Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C54: Jamming, Fracture, and Deformation |
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Sponsoring Units: GSOFT GSNP Chair: Robert Hoy, Univ of South Florida Room: LACC 514 |
Monday, March 5, 2018 2:30PM - 2:42PM |
C54.00001: Theory and computation of fracture formation in polymer electrolyte membranes Yule Wang, Michael Eikerling We present a physical-statistical model of fracture formation in polymer electrolyte membranes. The membrane is considered as a cross-linked network of ionomer bundles. Water that fills the pore spaces between bundles causes stress on them, incurring bundle breakage. The problem of fracture formation and propagation is mapped onto a bond percolation problem. It was solved previously for the random percolation case of the weak-stress regime (Melchy and Eikerling, J. Phys. Condens. Matter 27, 325103, 2015). This work focuses on the high-stress regime where breakage events become correlated because of the stress redistribution upon bundle breakage. We have implemented a rejection-free kinetic Monte Carlo method to simulate correlated bundle breakage events on regular lattices. Fracture rates of bundles are expressed through an exponential breakdown rule. So far, we have evaluated a global power-law stress redistribution scheme. Using this approach, we study the effect of the initial stress, correlation length and lattice anisotropy on fracture propagation. Different regimes or fracture propagation corresponding to random and correlated percolation are identified and the impact of relevant parameters transitions between these regimes as well as on the time to fracture is analyzed. |
Monday, March 5, 2018 2:42PM - 2:54PM |
C54.00002: Shear banding in drying colloidal films studied using scanning micro small angle x-ray scattering Bin Yang, Nathan Smith, Andreas Johannes, Manfred Burghammer, Michael Smith Shear banding is an important phenomenon observed in a range of fields in materials science, such as bulk metallic glasses, granular, complex fluids. A particularly challenging task is to relate the changes in material microstructure to the observed shear banding. In this study we exploit the capabilities of the scanning micro small angle x-ray scattering technique at the European Synchrotron and Radiation Facitility (ID13) to explore how the maximum strain magnitude and direction, and the local film density vary spatially around shear bands formed in a drying colloidal film. We also show how reducing the film growth rate can lead to crystalline microstructure which appears to suppress shear band formation.In these samples, the scanning micro-SAXS technique is used to follow changes that occur as a colloidal film forms and undergoes compaction in real time. |
Monday, March 5, 2018 2:54PM - 3:06PM |
C54.00003: Drying patterns in a confined hyperelastic hydrogel Baudouin Saintyves, Louis Zolla, L Mahadevan, Irmgard Bischofberger In many constraint systems, from paintings to muddy soils, the evaporation of a solvent leads to the formation of complex drying patterns. Commonly these patterns are characterized by brittle straight cracks. Here, we report very different structures that emerge during the drying of thin films of hyperelastic hydrogels confined between two glass plates. The evaporation front advances by intermittent bursts, leading either to a growing front of bubbles or finger-like structures. We discuss how these morphologies result from the interplay between the drying-induced flow inside the gel and the gel’s non-linear elasticity. |
Monday, March 5, 2018 3:06PM - 3:18PM |
C54.00004: Effect of solid surface tension on crack nucleation in soft gels Shruti Rattan, Alfred Crosby Solid surface tension has been shown to affect the deformation, adhesion, and crack propagation in soft solids. However, the effect of surface tension on crack nucleation, in particular, has been largely ignored. We study the effect of surface tension on the critical force to puncture (Pc ) a soft acrylic copolymer gel with spherically-tipped indenters of tip radii comparable to or smaller than the characteristic length scale Ys/E. Previously, we have shown that the size of a fracture process zone beneath an indenter tip is governed by the tip radius, R, and is critical in determining the failure regime, i.e. whether it is limited by a critical stress or a critical energy. In an energy-limited regime, Pc scales linearly with R. However, this scaling relation is derived assuming that the resistance to deformation and failure during puncture is mainly contributed by the elasticity of the gel, while surface tension was not taken into account in the analysis. Our results lead us to identify the characteristic material length scale, Ys /E and have demonstrated a new and interesting way in which soft materials resist failure at micron length scales. |
Monday, March 5, 2018 3:18PM - 3:30PM |
