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Abstract: A modified distinct element Voronoi method, which we refer to as “Trigon” logic, is proposed for use in both two and three-dimensional modelling. A rock material is represented by an assembly of triangular (in 2D) or tetrahedral (in 3D) blocks bonded through their contacts. To evaluate the ability of the proposed Trigon logic in simulating brittle fracture, a series of unconfined and confined compression and Brazilian tests is performed illustrating the sample failure mechanisms under two and three dimensional conditions. Typical failure fracture patterns (damage) experienced by brittle rock are explicitly simulated including axial tensile failure under unconfined compression, shear failure under confined compression, and tensile crack extension during indirect tension (Brazilian) testing. A ‘damage parameter’ D and a ‘damage intensity’ D 21 measure are proposed to allow the evaluation of the simulated rock damage in a more quantitative manner. At the mine opening scale, the process of roof shear failure is successfully captured using the Trigon logic. In comparison with the conventional Voronoi model, the proposed Trigon model is less mesh-sensitive and predicts both a more realistic friction angle and failure pattern under varied loading conditions. [Copyright &y& Elsevier]

Abstract: Tensile failure and fracture behaviour of parallel laid twisted flax fibre reinforced low melting polyethylene terephthalate (LPET) composites were investigated. The tensile failure results of the model specimens were compared with AE results in terms of amplitude, energy and counts. The failure results of the flax fibre LPET composites exhibited mainly matrix crack initiation as a brittle failure for low, medium and high fibre contents. Since the composites at high fibre contents have higher porosity content, they show higher strain to failure, higher variation in the tensile results and have different appearances on their fracture surfaces than those of the composites at low and medium fibre contents. [Copyright &y& Elsevier]

The ultimate stress and strain of polyisoprene rubber were studied by numerical simulations of three-dimensional random networks, subjected to tensile strains high enough to cause chain rupture. Previously published molecular chain force extension models and a numerical network construction procedure were used to perform the simulations for network crosslink densities between 2 × 1019 and 1×1020 cm-3, corresponding to experimental dicumyl-peroxide concentrations of 1-5 parts per hundred. At tensile failure (defined as the point of maximum stress), we find that the fraction of network chains ruptured is between 0.1% and 1%, depending on the crosslink density. The fraction of network chains that are taut, i.e. their end-to-end distance is greater than their unstretched contour length, ranges between 10% and 15% at failure. Our model predicts that the theoretical (defect-free) failure stress should be about twice the highest experimental value reported. For extensions approaching failure, tensile stress is dominated by the network morphology and purely enthalpic bond distortion forces and, in this regime, the model has essentially no free parameters. The average initial chain tortuosity (t) appears to be an important statistical property of rubber networks; if the stress is scaled by t and the tensile strain is scaled by t-1, we obtain a master curve for stress versus strain, valid for all crosslink densities. We derive an analytic expression for the average tortuosity, which is in agreement with values calculated in the simulations. [ABSTRACT FROM AUTHOR]

This paper showcases the authors’ predictions for the 13 challenging test cases of the third World Wide Failure Exercise. The cases involve the prediction of lamina biaxial stress–strain curves, matrix cracking and delamination in various cross-ply and quasi-isotropic laminates under uniaxial loading, variation of thermal expansion coefficient of a laminate with matrix cracking, bending of a general laminate, loading-unloading behaviour and the strength of various thin and thick laminates containing an open hole. The laminates were made of various glass and carbon fibre/epoxy materials. The constitutive model is based on plasticity theory, includes hydrostatic pressure effects and accounts for multiaxial load combination effects. The failure criteria distinguish between matrix failure, fibre kinking and fibre tensile failure. In-situ strengths are used for matrix failure. Propagation of failure takes into consideration the fracture energy associated with each failure mode and, for matrix failure, the accumulation of cracks in the plies. The model is used to make blind predictions of all test cases from the third World-Wide Failure Exercise. [ABSTRACT FROM AUTHOR]

