Structural damping, that is the presence of a velocity dependent dissipative term in the equation of motion, is rationalized as a thermalization process between a structure (here a beam) and an outside bath (understood in a broad sense as a system property). This is achieved via the introduction of the kinetic temperature of structures and formalized by means of an extended Lagrangian formulation of a structure in contact with an outside bath at a given temperature. Using the Nose-Hoover thermostat, the heat exchange rate between structure and bath is identified as a mass damping coefficient, which evolves in time in function of the kinetic energy/temperature history exhibited by the structure. By way of application to a simple beam structure subjected to eigen-vibrations and dynamic buckling, commonality and differences of the Nose-Hoover beam theory with constant mass damping models are shown, which permit a handshake between classical damping models and statistical mechanics-based thermalization models. The solid foundation of these thermalization models in statistical physics provides new insights into stability and instability for engineering structures. Specifically, since two systems are considered in (thermodynamic) equilibrium when they have the same temperature, we show in the case of dynamic buckling that a persistent steady-state difference in kinetic temperature between structure and bath is but indicative of the instability of the system. This shows that the kinetic temperature can serve as a structural order parameter to identify and comprehend failure of structures, possibly well beyond the elastic stability considered here.

VL - 85 UR - http://appliedmechanics.asmedigitalcollection.asme.org.libproxy.mit.edu/article.aspx?articleid=2680040 IS - 8 JO - J. Appl. Mech ER - TY - JOUR T1 - Thermodynamics, kinetics, and mechanics of cesium sorption in cement paste: A multiscale assessment JF - Physical Review Materials Y1 - 2018 A1 - Arayro, Jack A1 - Dufresne, Alice A1 - Zhou, Tingtao A1 - Katerina Ioannidou A1 - Franz-Josef Ulm A1 - Roland Jean-Marc Pellenq A1 - Béland, Laurent Karim KW - CALCIUM-SILICATE-HYDRATE; C-S-H; MOLECULAR-DYNAMICS; CRYSTAL-STRUCTURE; MONTE-CARLO; IMMOBILIZATION; RADIONUCLIDES; SIMULATIONS; RELAXATION; COLLOIDS AB -Cesium-137 is a common radioactive byproduct found in nuclear spent fuel. Given its 30 year half life, its interactions with potential storage materials-such as cement paste-is of crucial importance. In this paper, simulations are used to establish the interaction of calcium silicate hydrates (C-S-H)-the mam binding phase of cement paste-with Cs at the nano- and mesoscale. Different C-S-H compositions are explored, including a range of Ca/Si ratios from 1.0 to 2.0. These calculations are based on a set of 150 atomistic models, which qualitatively and quantitatively reproduce a number of experimentally measured features of C-S-H-within limits intrinsic to the approximations imposed by classical molecular dynamics and the steps followed when building the models. A procedure where hydrated Ca2+ ions are swapped for Cs1+ ions shows that Cs adsorption in the C-S-H interlayer is preferred to Cs adsorption at the nanopore surface when Cs concentrations are lower than 0.19 Mol/kg. Interlayer sorption decreases as the Ca/Si ratio increases. The activation relaxation technique nouveau is used to access timescales out of the reach of traditional molecular dynamics (MD). It indicates that characteristic diffusion time for Cs1+ in the C-S-H interlayer is on the order of a few hours. Cs uptake in the interlayer has little impact on the elastic response of C-S-H. It leads to swelling of the C-S-H grams, but mesoscale calculations that access length scales out of the range of MD indicate that this leads to practically negligible expansive pressures for Cs concentrations relevant to nuclear waste repositories.

The ability to measure the bulk dynamic behavior of soft materials with combined time and frequency resolution is instrumental for improving our fundamental understanding of connections between the microstructural dynamics and the macroscopic mechanical response. Current state-of-the-art techniques are often limited by a compromise between resolution in the time and frequency domains, mainly due to the use of elementary input signals that have not been designed for fast time-evolving systems such as materials undergoing gelation, curing, or self-healing. In this work, we develop an optimized and robust excitation signal for time-resolved mechanical spectroscopy through the introduction of joint frequency- and amplitude-modulated exponential chirps. Inspired by the biosonar signals of bats and dolphins, we optimize the signal profile to maximize the signal-to-noise ratio while minimizing spectral leakage with a carefully designed modulation of the envelope of the chirp, obtained using a cosine-tapered window function. A combined experimental and numerical investigation reveals that there exists an optimal range of window profiles (around 10% of the total signal length) that minimizes the error with respect to standard single-frequency sweep techniques. The minimum error is set by the noise floor of the instrument, suggesting that the accuracy of an optimally windowed-chirp (OWCh) sequence is directly comparable to that achievable with a standard frequency sweep, while the acquisition time can be reduced by up to 2 orders of magnitude, for comparable spectral content. Finally, we demonstrate the ability of this optimized signal to provide time- and frequency-resolved rheometric data by studying the fast gelation process of an acid-induced protein gel using repeated OWCh pulse sequences. The use of optimally windowed chirps enables a robust time-resolved rheological characterization of a wide range of soft materials undergoing rapid mutation and has the potential to become an invaluable rheometric tool for researchers across different disciplines.

