Particle Flow Code Research Articles

Direct tensile mechanical properties and damage fracture mechanisms of thermally damaged coal measures sandstone under high strain rate loading

The direct tensile mechanical properties of highly thermally damaged rock under high strain rate loads are a key basic mechanical problem that must be solved for the stability and effective control of surrounding rock stability in the combustion control area of an underground coal gasification project. In this paper, a two-dimensional equivalent grain-based model (GBM) unit and Split Hopkinson Tension Bar (SHTB) numerical model for thermally damaged coal-measures sandstone were constructed using particle flow code (PFC). The comparison with the experimental results shows that the numerical method can accurately describe the dynamic direct tensile behavior of thermally damaged sandstone. The following conclusions are drawn: The dynamic tensile strength of thermally damaged sandstone shows a significant strain rate-strengthening effect. With the increase in thermal damage temperature, the dynamic tensile strength of sandstone increases first and then decreases. The generation and development of hot cracks in a high-temperature environment is the fundamental reason for the change in macroscopic mechanical properties. The dynamic tensile failure process of thermally damaged sandstone can be divided into four stages: the dynamic damage initiation, the dynamic damage development, the damage crack penetration, and the sample dynamic fracture. With the increase in impact velocity, the initial kinetic energy, elastic strain energy, dissipated energy, and elastic strain energy ratio in the failure process of thermally damaged sandstone gradually increase. In contrast, the dissipated energy ratio gradually decreases. As the heat treatment temperature increases, the elastic strain energy and the ratio of elastic strain energy increase first and then decrease.

Simulation study on load-bearing capacity and broken characteristics of the loose gangue based on Talbot theory

In order to investigate the axial compression failure characteristics of gangue with varying particle sizes under the Talbot distribution, the Talbot gradation coefficient is constrained within the range of 0.3–0.7, the loading strain rate is set to 1kN / s, and different colors are employed to differentiate the particle sizes within the mixed gangue during the loading process. The test results demonstrate that the selected gangue materials have an elastic modulus of 13.62 GPa and a compressive strength of 73.25 MPa. Furthermore, the changes in gradation between different particle sizes are visually expressed and analyzed using the gangue breakage rate. It is concluded that the ‘large sized particle’ gangue tends to be broken locally, while the ‘small sized particle’ gangue tends to be broken as a whole. The initial porosity distribution of the mixed gangue is calculated to establish the initial model of the gangue sample using the Particle Flow Code program. Numerical simulation experiments related to uniaxial compression were implemented. The strain change characteristics observed in the actual uniaxial compression test are further validated. Analyzing the simulation results, it was observed that the force chain propagates from the top to the bottom, causing cracks to form in the same direction. Moreover, the transmission area of force chain gradually decreases, resulting in the highest compaction degree at the top and the loosest at the bottom of the compressed model.

Investigation on the effects of asphalt mixes and their combinations on asphalt mix shear strength by 3D discrete element method

ABSTRACT This study investigated the effects of asphalt mixes and their combinations on the asphalt mix shear strength (SS) by a three-dimensional (3D) discrete element method (DEM). The uniaxial penetration test (UPT) for testing the asphalt mix SS was succinctly depicted. 3D micromechanical models used to predict the asphalt mix SS were built through X-ray computed tomography and discrete particle model implemented in particle flow code in three dimensions (PFC3D). The effects of asphalt mixes and their combinations on the SS were analysed on the basis of these models. Simulation results were proven against laboratory UPTs. As shown, the presented simulation results were in good agreement with the laboratory results, and the deviations between them were below approximately 6%, and most were less than 4%. The SS of high modulus asphalt mixes was greater than that of SBS-modified asphalt mixes and rubber-modified asphalt mixes. The asphalt mix SS increased with the increase in aggregate size, and it decreased with the rise in void content. The double-layer asphalt mix SS was more remarkably influenced by the bottom asphalt layer. The crumb rubber content also affected the rubber-modified the asphalt mix SS. The rubber-modified asphalt mix SS increased with the rise in crumb rubber content.

