Tensile Microcracks Research Articles

Excavation-induced cracking of clastic rock: A true triaxial instantaneous unloading study with varied levels of initial damage

The cracking and squeezing problem in deep engineering significantly constrains project construction. To reveal the cracking mechanism of rock under instantaneous unloading, a self-designed rigid true triaxial experimental apparatus, equipped with groundbreaking electromagnetic unloading and high-speed camera functions, was utilized to conduct instantaneous unloading tests on clastic rock with varied levels of initial damage. The results indicate that samples undergo severe and irreversible lateral dilation during instantaneous unloading, with dilational strain rapidly increasing at higher initial damage levels. Instantaneous unloading induces the generation of numerous tensile microcracks, which propagate and coalesce rapidly. The crack propagation speed, as well as the length and width of the cracks at failure, increase with the rise of the initial damage level. The samples eventually exhibit two distinct macroscopic fracture zones: a tensile fracture zone and a tensile-shear mixed fracture zone. Analysis of acoustic emission hits and energy indicates that higher initial damage levels correspond to greater unloading damage, with a higher overall proportion of tensile cracks throughout the experiment. Under the induction of blasting excavation, radial stress is rapidly unloaded, and dense tensile cracks emerge in the immediate surrounding rock, leading to cracking and squeezing towards the free face. Importantly, higher initial damage levels intensify the squeezing effect. The self-developed equipment serves as a crucial technical tool for researching excavation unloading, and the insights derived from this study provide valuable guidance for understanding cracking mechanisms and informing the construction of deep engineering projects.

Numerical simulation of failure and micro-cracking behavior of non-persistent rock joint under direct shear

Persistency, as a key geometric parameter of joints, significantly affects shear strength parameters of jointed rock mass. A good understanding of how persistency affects shear behavior of joint is therefore crucial for better evaluation of stability of rock slope. To investigate the failure and micro-cracking behavior of non-persistent rock joint under direct shear, a novel Voronoi generation algorithm is first used to establish an improved grain-based model (GBM) of granite which considers the shape of feldspar. The calibrated model is then used to simulate the direct shear test of numerical models possessing different joint persistency under various normal stresses. The results reveal that the developed micro-cracks generally increase rapidly when the shear strain reaches to a value approximately 50 % of the peak shear strain and the grain boundary tensile micro-crack is dominant among these initiated micro-cracks. Micro-cracks generally initiate at the ends of rock bridge, and then gradually propagate to the central of rock bridge, forming en-echelon fractures. The failure mode of numerical model is closely related to the generated en-echelon fractures. An increase in both joint persistency and normal stress can lead to a shear failure. The finding in this study provides an important basis for understanding the mechanical behavior and failure mechanism of jointed rock mass.

Tensile Microcracking Behavior of Granites After High Temperature Treatment by Considering the Effect of Grain Size and Mineralogical Composition

Tensile Microcracking Behavior of Granites After High Temperature Treatment by Considering the Effect of Grain Size and Mineralogical Composition

A microscopic approach to brittle creep and time-dependent fracturing of rocks based on stress corrosion model

A brittle creep and time-dependent fracturing process model of rock is established by incorporating the stress corrosion model into discrete element method to analyze the creep behavior and microcrack evolution in brittle rocks at a micro-scale level. Experimental validation of the model is performed, followed by numerical simulations to investigate the creep properties and microcrack evolution in rocks under single-stage loading, multistage loading, and confining pressure, at various constant stress levels. The results demonstrate that as the stress level increases in single-stage creep simulations, the time-to-failure progressively decreases. The growth of microcracks during uniaxial creep occurs in three stages, with tensile microcracks being predominant and the spatial distribution of microcracks becoming more dispersed at higher stress levels. In multi-stage loading-unloading simulations, microcracks continue to form during the unloading stage, indicating cumulative damage resulting from increased axial stress. Additionally, the creep behaviour of rocks under confining pressure is not solely determined by the magnitude of the confining pressure, but is also influenced by the magnitude of the axial stress. The findings contribute to a better understanding of rock deformation and failure processes under different loading conditions, and they can be valuable for applications in rock mechanics and rock engineering.