C54.00005: The Topology and Mechanics of Fracture Surface Pattern Formation Itamar Kolvin, Gil Cohen, Jay Fineberg Fracture of brittle materials often results in the formation of structure on crack faces whose origin has long remained obscured. The difficulty lies in seeing how microscopic structures are formed by rapid cracks. To overcome this difficulty, we study soft brittle gels where crack speeds are much lower than in hard materials. Much below the shear wave speed, cracks form facetted surfaces deliniated by steps. At higher speeds, facets give way to micro-branches, frustrated cracks that branch off the main crack. We directly visualize the leading edge of the crack, the crack front, as it forms surface structure. In the faceting regime, steps induce long-ranged deformation along the crack front. Since surface formation costs energy, steps imply locally increased dissipation. We show that steps persist due to topological defects of the crack front, that quantitatively link local dissipation increases at steps to crack front deformation. We also show that crack front curvature may feed back to deflect step paths via nonlinear focusing of crack fronts, causing steps to converge to form a micro-branch. Thus, our results supply the basis for a unified picture of pattern formation on fracture surfaces. |
Monday, March 5, 2018 3:30PM - 3:42PM |
C54.00006: Identifying structural signatures of shear banding in polymer nanopillars Robert Ivancic, Robert Riggleman Recent studies (Schoenholz, 2016) have used machine learning methods to identify a field, ''softness," that characterizes the local structure around particles and is highly correlated with particle-level dynamics in glassy materials. While softness is a good predictor of local particle rearrangements, its ability to predict large length and timescale phenomena such as material failure remains unknown. Using machine learning methods, we identify mesoscale defects that lead to shear banding in confined polymer pillars well below the glass transition temperature. We successfully apply these methods to pillars of diameters of 12.5, 25, 50, and 100 particle diameters. Our results show that the primary structural features responsible for shear banding on this scale are small, particle sized fluctuations in the pillar diameter. Interestingly, we note the importance of mean softness in identifying shear band planes grows as a function of pillar diameter suggesting these features play a large role in material failure for bulk-like systems. We find that shear band planes are softer in the pillar’s interior but less soft on their boundaries than other planes prior to deformation. |
Monday, March 5, 2018 3:42PM - 3:54PM |
C54.00007: Characterization of the Ductile to Brittle Transition in Model Polymer Glasses Emily Lin, Robert Riggleman While recent studies have developed a deep understanding of the origins of localized plastic rearrangements in disordered solids, the organization of these rearrangements into failure modes remains poorly understood. At low temperatures, glassy systems are known to exhibit brittle failure after these localized rearrangements, referred to as shear transformation zones, organize into shear bands and fracture. One reason for the poor understanding of failure is the lack of simple molecular models that exhibit a ductile-to-brittle transition. Molecular Dynamics (MD) simulations using common model glass-formers, which are generally based on the conventional Lennard Jones (LJ) pair potential, have been shown to have significant ductility even at temperatures far below the glass transition temperature. In this work, I will describe our efforts to design a model system that exhibits a ductile-to-brittle transition that is accessible to MD simulations. Following ideas proposed by Falk and co-workers, we modify the LJ potential to a form that increases the elastic modulus of the material and leads to brittle failure at temperatures far below the glass transition. I will describe the characteristics that lead to brittle failure and how they depend on temperature and sample history. |
Monday, March 5, 2018 3:54PM - 4:06PM |
C54.00008: Three-Dimensional Continuum-Level Simulation of Shear Banding in Metallic Glasses Nicholas Boffi, Christopher Rycroft We simulate a three-dimensional continuum-level elasto-plastic model of a bulk metallic glass based on the shear transformation zone (STZ) theory of amorphous plasticity. The simulation utilizes a new projection method valid in the quasi-static limit based on a mathematical correspondence between the Navier-Stokes equations for incompressible fluid flow and the equations of quasi-static hypoelastoplasticity. We test the method by simulating three-dimensional shear band nucleation and growth in materials undergoing simple shear. We also present a variation of the method based on a coordinate transformation that enables direct mapping between continuum-level boundary conditions and the Lees-Edwards boundary conditions that are frequently imposed in molecular dynamics simulations, enabling direct comparisons between continuum and discrete simulation. |