Numerically investigation of the neutron irradiation swelling on the mechanical and structural evolution of beryllium pebble bed

Pebble bed is employed in the blanket system to achieve neutron multiplication and tritium breeding. Neutron irradiation among the pebble bed is inevitable when the tokamak device is in normal operation. Particles could be swelled and the formation of bubbles or pore on the surface could aggravate the roughness of particles the surface under irradiation. It has great impact of pebble bed structure. In this paper, the mechanical and structure characteristics of the Beryllium (Be) pebble bed in the Helium Coolant Ceramic Breeding (HCCB) blanket were investigated. Furthermore, the evolution process of the pebble bed with mesoscale is analyzed as well. The Discrete Element Method (DEM) with Particle Flow Code (PFC) software is utilized to simulate the variation of particles under neutron irradiation with different roughness of particle surfaces and irradiation doses. Meanwhile, 0, 1%, 2%, 4% and 6% irradiation of single particle in radius direction with 0, 0.1, 0.3 and 0.5 friction coefficients were assumed. The evolution of the local packing factor, coordination number, contact force and force chain are given, and the fabric analysis is also given. The results show that the irradiation swelling of the particles and the deterioration of the surface have great impacts on the structure and mechanical properties of the pebble bed under neutron irradiation. Particle swelling could improve the packing factor and coordination number of the pebble bed, and the contact force distribution. The deterioration of the particle surface reduces the coordination number and shortens the force chain length. This paper presents the working condition under neutron irradiation which could provide reference for the design and evaluate of the blanket pebble bed.

Transfer mechanism and criteria for static–dynamic failure of granite under true triaxial unloading test

In the construction procedure of infrastructure, the excavation disturbance significantly changes the initial stress state of rocks in deep geotechnical engineering, and a rock burst would occur accompanied by the ejection of rock block. In the present study, a true triaxial unloading test and its corresponding numerical analysis based on the three-dimensional particle flow code (PFC3D) are conducted on granite specimens to acquire characteristic information of static and dynamic failure during unloading and to explore the criteria for determining static and dynamic failure. The results indicate that the failure modes of granite under true triaxial unloading can be classified into three categories: ① no obvious coalesced crack, ② a main coalescence shear crack, and ③ a main coalescence shear crack with a V-shaped pit. The instantaneous kinetic energy in static failure is characterized by a sharp increase and sporadic sharp increases, while that in dynamic failure shows a sharp increase, a short quiet period, and a substantial increase. The critical stress combination for the transformation of static–dynamic failure of granite is proposed, which is consistent with the experimental results. The number of shear cracks gradually increases in static failure, while it suddenly increases in dynamic failure. During the transformation from static failure to dynamic failure, the ratio of a tensile crack number to that of shear crack number decreases. The proportion of shear cracks in particle bonding increases, while the trend of tensile failure weakens. However, tensile failure remains predominant in granite, accompanied by shear failure.

Analysis of Fracture Characteristics of Ore Rock Based on GMTS Criterion

In this paper, the Tongkaiyu ore-rock is taken as the research object, aiming to analyze the fracture characteristics of the mixed type I, II and I–II faults in ore-rock by using the appropriate fracture criterion, and analyze the fracture development mode. Considering the non-singular term (T stress) in the formula of stress field sequence at crack tip, the generalized maximum tangential stress (GMTS) criterion is established, which is consistent with the boundary conditions of the central straight Brazilian disk (CSTBD) specimen. Through CSTBD splitting test and three-dimensional particle flow code (PFC3D) numerical simulation, the Angle between the central direct joint and the loading direction was changed, and the fracture toughness and fracture development law of type I, type II and type I–II mixed fracture were obtained. The theoretical and simulation results show that, compared with the maximum tangential stress (MTS) criterion, the GMTS criterion has an advantage in judging the fracture characteristics of mixed types I, II and I–II in ore rock.

Mechanical Properties of Composite Silty Soil Modified with Cement and Zirconia-Based Nanopowder.

This study assessed the modification effects of zirconia-based nanopowder and cement contents and curing age on the mechanical properties of silty soil. The orthogonal test design was applied to derive the best combination of each influencing factor using the lateral unconfined compressive test. Two-dimensional particle flow code (PFC2D) distinct-element modeling software was also used to fit and analyze the test curves, as well as simulate the triaxial test with the derived parameters. The test results reveal the optimal combination of 20% cement, 2% zirconia-based nanopowder, and 28 d curing age. The extreme difference table was used to plot the orthogonal trend diagram, and cement content was found to be the most significant factor controlling the silty soil strength. The maximum peak stress was 2196.33 kPa under the optimum combination of factors, which could be obtained through the index estimation, and these results were experimentally verified. According to the predicted strength envelope, the cohesive force of nanopowder-cement-modified silty soil in the optimal proportion was 717.11 kPa, and the internal friction angle was 21.05°. Nano zirconium dioxide will accelerate the hydration reaction of cement, the flocculent structure produced by the hydration of cement and soil particles connected to each other, play the role of filling and anchoring, and thus increase the strength of the nano-zirconium dioxide, and the optimal dosage of nano-zirconium dioxide is 2%.

