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DRAC

Software

Code based on the FEM

DRAC is a code based on the Finite Element Method (FEM) and has been developed entirely within the Mechanics of Materials reserArch group (MECMAT) of the Universitat Politècnica de Catalunya (UPC).

DRAC is a non-linear Finite Element (FE) code that includes cohesive zero-thickness interface elements to geomechanics and concrete mechanics problems, including the effects of discontinuities and contact surfaces.

DRAC has several features that make it very versatile and robust. First, it incorporates thermo-hydro-mechanical coupling (monolithic approach), as well as the possibility of chemical coupling (THM-C) using a staggered strategy. Thisallows us to perform a detailed analysis of thermal, hydraulic and mechanical processes in complex geomechanical systemsincluding the study of multiphase flows in porous media. in the code is equipped with High Performance Computing (HPC) capabilities in a MPI parallel environment, which ensures efficient data processing and large-scale simulations.

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Constitutive models

The material library includes models for continuum, interface and rod element material behavior, with elastic, elasto-plastic, visco-elastic, damage mechanics and fracture mechanics-based constitutive laws.

Discrete fractures: Zero-thickness interface elements

Zero-thickness interface elements are a type of elements that are inserted between the regular continuum elements to represent discrete fractures,

HPC capabilities

Drac has been developed using MPI parallel programming to ensure in High Performance Computing (HPC) capabilities.

Coupling between physical processes

Solving non-linear coupled THM problems using monolithic approach and iterative strategies.

Configurational Mechanics

Configurational mechanics is a branch of mechanics that focuses on the analysis of material configurations and their effects on the energy stored in the system. A branch of DRAC incorporates these concepts in a procedure for the re-orientation of interface elements, in order to represent the development of cracking along non-preestablished paths.

Chemical degradation

A chemical degradation process modeled through diffusion-reaction model. This model is coupled in staggered approach with the monolithic THM code.

Induced seismicity

The injection of fluids into a geological environment (e.g., CO2 injection, hydraulic fracturing) can trigger seismic events, including the reactivation of faults. These phenomena are tackled using a visco-plastic version of the fracture-mechanics interface law together with a visco-plastic relaxation algorithm. The strategy developed makes it possible to detect the onset of instability, transit smoothly during the instability event and quantify the energy released in the process, all in a quasi-static analysis context.

Multiphase

Modelling multiphase flow in the rock mass using discrete or smeared fractures, with two (liquid+gas) fluid phases, and two (water and dry gas) species. The gas species may be an ideal gas, or a non-ideal gas including super-critical regime.

Constitutive models

Library of constitutive models for modelling stress-strain behaviour.

ModelsConstitutius

Crack Laws:
(a) Hyperbolic Cracking Surface F and Plastic Potential Q
(b) Evolution of Cracking Surface
(c) Modes of Fracture
(d) Softenintg Laws for tensile strength and cohesion

Discrete fractures: Zero-thickness interface elements

Zero-thickness Interface Elements (ZTIEs) are a special type of elements with zero-thickness that may be inserted between the standard continuum elements with the purpose of representing the opening-sliding of discrete fractures (Goodman, 197?, Gens et al. 1995, Segura and Carol 2008). The main kinematic variable of a ZTIE is the relative displacement,. defined as the difference between the (absolute) displacements of the same point on the surfaces of the two adjacent continuum elements.The element integration is performed over the mid-plane surface of the ZTIE, which is defined as shown in the figure below.

fracturas discretas

An example of a quadrilateral zero-thickness interface element placed between two hexahedral continuum elements. The mid-plane surface is the shaded area.

HPC capabilities

Parallel programming using MPI libraries, leading to High Performance Computing (HPC) capabilities.

Publications in this area: Garolera, D. (2017), Pérez, A. (2019).

capacidades HPC

Speed up* curve for progressively finer meshes of the same physical problem in linear elasticity.

Coupling between physical processes

DRAC solves coupled HM problems using both monolithic and staggered strategies. It solves using Darcy flow on continuum elements, and a conceptually similar approach involving longitudinal and transverse flow is used for zero-thickness interface elements via the cubic law.

Publications in this area: Segura and Carol 2008 part I, Segura and Carol 2008 part II, Garolera 2017, Pérez 2019.

AcoplamentsFisics

Evolution of σ_y in the continuum (MPa), and normal relative opening displacement (r_n) on the open interfaces (mm). Contours plotted over deformed mesh (magnification ×5000).

THM_esquema

Thermal, Hydraulic and Mechanical (THM) coupling diagram.

Configurational Mechanics

Configurational mechanics is a branch of mechanics that focuses on the analysis on the influence of material configuration (initial geometry and location of cracks defects, etc.) on the global energy stored in the structure. It is based on the concept of configurational (or material) forces, which are obtained by integration of Eshelby’s energy-momentum tensor (“configurational stresses”) and relate changes in global elastic energy to alterations in the original coordinates of the mesh nodes. By establishing an iterative procedure in which the initial nodal coordinates are progressively readjusted according to these configurational forces, it becomes possible to predict crack trajectories that minimize global structural energy.
DRAC implements such a technique, and allows the modification of the structural geometry (configuration) of the finite element mesh to reproduce more realistic crack paths
(Crusat 2019, Crusat et al. 2021).

MecanicaConfiguracional

Evolution and reorientation (based on the theory of configurational mechanics) of cracks in a concrete beam during the fracture process.

Environmental degradation

The FE code DRAC includes a diffusion reaction model to simulate specific physical or chemical environmental degradation processes occurring in cement and concrete, such as drying shrinkage, external sulfate attack (ESA), Alkali-Siclica reaction (ASR) or acid attack. Physical or Chemical volume changes or degradation, obtained as the result of a coupled diffusion or diffusion-reaction analysis, is introduced into the analysis via material volumetric expansions or via reduction of the mechanical strength parameters constitutive law of the zero-thickness interface elements (Liaudat et al. 2018, Liaudat et al. 2018, Barandiarán et al. 2019).

This analysis may be combined with a meso-level representation of the material including explicitly the aggregate particles or heterogeneities and the interfaces between various material components. The strategy leads to remarkably realistic results, such as the “onion-peel cracking” observed experimentally in ESA lab experiments (figure).

 

ataque químico

A 3D simulation cross-section of a concrete block undergoing ESA chemical attack

Induced seismicity

Human activities such as CO₂ geological injection, hydraulic fracturing, geothermal energy extraction, mining, etc. may induce seismic events, such as fault reactivation. These episodes are highly unstable, which makes their modeling using traditional numerical approaches difficult since iterative procedures may not easily converge when the instability event is approached. To control this iterative process, DRAC incorporates an energy-based visco-plastic work softening model for zero-thickness interface elements, which is solved by a stress-driven integration strategy. This fracture-mechanics interface law is used in conjunction with a visco-plastic relaxation algorithm. The overall strategy developed makes it possible to detect the onset of instability, transit smoothly during the instability event and quantify the energy released in the process, all in a quasi-static analysis context.

Publications in this area: Jaqués 2022.

InducedSeismicityDomain
InducedSeismicityResults

Instability induced by fluid injection at a single point. The image on the left shows the finite element mesh used in this simulation, while the image on the right shows the elastic energy at the end of the process.

Multiphase

Modelling multiphase flow of two fluid phases through fractured rock, considering a liquid and a gas phase; and two species: water and dry gas. In addition, it is considered that the liquid can evaporate and the gas may get dissolved also in the water, which means that each of the two phases may be composed of both species.
Own research related to this area; Barandiarán et al. 2022.

Evolution of the gas pressure, the water pressure and the water saturation along a porous medium with a discontinuity in the centre.

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