Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session N02: From Statistical Physics to High Performance Materials IIFocus
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Sponsoring Units: GSNP DSOFT DPOLY Chair: Ting Ge, University of South Carolina Room: Room 125 |
Wednesday, March 8, 2023 11:30AM - 12:06PM |
N02.00001: Simulations of Non-equilibrium Processes in Polymer-based Materials Invited Speaker: Arlette R Baljon Polymer-based materials range from adhesives to associative polymeric networks and mucus hydrogels. Non-equilibrium processes include stick-slip behavior, energy dissipation (as underlies adhesive strength), shear-induced structural transition, and memory effects. In this talk, I will discuss my contributions to the field over the years, starting with simulations I performed as a postdoctoral associate in Mark Robbins’ lab. Next, I will describe how these studies inspired my subsequent scientific work. |
Wednesday, March 8, 2023 12:06PM - 12:18PM |
N02.00002: Coarse-grained Molecular Dynamics Simulation of Epoxy Network Polymers Xi Hao, Chengyuan Wen, Gary D Seidel, Shengfeng Cheng Epoxy as an important type of thermosetting polymers has been widely used as matrix materials in polymer-based composites. To understand the thermomechanical response of such materials, we have developed a coarse-grained model of epoxy network polymers based on EPON resin 862 with diethyltoluenediamine (DETDA) as the curing agent, using a force-matching method coupled with chemistry-informed grouping of atoms into coarse-grained beads, direct Boltzmann inversion, and potential of mean force calculations between atomic groups. The model is used to construct a large epoxy network polymer and the mechanical moduli computed with the coarse-grained model are found to be close to the available experimental values and computational results from all-atom molecular dynamics simulations of a much smaller network. To further validate the coarse-grained model, the large coarse-grained network is backmapped to an all-atom one by replacing each coarse-grained bead with the corresponding atomic group. The mechanical properties of the large epoxy network with the all-atom representation are determined and compared with the results from the coarse-grained calculations. Furthermore, bond breakability is introduced to the coarse-grained network to enable the modeling of the fracture and crazing behavior of epoxy network polymers. |
Wednesday, March 8, 2023 12:18PM - 12:30PM |
N02.00003: Simulation of polymerization induced phase separation in model thermosets Mark J Stevens Polymerization induced phase separation (PIPS) in a three component thermoset is studied using molecular dynamics simulations of a new coarse-grained thermoset model. The system includes two crosslinker molecules, which differ in their glass transition temperatures (Tg) and chain length and thus have the potential for phase separation. One crosslinker has a high Tg corresponding to a rubbery behavior, and simulations were performed for a short length and a long length. The resin and other crosslinker have low Tg. A coarse- grained model is developed with these features and with interaction parameters determined so that for either rubbery crosslinker length, the system is in the liquid state at the cure temperature. Simulations of network formation were performed to study the effect of reaction rates on the the network structure particularly the phase composition. For sufficiently slow reaction rates, the long rubbery molecule exhibits PIPS into a bicontinuous array of nanoscale domains, but the short one does not, reproducing recent experimental results. The simulations demonstrate that the reaction rates must be slow enough to allow diffusion to yield phase separation. Particularly, the reaction rate corresponding to the secondary amine must be very slow, else crosslinking without phase separation occurs preventing PIPS.
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Wednesday, March 8, 2023 12:30PM - 12:42PM |
N02.00004: Computer Simulation of Fatigue Failure in Amorphous Solids Srikanth Sastry, Himangsu Bhaumik, Shivakumar Athani Fatigue failure occurs in solids upon cyclic loading, after a number of cycles of stress/strain. The number of cycles to failure grows larger as a limiting amplitude of stress/strain, the fatigue limit, is approached from above. Although yielding behaviour of amorphous solids under cyclic deformation has been investigated through computer simulations actively in recent years (e. g., [1]), the corresponding phenomenon of fatigue failure, and more specifically, the dependence of the number of cycles to failure and the mechanisms thereof, have received relatively less attention. We perform cyclic shear deformation of glasses over a range of strain amplitudes and investigate the dependence of the number of cycles to failure on the amplitude of deformation. We perform a detailed investigation of the spatio-temporal variation of the stresses, energies and structure, and interpret results in terms of recent theoretical approaches to understanding yielding and fatigue failure in amorphous solids [2,3]. |
Wednesday, March 8, 2023 12:42PM - 12:54PM |