Particle Flow Code modeling of the mechanical behavior of layered rock under uniaxial compression

Particle Flow Code modeling of the mechanical behavior of layered rock under uniaxial compression

Study on compaction characteristics and discrete element simulation for rubber particle-loess mixed soil

The rapid surge in traffic volume in China has resulted in a substantial accumulation of waste tires. By harnessing the lightweight and deformable characteristics of tire rubber particles, they are combined with soil to form rubber particle-loess mixed soil, which is progressively being embraced in civil engineering as a pivotal approach towards attaining green and sustainable development. In this study, waste tire rubber particles were integrated into loess to generate rubber particle-loess mixed soil, and compaction tests were conducted to investigate its compaction characteristics. Furthermore, PFC3D (Particle Flow Code 3D) was utilized for simulating the bearing ratio test of rubber particle-loess mixed soil, thereby validating the feasibility of numerical simulation for calculating CBR (California bearing ratio) values and exploring the relationship between micromechanical characteristics and macroscopic characteristics of such mixtures. The findings indicate that the maximum dry density of rubber particle-loess mixed soil significantly decreases with an increasing content of rubber particles. The utilization of PFC3D discrete element software proves efficacious in examining the bearing capacity of this mixture. Notably, when 20 mesh rubber particles constitute 20% by volume, the CBR value reaches its pinnacle and exhibits optimal bearing capacity. From a micromechanical perspective, the variation in internal porosity of rubber particle-loess mixed soil is positively associated with changes in macroscopic optimal water content, and negatively associated with changes in macroscopic CBR value. incorporating rubber particles enhances resistance against external forces while diminishing deformation within loess. This study provides a guidance for the efficient utilization of waste tires and the improvement of loess’s characteristics.

Correlations between mineral composition and mechanical properties of granite using digital image processing and discrete element method

This study investigated the correlations between mechanical properties and mineralogy of granite using the digital image processing (DIP) and discrete element method (DEM). The results showed that the X-ray diffraction (XRD)-based DIP method effectively analyzed the mineral composition contents and spatial distributions of granite. During the particle flow code (PFC2D) model calibration phase, the numerical simulation exhibited that the uniaxial compressive strength (UCS) value, elastic modulus (E), and failure pattern of the granite specimen in the UCS test were comparable to the experiment. By establishing 351 sets of numerical models and exploring the impacts of mineral composition on the mechanical properties of granite, it indicated that there was no negative correlation between quartz and feldspar for UCS, tensile strength (σt), and E. In contrast, mica had a significant negative correlation for UCS, σt, and E. The presence of quartz increased the brittleness of granite, whereas the presence of mica and feldspar increased its ductility in UCS and direct tensile strength (DTS) tests. Varying contents of major mineral compositions in granite showed minor influence on the number of cracks in both UCS and DTS tests.

Discrete element study on mechanical response and pressure relief effect of rock containing variable hole

The traditional large-diameter hole pressure relief technology has the disadvantages of insufficient pressure relief in deep surrounding rock and damage to shallow surrounding rock. Therefore, an improved reaming hole pressure relief technology is proposed. A series of compression tests are conducted on numerical models with various hole parameters using the three-dimension particle flow code (PFC3D) to reveal the pressure relief mechanism of reaming hole technology. Firstly, numerical models for the pre-holed specimen are constructed, and the micro-parameters are calibrated based on the experimental results. Then, the evolution of crack initiation, propagation, and coalescence for pre-holed specimens are investigated by confined compression simulations in PFC3D. The relationships among the cracking process, stress-strain curve, and number of micro-crack are analyzed in detail. Finally, the spatial-temporal evolution of the stress field around the hole is discussed from the stress and force chain perspective. The research results could serve as a foundation for evaluating the pressure relief effect through stress and fracture monitoring methods on the surrounding rock mass.

DEM analysis of crushing evolution in cemented granular materials during pile penetration

The present study demonstrates that the 3D discrete-element method provides a practical model approach to visualize the cemented grain crushing evolution under pile penetration. A combined method using a rigorous breakage criterion based on octahedral shear stress (OSS) was implemented in the particle flow code PFC3D. First, the pile penetration is simulated by considering the grains as uncrushable with a screening of highly stressed grains exceeding the threshold defined by the OSS failure condition. Then, the simulation is repeated where crushable agglomerates replace the highly stressed grains. This method is more accurate than the replacement method and more efficient than the agglomerate method. A quarter of the numerical model was considered after validation to achieve an acceptable computational time for parametric studies. Parametric investigations were performed on the effects of particle crushing, boundary conditions, pile tip shape, and pile penetration velocity on the penetration resistance behavior. In agreement with the observations of the physical calibration chamber, the present results indicate that the proposed modeling approach is reliable in reproducing the concentration of crushed granular material particles in the vicinity of the pile tip and shaft. In addition, grain crushing has been found to reduce both penetration and shaft resistance.

