Wancheng Zhu

NUMERICAL SIMULATION ON SHEAR FRACTURE PROCESS OF CONCRETE USING MESOSCOPIC MECHANICAL MODEL

Abstract The numerical simulation of the damage and fracture processes of concrete structures has evolved considerably in the past years. In this contribution, a newly proposed mechanical model is used to simulate the fracture behavior of double-edge notched (DEN) and double central notched (DCN) concrete specimens loaded in shear. In this numerical model, the concrete is assumed to be a three-phase composite composed of matrices, aggregates and matrix–aggregate interfaces. An elastic finite element program is employed as the basic stress analysis tool while the elastic damage mechanics is used to describe the constitutive law of meso-level element. The maximum tensile strain criterion and Mohr–Coulomb criterion are utilized as damage thresholds. The heterogeneous stress field is obtained from numerical simulation, thus it is found that heterogeneity of mechanical properties has significant effect on the stress distribution in concrete. The crack propagation processes simulated with this model shows good agreement with those of experimental observations. It has been found that the shear fracture of concrete observed at the macroscopic level is predominantly caused by tensile damage at the mesoscopic level.

FRACTURE SPACING IN LAYERED MATERIALS: A NEW EXPLANATION BASED ON TWO-DIMENSIONAL FAILURE PROCESS MODELING

Opening-mode fractures in layered materials, such as sedimentary rocks, pavement, functionally graded composite materials or surface coating films, often are periodically distributed with spacings scaled to the thickness of the fractured layer. The current general explanation is that when the fracture spacing to layer thickness ratio changes from greater than to less than critical values the normal stress acting perpendicular to the fractures changes from tensile to compressive. This stress state transition is believed to preclude further infilling of fractures, and the critical fracture spacing to layer thickness ratio at this point defines a lower limit, called fracture saturation. To better understand the controls on fracture spacing, we have investigated the problem using a progressive fracture modeling approach that shares many of the natural kinematic features, such as fracture nucleation, fracture infilling and fracture termination. As observed experimentally, our numerical simulations demonstrate that fracture spacing initially decreases as extensional strain increases in the direction perpendicular to the fractures, and at a certain ratio of fracture spacing to layer thickness, no new fractures nucleate (saturated). Beyond this point, the additional strain is accommodated by further opening of existing fractures: the spacing then simply scales with layer thickness, creating fracture saturation. An important observation from our fracture modeling is that saturation may also effectively be achieved by the interface delamination and throughgoing fracturing, which inhibit additional layer-confined fracturing. We believe that these processes may serve as another mechanism to accommodate additional strain for a fracture saturated layer. Because interface debonding stops the transition of stress from the neighboring layers to the embedded central layer, which may preclude further infilling of new fractures, our fracture modeling approach predicts a larger critical length scale of fracture spacing than that predicted by a stress analysis approach based on stress transition theory. Numerical simulations also show that the critical value of the fracture spacing to layer thickness ratio is strongly dependent on the mechanical disorder in the fractured layer. The spacing to thickness ratio decreases with increasing heterogeneity of the mechanical properties.

Fracture evolution around pre-existing cylindrical cavities in brittle rocks under uniaxial compression

The development of fracture around pre-existing cylindrical cavities in brittle rocks was examined using physical models and acoustic emission technique. The experimental results indicate that when granite blocks containing one pre-existing cylindrical cavity are loaded in uniaxial compression condition, the profiles of cracks around the cavity can be characterized by tensile cracking (splitting parallel to the axial compression direction) at the roof−floor, compressive crack at two side walls, and remote or secondary cracks at the perimeter of the cavity. Moreover, fracture around cavity is size-dependent. In granite blocks containing pre-existing half-length cylindrical cavities, compressive stress concentration is found to initiate at the two sidewalls and induce shear crack propagation and coalescence. In granite blocks containing multiple parallel cylindrical cavities, the adjacent cylindrical cavities can influence each other and the eventual failure mode is determined by the interaction of tensile, compressive and shear stresses. Experimental results show that both tensile and compressive stresses play an important role in fracture evolution process around cavities in brittle rocks.

Impact of Gas Adsorption Induced Coal Matrix Damage on the Evolution of Coal Permeability

It has been widely reported that coal permeability can change from reduction to enhancement due to gas adsorption even under the constant effective stress condition, which is apparently inconsistent with the classic theoretical solutions. This study addresses this inconsistency through explicit simulations of the dynamic interactions between coal matrix swelling/shrinking induced damage and fracture aperture alteration, and translations of these interactions to permeability evolution under the constant effective stress condition. We develop a coupled coal–gas interaction model that incorporates the material heterogeneity and damage evolution of coal, which allows us to couple the progressive development of damage zone with gas adsorption processes within the coal matrix. For the case of constant effective stress, coal permeability changes from reduction to enhancement while the damage zone within the coal matrix develops from the fracture wall to further inside the matrix. As the peak Langmuir strain is approached, the decrease of permeability halts and permeability increases with pressure. The transition of permeability reduction to permeability enhancement during gas adsorption, which may be closely related to the damage zone development in coal matrix, is controlled by coal heterogeneity, external boundary condition, and adsorption-induced swelling.

Microseismicity Induced by Fault Activation During the Fracture Process of a Crown Pillar

Shirengou iron mine in Hebei Province, China is now under transition from open pit to underground mining. During this process, the unstable failure risk of crown pillar is growing as a result of underground mining, fault activation and water seepage. To monitor the stability of the crown pillar, a microseismic monitoring system was equipped in 2006. Based on temporal and spatial distribution of microseismic events and deformation mechanism, it was found that it is the propagation of the buried fault F15 that causes the failure of the crown pillar, resulting in increased water seeping into the underground drifts. By analyzing the temporal changes in multiple microseismic parameters during the fracture process of the crown pillar, it was found that several distinct abnormalities in the microseismic data such as a rapid decrease in the b value, a sharp increase in energy release, an abnormal increase in apparent stress and a low dominant frequency, could be judged as the signal of an increasing risk. Therefore, the microseismic monitoring has been proven to be a suitable method for understanding damage and fracture process of the crown pillar during the transition from open pit to underground mining.