Abstract: This paper deals with the problem of rapid ascent mechanism of kimberlite magmas with a multi-directional approach: 1) the kinetics of diamond–graphite transition; 2) settling velocity of diamond phenocrysts in magmas and 3) formation of ruptures required for magma ascent with a high speed. Based on the diamond–graphite transition, we present an estimate of the ascent rates from high pressure and temperature experiments using Walker-type multi-anvil apparatus. The experiments were conducted with diamond, placed within a synthetic kimberlitic assemblage, keeping an initial pressure and temperature of 6GPa and 1350°C, respectively. It was observed that the volume fraction of diamond to graphite conversion strongly depended on the ascent rates. Using electrical resistivity and X-ray diffraction studies, we measured the degree of graphitization as a function of the ascent rate (u). For u <3m/s, diamond underwent almost complete graphitization (conversion>90%), whereas it remained nearly intact (conversion<10%) when u >10m/s. Our theoretical calculations of the settling velocity of mantle xenoliths again confirm that diamond can exist when u >3m/s. We performed numerical experiments with finite element (visco-elastic) models to analyze the dynamics of tensile failure at the tip of magma pools, leading to dilatational vertical fractures for magma transport. Considering the tensile strength of mantle in the order of 0.5kb, our models show this failure process as a function of the critical shape (Ar : ratio of vertical and horizontal dimensions) and density contrast (Δρ) of magma pools. The critical Δρ is estimated nearly 200kg/m3 when Ar is very large (>4). [Copyright &y& Elsevier]

This work reports four different modes of failure during startup uniaxial extension of entangled polymer melts: Capillary, tensile decohesion, shear-yielding-induced necking, and rupture. Emphasis is placed on the identification of the critical condition separating tensile failure from necking and the molecular mechanisms for each type of failures. When Weissenberg number Wi is not vanishingly small, a startup extension terminates in a rupture-like failure where nonuniform extension takes place in a sharply localized manner. This decohesion event reflects the tensile yielding of the entanglement network that occurs due to insufficient intermolecular gripping force to balance the growing intrachain elastic retraction force. At higher rates, the failure mode switches from the tensile decohesion to necking as the entangled melts experience a Cauchy stress level in excess of twice the elastic plateau modulus (2GN0). Since the minimum stress to produce shear yielding is on the order of GN0, in these high-rate extension tests the melts have the option to undergo shear yielding. Birefringence and particle-tracking velocimetric observations were carried out to reveal the first evidence for shear deformation as a precursor to the nonuniform extension: The necessarily localized shear yielding initiates nonuniformity of the stretched specimen sometimes termed 'necking.' [ABSTRACT FROM AUTHOR]

Abstract: This paper investigates different tensile fracture initiation criteria on the plane-strain configuration of a defect-free openhole wellbore. It aims to model the effect of the internal pressurization of a wellbore drilled in the direction of one of the principal stresses. We compare a brittle initiation model that accounts for both the necessary stress and energy conditions (mixed criteria) for the development of a fracture with a bilinear cohesive zone model which can degenerate either to the Dugdale rectangular softening model or the linear softening model. Moreover, we propose an approximation of the mixed criteria which amounts to solving a single scalar nonlinear equation. The effect of the wellbore size on the load at fracture initiation observed experimentally is well reproduced by these different models which all provide a similar response. The size effect on tensile failure is governed by the ratio between a material lengthscale over a structural lengthscale : . The material lengthscale is defined as the square of the ratio between the material fracture toughness over its tensile strength , while the structural lengthscale is here simply the wellbore radius . For small values of (i.e., ), tensile crack initiation is dominated by strength with vanishingly small size effects, while for large value of (), crack initiation is dominated by energy requirements and the initiation pressure increases with . The far-field stresses modify the transition between a strength-dominated to an energy-dominated tensile failure as a function of . A higher mean compressive far-field stress tends to make the failure more strength-driven, while higher differential far-field compressive stress promotes an energy-driven tensile failure. [Copyright &y& Elsevier]