Dynamic behavior of railway Vehicle-Ballasted track system with unsupported sleepers based on the hybrid DEM-MBD method

In order to understand dynamic effect of the unsupported sleepers on railway substructure at the meso-macroscopic scales, this paper develops a dynamics analysis framework combining the discrete element method (DEM) and the multi-body dynamics method (MBD). Firstly, a DEM model of ballasted tracks is built by the PFC software (The full name is Particle Flow Code, which is based on the general discrete element (DEM) framework and code software) for capturing meso-macro mechanical behavior, while a MBD model of electric locomotive is presented for simulating accurately dynamic wheel loads. A numerical simulation program is compiled by the FISH Language to implement two-way coupling calculation between the DEM model and the MBD model. Then, experimental validation is carried out by comparing the dynamic response results obtained from the ShuoHuang Heavy-haul Railway Line field dynamics test with the simulation results adopting a hybrid DEM-MBD model and the classical railway vehicle-track hybrid dynamics model (VTCD model) known as Zhai’s model. Finally, the hybrid DEM-MBD model is used to simulate dynamic responses of the locomotive and ballasted tracks considering 0 ∼ 5 pieces of unsupported sleepers. Simulated results show that the acceleration of 1st and 2nd wheelset begin to increase when running to the position of one sleeper-space in front of hanging area and increase to the maximum when running to the edge of hanging area. In addition, it is necessary to pay attention to the variation of acceleration amplitude near the characteristic frequency excited by the distance between bogie pivot centers. The contact stress in well-supported area adjacent to both ends of hanging area increases greatly when the number of unsupported sleepers is greater than 3.

Study on Muck Pile Shape of Open-Pit Bench Blasting Based on PFC

To study the influence of different resistance lines and rows delay intervals on the shape of the muck pile and the throwing distance of open-pit bench blasting based on the open-pit bench blasting test, two-dimensional particle flow code (PFC2D) is used for numerical simulation. The experimental and simulation data were compared and analyzed, and the error value was acceptable. Meanwhile, different resistance lines and row delay intervals are used to conduct numerical simulation analysis. The results show that the height change of the muck pile is directly proportional to the resistance line, and the rock throwing distance is inversely proportional to the resistance line change. The best resistance line is 1.75 m in the actual working condition, according to the effect of the muck pile. By fitting the relationship between the resistance line and rock throwing distance and blasting muck pile’s height, it is found that the relationship is nonlinear. By simulating different row delays, the optimal row delay time difference in the bench blasting model is determined to be 25 ms under similar working conditions. It provides a guiding significance for the blasting parameter design under similar working conditions.

A pipe domain seepage model based on outsourcing Voronoi network with particle flow code

A pipe domain seepage model based on outsourcing Voronoi network with particle flow code

Comparative Study of the Rock-breaking Mechanism of a Disc Cutter and Wedge Tooth Cutter by Discrete Element Modelling

The operation of a shield tunnel boring machine (TBM) in a high-strength hard rock stratum results in significant cutter damage, adversely affecting the thrust and torque of the cutter head. Therefore, it is very important to carry out the research on the stress characteristics and optimize the cutter parameters of cutters break high-strength hard rock. In this paper, the rock-breaking performance of cutters in an andesite stratum in the tunnel of Qingdao Metro Line No. 8 was investigated using the discrete element method and theoretical analysis. The rock-breaking processes of a disc cutter and wedge tooth cutter were simulated by software particle flow code PFC3D, and the rock-breaking degree, stress of the cutter, and rock-breaking specific energy were analyzed. The rock damage caused by the cutter in a specific section was divided into three stages: the advanced influence, crushing, and stabilizing stages. The rock-breaking degree and the tangential and normal forces of the wedge tooth cutter are larger than that of the disc cutter under the same conditions. The disc cutter (wedge tooth cutter) has the highest rock-breaking efficiency at a cutter spacing of 100 mm (110 mm) and a penetration depth of 8 mm (10 mm), and the rock-breaking specific energy is 11.48 MJ/m3 (12.05 MJ/m3). Therefore, two types of cutters with different penetration depths or cutter spacing should be considered. The number of teeth of wedge tooth cutters can be increased in hard strata to improve the rock-breaking efficiency of the shield. The research results provide a reference for shield cutterhead selection and cutter layout in similar projects.

