Search

The characteristics of structural discontinuities in the subsurface environment often play a key role in the overall behaviour of such systems and their response to externally imposed conditions. Rock joints are one of such features that constitute the heterogeneity of rock masses. Akin to other forms of discontinuities, the characteristics of rock joints affect the performance of their parent rock masses, which are constituents of rock formations. The fracturing process is one of such key geo-mechanical phenomena that is inevitably influenced by pre-existing joints. A numerical technique implemented via a discrete element method (DEM) is herein adopted to evaluate two fundamental properties that control the shear and dilatancy responses of discontinuities. Though these properties are also assessed in isolation, their interdependency, which is a dominant factor, is investigated. As joint frictional resistance increases, it escalates the potential of the joint to attenuate the rate of fracture growth. On the other hand, an increase in joint dilatancy increases the intensity of fracturing. The impact of joint frictional resistance is more pronounced at high friction magnitudes, and in this range, the predominant influence of joint friction overwhelms any effect of joint dilatancy. Contrarily, at low joint frictional resistance, contributions from even a small magnitude of joint dilatancy increases the degree of fracturing. The inter-relationship between joint friction and dilatancy has influencing implications that govern the performance of rock masses. An inquiry into their combined contributions provides information prerequisite for a more accurate estimation and appraisal of fracture behaviour in underground systems. [ABSTRACT FROM AUTHOR]

The fracturing phenomenon within the reservoir environment is a complex process that is controlled by several factors and may occur either naturally or by artificial drivers. Even when deliberately induced, the fracturing behaviour is greatly influenced by the subsurface architecture and existing features. The presence of discontinuities such as joints, artificial and naturally occurring faults and interfaces between rock layers and microfractures plays an important role in the fracturing process and has been known to significantly alter the course of fracture growth. In this paper, an important property (joint friction) that governs the shear behaviour of discontinuities is considered. The applied numerical procedure entails the implementation of the discrete element method to enable a more dynamic monitoring of the fracturing process, where the joint frictional property is considered in isolation. Whereas fracture propagation is constrained by joints of low frictional resistance, in non-frictional joints, the unrestricted sliding of the joint plane increases the tendency for reinitiation and proliferation of fractures at other locations. The ability of a frictional joint to suppress fracture growth decreases as the frictional resistance increases; however, this phenomenon exacerbates the influence of other factors including in situ stresses and overburden conditions. The effect of the joint frictional property is not limited to the strength of rock formations; it also impacts on fracturing processes, which could be particularly evident in jointed rock masses or formations with prominent faults and/or discontinuities. [ABSTRACT FROM AUTHOR]

This study proposed a novel approach based on the 3D discrete element method (DEM) to simulate the progressive delamination in unidirectional carbon fibre reinforced polymer (CFRP) composite laminates. A hexagonal packing strategy was used for modelling 0 ∘ representative plies, the interface between different plies was modelled with one bond and seven bonds following the conservation of energy principle and a power law. The number of representative layers and the stiffness of bonds within these layers were calibrated with a comparison of results obtained from finite element method and theoretical analysis. DEM simulations of delamination with both interface models were conducted on unidirectional composites for double cantilever beam (DCB), end-loaded split (ELS) and fixed-ratio mixed-mode (FRMM) tests. It was found that the seven-bond interface model has a better agreement with experimental data in all three tests than the one-bond interface model by adopting the proposed seven-bond arrangement in terms of the progressive delamination process. The main advantages of the present interface model are its simplicity, robustness and computational efficiency when elastic bonds are used in the DEM models. • 3D discrete element model with packed particles for anisotropic composite laminae. • A novel bond approach with discrete element method for progressive delamination. • Energy conservation principle based bond properties for interlaminar interface. • Mode-I, mode-II and mix-mode fracture are captured with progression failure. [ABSTRACT FROM AUTHOR]

The mechanical behaviour of unidirectional fibre-reinforced polymer composites subjected to transverse tension was studied using a two dimensional discrete element method. The Representative Volume Element (RVE) of the composite was idealised as a polymer matrix reinforced with randomly distributed parallel fibres. The matrix and fibres were constructed using disc particles bonded together using parallel bonds, while the fibre/matrix interfaces were represented by a displacement-softening model. The prevailing damage mechanisms observed from the model were interfacial debonding and matrix plastic deformation. Numerical simulations have shown that the magnitude of stress is significantly higher at the interfaces, especially in the areas with high fibre densities. Interface fracture energy, stiffness and strength all played important roles in the overall mechanical performance of the composite. It was also observed that tension cracks normally began with interfacial debonding. The merge of the interfacial and matrix micro-cracks resulted in the final catastrophic fracture. [ABSTRACT FROM AUTHOR]

Abstract: A particle discrete element method (DEM) was employed to simulate transverse cracking in laminated fiber reinforced composites. The microstructure of the laminates was modeled by a DEM model using different mechanical constitutive laws and materials parameters for different constituents, i.e. fiber, matrix and fiber/matrix interface. Rectangular, hexagonal and random fiber distributions were simulated to study the effect of fiber distribution on the transverse cracking. The initiation and dynamic propagation of transverse cracking and interfacial debonding were all captured by the DEM simulation, which showed similar patterns to those observed from experiments. The effect of fiber volume fraction was also studied for laminae with randomly distributed fibers. It was found that the distribution and volume fraction of fibers affected not only the transverse cracking path, but also the behavior of matrix plastic deformation and fiber/matrix interface yielding in the material. [ABSTRACT FROM AUTHOR]