Monday, March 5, 2018 4:06PM - 4:18PM |
C54.00009: A Green’s Function Method for Solving Dynamical Contact Problems Joseph Monti, Lars Pastewka, Mark Robbins Resolving atomic scale details while capturing long-range elastic deformation is the principal difficulty when solving contact mechanics problems with computer simulations. Fully atomistic simulations must consider large blocks of atoms to support long wavelength deformation modes, meaning that most atoms are far removed from the region of interest. We have developed a numerically efficient dynamic Green’s function technique to remedy the inability of simulations to treat realistic, time-evolving, elastic solids. Our method utilizes pre-computed Green’s functions that exactly reproduce elastic interactions without retaining the atomic degrees of freedom in the bulk. We invoke physical insights about the attenuation of subsurface waves as a power of wavevector to limit the number of crystalline layers required to accurately model wave propagation, without implementing arbitrary damping. We apply the method to single asperity sliding to study the velocity dependence of friction and present our preliminary findings. This geometry is typical of friction force and atomic force microscopy, and our results are suitable for comparison to experimental work in the field, where a detailed atomistic description of dynamics is lacking. |
Monday, March 5, 2018 4:18PM - 4:30PM |
C54.00010: Compressing binary crystals into glasses Huijun Zhang, Yilong Han A polycrystal transforms into a glass when its grain size is small enough. However, the boundary between polycrystal and glass is unclear partly because ultrafine-grained nanocrystals are unstable and undergo grain coarsening constantly. Here we propose a simple approach to induce the polycrystal-glass transition by compressing binary single crystals composed of hard and soft disks into polycrystals and further into glasses. At low pressure, both types of disks have the same effective diameter and form a crystal. At high pressures, soft disks are compressed into a smaller size, resulting in a glass. The full spectrum of grain sizes enables the study of the polycrystal-glass crossover. We observed rich structural, mechanical, dynamical and thermodynamic features at the polycrystal-glass boundary, which could serve as criteria for the polycrystal-glass transition. These analyses reveal the Hall-Petch and inverse-Hall-Petch behaviors in polycrystals and identify three different glass regimes. Such a crystal-to-glass transition is expected to occur widely in alloys when the pressure is high enough. These results cast new light on the fabrication and theories of both polycrystals and glasses. |
Monday, March 5, 2018 4:30PM - 4:42PM |
C54.00011: Transients and Shear Thinning due to Material Acceleration during Rapid Forced Intrusions into Granular Materials Christian Hubicki, Jennifer Rieser, Jeffrey Aguilar, Andras Karsai, Daniel Goldman The dynamics of forced intrusions are central to biological or robotic agents that excavate or locomote atop granular materials. We examine velocity-dependent effects of rapid constant-speed intrusions (up to 75 cm/s) into granular media. Using a force-instrument robotic arm, we vertically plunged rigid intruders into a bed of loosely packed poppy seeds at a constant speed, v, from 1 to 75 cm/s. For slow intrusions, force increased monotonically with intrusion depth. As v increased, a transient force developed over a characteristic time; excess work scaled with v2. Particle imaging (with the intruder against a clear side-wall) indicates that the transient forces correlate with the formation and rapid acceleration of a jammed structure beneath the intruder [Aguilar & Goldman. 2015. Nat Phys]. Once the transient decayed, force increased with depth across tested speeds. Surprisingly, in the steady state regime, force per unit depth decreased as v increased (18% from 1-75 cm/s) analogous to shear thinning in particulate suspensions. Imaging during the steady state shows grains above the intruder in a state of partial free-fall. A model incorporating free-fall suggests shear thinning results from reduced lithostatic pressure for rapid constant-speed intrusions. |
Monday, March 5, 2018 4:42PM - 4:54PM |
