Conference Agenda

Natural cracks in sedimentary rocks such as shale are potential weak paths for hydraulic fracturing to create fracture networks. The mechanism of formation of natural cracks in sedimentary rocks in the geologic past is an important problem to be understood. Why are the natural cracks roughly parallel and equidistant, and why is the crack spacing in the order of 10 cm rather than 1 cm or 100 cm? Here it is proposed that fracture mechanics must be coupled with the diffusion of pore fluid and solute ions to answer these questions. Parallel equidistant natural cracks are considered to develop in a subcritical manner driven by shear deformation and governed by the Charles-Evans law. Shear dilatancy in the fracture process zones (FPZ) induces a drop in the concentration of ions that increases the material fracture energy, and a drop in pore pressure that increases the resistance to frictional sliding. Both processes will lead to a decrease of the fracture propagation rate, and such an impact will be counteracted by the recharge of pore fluid and ions from the rock matrix to the fracture. We study the steady-state propagation and periodic cracks and derive an analytical solution of the crack spacing as a function of the properties of the rock, the solvent and solute, together with the imposed far-field deformation.

This study examines the essential mechanical and diffusion properties of fiber-reinforced concrete, limestone rock, and their combination with varying interface inclination angles. It focuses on the fracture characteristics, flow behavior, and heat transfer of these composites in the context of hydro-thermal-mechanical (HTM) coupling under triaxial compression. The research is divided into two parts. The first part focuses on studying Hydro-thermal-mechanical (HTM) coupling through an extensive experimental investigation. Composite specimens made of fiber-reinforced concrete and limestone rock were fabricated into cylindrical samples with different interface angles of 0°, 15°, 30°, and 45°. Various combinations of hydro-mechanical (HM) tests, thermal-mechanical (TM) tests, and HTM tests were conducted under four different confining pressures (0 MPa, 3 MPa, 6 MPa, and 9 MPa), five water pressures (1 MPa, 2 MPa, 3 MPa, 4 MPa, and 5 MPa), and two temperatures (50◦C and 80◦C). The results indicated that water pressure slightly weakened the strength of the composite and increased its permeability. Temperature had a significant effect, greatly reducing both the strength and elastic modulus of the composite. Meanwhile, confining pressure enhanced the peak stress and deformation capacity while suppressing permeability. The second part of the study focuses on mesoscopic modeling, which has been calibrated and validated against experimental results. This mesoscale model uses a discrete element method that combines the Lattice Discrete Particle Model (LDPM) with the Flow Lattice Model (FLM). This study simulates the damage characteristics, fluid flow, and heat flux of composites under various combined conditions, including confining pressure, water pressure, and temperature. The results include the stress-strain response, failure modes, cumulative fluid volume, penetration depth, and the non-uniform distribution of heat.

Most of the physical phenomena in engineering problems occur under non-isothermal conditions. The occurrence of some physical phenomena or chemical reactions can lead to local temperature changes and, consequently, to heat transfer and even local phase changes in the fluid. The need to consider the effect of heat transfer and phase changes in the fluid becomes critical in analyses of many multi-field problems in porous and fractured materials. A novel DEM-based pore-scale 3D thermo-hydro-mechanical (THM) model of two-phase fluid flow and heat transfer in fluid and solids is based on a direct numerical simulation approach. The model's original concept is based on the notion that in a physical system, two domains coexist: the 3D discrete (solid) domain and the 3D continuous (fluid) domain. Both domains are discretized into a coarse mesh of tetrahedra. The THM model was validated by comparing the numerical results with the analytical solution of the classic 1D heat transfer problem. Numerical calculations were carried out for bonded granular specimens imitating concrete with a 3D DEM fully coupled with 3D CFD (based on a fluid flow network) and 3D heat transfer that linked discrete mechanics with fluid mechanics and heat transfer at the meso-scale. The heat transfer was related to the fluid (diffusion and advection) and bonded particles (conduction). Bonded particle assemblies with random grain distribution were considered. Perfect accordance was obtained between numerical and analytical outcomes. In addition, the effects of a macro-crack in the specimen on the distribution of fluid pressure, density, velocity, and temperature were studied.

The Lattice Discrete Particle Model (LDPM) is a discrete mesoscale model of concrete that can accurately describe the macroscopic behavior of concrete during elastic, fracturing, softening, and hardening regimes. The LDPM formulation is obtained by modeling the interaction among coarse meso-scale aggregate pieces between polyhedral cells (each containing one aggregate particle) whose external surfaces are defined by sets of triangular facets. At each facet, a vectorial form of constitutive model is used to simulate physical mechanisms such as tensile fracture, cohesion, friction, etc. LDPM has been calibrated and validated extensively through the analysis of a large variety of experimental tests. Numerical results show that it can reproduce with great accuracy the response of concrete under uniaxial and multiaxial stress states in both compression and tension and under both quasi-static and dynamic loading conditions. In this presentation, we will give an overview of recent implementations of LDPM in various computational platforms. LDPM was implemented in the following software packages: Abaqus/Explicit via user subroutine; Project Chrono, a physics-based modeling and simulation infrastructure based on a platform-independent open-source design; Cast3m a multi-physics software developed by CEA; Open Academic Solver, an open-source software developed at Brno University; JAX-LDPM, an open-source GPU-based software in active development by researchers from the Hong Kong University of Science and Technology; and FE-MultiPhys, developed at Virginia Tech. The different implementations will be compared by simulating typical failure tests for concrete, including, unconfined compression test, three-point bending test, and direct tensile test. Finally, the presentation will provide a vision for future LDPM developments that will likely be implemented in these software packages.

Crack characterization in reinforced concrete structures, such as the containment walls of 1300 MWe nuclear power plants, is critical for accurately estimating air leakage. Traditional modeling strategies, such as Poiseuille’s law applied to a simplified geometry, rely on indirect parameters like a tortuosity coefficient, which is difficult to predict and has limited validity, leading to increased uncertainty. This study presents a novel post-processing tool based on the Beam-Particle simulation approach, capable of detecting micro-crack paths and constructing macro-crack geometries using graph theory. The generated macro-crack geometry can be integrated into computational fluid dynamics (CFD) simulations for more accurate airflow predictions or by calibrating simplified approaches like Poiseuille’s law based on numerically obtained crack characteristics. Validation against optical measurements from Brazilian splitting tests demonstrates the tool’s potential to advance simplified modeling and enhance detailed crack characterization, opening new possibilities for improved air leakage predictions.

Thin-walled structures made of concrete, or other forms of cement-based composites, are common within the civil infrastructure. In many situations, such structures experience out-of-plane loading, which can lead to various forms of distributed cracking depending on the boundary conditions. Discrete mechanical models are appropriate for simulating such cases of distributed fracture. However, their applications toward modeling the out-of-plane behavior of thin-walled structures are few. One main difficulty involves the large computational expense associated with three-dimensional discretizations of the structure, which are typically needed to capture crack propagation through the wall thickness. In this research, an extension of the Voronoi-cell lattice model (VCLM) is proposed to simulate the behavior of planar structural elements subjected to out-of-plane loading. Based on a two-dimensional network of nodes, a layered assembly of the element cross-sections provides a three-dimensional description of section behavior. With sufficiently fine discretization of the planar
structure, this layered VCLM is shown to be elastically uniform for combined membrane and flexural loadings. Compared to corresponding three-dimensional discretizations, computational expense is greatly reduced, thus extending the range of modeling applications. Other capabilities of the layered VCLM, and the consequences of mesh resolution of the lattice structures, are demonstrated through elastic stress analysis and fracture analyses of grid-reinforced cement-based composites.