Meso-mechanical Properties Research Articles

Numerical simulation of the meso-mechanical properties of double cruciform fissures rocks after freeze–thaw cycles

Numerical simulation of the meso-mechanical properties of double cruciform fissures rocks after freeze–thaw cycles

Investigation of meso-mechanical properties of Jinping dolomitic marble based on flat-joint model

Brittle rock has the mechanical characteristics of a high ratio of uniaxial compressive strength to tensile strength, brittle-ductile transition, and so on. The mineral meso-structure of brittle rock shows the interlocking behavior and there are some microcracks between the mineral grains, so it is difficult to analyze the mechanical characteristics accurately with a regular constitutive model. The previous numerical models have some limitations when applied to brittle rock, that they cannot provide sufficient internal interlocking and are incapable of considering the microcracks. These limitations are addressed by using the updated flat-joint contact model to join notional polygonal particles with an initial gap and increasing the particle coordination number by adding flat-joint contacts to more particles that are within some non-zero distance of one another via the range coefficient contact identification method. A comparison of experimental and numerical results confirms that the updated flat-jointed model provides a good match with the mechanical properties of brittle rock. In this paper, we investigate the influence of mineral meso-structure (micro-cracks, range coefficient, and radius multiplier) on the meso-mechanical properties of Jinping dolomitic marble via the updated flat-joint contact model under direct-tension tests and compression tests. It is found that the updated model can reproduce the microstructure effect of brittle rock and can better simulate the mechanical characteristics of the brittle rock than conventional models, such as high strength ratio, brittle-ductile transition, initial nonlinearity, and different modulus in compression and tension.

DEM simulation of effects of particle size distribution on meso-mechanical properties of slip zone soil

Particle Size Distribution (PSD) exerts a substantial influence on the mechanical properties of geological materials such as rocks and soils, which can be viewed at a microscale as an assembly of discrete particles. An exploration into the effects of particle gradation on the properties of these materials provides valuable insights into their nature. In the study, the Discrete Element Method (DEM) was used to conduct numerical shear tests on eight distinct groups of slip zone soil, each characterized by a different particle gradation. The aim was to examine the meso-mechanical properties and shear evolution laws of slip zone soil numerical samples with both optimal and sub-optimal PSDs. Findings underscore the pivotal role that PSD plays in various aspects, including dilatancy, the evolution of the displacement field, the network of contact force chains, the principal stress, and the distribution of normal and tangential contact forces within the slip zone soil. It was observed that the network of contact force chains in the numerical samples with an optimal PSD was more complex than in those samples with a sub-optimal PSD. Additionally, the distribution of principal stresses before and after shear was more uniformly balanced. This particle size-based study offers significant reference value for future investigations into the impact of PSD on the macroscopic and meso-mechanical properties of slip zone soil. By augmenting this knowledge, a more comprehensive understanding of the fundamental behavior of these materials can be attained, leading to improved prediction and management of geological risks.

Study on the influence of coarse aggregate morphology on the meso-mechanical properties of asphalt mixtures using discrete element method

To investigate the influence of coarse aggregate morphologies on the performance of asphalt mixture, three discrete element models with different coarse aggregate morphologies were proposed in this study. Skeleton contact characteristics, skeleton contact stress, coarse aggregate movement characteristics, interfacial stress distribution, and micro-crack development characteristics with different coarse aggregate morphologies were compared and analyzed. The results indicate that there is often 40%–50% of compressive stress at the coarse aggregate contact point, 10%–20% of tensile stress, and 30%–40% of tensile and compressive mixed stress. Aggregate morphology has a more significant impact on the contact area than it does on the type of stress at the contact point or the number of main skeleton contact points. Contact stress, coarse aggregate movement, and crack development in the asphalt mixture are all greatly influenced by coarse aggregate morphologies. More needle-flake aggregates result in less contact stress, more coarse aggregate movement displacement, and faster asphalt mixture cracking. Shear stress makes up the majority of the micro-cracks that are caused by induced stress in asphalt mixture; tensile stress micro-cracks are quite minor, making up around 10% of the total number of micro-cracks. In order to increase the aggregate–asphalt interface qualities and the crack resistance of the asphalt mixture, it is important to avoid using too much needle-flake coarse aggregate. The findings can provide guidance for the choice of raw materials and mix ratios for asphalt pavement design, improving the asphalt mixture's meso-mechanics and macroscopic road performance.