This study presents a 3D Discrete Element Method (DEM) model for the progressive failure analysis of Carbon Fiber Reinforced Polymer (CFRP) composite materials subjected to uniaxial tensile loading. Particles in the model are packed and bonded in regular patterns (hexagonal or square). The relationship between the bond stiffness and material properties is established based on the average strain energy method. The random distribution of bond strengths calibrated from experiments with a variation of 30% and 10% following a normal distribution law is assigned to the bonds in 0∘ and 90∘ plies to capture random cracks, respectively. Tsai-Hill failure criterion is utilized for the calibration of bond strength of ± 45 ° plies to predict their failures in composite laminates. Quantitative and qualitative analyses were conducted for predicting the damage initiation and propagation of the cross-ply and Quasi-Isotropic (QI) composite laminates under tensile loading, respectively. Two interface stiffnesses were utilized in the failure prediction of cross-ply composite laminates, and it was found that the numerical results with the interface stiffness calculated from fracture energy are in good, quantitative agreements with the experiments. All the four stages of the failure process of QI composite laminates are well captured by the 3D DEM model, including isolated cracks, inner delamination cracks, outer delamination cracks and final failure. [ABSTRACT FROM AUTHOR]

Abstract Outcrop mapping of a chalk cliff and wavecut platform in Thanet, Southeast England show a complex fracture pattern that seems to be controlled by meso-scale strike-slip faults within the chalk. The response of these faults to changes to in situ stress and fluid pressure is thought to control the nucleation and propagation of fractures in the chalk. In this study the DEM (Discrete Element Method) technique has been employed as a follow up to previous field and numerical (boundary and finite element method) investigations to ascertain the role of the faults in the initiation and nucleation of fractures The role of fluid pressure, in-situ stress, and fault geometry are recognised as focal factors. The generation of localised areas of tensile stresses due to fluid pressure and stress perturbations have been shown to cause the initiation of fractures around the fault bends. For releasing bends, localised tensile stresses tend to occur along the central segment of the fault bend, whereas for restraining bends, tensile stresses are more likely to develop on the outer edges of the fault bend. The dissimilarity in the fracturing process due to differences in the geometry of pre-existing faults demonstrates the significance of both fault geometry and fluid behaviour in controlling fracturing. Highlights • We investigated the behaviour of faults at a chalk cliff and wavecut platform at Thanet, UK. • The discrete element numerical technique was implemented. • Fluid pressure, in-situ stresses and fault geometry greatly influence fracture proliferation. • The orientation of propagating fractures is found to be perpendicular to the fault plane. • Fluid pressure and stress perturbations are shown to initiate fracturing around fault bends. [ABSTRACT FROM AUTHOR]

Highlights • A parallel-bonded chemical corrosion (PCE) model for modelling corroded rocks (limestone) is developed. • Nuclear Magnetic Resonance (NMR) and triaxial compression tests are carried to determine the damage law of the PCE model. • The PCE model is implemented in Discrete Element Method (DEM) and calibrated by experiments. • The PCE model is applied to investigate the chemical effects on crack propagation in rocks. Abstract The mechanical behaviours of rock mass are influenced by the presence of cracks at the microscopic and macroscopic levels. When coupled with corrosion by chemical ions in ground water, these cracks can cause instabilities and fragmentation near the excavated surface of underground structures, such as shield tunnels, etc. This paper presents the development of a parallel-bonded chemical corrosion (PCC) model for modelling corroded rocks (limestone). The model extends the bonded-particle model (BPM) by adding a chemically induced damage law to the particle bond. The damage law of the PCC model is derived from Nuclear Magnetic Resonance (NMR) and triaxial compression tests. The PCC model is validated with experimental results and is capable of simulating the micro-damage evolution process as well as predicting the macro-mechanical degradation caused by the chemical corrosion. It is then applied to investigate chemical effects on crack initiation, propagation, coalescence, and the mechanical properties of the limestone containing pre-existing flaws. Microscale correlations are derived linking the crack propagation process, flaw distribution and the effects of chemical corrosion. [ABSTRACT FROM AUTHOR]

Abstract: Discrete element method (DEM) was employed in the research works presented in this paper to simulate the microscopic machining process of ceramics. A densely packed particle assembly system of the polycrystalline SiC has been generated in DEM software package PFC2D using bonded-particle model (BPM) in order to represent for the ceramic part numerically. Microscopic mechanical properties of SiC were calibrated by comparing the numerical tests in PFC2D with the equivalent experimental results, and introduced into the subsequent modeling of the ceramic machining process. The dynamic process of initiation and propagation of the micro-cracks under various machining conditions has been explicitly modeled in the DEM simulations. The numerical results from DEM modeling agreed well with the experimental observations and theoretical predictions. Rational relations between cracking damage of ceramics and cutting conditions have been established based on the analysis of simulation results. A generalized model of defining the range of inelastic zone has also been developed based on the numerical results. Moreover, this study has demonstrated the advantage of DEM model in its capability of revealing the mechanical details of machining process at micro-scale. [Copyright &y& Elsevier]

This paper presents an assessment of three different particle based approaches for 3D modelling of fibre reinforced polymer (FRP) composite laminates with anisotropic elasticity, namely 3D Discrete Lattice model, 3D Hexagonal Close Packing model and Extended 2D Hexagonal and Square Packing model. These approaches are compared and evaluated against experimental results using a 0 ∘ ply lamina case. It has been confirmed that the Extended 2D Hexagonal and Square modelling approach in Discrete Element Method (DEM) is capable of modelling 3D composite laminates with better efficiency. Angle-ply lamina and two different laminates are modelled with the chosen particle approach. Good agreements between DEM, Finite Element and theoretical results prove the capability of this developed DEM approach for modelling the elastic behaviour of general FRP composite lamina and laminates. [ABSTRACT FROM AUTHOR]