arXiv subjects

Enrico Masoero

Publications and source records attributed to Enrico Masoero.

10 recordsLinked to original sources

arXiv2025

Nanomechanics and Pore Structure of Sodium and Potassium Geopolymer Gels: Experiments, Molecular Dynamics and Coarse-Grained Simulations

The link between composition, microstructure and mechanics of NASH and KASH gels is elusive, even in pure metakaolin-based geopolymers. This article exploits molecular mechanics, coarse-grained nanomechanics and micromechanics, to interpret new experimental results from microscopy, porosimetry and nanoindentation. KASH displays a finer nanogranular structure than NASH (3 vs 30 nm particle diameters, 5 vs 50 nm average pore diameters), higher skeletal density (2.3 vs 2.02 g/cm$^3$), nanoindentation moduli (9.21 vs 7.5 GPa) and hardness (0.56 vs 0.37 GPa) despite a higher total porosity (0.48-0.53 vs 0.38). This suggests a stiffer and stronger solid skeleton for KASH, confirmed through predictive molecular dynamics simulations on recent and new models of NASH and KASH. The atomistic simulations inform mechanical interactions for new, coarse-grained, particle-based models of NASH and KASH. The resulting simulations predict the nanoindentation result that KASH is stiffer than equally porous NASH and the impact of formation eigenstresses on elastic moduli.

cond-mat.mtrl-sci↗

arXiv2025

Nanoparticle simulations of logarithmic creep and microprestress relaxation in concrete and other disordered solids

Bažant's microprestress theory relates the logarithmic basic creep of concrete to power-law relaxation of heterogeneous eigenstresses at the nanoscale. However, the link between material chemistry, nanostructure, and microprestress relaxation, is not understood. To approach this, we use a simple model of harmonically interacting, packed nanoparticles, relaxing with and without external stress. Microprestresses are related to per-particle virial stress heterogeneities. Simulation results show that logarithmic creep and power-law microprestress relaxation emerge from generic deformation kinetics in disordered systems, which can occur in various materials and at various scales. When the interactions are matched to some mechanical properties of C--S--H at the 100 nm scale, the predicted microprestresses have similar magnitude as in Bažant's theory. The ability of our simulations to quantitatively link stress relaxation with nanostructure and chemistry-dependent interactions, provides a pathway to better characterise, extrapolate, and even engineer the creep behaviour of traditional and new concretes.

cond-mat.mtrl-sci↗

arXiv2024

MASKE: A kinetic simulator of coupled chemical and mechanical processes driving microstructural evolution

The microstructure of materials evolves through chemical reactions and mechanical stress, often strongly coupled in phenomena such as pressure solution or crystallization pressure. This article presents MASKE: a simulator to address the challenge of modelling coupled chemo-mechanical processes in microstructures. MASKE represents solid phases as agglomerations of particles whose off-lattice displacements generate mechanical stress through interaction potentials. Particle precipitation and dissolution are sampled using Kinetic Monte Carlo, with original reaction rate equations derived from Transition State Theory and featuring contributions from mechanical interactions. Molecules in solution around the solid are modelled implicitly, through concentrations that change during microstructural evolution and define the saturation indexes for user-defined chemical reactions. The structure and implementation of the software are explained first. Then, two examples on a nanocrystal of calcium hydroxide address its chemical equilibrium and its mechanical response under a range of imposed strain rates, involving stress-driven dissolution and recrystallization. These examples highlight MASKE's distinctive ability to simulate strongly coupled chemo-mechanical processes. MASKE is available, open-source, on GitHub.

cond-mat.other↗

arXiv2022

Coarse-graining in time with the Functional Renormalisation Group: Relaxation in Brownian Motion

We apply the functional Renormalisation Group (fRG) to study relaxation in a stochastic process governed by an overdamped Langevin equation with one degree of freedom, exploiting the connection with supersymmetric quantum mechanics in imaginary time. After reviewing the functional integral formulation of the system and its underlying symmetries, including the resulting Ward-Takahashi identities for arbitrary initial conditions, we compute the effective action $Γ$ from the fRG, approximated in terms of the leading and subleading terms in the gradient expansion: the Local Potential Approximation and Wavefunction Renormalisation respectively. This is achieved by coarse-graining the thermal fluctuations in time resulting in e.g. an effective potential incorporating fluctuations at all timescales. We then use the resulting effective equations of motion to describe the decay of the covariance, and the relaxation of the average position and variance towards their equilibrium values at different temperatures. We use as examples a simple polynomial potential, an unequal Lennard-Jones type potential and a more complex potential with multiple trapping wells and barriers. We find that these are all handled well, with the accuracy of the approximations improving as the relaxation's spectral representation shifts to lower eigenvalues, in line with expectations about the validity of the gradient expansion. The spectral representation's range also correlates with temperature, leading to the conclusion that the gradient expansion works better for higher temperatures than lower ones. This work demonstrates the ability of the fRG to expedite the computation of statistical objects in otherwise long-timescale simulations, acting as a first step to more complicated systems.

cond-mat.stat-mech↗

arXiv2021

Functional Renormalisation Group for Brownian Motion I: The Effective Equations of Motion