N02.00005: The Fundamental Physics of the Onset of Frictional Motion: How does friction start? Jay Fineberg Recent experiments have demonstrated that rapid rupture fronts, akin to earthquakes, mediate the transition to frictional motion. Moreover, once these dynamic rupture fronts (“laboratory earthquakes”) are created, their singular form, dynamics and arrest are well-described by fracture mechanics. Ruptures, however, need to be created within initially rough frictional interfaces, before they are able to propagate. This is the reason that “static friction coefficients” are not well-defined; frictional ruptures can nucleate for a wide range of applied forces. A critical open question is, therefore, how the nucleation of rupture fronts actually takes place. We experimentally demonstrate that rupture front nucleation is prefaced by slow nucleation fronts. These nucleation fronts, which are self-similar, are not described by our current understanding of fracture mechanics. The nucleation fronts emerge from initially rough frictional interfaces at well-defined stress thresholds, evolve at characteristic velocity and time scales governed by stress levels, and propagate within a frictional interface to form the initial rupture from which fracture mechanics take over. These results are of fundamental importance to questions ranging from earthquake nucleation and prediction to processes governing material failure. |
Wednesday, March 8, 2023 12:54PM - 1:06PM |
N02.00006: A nonlocal contact model for adhesive elastic-plastic particles William R Zunker, Kenneth N Kamrin We present a contact model able to capture the response of interacting adhesive elastic-plastic particles. The model is built upon the Method of Dimensionality Reduction which allows the problem of a 3D axisymmetric contact to be mapped to a semi-equivalent 1D problem of a rigid indenter penetrating a bed of independent Hookean springs. Plasticity is accounted for by continuously varying the 1D indenter profile subject to a constraint on the contact stress. By considering the incompressible nature of this plastic deformation, the contact model is also able to account for the nonlocal effects of neighboring contacts, including formation of new contacts from outward displacement of the free surface. JKR type adhesion is recovered easily by simply allowing the springs to ‘stick’ to the 1D indenters surface. Additionally, we account for the rapid stiffening in the force-displacement curve under high confinement (e.g. during powder compaction) by allowing a superimposed bulk elastic response to be switched on. To validate the model, we compare it to finite element simulations of adhesive elastic-plastic contact. These comparisons show that the proposed contact model is able to accurately capture plastic displacement at the contact, contact stress, particle volume, contact radius, and force as a function of displacement under a variety of complex loadings. |
Wednesday, March 8, 2023 1:06PM - 1:18PM |
N02.00007: Spontaneous Friction Switches in Structural Superlubricity Mehmet Z Baykara, Wai H Oo We present results of atomic force microscopy based sliding experiments performed on gold nanoislands on graphite, a material system that exhibits structural superlubricity under ambient conditions [1]. Our measurements reveal a previously undiscovered effect in structural superlubricity: spontaneous switches (i.e., jumps) between two well-defined friction branches. Repeated experiments on the same sample system show that the switching effect disappears in a few weeks, pointing toward a significant influence of contamination. Our work is indicative of non-trivial energy dissipation mechanisms in this elusive ultra-low friction regime. |
Wednesday, March 8, 2023 1:18PM - 1:30PM |
N02.00008: Computational Approaches for Studying the Nucleation of Voids at the Nanoscale. Vicente Munizaga, Michael L. L Falk Solid state precipitation and growth of intermetallic phases from metal solid solutions is important for the design and manufacturing of alloys. Intermetallic precipitates, particularly on the nanoscale, are one of the most effective methods to strengthen metals. It has been demonstrated that in Mg alloys this microstructure can result from the defects generated by mechanical deformation. Vacancies and voids are commonly found among these defects and may play a crucial role in the nucleation process, as they change the composition in their vicinity by attracting or repelling solute. |
Wednesday, March 8, 2023 1:30PM - 1:42PM |
N02.00009: Slip Avalanches on a Conical Bead Pile With Cohesion Susan Y Lehman, Karin A Dahmen A conical bead pile subject to slow driving is used as a model critical system to experimentally investigate the distributions of avalanche sizes and time between events. The pile is composed of roughly 20 000 steel beads, 3 mm in diameter, and driven by adding one bead at a time to the pile apex. We record the changes in pile mass over tens of thousands of bead drops to characterize the distribution of avalanche sizes. The experimental results match well with a mean-field model of slip avalanches [Dahmen et al., Nat Phys 7, 554 (2011)], as shown recently with particular emphasis on the effect of cohesion [Lehman et al., Gran Matt 24:35 (2022)]. The effects of cohesion on the statistics and the time series properties of the experimental avalanches correspond closely to the effect of dynamic weakening on the slip avalanches in this simple model used both for earthquakes and the plastic deformation of brittle solids. In addition to the avalanche size data, we also record the activity on the pile directly with a camera located above the pile. Here we extend the comparison between the experiment and the model to characteristics of individual avalanches, such as avalanche duration and extent on the pile, by using the particle image velocimetry to analyze the avalanche motion. |
Wednesday, March 8, 2023 1:42PM - 1:54PM |