Brittle Damage Processes Around Equi‐Dimensional Pores or Cavities in Rocks: Implications for the Brittle‐Ductile Transition

AbstractIn the Earth's upper crust, rocks deform mostly by means of brittle fracturing processes. At the micro‐scale these processes involve the formation and growth of microcracks in the vicinity of defects such as open fissures, pores and other cavities. Large defects can produce strong enough perturbations of the stress field to activate and/or intensify brittle damage around them. Here we considered the ideal cases of smooth cylindrical and spherical pores inside an infinite solid body subjected to remote triaxial compressive stresses (i.e., the intermediate and minimum principal stresses are assumed equal). We first established the resulting local stress field around one of those large pores and then verified whether certain brittle damage processes could be activated in these conditions. We mainly considered the formation of tensile microcracks and micro shear bands. The former requires the presence of tensile stresses in some regions around the pore, while the latter needs sufficiently large shear stresses on pre‐existing optimally inclined microcracks to overcome their frictional resistance to sliding. We find that shear driven deformation remains localized in the vicinity of the pore. On the other hand, dilatational, tensile cracks can propagate large distances away from the pore but cannot form at and above some threshold ratio of the least to the largest principal stresses. Faulting associated with interacting tensile cracks is therefore suppressed with increasing depth. Our analysis leads to conclusions generally consistent with published experimental observations and provides some clues to discuss the physical cause of the brittle‐ductile transition in rocks.

Uniaxial constitutive models for wood under cyclic compression loads with varying loading orientations

A desirable objective is to develop a unified constitutive model for wood that can effectively capture its uniaxial cyclic compressive behaviors. To achieve this goal, quasi-static cyclic compression tests have been conducted to supplement the limited cyclic experimental data available. Observation of deformation within the cell structure led to the concluded that material failure resulted from the coalescence of shear or tensile microcracks and the rearrangement of cell structures in regions under compressive stress. Based on the experimental results, a secant stiffness degradation-based and an empirical-based unified constitutive model for wood were proposed. Comparison between the predicted and experimental results indicated that the two proposed models could accurately capture the nonlinear characteristics of wood.

Effect of thermal treatment on microcracking characteristics of granite under tensile condition based on bonded-particle model and moment tensor

It is known that the heterogeneity caused by thermally induced micro-cracks and thermal stress can affect the mechanical behavior of granite. The laboratory-scale tests have the intrinsic limitation of non-repeatability and lack of effective methods to characterize the interaction effect between thermal micro-cracks and thermal stresses. In this study, we demonstrate how advancements in particle bonded model and moment tensor can help better understand the roles of high temperature in weakening granite and thermally induced cracking process in Brazilian test. Our results show that the types of micro-cracks (intergranular, intragranular, and transcrystalline ones) are related to their thermal expansion coefficients of mineralogical compositions. The intergranular tensile micro-cracks are predominant during the heating and heating–cooling processes. An obvious weakening of granite and non-central initiation is associated with the heterogeneity caused by the thermal damage and thermal stress. We also quantitatively evaluate the thermal damage based on orientation distribution, b-value, and nature of the sources, which gives a new microcracking perspective on tensile characteristics subjected to high temperature.