This paper presents a pressure-dependent three-dimensional constitutive law to predict failure for laminated composites. The nonlinear constitutive response in shear and in the transverse and through-the-thickness directions, which is measured experimentally, is incorporated directly into the model. In addition, secant stiffnesses are dependent on the state of hydrostatic pressure and on the general state of strain. The failure criteria distinguish between matrix failure, fibre kinking and fibre tensile failure. In-situ strengths are used for matrix failure. Propagation of failure takes into consideration the fracture energy associated with each failure mode and, for matrix failure, the accumulation of cracks in the plies. A detailed discussion is undertaken of the mismatch between the available experimental data and the physical properties required to characterise the constitutive response up to final failure. The model is employed to make blind predictions of the triaxial failure envelopes and stress–strain curves of all 12 test cases provided by the organisers of the second World-Wide Failure Exercise. [ABSTRACT FROM AUTHOR]

Because of applications of composites in space and in low temperature equipment, low temperature mechanical properties of glass fiber-reinforced epoxy have to be assessed. Experimental or analytical investigation on the tensile failure behavior of glass/epoxy laminated composite with/or without stress concentration subjected to thermo-mechanical static loadings at low temperatures has not been done yet. In the present work, a model was developed to perform the progressive failure analysis of quasi isotropic composite plates at low temperatures. The initial failure load is calculated by means of an elastic stress analysis. The load is increased step by step. For each given load, the stresses are evaluated and the appropriate failure criterion is applied to inspect for possible failure. For the failed element, material properties are modified according to the failure mode using a non-zero stiffness degradation factor. Then, the modified Newton-Raphson iteration is carried out until convergence is reached. This analysis is repeated for each load increment until the final failure occurs and the ultimate strength is determined. The present method yields results in a reasonable agreement with the experimental data at room temperature and −60°C. The effect of low temperature on the failure mechanism of the plates was also determined. [ABSTRACT FROM AUTHOR]

Abstract The intent of this research is to determine the tensile strength and fracture behavior of annealed and chemically strengthened aluminosilicate glass under static and dynamic loading conditions using Brazilian disk (BD) tests. Owing to high brittleness and very small failure strain of glass, this test method is preferred to indirectly measure the tensile strength by compressive loading of cylindrical specimens diametrically instead of using direct tensile test. In this study, based on the BD principle, the static tensile strength is measured by using the universal testing machine and a modified SHPB with pulse shaper technique is used to measure the dynamic tensile strength of two types of glasses. The synchronous high-speed photography in both static and dynamic tests is used to capture the damage/crack initiation, crack pattern, and failure process of BD specimens. Also, the strain-time history was recorded from gauges directly embedded on BD specimens in both tests. The test results showed that two types of glasses under investigation are the loading rate-sensitive. In the case of annealed glass (AG), the average dynamic tensile strength was increased three times from the static strength and a substantial increase in tensile strength of chemically strengthened glass (CSG) was observed at high loading rate. The high-speed images and collected glass debris during BD tests are analyzed and discussed to explain the rate-sensitivity, fracture mode and tensile failure process of two types of glasses. The high-speed video suggested that the main central crack caused the splitting or failure in AG and in case of CSG the main central crack along with multiple cracks the splitting or failure occurred. Finally, the scanning electron microscope (SEM) images of collected glass residue of (AG) and (CSG) was also studied to illustrate the brittle fracture surface, crack pattern and deformation mechanism in two types of glasses. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents fatigue results for sandwich beams that exhibit a transition in failure mode, from core shear failure to face laminate tensile failure, as function of load amplitude only. The basis of this are fatigue tests of foam cores in shear and tensile tests on composite laminates. These results show that the slopes of the stress–life (S–N) relation are different for the core and laminates. By using the obtained stress–life relations, a simple design scheme is given for sandwich beams which are anticipated to have a transition of failure mode for a particular load level. Two designs are manufactured and tested in fatigue under constant amplitude loading. The results clearly show the aim of investigation with transitions in failure modes giving a structural stress–life diagram a bi-linear shape. For high load and small number of cycles to failure, the beams fail by core shear fracture while for lower loads, and large number of cycles to failure the beams fail by face sheet tensile failure. [ABSTRACT FROM AUTHOR]

