inversely related to the degree of softness of localized structures in a glass, determined by a coupling between internal stresses, anharmonicity and low-frequency vibrational modes. These anomalously large values follow a fat-tailed distribution, with a universal exponent related to the recently established universal ω4 density of states of quasi-localized low-frequency vibrational modes. This power-law tail manifests itself by highly localized spots which are significantly softer than their surroundings. These soft spots are shown to be susceptible to plastic rearrangements under external driving forces, having predictive powers that surpass those of the normal-modes-based approach. These results offer a general, system/model-independent, physical-observable-based approach to identify structural properties of quiescent glasses and to relate them to glassy dynamics. |
Tuesday, March 6, 2018 4:06PM - 4:18PM |
H48.00007: Thermalized formulation of soft glassy rheology Robert Hoy We present a version of soft glassy rheology that includes thermalized strain degrees of freedom. It fully specifies systems' strain-history-dependent positions on their energy landscapes and therefore allows for quantitative analysis of their heterogeneous yielding dynamics and nonequilibrium deformation thermodynamics. As a demonstration of the method, we illustrate the very different characteristics of fully-thermal and nearly-athermal plasticity by contrasting systems’ evolution under thermalized vs. nonthermalized plastic flow rules. |
Tuesday, March 6, 2018 4:18PM - 4:30PM |
H48.00008: Average dynamics, fluctuating relaxation rates, and dynamic susceptibility in glass-forming liquids Horacio Castillo We systematically examine the consequences of the assumption that dynamic heterogeneity in glass-forming liquids can be described in terms of locally fluctuating relaxation rates. We find that under mild assumptions, the time dependence of the dynamic susceptibility χ4(t) for times of the order of the α-relaxation time τα is completely determined by the average dynamics, except for an overall scale factor. In particular, the shape and location (but not the overall scale) of the peak in χ4(t) can be predicted from knowledge of the average relaxation function F0(t). |
Tuesday, March 6, 2018 4:30PM - 4:42PM |
H48.00009: Fine- and Coarse-Grained Modeling of Yielding and Strain Hardening in Glassy Polymers Ronald Larson, Weizhong Zou, Robert Hoy We present both molecular dynamics (MD) simulations and coarse-grained Brownian dynamics modeling of glassy polymers under uniaxial extensional flow. The MD simulations show that individual Kremer-Grest chains collapse into folded states at Hencky strains of order 2, which then undergo unraveling upon further straining, analogous to that in extensional flows of dilute polymer chains. This motivates a coarse-grained picture that divides the stress into a segmental mode governing monomer friction, and a “polymer” mode contributed by the configuration of polymer chains. The Brownian dynamics model uses a schematic model of yielding of the segmental mode, analogous to that of small-molecule glasses (Fielding, et al. Phys. Rev. Lett., 108.048301, 2012), while the polymer mode is represented by finitely extensible bead-spring chains whose bead drag coefficient is proportional to the viscosity of the segmental mode. This produces behavior consistent with experimental work of the Ediger group (Lee et al., Science, 323, 231-324, 2009), and produces folded states similar to those observed in the MD simulations. The modeling provides strong evidence that strain hardening arises from the high tensions in the folded polymer strands, consistent with earlier suggestions of Fielding et al. |
Tuesday, March 6, 2018 4:42PM - 4:54PM |
H48.00010: Local fluctuations in the relaxation rate in glassy systems: probing dynamical heterogeneity and its exchange time Rajib Pandit, Elijah Flenner, Horacio Castillo We numerically study the equilibrium dynamics of a glass-forming binary hard-sphere mixture in three dimensions, for different packing fractions. We provide a quantitative description of dynamical heterogeneity in terms of fluctuations in the local relaxation rates. We extract a newly defined two-point correlator, χ2Φ(t), that probes those fluctuations. The time decay of χ2Φ(t) is characterized by a certain time scale τex, which we identify as the exchange time of the slow and fast regions. We calculate the exchange time τex as a function of packing fraction, and we find that in the vicinity of the glass transition, τex is an order of magnitude longer then the α-relaxation time, τα. |
Tuesday, March 6, 2018 4:54PM - 5:06PM |
H48.00011: Granular Plasticity in Triaxial Compression Experiments: Nonuniversal Stress Fluctuations and Particle Shape Dependence Kieran Murphy, Heinrich Jaeger A disordered network of particles in a granular material, when compressed past yielding, can quietly rearrange or violently erupt into restructuring cascades. By varying particle shape we explore a wide range of plastic deformation behavior in triaxial compression experiments on 3D-printed and laser cut particles. Sudden stress drops are shown via X-ray imaging to be localized particle slip events. We calculate from stress-strain data each particle shape’s friction angle, a dimensionless measure of shear strength, and volatility, a method for quantifying fluctuations in financial mathematics. Qualitative regions emerge in the parameter space suggesting distinct microstructural origins of particle rearrangement. For all shapes the magnitude of relaxation events appears to follow a truncated power law distribution, with particle shape driving both the power law exponent and the location of the exponential roll off. We find a nonuniversal range of exponents which are correlated with the particle shape’s ratio of the radii of circumscribed and inscribed spheres. Finally, we discuss extensions to interface depinning and other popular frameworks which better explain our results in the broader context of amorphous plasticity. |
Tuesday, March 6, 2018 5:06PM - 5:18PM |
H48.00012: Active Microrheology in an Emulsion Glass Nesrin Senbil, Chi Zhang, Frank Scheffold Active microrheology can probe the non-linear properties of a colloidals on the microscale and thus provide insight into the microscopic origins of macroscopic viscoelasticity and yielding. Here we apply this method to uniform oil-in-water emulsions that show nearly hard-sphere behavior with an experimentally confirmed glass and a jamming transition at volume fractions of 59% and 64%, respectively. In our experiments, we use a laser tweezer to apply a constant force on a 2 micrometer-size polystyrene probe, with the aim to pull it through the emulsion host composed of oil droplets with the same diameter. We observed the trajectory of the probe, released from one cage and moving to the next, within the glassy phase, provided the applied force is high enough to break the cage and delocalize the particle. The force required for this transition can be defined as a critical force . We compare our results with theoretical work based on mode-coupling theory. We find agreement with MCT-results at packing fractions close to glass transition. However, as the volume fraction is increased beyond 61% our experimental results indicate much more pronounced increase of the critical force. Our results will provide critical input for the nonlinear viscoelastic response of dense colloidal systems. |
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