Evaluation of using oyster shell as a complete replacement for aggregate to make eco-friendly concrete

The objective of this study is to investigate the feasibility of using oyster shells (OS) as a complete replacement for concrete aggregate, which provides a way to make eco-friendly concrete for the recycling of waste OS and the conservation of natural resources. In this study, the mineral composition of OS was analysed, and a series of experiments were carried out to investigate the effects of curing water, mineral admixtures, and steel fiber (STF) incorporation on the strength of all-OS aggregate concrete (AOSAC), and scanning electron microscopy (SEM) and the finite element method (FEM) were used to investigate the microstructure and meso-mechanical properties of AOSAC. In the FEM simulation, it was found that the main cracks of AOSAC under compression damage develop obliquely from the top and bottom corner to the interior, forming a double inverted triangle region on the surface, and the concrete outside the region quits working. The sequence of damage was observed in the internal slices of the model as interface transition zone (ITZ), OS aggregate, and mortar. In the tests, it was found that seawater curing introduces additional hydration reactions that leads to cracking within the concrete due to the generation of expansive hydrates resulting in reduced strength. The incorporation of fly ash (FA) can alleviate this problem and inhibit the generation of expansion hydrates. The incorporation of STF equivalent to 0.5% by volume results in the maximum increase in the strength of AOSAC, and the AOSAC developed was found to be fully strong enough for structural engineering purposes.

Internal corrosion characteristics of endogenous sulfate introduced by recycled aggregates in recycled aggregates concrete: Insights into the macro-mechanical and meso-mechanical properties

The old cement paste adhering to recycled aggregates (RA) carries some sulfate corrosion media because of the external sulfate attack occurred in parent concrete. These sulfate corrosion media would be introduced into recycled aggregates concrete (RAC) that is prepared using RA. For studying the effects of the internal sulfate corrosion media on RAC, different Na2SO4 solutions (0%, 1%, 5% and 8%) were used to soak RA for 120 days. The effects of the soaked RA on macro-mechanical properties and meso-mechanical properties of RAC were explored. It was found that the endogenous sulfate ions diffused from RA into the new cement paste to react with cement hydration products (such as Ca(OH)2) to generate corrosion products, ettringite and gypsum. The moderate corrosion products could fill pores and cracks at interfacial transition zone (ITZ) to enhance the early compressive strength and flexural strength of RAC. However, as the erosion level of RA and curing time of concrete increased, much corrosion products caused the obvious deterioration in strength of RAC and microhardness of ITZ, as well as the increase of thickness of ITZ. For easing the comprehensive evaluation of microhardness and thickness of ITZ, a new parameter (Δζ) was proposed. The smaller the Δζ, the higher the quality of ITZ.

Research on the macro- and meso-mechanical properties of frozen sand mold based on Hertz-Mindlin with Bonding model

In this study, macro- and meso-mechanical properties of frozen sand molds were discussed based on the Hertz-Mindlin with Bonding (HMB) model. Plackett-Burman, steepest ascent, and central composite designs were utilized to propose a parameter calibration methodology. The effects of mesoscopic parameter variations on the compressive strength and average gradient of stress-strain were investigated through response surface method analysis. Results showed that the relative error between the simulated and measured repose angle is 3.1% under calibrated intrinsic contact parameters. The compressive strength and average stress-strain gradient primarily depend on the normal and shear stiffness per unit area, as well as the particle size and porosity of the silica sand. Furthermore, taking load-displacement curves of three frozen sand molds with different geometric characteristics as the target value, the reliability and effectiveness of the frozen sand mold HMB model were verified through uniaxial compression tests and discrete element simulations.