VL - 8 UR - https://link.aps.org/doi/10.1103/PhysRevX.8.041042 IS - 4 JO - Phys. Rev. X ER - TY - JOUR T1 - Two-dimensional numerical simulation of chimney fluidization in a granular medium using a combination of discrete element and lattice Boltzmann methods JF - Physical Review E Y1 - 2018 A1 - Jeff Ngoma A1 - Pierre Philippe A1 - Stéphane Bonelli A1 - Farhang Radjaï A1 - Jean-Yves Delenne AB -We present here a numerical study dedicated to the fluidization of a submerged granular medium induced by a localized fluid injection. To this end, a two-dimensional (2D) model is used, coupling the lattice Boltzmann method (LBM) with the discrete element method (DEM) for a relevant description of fluid-grains interaction. An extensive investigation has been carried out to analyze the respective influences of the different parameters of our configuration, both geometrical (bed height, grain diameter, injection width) and physical (fluid viscosity, buoyancy). Compared to previous experimental works, the same qualitative features are recovered as regards the general phenomenology including transitory phase, stationary states, and hysteretic behavior. We also present quantitative findings about transient fluidization, for which several dimensionless quantities and scaling laws are proposed, and about the influence of the injection width, from localized to homogeneous fluidization. Finally, the impact of the present 2D geometry is discussed, by comparison to the real three-dimensional (3D) experiments, as well as the crucial role of the prevailing hydrodynamic regime within the expanding cavity, quantified through a cavity Reynolds number, that can presumably explain some substantial differences observed regarding upward expansion process of the fluidized zone when the fluid viscosity is changed.

VL - 97 UR - https://link.aps.org/doi/10.1103/PhysRevE.97.052902 IS - 5 JO - Phys. Rev. E ER - TY - JOUR T1 - Temperature distribution and heat radiation of patterned surfaces at short wavelengths JF - Physical Review E Y1 - 2017 A1 - Emig, Thorsten AB -We analyze the equilibrium spatial distribution of surface temperatures of patterned surfaces. The surface is exposed to a constant external heat flux and has a fixed internal temperature that is coupled to the outside heat fluxes by finite heat conductivity across the surface. It is assumed that the temperatures are sufficiently high so that the thermal wavelength (a few microns at room temperature) is short compared to all geometric length scales of the surface patterns. Hence the radiosity method can be employed. A recursive multiple scattering method is developed that enables rapid convergence to equilibrium temperatures. While the temperature distributions show distinct dependence on the detailed surface shapes (cuboids and cylinder are studied), we demonstrate robust universal relations between the mean and the standard deviation of the temperature distributions and quantities that characterize overall geometric features of the surface shape.

VL - 95 UR - http://link.aps.org/doi/10.1103/PhysRevE.95.052104 IS - 5 JO - Phys. Rev. E ER - TY - JOUR T1 - Three-dimensional bonded-cell model for grain fragmentation JF - Computational Particle Mechanics Y1 - 2017 A1 - Cantor, D A1 - Emilien Azéma A1 - Philippe Sornay A1 - Farhang Radjaï KW - Bonded-cell model; Fragmentation; Discrete element method; Contact dynamics method; Voronoi cell; Weibull statistics AB -We present a three-dimensional numerical method for the simulation of particle crushing in 3D. This model is capable of producing irregular angular fragments upon particle fragmentation while conserving the total volume. The particle is modeled as a cluster of rigid polyhedral cells generated by a Voronoi tessellation. The cells are bonded along their faces by a cohesive Tresca law with independent tensile and shear strengths and simulated by the contact dynamics method. Using this model, we analyze the mechanical response of a single particle subjected to diametral compression for varying number of cells, their degree of disorder, and intercell tensile and shear strength. In particular, we identify the functional dependence of particle strength on the intercell strengths. We find that two different regimes can be distinguished depending on whether intercell shear strength is below or above its tensile strength. In both regimes, we observe a power-law dependence of particle strength on both intercell strengths but with different exponents. The strong effect of intercell shear strength on the particle strength reflects an interlocking effect between cells. In fact, even at low tensile strength, the particle global strength can still considerably increase with intercell shear strength. We finally show that the Weibull statistics describes well the particle strength variability.