Increasing the damage and crack porosity in crustal rocks can result in significant changes to various key physical properties, including mechanical strength, elastic and mechanical anisotropy, and the enhancement of transport properties. Using a Non-Interactive Crack Effective Medium (NIC) theory as a fundamental tool, we show that elastic wave dispersion can be inverted to evaluate crack density as a function of temperature and is compared with optically determined crack density. Further, we show how the existence of embedded microcrack fabrics in rocks also significantly influences the fracture toughness (KIC) of rocks as measured via a suite of tensile failure experiments (chevron cracked notch Brazilian disk). Finally, we include fluid flow in our analysis via the Guéguen and Dienes crack porosity-permeability model. Using the crack density and aspect ratio recovered from the elastic-wave velocity inversion, we successfully compare permeability evolution with pressure with the laboratory measurements of permeability. [ABSTRACT FROM AUTHOR]

The rock fragmentation process induced by a single button-bit, two neighboring button-bits, and multiple button-bits are numerically studied using the rock and tool interaction code (R-T2D). Through this study, a better understanding of the bit-rock fragmentation mechanisms is gained. It is found that side crack is initiated from the crushed zone or bifurcated from Hertzian crack to propagate approximately parallel to the free rock surface but in a curvilinear path driven by the tensile stress associated with the expansion of the crushed zone during the loading process. In the crushed zone, the mechanism of side crack is mixed tensile and shear failure, but outside the crushed zone, the dominant mechanism of side crack is tensile failure. A semiempirical and semitheoretical relationship among the side crack length, the drilled rock property, and the drilling force is formulated to approximately predict the side crack length. In the simultaneous loading, the interaction and coalescence of side cracks induced by the neighboring button-bits with an optimum line spacing enable formation of largest rock chips, control of the direction of subsurface cracks and a minimum total specific energy consumption. A formula is derived to determine the optimum line spacing on the basis of the drilled rock properties, the diameter and shape of the button-bit, and the drilling conditions. In the rock fragmentation by multiple button-bits, most of the rock between the neighboring button-bits is chipped as a result of the coalescence of side cracks. In the remaining rock, the intensely crushed zones and significant extensional cracks are observed adjacent to the sidewall and the inside of the borehole. Fragment side distribution shows more than 80% of the fragments are fines in the crushed zones as well as the cracked zones, the large fragments be indeed observed, which are the big chips caused by the coalescence of side cracks. [ABSTRACT FROM AUTHOR]

A multiscale virtual internal bond (VIB) model for isotropic materials has been recently proposed to describe material deformation and fracture under quasi-static and dynamic loading situations. Fracture simulation using an isotropic VIB model is made possible by incorporating a cohesive type law, inspired by atomistic-level interaction among particles into a hyperelastic framework at the continuum level. Thus, fracture is built directly into the constitutive formulation. The numerical implementation of the material model for brittle materials into a finite-element scheme and the determination of model parameters has been previously researched. In this paper, the VIB model is applied to the dynamic fracture simulation of plain concrete. The experimental study on the dynamic tensile failure of concrete by Gran et al. is used to determine the VIB model material parameters for plain concrete. Simulations and results of comparisons with the concrete–VIB model with the experimental results are presented in this paper. [ABSTRACT FROM AUTHOR]

This paper presents a new 1-D non-local damage-plasticity deformation model for ductile materials. It uses the thermodynamic framework described in Houlsby and Puzrin (2000) and holds, nevertheless, some similarities with Lemaitre’s (1971) approach. A 1D finite element (FE) model of a bar fixed at one end and loaded in tension at the other end is introduced. This simple model demonstrates how the approach can be implemented within the finite element framework, and that it is capable of capturing both the pre-peak hardening and post-peak softening (generally responsible for models instability) due to damage-induced stiffness and strength reduction characteristic of ductile materials. It is also shown that the approach has further advantages of achieving some degree of mesh independence, and of being able to capture deformation size effects. Finally, it is illustrated how the model permits the calculation of essential work of rupture (EWR), i.e. the specific energy per unit cross-sectional area that is needed to cause tensile failure of a specimen. [ABSTRACT FROM AUTHOR]