Effects of hydrate distribution heterogeneity on the mechanical properties of hydrate-bearing sediments with consideration of the coalescence phenomenon: Insights from DEM simulations

The heterogeneous distribution of hydrates in natural or synthetic hydrate-bearing sediments (HBSs) has definitively proven its influences on hydrate dissociation and related mechanical responses, which can alter hydrate production strategies. However, due to the limitations of experimental conditions, applicable hydrate heterogeneity characterization and corresponding evaluation of the mechanical properties of HBSs remain lacking. In this study, discrete element modelling of numerical HBSs with different hydrate heterogeneous distributions considering the coalescence phenomenon is performed. The hydrate heterogeneity degree is quantified through image recognition, and the constructed HBSs are subjected to biaxial compression tests. The results show that for two HBSs with the same hydrate saturation above 10 % but highly different hydrate heterogeneity degrees exceeding 0.1, the peak strength, secant modulus and maximum dilation angle of the strongly heterogenous HBSs are lower than those of the weakly heterogenous HBSs. In addition, the weakly heterogeneous HBSs exhibit a more uniform contact force chain structure, more hydrates participate in loading support, and more particle rearrangements, which are the meso-mechanisms for the above macro-mechanical behaviors. Combined with the consideration of the hydrate morphology and cementing ratio, the mechanical properties of HBSs fluctuate due to the randomness of hydrate coalescence but generally show a decreasing trend with increasing hydrate heterogeneity degree. These findings provide an important supplement for understanding the macro- and meso-mechanical properties of HBSs with different hydrate heterogeneity degrees, which indicates that we should precisely describe hydrate coalescence in laboratory experiments and numerical simulations and consider its representativeness in field applications.

Influence of MgAl-LDHs/TiO2 composites on the mechanical and photocatalytic properties of cement pastes and recycled aggregates

To study the effects of MgAl-LDHs/TiO2 on the properties of cement paste and recycled aggregates, MgAl-LDO (MgAl-LDHs calcined products) and TiO2 were mixed during the reconstruction of MgAl-LDHs laminates based on the memory effects of layered double hydroxides (LDHs), and then MgAl-LDHs/TiO2 was synthesized. The performance changes were characterized with hydration heat tests, the microscopic morphologies of the hydration products, and the physical and mechanical properties of the cement paste. The effect of the MgAl-LDHs/TiO2 on the multi-interfacial transition zone (ITZ, the interface between the old paste and new paste) of the recycled aggregates was characterised with the micromorphology, meso-mechanical properties, and element diffusion behaviour. The photocatalytic activity of MgAl-LDHs/TiO2 loaded on recycled aggregate was evaluated with the degradation of methylene blue under sunlight. The results showed that the addition of MgAl-LDHs/TiO2 promoted the initial hydration of ordinary Portland cement (OPC) and induced and promoted the growth and formation of crystalline cement hydration products. The initial setting time and final setting time were shortened, and the compressive strength of the hardened paste was improved when MgAl-LDHs/TiO2 was added to the cement paste. The MgAl-LDHs/TiO2 compacted the recycled aggregates and improved the structural properties of the ITZ. The MgAl-LDHs/TiO2 altered diffusion within the ITZ. The total efficiency for the degradation of methylene blue by the 5–10 mm MgAl-LDHs/TiO2 loaded on recycled aggregate reached 82.2% after 80 min under simulated sunlight.

Analysis on the Shear Behavior of Steel-Asphalt Interface Using Discrete Element Method on the Mesoscopic Scale

Shear failure is a major distress in steel bridge deck pavement (SBDP). To investigate the shear behavior of steel-asphalt interface (SAI), experimental study and numerical analysis were proposed and performed. First, the direct shear tests at two temperatures of 30°C and 60°C were carried out to provide verification for numerical analysis. Second, the virtual tests were conducted under the same condition as laboratory test using discrete element (DE) method to evaluate the shear behavior of SAI and analyze the influence of temperature on the development of interlaminar mesocracks. Third, factors affecting the interlaminar mesomechanical properties, including interface roughness and interfacial bubbles, were proposed and analyzed at 30°C. Results show that with the increase of shear displacement lots of interlaminar mesocracks occur rapidly with the decreasing of shear stress. The mesocracks occur more easily under high temperature. The shear strength of SAI increases at first and then decreases with the increase of interface roughness and decreases with the increase of interfacial bubbles.