Rate sensitive plasticity-based damage model for concrete under dynamic loading

In this manuscript, a rate-sensitive plasticity-based damage model for concrete subjected to dynamic loads is presented. The developed rate-sensitive damage model incorporates the key experimental evidence related to strain rate and damage rate. With increasing strain rates, the model is able to predict a decrease in the rate of damage evolution due to artificial stiffening effects together with a final higher level of damage. The major contribution of the work is to include the effect of damage rate, strain rate and also to consider all the physical mechanisms of damage. This is achieved by using a power law model to relate the rate of damage to the equivalent plastic strain rate. The damage parameter considers hydrostatic, tension and compression damage. Such a damage definition helps in the prediction of pulverized damage due to a loss in cohesive strength at increased hydrostatic stress, shear-induced compressive damage and tensile microcracking. Strong volumetric deformation of the material that includes hydrostatic and compaction damage is also considered in the model. Because hydrostatic damage accounts for the reduction in stiffness and compaction damage accounts for the increase in stiffness under pure compressive loading. A strain rate-dependent failure surface is considered and with increasing strain rate there is an increase in the size of the failure surface which is capped at an upper limiting value. The incremental effective stress-strain relationship is defined in terms of rate of damage, accumulated damage and viscosity parameters reflecting the inherent inertial, thermal and viscous mechanisms respectively. The stress-strain relationship in the model also accounts for stress reversals that occur due to interference of an incident and reflected wave, by including a Heaviside function. The developed model is implemented in LS-DYNA using vectorized UMAT. Verification, validation and parameterization of the developed model have been made through several numerical analyses.

Revealing the failure mechanism of high strength seawater coral aggregate reinforced concrete slabs based on acoustic emission technology: Parameter analysis and DBN-BPNN classification

In this study, high strength seawater coral aggregate concrete (SCAC) with a compressive strength of 80 MPa was prepared, and seawater coral aggregate reinforced concrete slab (SCARCS) was designed with epoxy-coated steel bars. Combining with the AE technology, monotonic flexural static tests were conducted on SCARCS specimens with three varying reinforcement ratios (i.e., 0.85%, 1.13%, and 1.41%). Results showed that the increasing the reinforcement ratio led to an increase in peak bearing capacity and a decrease in center deflections at the peak. The loading process can be divided into four stages based on the varying magnitudes of the b-value and Ib-value. The primary mode of failure was tensile microcracks at various stages, and the ratio of shear microcracks reached its maximum value in the final stage of loading. Besides, based on the signal intensity analysis, it can be observed that the scale and complexity of cracks propagation inside the SCARCS specimen increased as the reinforcement ratio rose. In addition, back propagation neural network (BPNN) classification model optimized by deep belief network (DBN) algorithm was proposed to predict the damage degree of SCARCS during loading, and the average prediction accuracy of the proposed model was shown to be over 89%. The classification outcomes demonstrated that with an increase in the reinforcement ratio of SCARCS, the proportion of AE signals in Cluster 3 increased, while the proportion of AE signals in Cluster 2 dropped. Notably, both the average frequency and energy presented an ascending trend. The findings of this study provided valuable insights for designing and assessing the safety of SCAC structures for essential island and reef engineering infrastructure.

Experimental and numerical study of disk specimen with rough structural plane under splitting test

To study the effects of the anisotropic matrix and structural planes on the splitting strength and failure mode of rocks, Brazilian splitting tests were carried out with seven different loading angles on specimens of rock-like materials with rough structural planes. The surface strains of the samples during the failure process were monitored and analysed with the help of a high-speed camera and digital image correlation (DIC) technology. The test results showed that the Brazilian splitting strength (BSS) decreased gradually with an increased loading angle. According to the crack morphology, the samples showed three failure modes, and the structural plane and the loading angle (θ) had an important effect on the failure mode. When θ < 75°, the sample failure was mainly affected by the matrix, and when θ > 75°, the sample failure was mainly controlled by the structural plane. The numerical simulation of the sample with a structural plane was carried out by the PFC2D particle flow program, the micro parameters were calibrated using a back propagation (BP) neural network model. The internal cracks of the sample under a splitting load were mainly matrix tensile microcracks and structural plane shear microcracks, and the tensile microcracks in the side with the weak matrix appeared significantly earlier than those in the side with the strong matrix. With increasing loading angle, the proportion of tensile microcracks in the matrix increased, while the proportion of shear microcracks in the matrix decreased, especially in the weak matrix. The microcracks at the structural plane mainly changed from tensile microcracks to shear microcracks, and the development degree of microcracks along the structural plane was more significant than that on the weak matrix with increasing loading angle. The results of the study can provide a reference for rock stability evaluation and utilization.

