Papers by Andrey P Jivkov
Abstract Corrosion crack nucleation and growth are modelled as a moving boundary problem. The mod... more Abstract Corrosion crack nucleation and growth are modelled as a moving boundary problem. The model incorporates three physical processes––dissolution, passivation and straining––into a continuum mechanical framework. The dissolution triggers surface advance; the passivation restrains the access of the environment to bare metal; the deformation causes for passivity breakdown. Plane cracks nucleating from surface pits in an elastic–plastic material body under fatigue load are considered. The problem is solved using a FEM program and a moving boundary tracking procedure. The model simulates how cracks form and grow in a single continuous course. The geometry of the developed cracks is found independent of the initial pit size. Plasticity is found to influence the curvature at the tip of the nucleated corrosion cracks. The most important evolution length parameter, the width of the corrosion crack, is found to depend on the size constraints of the tracking procedure. It is concluded that the model is deficient for determining all length scales observed in reality. Physical processes to be considered in an advanced model are proposed and discussed.
Structural control & health monitoring, Dec 9, 2021
Acoustic emission (AE) is a useful method for recording fracture processes in concrete. In this w... more Acoustic emission (AE) is a useful method for recording fracture processes in concrete. In this work, AE data are recorded during three‐point bending tests to fracture of hydraulic concrete. First, AE data are used to analyze concrete's damage development using hits distribution, b‐value, Ib‐value, and average frequency versus RA value. Second, clustering analysis of AE signals is performed by hierarchical clustering. Third, a support vector machine model based on the gray wolf optimization algorithm is proposed to quantify the degree of damage. Via b‐value analysis it is shown that the fracture process of hydraulic concrete can be divided into three stages: microcracks nucleation; microcracks coalescence into macrocracks (macrocrack nucleation); and macrocrack propagation. Further, it is shown that rise time, ringdown counts, energy, duration, amplitude, and central frequency can be used to characterize the failure modes. Specifically, it is found that microcrack nucleation stage is dominated by tensile failures, macrocrack nucleation stage is characterized by rapid increase of shear failures, which become dominant over tensile failures, and macrocrack propagation stage is dominated by shear failures. Via hierarchical cluster analysis, it is found that the fracture process can be divided into three clusters, which corresponds to the three stages obtained via b‐value analysis. Finally, the proposed support vector machine model based on gray wolf optimization is found to predict the degree of damage in excellent agreement with experiment. This offers an effective practical method for damage assessment by combining AE with machine learning.
Siam Journal on Applied Mathematics, 2021
The boundary value problem on a weighted graph relevant to the static analysis of truss structure... more The boundary value problem on a weighted graph relevant to the static analysis of truss structures with linear elastic bars is presented along with strategies for assembling the corresponding algeb...
Expert Systems With Applications, Sep 1, 2023
ACS Applied Materials & Interfaces, Jul 10, 2023
arXiv (Cornell University), Jul 13, 2022
Diffusion of ions due to temperature gradients (known as thermal diffusion) in charged nanochanne... more Diffusion of ions due to temperature gradients (known as thermal diffusion) in charged nanochannels is of interest in several engineering fields, including energy recovery and environmental protection. This paper presents a fundamental investigation of the thermal diffusion of sodium chloride in charged silica nanochannels performed by molecular dynamics (MD). The results reveal the effects of nanoconfinement and surface charges on the sign and magnitude of the Soret coefficient. It is shown that the sign and magnitude of the Soret coefficient are controlled by the structural modifications of the interfacial solutions. These modifications include the ionic solvation and hydrogen bond structure induced by the nanoconfinement and surface charges. The results show that both nanoconfinement and surface charges can make the solutions more thermophilic. Furthermore, the thermal diffusion of solutions in boundary layers is significantly different from that of solutions in bulk fluid, contributing 2 to the overall difference between the thermal diffusivity of pore fluid and that associated with bulk fluid. The findings provide further understanding of thermal diffusion in nano-porous systems. The proposed MD simulation methodology is applicable to a wider category of coupled heat and mass transfer problems in nanoscale spaces.