We report tensile failure experiments on polyurethane (PU) foams. Experiments have been performed by imposing a constant strain rate. We work on heterogeneous materials for whom the failure does not occur suddenly and can develop as a multistep process through a succession of microcracks that end at pores. The acoustic energy and the waiting times between acoustic events follow power-law distributions. This remains true while the foam density is varied. However, experiments at low temperatures (PU foams more brittle) have not yielded power-laws for the waiting times. The cumulative acoustic energy has no power-law divergence at the proximity of the failure point which is qualitatively in agreement with other experiments done at imposed strain. We notice a plateau in cumulative acoustic energy that seems to occur when a single crack starts to propagate. [ABSTRACT FROM AUTHOR]

The propagation of cracks on a small scale can result in dynamic disasters, such as rock bursts induced by large-scale fractures in coal seams. To gain insights into the mechanism of fracture-induced disasters in deep coal seams, this study investigates the micro-process and size effect of crack propagation in coal specimens featuring different sizes of center-crack circular discs (CCCD). We conducted Brazilian splitting tests on coal samples with varying dimensions and employed acoustic emission (AE) and digital image correlation (DIC) techniques to quantitatively describe the crack propagation process. Additionally, coal specimens of different diameters were compared for the size effect characteristics of different fracture parameters during the tensile failure process. The results show that as the specimen size increases, quasi-brittle failure characteristics become pronounced, tensile strength gradually decreases and stabilizes, while fracture toughness and fracture process zone length gradually increase and tend to stabilize. The macroscopic cracks in the coal specimens generally extend from the centre of the specimen to both ends along the precracks. Large energy events are mainly concentrated at the lower end of the precracks. As coal specimen size increases, the number of AE events decreases, while the proportion of large energy events gradually increases, and the critical crack opening quantity exhibits a linear increase with specimen size. By conducting the fracture size effect experiment on coal specimens, we extend the relevant mechanical parameters from laboratory-scale tests to field engineering scales. This provides fundamental parameters for the analysis of large-scale fracture process of in-situ coal based on fracture mechanics. [ABSTRACT FROM AUTHOR]

Hydraulic and pneumatic fracturing have been used to improve the effectiveness of most in situ remediation methods for contaminated sites underlain with unfavorable low-permeability soils. This paper presents results of a laboratory experimental investigation to characterize the mechanisms related to the initiation pressure and growth of fractures stimulated from vertical and horizontal wells. The mechanisms of fracture in low-permeability soils appeared to be of a tensile failure mechanism enhanced by the generation of pore pressure as the soil around the well was being sheared due to the radial-tangential stress difference imposed by the injected pressure. The impacts of initial fracture slots on fracture orientation and initiation pressure were also investigated. The test results have demonstrated that the presence of initial fracture slots could reduce the injection pressure required to initiate fracture in the well. The initial slot, however, did not necessarily control the orientation of the propagating fracture. The effect of imposed stresses in the soil was evaluated also and was found to influence the orientation and propagation of fracture. Smaller stress contrast favored multiple deviated fractures, whereas larger stress contrast favored distinct fractures.Key words: soil fracturing, laboratory test, low-permeability soil, fracture propagation.La fracturation hydraulique et pneumatique a été utilisée pour améliorer l'efficacité de la plupart des méthodes in situ de rémédiation pour les sites contaminés reposant sur des sols défavorables de faible perméabilité. Cet article présente les résultats d'une étude expérimentale en laboratoire pour caractériser les mécanismes reliés à la pression d'amorce et à la croissance des fractures activées à partir de puits verticaux et horizontaux. Les mécanismes de fracture dans les sols de faible perméabilité semblent appartenir à un mécanisme de rupture en traction favorisé par la génération de pression interstitielle lorsque le sol sur le pourtour du puits est en cisaillement dû à la différence entre les contraintes radiale et tangentielle appliquées par la pression d'injection. Les impacts de fentes initiales de fracture sur l'orientation de la fracture et sur la pression d'amorce ont aussi été étudiés. Les résultats des essais ont démontré que la présence de fentes initiales de fracture pouvait réduire la pression d'injection requise pour amorcer la fracture dans le puits. Cependant, la fente initiale ne contrôlait pas nécessairement l'orientation de la fracture en progression. L'on a aussi évalué l'effet des contraintes appliquées dans le sol et l'on a trouvé qu'elles influençaient l'orientation et la propagation de la fracture. Un plus faible contraste de contraintes favorisait des fractures multiples déviées alors qu'un plus grand contraste de contraintes favorisait des fractures distinctes. Mots clés : fracturation du sol, essai de laboratoire, sol de faible perméabilité, propagation de fracture.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]