VL - 4 UR - http://link.springer.com/10.1007/s40571-016-0129-0 IS - 4 JO - Comp. Part. Mech. ER - TY - JOUR T1 - A time-dependent atomistic reconstruction of severe irradiation damage and associated property changes in nuclear graphite JF - Carbon Y1 - 2017 A1 - Baptiste Farbos A1 - Freeman, Helen A1 - Hardcastle, Trevor A1 - Da Costa, Jean-Pierre A1 - Brydson, Rik A1 - Scott, Andrew J. A1 - Weisbecker, Patrick A1 - Germain Christian A1 - Gérard L. Vignoles A1 - Jean-Marc Leyssale KW - TRANSMISSION ELECTRON-MICROSCOPY; MOLECULAR-DYNAMICS SIMULATIONS; HIGHLY ANISOTROPIC PYROCARBONS; THERMAL-CONDUCTIVITY; NEUTRON-IRRADIATION; CARBON; GRAPHENE; DEFECTS; ENERGY; NANOSTRUCTURE AB -Wheat grains can be considered as a natural cemented granular material. They are milled under high forces to produce food products such as flour. The major part of the grain is the so-called starchy endosperm. It contains stiff starch granules, which show a multi-modal size distribution, and a softer protein matrix that surrounds the granules. Experimental milling studies and numerical simulations are going hand in hand to better understand the fragmentation behavior of this biological material and to improve milling performance. We present a numerical study of the effect of granule size distribution on the strength of such a cemented granular material. Samples of bi-modal starch granule size distribution were created and submitted to uniaxial tension, using a peridynamics method. We show that, when compared to the effects of starch-protein interface adhesion and voids, the granule size distribution has a limited effect on the samples’ yield stress.

VL - 120 UR - https://linkinghub.elsevier.com/retrieve/pii/S0008622317304542 JO - Carbon ER - TY - JOUR T1 - Topological Control on the Structural Relaxation of Atomic Networks under Stress JF - Physical Review Letters Y1 - 2017 A1 - Mathieu Bauchy A1 - Wang, Mengyi A1 - Yu, Yingtian A1 - Wang, Bu A1 - Krishnan, M. Anoop A1 - Enrico Masoero A1 - Franz-Josef Ulm A1 - Roland Jean-Marc Pellenq AB -Upon loading, atomic networks can feature delayed irreversible relaxation. However, the effect of composition and structure on relaxation remains poorly understood. Herein, relying on accelerated molecular dynamics simulations and topological constraint theory, we investigate the relationship between atomic topology and stress-induced structural relaxation, by taking the example of creep deformations in calcium silicate hydrates (C─S─H), the binding phase of concrete. Under constant shear stress, C─S─H is found to feature delayed logarithmic shear deformations. We demonstrate that the propensity for relaxation is minimum for isostatic atomic networks, which are characterized by the simultaneous absence of floppy internal modes of relaxation and eigenstress. This suggests that topological nanoengineering could lead to the discovery of nonaging materials.

VL - 119 IS - 3 JO - Phys. Rev. Lett. ER - TY - JOUR T1 - Transient dynamics of a 2D granular pile JF - The European Physical Journal E Y1 - 2015 A1 - Patrick Mutabaruka A1 - Krishna Kumar A1 - Kenichi Soga A1 - Farhang Radjaï A1 - Jean-Yves Delenne AB -

We investigate by means of Contact Dynamics simulations the transient dynamics of a 2D granular pile set into motion by applying shear velocity during a short time interval to all particles. The spreading dynamics is directly controlled by the input energy whereas in recent studies of column collapse the dynamics scales with the initial potential energy of the column. As in column collapse, we observe a power-law dependence of the runout distance with respect to the input energy with nontrivial exponents. This suggests that the power-law behavior is a generic feature of granular dynamics, and the values of the exponents reflect the distribution of kinetic energy inside the material. We observe two regimes with different values of the exponents: the low-energy regime reflects the destabilization of the pile by the impact with a runout time independent of the input energy whereas the high-energy regime is governed by the input energy. We show that the evolution of the pile in the high-energy regime can be described by a characteristic decay time and the available energy after the pile is destabilized.

Beyond a given threshold, an upward fluid flow at constant flowrate, injected through a small size section, is able to generate a fluidization along a vertical chimney over the entire height of a granular assembly. Fluidization is first initiated in the immediate vicinity of the injection hole and then the fluidized zone grows gradually until reaching the upper surface of the granular packing. In this work, we present numerical results on the kinetics of chimney fluidization in an immersed granular bed produced with two-dimensional simulations coupling the Discrete Element and Lattice Boltzmann Methods (DEM-LBM). A parametric study is carried out with 11 different sets of physical parameters and analyzed based on spatio-temporal diagrams. Then a dimensional analysis allows finding general scaling laws for both threshold and growth rate of the fluidized zone by use of two dimensionless numbers, namely Reynolds and Archimedes numbers, while quite simple empirical relationships can also be proposed.