This work describes a local approach to cleavage fracture (LAF) incorporating the statistics of m... more This work describes a local approach to cleavage fracture (LAF) incorporating the statistics of microcracks to characterize the cleavage fracture toughness distribution in structural steels. Fracture toughness testing conducted on standard compact tension C(T) specimens for a 22NiMoCr37 pressure vessel steel provides the cleavage fracture resistance data needed to determine the measured toughness distribution. Metallographic examination of etched surfaces for the tested steel also provides the distribution of carbides, which are assumed as the Griffith fracture-initiating particles, dispersed in the material from which the cleavage fracture toughness distribution is predicted. Overall, the analyses conducted in the present work show that LAFs incorporating the statistics of microcracks are a viable engineering procedure to describe the dependence of fracture toughness on temperature in the DBT region for ferritic steels.
Computer Physics Communications, Oct 1, 2022
A formulation of elliptic boundary value problems is used to develop the first discrete exterior ... more A formulation of elliptic boundary value problems is used to develop the first discrete exterior calculus (DEC) library for massively parallel computations with 3D domains. This can be used for steady-state analysis of any physical process driven by the gradient of a scalar quantity, e.g. temperature, concentration, pressure or electric potential, and is easily extendable to transient analysis. In addition to offering this library to the community, we demonstrate one important benefit from the DEC formulation: effortless introduction of strong heterogeneities and discontinuities. These are typical for real materials, but challenging for widely used domain discretization schemes, such as finite elements. Specifically, we demonstrate the efficiency of the method for calculating the evolution of thermal conductivity of a solid with a growing crack population. Future development of the library will deal with transient problems, and more importantly with processes driven by gradients of vector quantities.
International Journal of Heat and Mass Transfer, Apr 1, 2022
A wide range of natural and industrial processes involve heat and mass transport in porous media.... more A wide range of natural and industrial processes involve heat and mass transport in porous media. In some important cases the transported substance may undergo phase change, e.g. from liquid to solid and vice versa in the case of freezing and thawing of soils. The predictive modelling of such phenomena faces physical (multiple physical processes taking place) and mathematical (evolving interface with step change of properties) challenges. In this work, we develop and test a non-local approach based on bond-based peridynamics which addresses the challenges successfully. Our formulation allows for predicting the location of the interface between phases, and for calculating the temperature and pressure distributions within the saturated porous medium under the conditions of pressure driven water flow. The formulation is verified against existing analytical solutions for 1D problems, as well as finite element transient solutions for 2D problems. The agreement found by the verification exercise demonstrates the accuracy of the proposed methodology. The detailed coupled description of heat and hydraulic processes can be considered as a critical step towards a thermo-hydro-mechanical model, which will allow, for example, description of the hydrological behaviour of permafrost soils and the frost heave phenomenon.
Cement and Concrete Research, Jun 1, 2020
Applied Numerical Mathematics, Nov 1, 2021
We present a systematic description and comparison of the Finite Element Method (FEM) with the re... more We present a systematic description and comparison of the Finite Element Method (FEM) with the relatively new Virtual Element Method (VEM) for solving boundary value problems in linear elasticity, including primal and mixed formulations. The description highlights the common base and the essential difference between FEM and VEM: discretisation of the same primal (Galerkin) and mixed weak formulations and assembly of element-wise quantities, but different approaches to element shape functions. The mathematical formulations are complemented with detailed description of the computer implementation of all methods, including all versions of VEM, which will benefit readers willing to develop their own computational framework. Numerical solutions of several boundary value problems are also presented in order to discuss the weaker and stronger sides of the methods.