Typical fracture modes of metal cylindrical shells under internal explosive loading

The competition and coupling of various fracture modes, including tensile fracture, shear fracture and spallation, exist in explosively-driven metal shells. However, the fracture mechanisms and influencing factors of different fracture modes are still unclear. In this study, three typical fracture modes of explosively-driven metal shells–tensile fracture, tensile-shear fracture and shear fracture are numerically revealed by using both the maximum principal strain and the Johnson-Cook cumulative-damage failure criterion. The selection of failure criteria is explained in detail. The fracture strain and fragment morphology are in good agreement with the experimental results. Combined with failure criterion, the numerical results are analyzed from the perspective of pressure, stress and strain. It is found that the formation of different fracture modes depends on the competition between the shear failure formed in the inner surface of the shell and the tensile micro-cracks formed in the middle of the shell. To further investigate the influencing factors of different fracture modes, three dimensionless parameters affecting the fracture modes are proposed based on dimensional analysis. In addition, by analyzing 29 sets of experimental data, we propose the dimensionless equations that can control different fracture modes. And the distribution diagrams of three typical fracture modes (tensile, tensile-shear and shear fracture) of explosively-driven metal shells are determined. Furthermore, we propose a parameter that can effectively distinguish adiabatic shear fracture from non-adiabatic shear fracture.

Spatio-Temporal Distribution of the Sources of Acoustic Events in Notched Fiber-Reinforced Concrete Beams under Three-Point Bending.

The acoustic activity, generated in notched, beam-shaped concrete specimens, loaded under three-point bending, is studied in terms of the position of the sources of acoustic events, and the frequency of their generation. Both plain specimens (without any internal reinforcement) and specimens reinforced with various types of short fibers were tested. The target of the study is to investigate the existence of indices that could be considered as pre-failure indicators of the upcoming fracture. In addition, an attempt is undertaken to classify the damage mechanisms activated to tensile or shear nature. Considering comparatively the spatio-temporal evolution of the position of the acoustic sources and the respective temporal evolution of the frequency of generation of acoustic events, it was concluded that for relatively low load levels the acoustic sources are rather randomly distributed all over the volume of the specimens. As the load increases toward its maximum value, the acoustic sources tend to accumulate in the immediate vicinity of the crown of the notch and the average distance between them approaches a minimum value. When this minimum value is attained, the load is maximized and the generation frequency of the acoustic events increases rapidly. The simultaneous fulfillment of these three conditions is observed a few seconds before the onset of propagation of the catastrophic macrocrack for all classes of specimens tested, providing a kind of warning signal about the upcoming fracture. Moreover, the classification of the damage mechanisms to tensile and shear ones revealed a crucial difference between the plain and the reinforced specimens after the maximization of the load applied. Indeed, while for the plain specimens, the prevailing damage mechanism is tensile microcracking, for the reinforced specimens a balance between tensile and shear damage mechanisms is observed after the load applied has attained its peak and starts decreasing.

Experimental study on the effect of anchors on the creep mechanical properties and acoustic emission characteristics of fractured sandstone under uniaxial compression