Particle flow simulation of Brazilian splitting failure characteristics of layered shale

Shale reservoir is the research hotspot in the development of unconventional oil and gas resources. The properties of bedding plane have an important influence on the mechanical behavior of shale. According to the fracture characteristics of layered shale under Brazilian splitting load, the effects of bedding angle, bond strength ratio and natural fractures on the tensile strength and fracture pattern of layered shale in Brazilian splitting test were studied by particle flow code. The evolution mechanism of meso–macro fracture of shale is analyzed by combining the dynamic change process of particle mesoscopic force chain and crack. The results show that three types of fracture patterns are observed: splitting tensile failure of shale matrix, splitting tensile failure along bedding plane and tensile–shear composite failure along shale matrix and bedding plane. Mesoscopic cracks initiate at the top and bottom loading points of the disk. After the peak load, mesoscopic cracks propagate through and appear dense force chains until macroscopic failure occurs. The bond strength ratio of bedding plane affects the failure pattern and peak strength of the specimen. With the increase in bedding angle, the tensile strength decreases gradually. Considering the influence of natural fractures, the tensile strength of shale is lower, and the crack propagation is more complex. The research model and results provide theoretical basis for the mechanical properties evaluation and fracture pattern analysis of shale.

Study on the Mechanism of Squeezing Silt by Riprap of Rubble Mound Breakwater Based on the Particle Flow Code

Study on the Mechanism of Squeezing Silt by Riprap of Rubble Mound Breakwater Based on the Particle Flow Code

Investigations on the macro‐meso mechanical properties and energy dissipation mechanism of granite shear fracture under dynamic disturbance

AbstractTo explore the mechanical characteristics and failure mechanisms of rock shear fracture under dynamic disturbance, the laboratory shear test and numerical simulation based on particle flow code (PFC) were carried out, and the mesoscopic degradation characteristics and energy dissipation mechanisms of rock shear fracture were analyzed. The results show that: (1) The crack initiation stress of rock under the disturbance condition is relatively reduced by 20%–29%, and there is a “shear fracture weakening effect.” The frequent alternating effect of stress concentration and release at the post‐peak failure stage is more obvious. (2) The number of cracks in rock under shear action shows the evolution of “calm → low speed growth → rapid increase → decrease of amplification → stabilization” with the increase of shear displacement. Compared with the conventional shear test, more cracks, and wider debris zones are formed. The number of cracks increases by 53%–106%, and the number of debris increases by 31%–138%. (3) The energy change trend of disturbance shear fracture test is basically consistent with that of conventional shear test. With the increase of normal stress, the input energy, elastic strain energy and dissipation energy of rock failure point show a certain linear growth feature. Compared with the conventional shear test, the input energy and stored strain energy of the failure point in the disturbance shear test are smaller, with relative reduction of 16.5% and 22.1%. Namely, the normal frequent disturbance reduces the energy storage capacity and failure resistance of rock.

Influence of hole diameter on mechanical properties and stability of granite rock surrounding tunnels

Nowadays, the development and utilization of more and more engineering construction are closely related to granite. However, many granite rock masses in Qingdao contain natural hole defects, which have a great impact on the mechanical properties of granite. It may even cause instability of surrounding rock and safety accidents. Therefore, in this paper, we discuss the influence of the hole diameter on the mechanical properties and stability of granite rock surrounding tunnels. Uniaxial compression experiments were conducted on granite with different hole diameters, and monitoring was carried out using the acoustic emission system and the XTDIC (Xintuo 3D Digital Image Correlation) three-dimensional–full-field strain-measurement systems. The relationship between the strength, deformation, and hole size of granite was investigated. In addition, using the Yangkou tunnel as the prototype and the PFC2D (Particle Flow Code of 2D) particle-flow–numerical-simulation program, a working tunnel model with different hole sizes was established to simulate the influence of natural hole defect sizes on the stability of rock. The results show that: (1) with an increase in hole diameter, the uniaxial compressive strength and elastic modulus of the granite sample gradually decreased. The brittleness of the granite samples gradually decreased, and the ductility gradually increased. (2) Under the action of axial stress and with an increase in the hole diameter, the sample was more likely to produce a stress concentration around the hole defect, which increased the deformation localization band, development, and expansion, as well as the intersection degree. As a result, granite samples are more likely to develop new cracks. These cracks increase in number and size, reducing the compressive strength of the granite sample. (3) The size of the hole defects significantly affected the damage and mechanical properties of the model surrounding rock. When increasing the hole diameter, the defect area increased and the tensile stress concentration near the hole in the localized rock became more evident. In addition, the stability of the rock surrounding the tunnel was significantly reduced, and its bearing capacity was weakened, leading to easier crack initiation and rock damage.