Experimental application and accuracy assessment of 2D-DIC in meso-direct-shear test of sandy soil

The examination of the meso-mechanical properties of soils is fundamental to understand its macro-behaviour. This paper aims to evaluate the potential application of Digital Image Correlation (DIC), an optical image processing approach, in direct shear test (DST) for soils. Hence, a new shear box was designed to investigate the close behaviour of granular materials. The noise-floor and shear displacement accuracy were discussed to assess the reliability of DIC, besides the effectiveness of the new box was also examined. The distribution of the displacements, strains, and shear angle were measured under four different normal stresses. From the results, the immediate settlement, and the dilative behaviour of the sand during shearing were observed. Furthermore, the investigation revealed a strain concentration at the interface between the boxes, where a shear band was formed. The use of DIC opens up new ways in soil mechanics, overcoming some limitations of the conventional DST.

Numerical Simulation of Steel Fiber Pull-Out Process Based on Cohesive Zone Model and Unified Phase-Field Theory

In steel fiber reinforced concrete, the interface is a very complex and weak structure. It is because of the weak interface layer between the steel fiber and the matrix that the reinforcing and toughening properties of the steel fiber cannot be fully exerted. The interface bond performance is the core of the meso-mechanical properties of steel fiber reinforced concrete. To study its influence on the mechanical properties of steel fiber reinforced concrete, three-phase finite element models of steel fiber pull-out are established based on the cohesive zone model and unified phase-field theory by means of FEM in this paper. The interface bond is simulated by a zero-thickness cohesive element, and the pull-out process of steel fiber in the concrete matrix is analyzed to provide a basis for the fracture research of steel fiber reinforced concrete. In this paper, the influence of factors such as the embedment depth, length–diameter ratio, embedment angle, and interface properties of steel fibers on the pull-out mechanical properties of steel fibers are considered, and the relevant finite element models are established to conduct numerical simulations of the pull-out process of steel fibers. The numerical simulation results are in good agreement with the experimental results, and this verifies the reliability of the model. The results show that the steel fiber pull-out finite element model established by the cohesive zone model and phase-field regularized cohesive zone model (PF-CZM) has a certain reliability; the peak pull-out load of the steel fiber increases with an increase in the embedment depth of the steel fiber, and decreases with an increase in the length–diameter ratio and embedment angle of the steel fiber; by controlling the strength of the interface layer and the concrete matrix, the reinforcement effect of the steel fiber on the concrete matrix can be improved; and the PF-CZM has a good characterization of the damage and failure evolution process of the concrete matrix.

Discrete element modelling of the uniaxial compression behavior of pervious concrete

Pervious concrete (PC) has been widely used in recent years. With increasing attention on PC, it promotes further understanding of its mechanism responsible for the resulted performance. The objective of this study is to propose a 3D discrete element method (DEM) model to simulate the uniaxial compression process and analyze the meso mechanical properties of PC. The user-defined ellipsoid composed of pebble discrete elements was used to simulate aggregate. In the DEM, the mechanical behaviors of materials were simulated with contact constitutive models of discrete elements. The linear contact model and parallel-bond model were employed to simulate the strength properties between two adjacent aggregates, and between cement paste and aggregate. The parallel-bond parameters were calibrated based on the stress-strain curve obtained from the laboratory test. The reliability of the method was verified by comparing the virtual results with the laboratory tests of PC with 6 groups of different aggregate sizes. It was proved that the DEM model established can be used to simulate the rising section of the stress-strain curve of PC accurately. The maximum absolute error of uniaxial compressive strength is 0.67 MPa, and the relative error is within 5%. By analyzing the number of contact points, the contribution rate of different sizes of aggregates to the contact force and the distribution of the force chain, it can be concluded that the coarse aggregate plays a major role in resisting compression load in PC, while fine aggregate mainly increases the number of contact points to achieve force transmission.