Tough hydrogels have emerged as a promising class of materials to target load‐bearing applications, where the material has to resist multiple cycles of extreme mechanical impact. A variety of chemical interactions and network architectures are used to enhance the mechanical properties and fracture mechanics of hydrogels making them stiffer and tougher. In recent years, the mechanical properties of tough, high‐performance hydrogels have been benchmarked, however, this is often incomplete as important variables like water content are largely ignored. In this review, the aim is to clarify the reported mechanical properties of state‐of‐the‐art tough hydrogels by providing a comprehensive library of fracture and mechanical property data. First, common methods for mechanical characterization of such high‐performance hydrogels are introduced. Then, various modes of energy dissipation to obtain tough hydrogels are discussed and used to categorize the individual datasets helping to asses the material's (fracture) mechanical properties. Finally, current applications are considered, tough high‐performance hydrogels are compared with existing materials, and promising future opportunities are discussed. [ABSTRACT FROM AUTHOR]

The structural stability of engineering rock mass under dynamic disturbance is directly associated with the fracture mechanics properties in engineering practice. Fully understanding the rock's fracture mechanical behavior and crack evolution caused by stress concentration at the crack tip in engineering rock mass under dynamic load can offer useful insight into the rock's dynamic fracture mechanism. A dynamic test using split-Hopkinson pressure bar (SHPB) test system was performed on a single prefabricated fissure sandstone centrally cracked Brazilian disk (CCBD) specimens. Based on the theory of fracture mechanics and one-dimensional stress wave theory, the dynamic crack initiation criterion of CCBD specimen is proposed, and the regression model of sandstone's dynamic fracture toughness under the coupling effect of fissure angle and strain rate is established by using response surface methodology (RSM). The influence of strain rate and fissure angle on stress wave characteristics, dynamic tensile mechanical behavior, and fracture mechanics characteristic was investigated in this study. The findings demonstrate that: (1) The fissure angle plays a pivotal role in determining the failure mode of sandstone. As the fissure angle increases, three distinct failure modes emerge in the sandstone specimens, while variations in strain rate have minimal impact on the fracture mode of these specimens. (2) Alterations in the fissure angle result in changes to the waveform of transmitted waves. When the fissure angle is below 30°, the transmitted wave exhibits "double peak" characteristics; when it exceeds 30°, a "single peak" waveform is observed. This phenomenon can be attributed to diffraction principles governing incident waves. (3) When the impact pressure is 0.2 MPa, the peak load initially exhibits an increase followed by a decrease, with the peak load reaching its maximum at a fracture angle of 60°; when the impact pressures are 0.3 and 0.5 MPa, there exists a negative correlation between the peak load and the fissure angle. (4) The influence of strain rate on sandstone's fracture resistance is predominant, with alterations in fissure angle exerting an auxiliary effect on this property. The research results can provide a theoretical and experimental basis for dynamic disaster prevention in urban underground space. [ABSTRACT FROM AUTHOR]