JF - IV International Conference on Particle-based Methods (PARTICLES 2015) PB - International Center for Numerical Methods in Engineering (CIMNE) CY - SEP 28-30 2015 Barcelona, SPAIN VL - PARTICLE-BASED METHODS IV-FUNDAMENTALS AND APPLICATIONS UR - https://hal.archives-ouvertes.fr/hal-01269325 ER - TY - JOUR T1 - Tensile strength and fracture of cemented granular aggregates JF - The European Physical Journal E Y1 - 2012 A1 - Rafik Affès A1 - Jean-Yves Delenne A1 - Yann Monerie A1 - Farhang Radjaï A1 - Vincent Topin AB -Cemented granular aggregates include a broad class of geomaterials such as sedimentary rocks and some biomaterials such as the wheat endosperm. We present a 3D lattice element method for the simulation of such materials, modeled as a jammed assembly of particles bound together by a matrix partially filling the interstitial space. From extensive simulation data, we analyze the mechanical properties of aggregates subjected to tensile loading as a function of matrix volume fraction and particle-matrix adhesion. We observe a linear elastic behavior followed by a brutal failure along a fracture surface. The effective stiffness before failure increases almost linearly with the matrix volume fraction. We show that the tensile strength of the aggregates increases with both the increasing tensile strength at the particle-matrix interface and decreasing stress concentration as a function of matrix volume fraction. The proportion of broken bonds in the particle phase reveals a range of values of the particle-matrix adhesion and matrix volume fraction for which the cracks bypass the particles and hence no particle damage occurs. This limit is shown to depend on the relative toughness of the particle-matrix interface with respect to the particles.

VL - 35 IS - 11 JO - Eur. Phys. J. E ER - TY - JOUR T1 - Thermodynamics of Water Confined in Porous Calcium-Silicate-Hydrates JF - Langmuir Y1 - 2012 A1 - Patrick A Bonnaud A1 - Qing Ji A1 - Benoit A. Coasne A1 - Roland Jean-Marc Pellenq A1 - Krystyn J. Van Vliet AB -Water within pores of cementitious materials plays a crucial role in the damage processes of cement pastes, particularly in the binding material comprising calcium-silicate-hydrates (C–S–H). Here, we employed Grand Canonical Monte Carlo simulations to investigate the properties of water confined at ambient temperature within and between C–S–H nanoparticles or “grains” as a function of the relative humidity (%RH). We address the effect of water on the cohesion of cement pastes by computing fluid internal pressures within and between grains as a function of %RH and intergranular separation distance, from 1 to 10 Å. We found that, within a C–S–H grain and between C–S–H grains, pores are completely filled with water for %RH larger than 20%. While the cohesion of the cement paste is mainly driven by the calcium ions in the C–S–H, water facilitates a disjoining behavior inside a C–S–H grain. Between C–S–H grains, confined water diminishes or enhances the cohesion of the material depending on the intergranular distance. At very low %RH, the loss of water increases the cohesion within a C–S–H grain and reduces the cohesion between C–S–H grains. These findings provide insights into the behavior of C–S–H in dry or high-temperature environments, with a loss of cohesion between C–S–H grains due to the loss of water content. Such quantification provides the necessary baseline to understand cement paste damaging upon extreme thermal, mechanical, and salt-rich environments.

Application of the fourth moment approximation (FMA) to the local density of states within a tight binding description to build a reactive, interatomic interaction potential for use in large scale molecular simulations, is a logical and significant step forward to improve the second moment approximation, standing at the basis of several, widely used (semi-)empirical interatomic interaction models. In this paper we present a sufficiently detailed description of the FMA and its technical implications, containing the essential elements for an efficient implementation in a simulation code. Using a recent, existing FMA-based model for C-Ni systems, we investigated the size dependence of the diffusion of a liquid Ni cluster on a graphene sheet and find a power law dependence of the diffusion constant on the cluster size (number of cluster atoms) with an exponent very close to −2/3, equal to a previously found exponent for the relatively fast diffusion of solid clusters on a substrate with incommensurate lattice matching. The cluster diffusion exponent gives rise to a specific contribution to the cluster growth law, which is due to cluster coalescence. This is confirmed by a simulation for Ni cluster growth on graphene, which shows that cluster coalescence dominates the initial stage of growth, overruling Oswald ripening.

VL - 84 IS - 8 JO - Phys. Rev. B ER -