A micro–macro constitutive model for rock considering breakage effects

The paper proposes a three-scale binary medium-based constitutive model on the basis of the meso structures and micro components to describe the elasto-plastic mechanical behavior of mudstone samples. Based on the breakage mechanism of geomaterials, mudstone samples are considered as two different materials (bonded and frictional elements) at mesoscales. From micro to meso scales, given the similar but different mineralogy composition and porosity of the bonded and frictional elements at microscale, as well as their separate mechanical characteristics, different homogenization methods are adopted to obtain their respective meso mechanical properties. At the mesoscale, in view of the unique meso structures and the continuous material transformation, the extended self-consistent scheme (SCS) is improved to be adaptable to elasto-plastic composites with varying meso components. With the consideration of the evolution form of the breakage ratio under the external loading being given based on the assumed strength distribution of the meso bonded elements, the mechanical relations between meso and macro scales are established. Finally, on the basis of the mean-field method and combined with the critical mechanical connections between different scales, the micro-meso-macro constitutive model for mudstone samples are proposed. The model validation shows that, with a few model parameters, the proposed model can well reflect the stress and deformation features of mudstone samples with complex micro-components.

Macro–meso mechanical properties of ballast bed during three-sleeper tamping operation

ABSTRACT The tamping operation is an effective means to quickly restore the status of the lines. The purpose is to explore the three-sleeper tamping operation mechanism maintenance. First, a model of the interaction between tamping picks and ballast particles was proposed. Then, the model of three-sleeper tamping operation was established by discrete element method (DEM) and multibody dynamics (MBD) coupling approach. Finally, the macro–meso properties of ballast particles was analysed. The results indicate that the tamping operation can considerably reduce the compactness of the ballast bed in the bottom of the sleeper and the crib area and also disturb the ballast shoulder area. In the penetration stage, there is a critical position of normal contact force of ballast particles. Moreover, the ballast at the bottom of the sleeper can move to either side of the sleeper during the rotation stage of the retracting, which is not conducive to the compaction.

Experimental study of the relationship between bond strength of aggregates interface and microhardness of ITZ in concrete

• A research on the acquisition of macro-mechanical data of the ITZ in the meso-mechanical calculation of concrete. • The mechanical properties of the ITZ with different mix ratios were measured, the relationship between the bond strength and the microhardness of the ITZ in concrete is established. • A two-parameter linear relationship between the splitting tensile strength of the interface bond and the age and the W/B ratio was established. • The priority order of the calculated value of the interface strength in the numerical analysis of concrete meso-mechanics is proposed. The problem of taking macro-mechanical data of the interface between aggregates and slurry in the calculation of meso-mechanics of concrete was studied in this paper. Through a systematic standard interface test study where the interfacial bond splitting tensile strength method and the interfacial transition zone (ITZ) microhardness method were used, mechanical properties were investigated at macro-scale (bond strength), and followed by ITZ microhardness at meso-scale, moreover the relationships between macro- and meso-mechanical properties were established. The results show that interfacial bond splitting strength of aggregates-slurry gradually increases with increasing hydration age and decreases with increasing water-binder ratio; the ITZ thickness decreases and the ITZ microhardness valley value increases with the extension of curing age. There is a certain correlation between macro-scale mechanical strength of interface and meso-scale mechanics of ITZ. There is a significant linear relationship between the splitting tensile strength of the interface bond and the splitting tensile strength of the cement slurry, so the interface strength can be calculated according to the splitting tensile strength of the slurry substrate. The interfacial bond splitting tensile strength and ITZ microhardness valley have different degrees of correlation. The interfacial bond splitting tensile strength has a particularly significant linear relationship with the microhardness value of the cement paste substrate. Therefore, the calculation value of the interface strength in the numerical analysis of concrete micromechanics should preferably adopt the linear relationship formula with the microhardness of the cement paste substrate.

Numerical simulation of mesomechanical properties of limestone containing dissolved hole and persistent joint

In order to reveal the complex mechanical properties and mesoscopic failure behavior of limestone rock mass containing both dissolved hole defect and persistent joint, numerical models are developed using the two-dimensional particle flow code (PFC2D) to carry out the uniaxial compressive strength (UCS) tests. The stress–strain, contact force chain, vertical stress field, and crack evolution characteristics of limestone samples containing circular holes and different dip angles persistent joints were analyzed. The results show that the UCS and elastic modulus of the rock samples can be reduced by the circular hole and the persistent joint, and the persistent joints with steep dip angles have a more obvious control effect on the mechanical properties of the rock samples. With the increase in loading stress level, the contact force chain in the hole-joint combination model gradually deflects slightly along the joint dip angle from the left and right sides of the hole and gradually spreads outward. Under uniaxial compression, the initiation cracks in persistent joints and rock blocks are primarily tensile cracks. The initiation positions first appear in the persistent joint, and the initiation positions in the rock block change from the upper and lower vertices of the circular hole to the junction below the joint with the increase of the joint dip angle. The crack initiation stress in the persistent joint is smaller than that in the corresponding rock block, and the control effect of the joint dip angle on the crack initiation stress of the whole rock sample is reduced.