We use the functional Renormalisation Group (fRG) to describe the in and out of equilibrium dynamics of stochastic processes, governed by an overdamped Langevin equation. Exploiting the connection between Langevin dynamics and supersymmetric quantum mechanics in imaginary time, we write down renormalisation flow equations for the effective action, approximated in terms of the Local Potential Approximation and Wavefunction Renormalisation. We derive \textit{effective equations of motion} (EEOM) from the effective action (EA) $Γ$ for the average position $\left\langle x\right\rangle$, variance $\langle \left(x- \langle x \rangle\right)^2\rangle$ and covariance. The fRG flow equations outlined here provide a concrete way to compute the EA and thus solve the derived EEOM. The obtained effective potential should determine directly the exact equilibrium statistics, name the position, the variance, as well as all higher order cumulants of the equilibrium Boltzmann distribution. This first paper of a two part series is mostly concerned with setting up the necessary formalism while in part two we will numerically solve the equations derived her and assess their validity both in and out of equilibrium.

cond-mat.stat-mech↗

arXiv2018

Capillary stress and structural relaxation in moist granular materials

We propose a theoretical framework to calculate capillary stresses in complex mesoporous materials, such as moist sand, nanoporous hydrates, and drying colloidal films. Molecular simulations are mapped onto a phase-field model of the liquid-vapor mixture, whose inhomogeneous stress tensor is integrated over Voronoi polyhedra in order to calculate equal and opposite forces between each pair of neighboring grains. The method is illustrated by simulations of moisture-induced forces in small clusters and random packings of spherical grains using lattice-gas Density Functional Theory. For a nano-granular model of cement hydrates, this approach reproduces the hysteretic water sorption/desorption isotherms and predicts drying shrinkage strain isotherm in good agreement with experiments. We show that capillary stress is an effective mechanism for internal stress relaxation in colloidal random packings, which contributes to the extraordinary durability of cement paste.

cond-mat.soft↗

arXiv2018

C-S-H gel densification: the impact of the nanoscale on self desiccation and sorption isotherms

The relationship between humidity and water content in a hydrating cement paste is largely controlled by the nanostructure of the C-S-H gel. Current hydration models do not describe this nanostructure, thus sorption isotherms and self-desiccation are given as constitutive inputs instead of being predicted from microstructural evolution. To address this limitation, this work combines a C-S-H gel description from nanoscale simulations with evolving capillary pore size distributions from a simple hydration model. Results show that a progressive densification of the C-S-H gel must be considered in order to explain the self-desiccation of low-alkali pastes. The impact of C-S-H densification on the evolution of microstructure and sorption isotherms is then discussed, including the effect of water-to-cement ratio, cement powder fineness, and curing temperature. Overall, this work identifies an area where nanoscale simulations can integrate larger-scale models of cement hydration and poromechanics.

cond-mat.soft↗

arXiv2015

Hierarchical structures for a robustness-oriented capacity design

In this paper, we study the response of 2D framed structures made of rectangular cells, to the sudden removal of columns. We employ a simulation algorithm based on the Discrete Element Method, where the structural elements are represented by elasto-plastic Euler Bernoulli beams with elongation-rotation failure threshold. The effect of structural cell slenderness and of topological hierarchy on the dynamic residual strength after damage $\ROne$ is investigated. Topologically \textit{hierarchical} frames have a primary structure made of few massive elements, while \textit{homogeneous} frames are made of many thin elements. We also show how $\ROne$ depends on the activated collapse mechanisms, which are determined by the mechanical hierarchy between beams and columns, i.e. by their relative strength and stiffness. Finally, principles of robustness-oriented capacity design which seem to be in contrast to the conventional anti-seismic capacity design are addressed.

cond-mat.soft↗

arXiv2015

Progressive Collapse Mechanisms of Brittle and Ductile Framed Structures

In this paper, we study the progressive collapse of 3D framed structures made of reinforced concrete after the sudden loss of a column. The structures are represented by elasto-plastic Euler Bernoulli beams with elongation-rotation failure threshold. We performed simulations using the Discrete Element Method considering inelastic collisions between the structural elements. The results show what collapse initiation and impact-driven propagation mechanisms are activated in structures with different geometric and mechanical features. Namely, we investigate the influence of the cross sectional size and reinforcement $α$ and of the plastic capacity $β$ of the structural elements. We also study the final collapse extent and the fragment size distribution and their relation to $α$, $β$ and to the observed collapse mechanisms. Finally, we compare the damage response of structures with symmetric and asymmetric reinforcement in the beams.

cond-mat.soft↗

arXiv2015

Creep of Bulk C--S--H: Insights from Molecular Dynamics Simulations

Understanding the physical origin of creep in calcium--silicate--hydrate (C--S--H) is of primary importance, both for fundamental and practical interest. Here, we present a new method, based on molecular dynamics simulation, allowing us to simulate the long-term visco-elastic deformations of C--S--H. Under a given shear stress, C--S--H features a gradually increasing shear strain, which follows a logarithmic law. The computed creep modulus is found to be independent of the shear stress applied and is in excellent agreement with nanoindentation measurements, as extrapolated to zero porosity.

cond-mat.mtrl-sci↗