This study conducts laboratory tests to investigate the creep and acoustic emission (AE) of unanchored and anchored fractured sandstone samples under uniaxial compression by taking the Triassic fine sandstone collected from the Xiaolangdi Reservoir area as the research object and ensuring the same testing conditions using an RLJW-2000 rock rheometer and a PCI-Ⅱ AE instrument. Moreover, this study identified the effects of anchors on the creep mechanics and AE characteristics of fractured sandstone samples by analyzing the strain, long-term strength, AE event count, AE rate, AE energy rate, and rising angle – average frequency (RA-AF) characteristics of each sample before and after anchoring. According to the study results: (1) Under the same stress level, the anchored sample's instantaneous strain, creep strain, and total strain are smaller than those of the unanchored sample. Anchors exerted the largest inhibitory effect on creep strain, followed by the total strain, while anchors inhibited instantaneous strain to the smallest extent. (2) Compared to the unanchored sample, the anchored sample experienced a significant increase (as high as 50%) in long-term strength. Anchor reinforcement significantly improves the long-term strength of samples. (3) Under the same stress level, the AE event count and cumulative AE event count of the anchored sample are smaller than those of the unanchored sample. Anchors significantly inhibited microcracks initiation, propagation, overlapping, and penetration in samples. (4) The maximum AE rate of the anchored sample is only 69.30% of that of the unanchored sample, whereas the peak AE energy rate of the anchored sample is only 23.21% of that of the unanchored sample. Anchors reduce the AE event count generated and the AE energy released by samples per unit of time, exerting a significant crack-arrest effect on them. (5) The presence of anchors changes the type and number of microcracks in the samples' creep process. Anchors greatly inhibit the generation of high-RA and low-AF AE signals and generate low-RA and high-AF ones. As a result, more tensile microcracks occur in samples under stress levels close to the failure stress level (especially the last stress level), which inhibit the formation of shear microcracks in samples and causes samples to be more prone to macroscopic tensile-shear failure. (6) The microcrack characteristics of samples characterized by RA-AF values are consistent with the macrocrack characteristics of samples. Therefore, RA-AF values can satisfactorily characterize the microcracks' variation characteristics in the sandstone samples' creep process before and after anchoring.

Characterizing the cracking process of various rock types under Brazilian loading based on coupled Acoustic Emission and high-speed imaging techniques

Tensile strength is one of the important mechanical properties of rock and rock-like materials and its accurate estimation and understanding of its failure mechanism are crucial for efficient design of structures on or within the rock masses. Amongst various approaches developed for estimating this property, Brazilian testing has received considerable attention analytically, numerically and experimentally. The test has been deployed for a wide range of materials such as rock, concrete, and ceramic, however its fracturing process at micro and macro scales has not yet been properly characterized. Thus, in this study, for a full understanding of the fracturing process of different rock types under indirect tensile loading, the coupled Acoustic Emission (AE) and high-speed imaging techniques were used to characterize the generated micro and macrocracks under Brazilian loading. The characteristics of AE parameters and the evolution of micro-macro cracks were investigated for three different rocks including sandstone, coal and granite at different sizes and loading configurations. A systematic framework was developed to exhibit the link between micro and macro fracturing processes under Brazilian loading. It was noted that the tensile microcracks are the first to initiate during the Brazilian failure with high intensity compared to shear microcracks. Transition in microcracking mechanism from tension to shear was evident in the samples with high brittleness. The tensile microcracks showed ascending trends with a decrease in geometrical or volumetric sample size for granite and sandstone, while its trend for coal was inconclusive due to its unique intrinsic fracture network or so-called “cleat network”. At macro scale level, single and multiple cracking mechanisms were observed in the tested samples where those with high shear microcracks mostly revealed failure under multiple cracking. Finally, some insights were provided into the transition from single tensile cracking to multiple tensile/shear cracking based on high-speed image data with high resolution.

Analysis of damage characteristics of steel fiber-reinforced concrete based on acoustic emission