N02.00010: Mechanical response of particulated granular materials Anne Xia, Dong Wang, Jerry Zhang, Mark D Shattuck, Corey S O'Hern Numerous prior studies have shown that the ensemble-averaged shear modulus of jammed packings of spherical particles increases with pressure p, ~pβ, where β=0.5 in the large-system limit. The growth of with increasing pressure is caused by pressure-induced particle rearrangements that lead to an increasing number of interparticle contacts. However, there are numerous applications for which it is desirable to design materials that can maintain their flexibility with small values of G, but possess large values of the bulk modulus B at high pressures. To design bulk materials with small values of G/B, we construct “particulated” granular metamaterials, which possess small numbers of grains confined within undercoordinated physical boundaries (or voxels) that are then connected to form a bulk structure. In this work, we employ discrete element method simulations to identify and measure the elastic moduli of all configurations for small numbers of monodisperse, frictionless spheres confined within single voxels with arbitrary shapes. We then relate the elastic moduli of the particle-filled single voxels to those for bulk systems composed of regular arrangements of the particle-filled single voxels. We show that since the particles are prevented from rearranging, we can design bulk particulated tessellations with small values of G/B < 10-3 even at large external pressures. |
Wednesday, March 8, 2023 1:54PM - 2:06PM |
N02.00011: Particulated Granular Metamaterials Dong Wang, Jerry Zhang, Weiwei Jin, Annie Xia, Nidhi Pashine, Rebecca Kramer-Bottiglio, Mark D Shattuck, Corey S O'Hern Granular materials display complex mechanical response. For example, the ensemble-averaged shear modulus <G> increases with pressure as P0.5 in the large-system limit when particles are allowed to rearrange during isotropic compression. In this work, we seek granular materials with shear moduli that decrease with increasing pressure even in the large-system limit. To do this, we design “particulated” granular metamaterials composed of multiple cubic “voxels” each containing jammed packings of a small number of spheres (N < 8). As shown in previous studies, G typically decreases linearly with P for a single voxel containing a small number of spheres with a slope that depends on the angle of the shear. We show that the behavior of G versus P for granular metamaterials made up of a large number of voxels (each with N < 8) is controlled by the ratio of the particle-particle and particle-wall interactions. In particular, we are able to achieve large particulated granular metamaterials for which G decreases with increasing P. |
Wednesday, March 8, 2023 2:06PM - 2:18PM |
N02.00012: Probing of the buckling of spherical shells with a distribution of imperfections Arefeh Abbasi, Fani Derveni, Yuexia L Lin, Pedro Reis The buckling capacity of shell structures is strongly sensitive to imperfections and accurately predicting the critical buckling condition requires a priori knowledge of the defects. A novel procedure has been recently proposed to predict the buckling capacity of thin cylindrical shells based on the poking of pre-compressed shells to estimate their resistance to buckling. Subsequent studies revealed that the poking location plays a crucial role; determining the buckling capacity is only possible if the probing is done near the location of the most significant defect. However, the defects are generally unknown and often difficult to identify. We investigate the buckling of spherical shells containing a random distribution of defects. Using finite element simulations, we perform a statistical analysis of these imperfect shells, poking them with various indenters and at random-chosen locations. Such shells are more realistic and practically relevant than the previous single-defect cases. We believe our results on the probabilistic characterization of the stability of randomly imperfect shells using a statistical poking technique can inform the design rules of thin-walled structures. |
Wednesday, March 8, 2023 2:18PM - 2:30PM |
N02.00013: Soft pump producing a rapid flow using shell buckling Byungho Lee, Anna Lee Soft pneumatic pumps have played a significant role in improving the portability and adaptability of soft robots. However, they also had difficulties actuating stiff pneumatic devices and inducing rapid motion. Here, we introduce a soft pump, overcoming these limitations by using the snap-through instability and finely controlling the performance using an electro-active polymer. The soft pump consists of a spherical shell and two open cylindrical channels attached to opposite sides of the shell. Electrodes are attached to the surface of the pump to induce electrical responsiveness. The soft pump is axially stretched at a constant velocity. As the total length of the pump reaches a critical value, the spherical shell buckles and generates rapid outflow. In addition, the applied voltage on the electro-active soft pump programs the maximum flow rate and the mechanical power by changing effective geometric parameters and material properties. We conduct the finite element method (FEM) and experiments to verify the actuating behavior and the reprogrammability of the soft pump. FEM is used to obtain the maximum volume of the exposure air for different geometric parameters, material properties, and applied voltages. Experiments are used to evaluate the performance of the soft pump running a pneumatic device controlling the applied voltage. This work presents the potential of the novel soft pump as an advanced actuator since the single pump can drive various pneumatic devices by reprogramming the maximum flow rate. |
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