C54.00012: Pulling an object out of a granular material: Effect of boundaries Yue Zhang, Payman Jalali, Robert Behringer We consider controlled failure experiments where a circular intruder is buried at a depth D in a quasi-two-dimensional granular material of photoelastic discs. Under gradually increasing upward pulling force, F, a steadily strengthening granular network forms to the point of failure. Once failure begins, the buried intruder is pulled rapidly out of the material. In a first study, the experiments were done in a very wide container, of width W so that the distance between the intruder and the side boundaries was big enough that boundary effects were minimal. In this case, as F was increased towards failure, a fan-shape network of force chains much wider than the intruder formed above the intruder. In a second study, we steadily decreased W and changed the friction on the sidewalls. For smooth walls, there was no change with decreasing W in the force at failure, F(D). However, for rough walls, there was a clear change in F(D). This talk will present details of the force networks and mechanical response for these experiments. Parallel studies on a 3D version of this experiment will be presented by Jalali et al. |
Monday, March 5, 2018 4:54PM - 5:06PM |
C54.00013: Measuring Hardness of Stable Glasses Using Nanoindentation Sarah Wolf, Yijie Jiang, Lisa Mariani, Tianyi Liu, Georgia Huang, Keyume Ablajan, Xiao Xia Liang, Philip Gilmartin, Thiago Toledo, Minyan Li, Patrick Walsh, Kevin Turner, Zahra Fakhraai It has been shown that the process of Physical Vapor Deposition, or PVD, can form glasses with enhanced stability compared to liquid-quenched glasses. This occurs when the substrate temperature is held close to 85% of the glass transition temperature and the deposition occurs sufficiently slowly. These stable glasses have been shown to exhibit higher density, lower enthalpy, and better kinetic stability over ordinary glasses. Given these exceptional properties, it is of interest to investigate other properties that may be affected by PVD. Some work has shown that the speed of sound and elastic moduli increase with increased stability for some glass formers. Here we directly measure mechanical properties of stable glasses using nanoindentation. We investigate the relationship between mechanical properties and stability of several stable glass-forming molecules with various shapes and aspect ratio. Elastic modulus and hardness of these molecules are measured as a function of deposition temperature. It is found that the molecular structure can play an important role in the stability and mechanics of PVD glasses. |
Monday, March 5, 2018 5:06PM - 5:18PM |
C54.00014: Modeling Shear-rate-gradient-driven Size-segregation in Dense, Bidisperse Granular Flows Down a Vertical Chute Daren Liu, David Henann Dense granular systems, consisting of particles of disparate sizes, segregate based on size during flow, resulting in complex, coupled segregation and flow patterns. There are two driving mechanisms of size-segregation during flow: (i) gravity and (ii) shear-strain-rate gradients. The mechanism of shear-rate-gradient-driven segregation has received less attention and, as such, is not as well understood. In this talk, we study this segregation mechanism in a flow configuration that eliminates gravity-driven segregation – gravity-driven flow down a long vertical chute with rough parallel walls. We perform two-dimensional discrete element method (DEM) simulations of vertical chute flow of dense, bidisperse granular systems. Specifically, we systematically study the effect of flow rate, chute width, fraction of large/small grains, and grain-size ratio on the segregation dynamics. This informs the development of continuum constitutive equations for both the shear-rate-gradient-driven size-segregation flux as well as the diffusion flux. When coupled with the nonlocal granular fluidity model – a recently-proposed, nonlocal continuum model for dense granular flow – we show that both the flow field and segregation dynamics may be simultaneously captured using this closed, coupled system. |
Monday, March 5, 2018 5:18PM - 5:30PM |
C54.00015: Isostaticity and the solidification of semiflexible polymer melts Christian Plaza-Rivera, Hong Nguyen, Robert Hoy Using molecular dynamics simulations of a tangent-soft-sphere bead-spring polymer model, we examine the degree to which semiflexible polymer melts solidify at isostaticity. Flexible and stiff chains crystallize when they are isostatic as defined by appropriate degree-of-freedom-counting arguments. Semiflexible chains also solidify when isostatic if a generalized isostaticity criterion that accounts for the slow freezing out of configurational freedom as chain stiffness increases is employed. The configurational freedom associated with bond angles (θ) can be associated with chains’ characteristic ratio C∞ = <1+cos(θ)> /<1-cos(θ)> . We find that the dependence of the average coordination number at solidification Z(Ts) on C∞ has the same functional form [Z = a - b*ln(C∞)] as the dependence of the average coordination number at jamming Z(φJ) on C∞ in athermal systems, suggesting that jamming-related phenomena play a significant role in thermal polymer solidification. |
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