The Influence of Fracturing Fluids on Fracturing Processes: A Comparison Between Water, Oil and SC-CO2

Conventional water-based fracturing treatments may not work well for many shale gas reservoirs. This is due to the fact that shale gas formations are much more sensitive to water because of the significant capillary effects and the potentially high contents of swelling clay, each of which may result in the impairment of productivity. As an alternative to water-based fluids, gaseous stimulants not only avoid this potential impairment in productivity, but also conserve water as a resource and may sequester greenhouse gases underground. However, experimental observations have shown that different fracturing fluids yield variations in the induced fracture. During the hydraulic fracturing process, fracturing fluids will penetrate into the borehole wall, and the evolution of the fracture(s) then results from the coupled phenomena of fluid flow, solid deformation and damage. To represent this, coupled models of rock damage mechanics and fluid flow for both slightly compressible fluids and CO2 are presented. We investigate the fracturing processes driven by pressurization of three kinds of fluids: water, viscous oil and supercritical CO2. Simulation results indicate that SC-CO2-based fracturing indeed has a lower breakdown pressure, as observed in experiments, and may develop fractures with greater complexity than those developed with water-based and oil-based fracturing. We explore the relation between the breakdown pressure to both the dynamic viscosity and the interfacial tension of the fracturing fluids. Modeling demonstrates an increase in the breakdown pressure with an increase both in the dynamic viscosity and in the interfacial tension, consistent with experimental observations.

Numerical Modeling of Jointed Rock Under Compressive Loading Using X-ray Computerized Tomography

As jointed rocks consist of joints embedded within intact rock blocks, the presence and geometrical fabric of joints have a great influence on the mechanical behavior of rock. With consideration of the actual spatial shape of joints, a numerical model is proposed to investigate the fracture evolution mechanism of jointed rocks. In the proposed model, computerized tomography (CT) scanning is first used to capture the microstructure of a jointed sandstone specimen, which is artificially fabricated by loading the intact sample until the residual strength, and then digital image processing (DIP) techniques are applied to characterize the geometrical fabric of joints from the CT images. A simple vectorization method is used to convert the microstructure based on a cross-sectional image into a layer of 3-D vectorized microstructure and the overall 3-D model of the jointed sandstone including the real spatial shape of the joints is established by stacking the layers in a specific sequence. The 3-D model is then integrated into a well-established code [three-dimensional Rock Failure Process Analysis, (RFPA3D)]. Using the proposed model, a uniaxial compression test of the jointed sandstone is simulated. The results show that the presence of joints can produce tensile stress zones surrounding them, which result in the fracture of jointed rocks under a relatively small external load. In addition, the spatial shape of the joints has a great influence on the fracture process of jointed rocks.

Numerical simulations of failure of brittle solids under dynamic impact using a new computer program - DIFAR

This paper presents a new computer program called DIFAR (or dynamic incremental failure analysis of rock) that can simulate fracture process of brittle rocks under dynamic impacts. The program is based on a linear elastic finite element method incorporated with a failure criterion for damage checking. Modulus is reduced once the failure criterion is satisfied. In addition, Weibull distribution of the modulus and strength of the elements are used for modeling the mesoscopic heterogeneity. The failure criterion is a Mohr-Coulomb type of condition with a tensile cut-off, in which strength parameters are functions of the strain rate. More importantly, the whole fracture process of rock fragmentation can be simulated, including initiation, propagation, and coalescence of microcracks. The program DIFAR has been used to simulate elastic wave propagation and nonlinear fragmentation, and validity and efficiency of this program is demonstrated. The program can be considered as a dynamic counterpart of the RFPA, a failure analysis program for static loads, developed at Northeastern University, China.

Numerical Simulation of Rock Creep Behavior with a Damage-Based Constitutive Law

AbstractTime-dependent creep behavior of rocks is crucial not only for assessing geophysical hazards, such as earthquake rupture and volcanic eruption, but also for analyzing the long-term stability of rock engineering structures, such as underground mines and underground excavations. In the present paper, multiple stress-stepping creep tests on green sandstone under uniaxial stress conditions were performed. Results from stress-stepping creep experiments show that creep strain rates are highly dependent on the level of applied stress. Then, a time-dependent creep model based on damage constitutive law at a mesoscale was proposed to model the time-dependent behavior of heterogeneous brittle rocks. In the proposed model, the rock heterogeneity is considered by assuming the rock parameters following a statistical Weibull distribution, and both the maximum tensile strain criterion and the Mohr-Coulomb criterion are used as two damage thresholds to control the rock damage. The damage-based creep model is impl...

Stress Analysis of a Borehole in Saturated Rocks Under in situ Mechanical, Hydrological and Thermal Interactions

Abstract A novel approach is developed to represent coupled thermal-hydraulic-mechanical (THM) behavior of porous systems that incorporates the non-isothermal free and forced convection of a single component fluid in a non-boiling thermoelastic medium. The three-way simultaneous coupling between the THM triplet is currently linear, but no restriction is placed on incorporating material nonlinearities. The coupled PDEs are solved in space by grid-adaptive finite elements. The model is validated against solutions for linear non-isothermal consolidation of a column. We demonstrate the utility of the model by analyzing the behavior of a deep wellbore in a themoelastic medium circulated by a pressurized, but chilled fluid. Model results illustrate the significant importance of the cross-couplings between individual THM processes for the evaluation of wellbore stability.