Steel fiber-reinforced concrete (SFRC) has been widely used as a building material worldwide. The macroscopic destruction of this material is a manifestation of the internal microdamage evolution. Hence, investigating the internal damage evolution process in concrete for monitoring in-service concrete has considerable significance in terms of safety. This study investigates the damage characteristics of SFRC with different steel fiber contents (0, 15, 30, 45, and 60 kg/m3) using Brazilian disk splitting tests and acoustic emission (AE) techniques. The changing trend of the characteristic parameters (counts and energy, respectively) of AE signals throughout the failure process of concrete is analyzed. Then, the moment tensor method is used to determine the temporal and spatial evolution of microcracks in SFRC. Research results show that the AE characteristic parameters are closely related to the internal damage of concrete specimens, and show different characteristics of AE signals in different failure stages. When the matrix cracks, the AE counts become denser, the values increase sharply, and the energy is closely related to fiber content (the lower the fiber content, the higher the energy value, and the minimum AE energy is 1.80e5 aJ (in SFRC60)). According to the moment tensor method, the cumulative microcracks are first appear to near the central axis and then dispersed to the surrounding area in the SFRC with the effect of steel fiber, effectively weakening the local damage. Based on the R-value method, the proportion analysis results of microcracks show that tensile damage is the primary destruction mechanism throughout the failure process of the Brazilian disk. The minimum proportion for tensile microcracks is 56.67%.

Laboratory study of fracture initiation and propagation in Barre granite under fluid pressure at stress state close to failure

Hydraulic fracturing is frequently used to increase the permeability of rock formations in energy extraction scenarios such as unconventional oil and gas extraction and enhanced geothermal systems (EGSs). The present study addresses uncertainties in the hydraulic fracturing process pertaining to EGSs in crystalline rock such as granite. Specifically, there is debate in the literature on the mechanisms (i.e. tensile and/or shear) by which these fractures initiate, propagate, and coalesce. We present experiments on Barre granite with pre-cut flaws where the material is loaded to high far-field stresses close to shear failure, and then the fluid pressure in the flaws is increased to move the Mohr's circle to the left and observe the initiation and propagation of fractures using high-speed imaging and acoustic emissions (AEs). We find that the hydraulic fractures initiate as tensile microcracks at the flaw tips, and then propagate as a combination of tensile and shear microcracks. AE focal mechanisms also show elevated levels of tensional microfracturing near the flaw tips during pressurization and final failure. We then consider a numerical model of the experimental setup, where we find that fractures are indeed likely to initiate at flaw tips in tension even at relatively high far-field stresses of 40 MPa where shear failure is generally expected.

Mechanical Properties and Failure Behavior of Dry and Water-Saturated Foliated Phyllite under Uniaxial Compression.

Phyllite is widely distributed in nature, and it deserves to be studied considering rock engineering applications. In this study, uniaxial compression tests were conducted on foliated phyllite with different foliation angles under dry and water-saturated conditions. The impacts of water content and foliation angle on the stress-strain curves and basic mechanical properties of the Phyllite were analyzed. The experimental results indicate that the peak stress and peak strain decrease first and then increase with increasing foliation angle as a U-shape or V-shape, and the phyllite specimens are weakened significantly by the presence of water. Moreover, an approach with acoustic emission, digital image correlation, and scanning electron microscopic is employed to observe and analyze the macroscopic and mesoscopic failure process. The results show that tensile microcracks dominate during the progressive failure of phyllite, and their initiation, propagation, and coalescence are the main reasons for the failure of the phyllite specimens. The water acts on biotite and clay minerals that are main components of phyllite, and it contributes to the initiation, propagation, and coalescence of numerous microcracks. Finally, four failure modes are classified as followed: (a) for the specimens with small foliation angles α = 0° or 30° (Saturated), both shear sliding and tensile-split across the foliation planes; (b) for the specimens with low to medium foliation angles α = 30° (Dry) or 45°(Saturated), shear sliding dominates the foliation planes; (c) for the specimens with medium to high foliation angles α = 45° (Dry) or 60°, shear sliding dominates the foliation planes; (d) for the specimens with high foliation angles α = 90°, tensile-split dominates the foliation planes.