Effect of Moisture on the Macro Failure Characteristics of Weakly Consolidated Mudstone: Mesomechanism

Mudstone, whose significant characteristic is water-weakening, widely exists in all kinds of geotechnical engineering, which brings great security risks to engineering safety. In this paper, through a series of macro and meso test methods, the macro and meso action mechanism of water on the weakly consolidated plane of mudstone was studied. In the uniaxial compressive strength test, the mudstone cementation plane was the main part of the fracture derived. Under the moist state, the strength of mudstone would decrease significantly, which was inversely correlated with relative moisture content. In the case of flooding, mudstone cracking occurred only along the cementation plane. Through ESEM+mapping test, results showed the humidification curing caused obvious damage to the cemented plane, where sodium and calcium were lost without a certain cause. The cemented plane was rich in kinds of clay minerals in mesoscale and presented an inhomogeneous state. The surface hardness data obtained from nanoindentation were discrete and difficult to quantitatively evaluate the mesomechanical properties. However, compared with the dry state, the data set of the moist state had a significant decline, showing a weakening effect. The mudstone is prone to fail along these weakly consolidated planes. Moisture intrudes into the bedding plane further deteriorating their consolidation.

Discrete element simulation on failure mechanical behavior of transversely isotropic shale under two kinds of unloading paths

Based on the discrete element method, a two-dimensional numerical model of shale with different bedding inclinations was established. Using this model, the macro-and meso-mechanical properties of bedded shale under different unloading paths and bedding inclination were studied. In this study, the damage mechanism of bedded shale during the unloading process is revealed. The simulation uses the displacement-controlled loading method, with a loading speed of 0.05 m∙s−1. The results indicate that the bedding inclination of the shale has a significant impact on both mechanical parameters and failure modes under unloading. The peak strength and the ultimate bearing capacity show a “U” shape trend under both unloading paths. Observations of maximum values are observed at β = 0° or β = 90° while observing the minimum values at β = 30°or β = 45°. The failure mode of shale specimens with β = 0° at lower initial confining pressure, the failure mode is splitting damage along the multiple bedding plans, which gradually changes to shear damage with increasing initial confining pressures crossing the weak surface. The failure mode of the specimens with β = 15–45° at different initial confining pressure is shear-slip damage along the bedding planes. And shale at β = 60–90° failure modes are all conjugate shear damage intersecting with the weak surface. The damage to the specimen caused by unloading and increasing the axial stress is greater than that caused by the unloading and constant axial strain. The effects of β = 0° and 90° on microcracking and stress distribution within the specimen are significantly smaller than for the other bedding inclinations.

Evaluation of macro- and meso-mechanical properties of concrete under the aggressiveness of landfill leachate

Current researches on the mechanical properties of concrete are mainly concentrated on the aggressiveness of sulfate, chloride ion, acid and freeze–thaw cycles. However, the evolution of concrete mechanical properties aggressived by landfill leachate remains to be revealed. As the micro-nano indentation test is a new method to measure the fine/micro mechanical behavior of materials, it is used in this research to evaluate the micro-mechanics of the landfill leachate of the concrete sample surface and different aggressived depths. In addition, the evolution of the macro-mechanical properties of concrete aggressived by the landfill leachate was studied through uniaxial compression tests. The results show that the hydration products with high elastic modulus are consumed by the aggressiveness of landfill leachate, which results in a large number of microscopic cracks on the surface of the concrete. Moreover, the longer the aggressive time, the shallower the aggressive depth, and the more severe the deterioration of the micromechanical properties of the sample surface. It is also notable that the uniaxial compressive strength of the concrete samples aggressived by landfill leachate showed a linear increase and then gradually decrease. Comparing with the sodium chloride and sodium sulfate solutions, the landfill leachate has the most significant weakening effect on the elastic modulus of concrete.