Cement and Concrete Research, 2021
One of the most effective and safe mechanisms for CO 2 storage in saline aquifers is solubility t... more One of the most effective and safe mechanisms for CO 2 storage in saline aquifers is solubility trapping by which CO 2 will be dissolved into the brine. Since the brine contained CO 2 is denser than the surrounding fluids, it would sink to the bottom of the rock formation over time, trapping CO 2 more securely. However, the rate of CO dissolution into the brine depends on active mass transfer mechanisms, such as convection and diffusion mechanisms, under storage conditions. Convective mixing significantly enhances the rate of CO 2 dissolution into the brine. In this paper, we are aiming to comprehensively study the effects of formation brine salinity and reservoir temperature on convection mechanism and on CO 2 storage in saline aquifers. For this objective, a series of high-pressure high-temperature experiments were performed in a specially designed PVT bead pack cell. Based on the pressure data, the effects of aforementioned parameters on CO 2 dissolution, diffusion coefficient, Rayleigh number and the onset time of convection were studied. To further investigate the effects of above parameters on convection mechanism and to quantify the effectiveness of convection mechanism on CO 2 dissolution, Sherwood number and the time (t ୗ୦୫ୟ୶), when it reaches its maximum value, were obtained in each experiment. Finally, a commercial software was used to model the convection mechanism and to study the effects of above parameters on density-driven fingers and convection mechanism. The results of both experimental and numerical studies revealed that decrease in brine salinity and temperature lead to increase in the Rayleigh number and Sherwood number.
Engineering Geology, Oct 1, 2022
Geotechnique, Jan 11, 2022
Understanding the desiccation-induced cracking in soil has been improved over the last 20-30 year... more Understanding the desiccation-induced cracking in soil has been improved over the last 20-30 years through experimental studies but the progress in predictive modelling of desiccation cracking has been limited. The heterogeneous structure of soils and the multi-physics nature of the phenomenon, involving emergence and propagation of discontinuities, make the mathematical description and analysis a challenging task. We present a non-local hydro-mechanical model for soil desiccation cracking capable of predicting crack initiation and growth. The model is based on the peridynamics (PD) theory. Attempts to model the soil desiccation cracking by PD are limited to a purely mechanical description of the process that involves calibration of the parameters. Differently, the model presented in this paper describes soil desiccation cracking as a hydro-mechanical problem, where moisture flow and deformation are coupled. This allows for investigating and explaining the mechanisms controlling the initiation and propagation of discontinuities. The model is applied and tested against two sets of experimental data to explain the typical features of drying-induced cracking of clays. The validations use experimental parameters (Young's modulus, water retention characteristics) and avoid calibrations to test the accuracy of the model. We demonstrate the correlations between the shrinkage of soil clay, changes in displacement fields and crack growth. Crack initiation, propagation and ultimate crack patterns simulated by the model are found to be in very good agreement with experimental observations. The results show that the model can capture realistically key hydraulic, mechanical and geometry effects on clay desiccation cracking.
This work proposes a model for corrosion driven crack growth. The model poses a moving boundary p... more This work proposes a model for corrosion driven crack growth. The model poses a moving boundary problem, where a chemical attack removes material from the body. The rate of the chemical attack is a function of the strain along the body surface. No crack growth criterion is needed for the analysis. A finite strain formulation is used and the material model is assumed hyperelastic. The problem is stated for a large body, containing a large crack. A low frequency cyclic loading is considered. Thus, corrosion is assumed to dissolve material with a rate, approximately proportional to the strain rate. The problem is solved using finite element method based program, enhanced with a procedure handling the moving boundary. Parametric studies are performed for a series of different initial shapes of the near-tip region. Presented results show that the crack growth rate is largely dependent on the initial crack geometry. For a set of initial shapes and load levels steady-state conditions of growth are achieved, while for the others the cracks show tendency to branch.
Uploads
Papers by Andrey P Jivkov
Microstructure representative models are generated with experimentally measured particle and pore size distributions and volume densities in two graphite grades. The results illustrate the effect of distributed porosity on the emerging stress-strain response and damage evolution. It is shown how the failure mode shifts from graceful, plastic-like, behaviour associated with substantial energy dissipation via distributed damage at lower porosities, to glass-like behaviour with negligible energy dissipation at higher porosities. Thus the work proposes a microstructure-informed methodology for integrity assessment of aging structures, where porosity increase is driven by environmental factors, such as radiation of nuclear graphite components.