Micromechanics of rock damage and its recovery in cyclic loading conditions

SUMMARY Under compressive stress, rock ‘damage’ in the form of tensile microcracks is coupled to internal slip on microscopic interfaces, such as pre-existing cracks and grain boundaries. In order to characterize the contribution of slip to the overall damage process, we conduct triaxial cyclic loading experiments on Westerly granite, and monitor volumetric strain and elastic wave velocity and anisotropy. Cyclic loading tests show large hysteresis in axial stress–strain behaviour that can be explained entirely by slip. Elastic wave velocity variations are observed only past a yield point, and show hysteresis with incomplete reversibility upon unloading. Irrecoverable volumetric strain and elastic wave velocity drop and anisotropy increase with increasing maximum stress, are amplified during hydrostatic decompression, and decrease logarithmically with time during hydrostatic hold periods after deformation cycles. The mechanical data and change in elastic properties are used to determine the proportion of mechanical work required to generate tensile cracks, which increases as the rock approaches failure but remains small, up to around 10 per cent of the net dissipated work per cycle. The pre-rupture deformation behaviour of rocks is qualitatively compatible with the mechanics of wing cracks. While tensile cracks are the source of large changes in rock physical properties, they are not systematically associated with significant energy dissipation and their aperture and growth is primarily controlled by friction, which exerts a dominant control on rock rheology in the brittle regime. Time-dependent friction along pre-existing shear interfaces explains how tensile cracks can close under static conditions and produce recovery of elastic wave velocities over time.

Loading rate dependence of staged damage behaviors of granite under uniaxial compression: Insights from acoustic emission characteristics

A series of laboratory-based uniaxial compression tests combined with acoustic emission (AE) monitoring was conducted to investigate the loading rate dependence of AE characteristics and the damage process of granite. Experimental results showed that the mechanisms of formation and evolution of cracks within the rock during loading can be characterized by AE parameters and real-time spatial AE locations. With increasing loading rate, the cumulative AE count decreased as a negative power function while the cumulative AE energy decreased almost linearly. Decreasing b-values indicate additional large-magnitude AE events and more concentrated distribution modes of fractures under high-speed loading conditions. Moreover, with increasing loading rate, the dominant failure mechanism was observed to transition from shear to tensile. A transition from tensile to shear crack modes during the damage process of each sample was noted to correspond with a decrease in b-values and the onset of crack nucleation, which may serve as a diagnostic precursor to rock failure. Crack nucleation patterns also exhibited rate dependence. Under low-speed loading conditions, nucleation was initiated at the unstable crack growth stage and was characterized by interactions between linked tensile microcrack arrays to form a shear macrorupture zone, whereas crack nucleation advanced to the stable crack growth stage under high-speed loading conditions, resulting in vertical splitting.

Damage evolution of concrete under tensile load using discrete element modeling

The objective of this study is to investigate the damage evolution process in the flattened Brazilian test of concrete, and assess the effect of the maximum aggregate size on the tensile strength of concrete using the discrete element method considering the heterogeneity of concrete. Random irregular coarse aggregates were generated in each gradation according to the theoretical content. Based on this, a series of discrete element numerical models of flattened Brazilian disks with aggregates of different gradations were established, including 5–10, 5–16, and 5–20 mm, and numerical simulations of flattened Brazilian tests were conducted. Before the peak load, the compressive force was concentrated near the flat loading ends, which can be considered as the driving force for the initiation of shear microcracks, and the tensile force distributed away from the loading ends induced the appearance of tensile microcracks in the mortar and the interface transition zones (ITZs). Tortuous macroscopic cracks formed owing to the accumulation of tensile microcracks at the center of the specimen when the peak load was reached. After the peak load, secondary cracks initiated and extended gradually, and shear failure began to occur in the mortar and the ITZs owing to the redistribution of the compressive force. The failure mechanism was correlated with the displacement trends of the particles. Tensile microcracks could be associated with the tensile displacement trend of the particles. Shear and tensile microcracks co-existed in the zones where the particles moved in a mixed-mode displacement trend. The tensile strength of the concrete decreased slightly with an increase in the maximum aggregate size in the flattened Brazilian tests, and the tensile strength of the concrete was overestimated by the flattened Brazilian tests.