It is very important to predict the blasting fragmentation size of fractured rock mass. Based on the geological conditions of practical engineering rock mass, a plane analysis model of bench blasting in layered rock mass considering microcrack structure was established. And the distribution law of stress in the bench blasting of layered rock mass with different occurrence and layer thickness is analyzed by using the CDEM numerical simulation method. Combined with the theory of rock fracture mechanics and relevant test results, this paper quantitatively studied the change laws of the angle for the cracks to propagate, speed and crack initiation length at different particle points in bench blasting and realized the time history analysis of the crack development and rock fragmentation in bench blasting. And then, this paper derived the quantitative relationship between the post-expansion cranny ratio and the blasting fragmentation from a theoretical point of view, and proposed the prediction method of the median fragmentation in bench blasting of layered rock mass. Combined with the bench blasting productive test of Changjiu (Shenshan) limestone mine, the largest sand aggregate production mine in the world, the correctness of the theoretical results is verified. The test results demonstrate that in a vertically inclined rock bench, the degree of rock fragmentation intensifies in the horizontal direction, and the overall fragmentation effect can be optimized by adjusting the blasting hole spacing; the blasting fragmentation prediction method proposed in the paper can accurately predict the median fragmentation of layered rock, which provides a novel method for predicting the effects of bench blasting. Highlights: The prediction method for the median fragmentation of layered bench rock mass blasting is obtained The quantitative relationship between the post-expansion cranny ratio and the median fragmentation of bench blasting is theoretically derived The distribution law of microcracks in bench rock mass is simulated by discrete fracture network model. The internal force distribution characteristics are analyzed by continuous medium discrete element method The propagation law and rock breaking process of microcracks and layered cracks in bench blasting are studied At the same time, the correctness of the prediction method is verified by the field test and production of Changjiu (Shenshan) limestone mine [ABSTRACT FROM AUTHOR]

Crossbow-related injuries resulting in serious and mortal consequences have increased in recent years, and although significant research exists for both injury and fatality on the human body, limited data exists on the lethality of the bolt and the failure modes of protective materials. This paper concerns itself with the experimental validation of four differing crossbow bolt geometries, their effects on material failure and potentially lethality. During this study, four different types of crossbow bolt geometries were tested against two protection mechanisms that differed in mechanical properties, geometry, mass and size. The results show that at 67 ms−1, ogive, field and combo tips do not provide lethal effect at 10-m range, whilst a broadhead tip will perforate both the para-aramid and a reinforced area of polycarbonate material consisting of two 3-mm plates at 63–66 ms−1. Although perforation was apparent with a more honed tip geometry, the chain mail layering within the para-aramid protection and friction caused by polycarbonate petalling on the arrow body reduced the velocity enough to demonstrate the materials under test are effective at withstanding crossbow attack. Subsequent calculation of the maximum velocity that arrows could achieve if fired from the crossbow within this study shows results close to the overmatch value of each material and therefore a requirement to advance the knowledge in this field to influence the development of more effective armour protection mechanisms. [ABSTRACT FROM AUTHOR]

Semiconductor advancements demand greater integrated circuit density, structural miniaturization, and complex material combinations, resulting in stress concentrations from property mismatches. This study investigates the failure in two types of interfaces found in chip packages: silicon–epoxy mold compound (EMC) and polyimide–EMC. These interfaces were subjected to quasi-static and fatigue loading conditions. Employing a compliance-based beam method, the tests determined interfacial critical fracture energy values, ( G I C ), of 0.051 N/mm and 0.037 N/mm for the silicon–EMC and polyimide–EMC interfaces, respectively. Fatigue testing on the polyimide–epoxy interface revealed a fatigue threshold strain energy, ( G t h ), of 0.042 N/mm. We also observed diverse failure modes and discuss potential mechanical failures in multi-layer chip packages. The findings of this study can contribute to the prediction and mitigation of failure modes in the analyzed chip packaging. The obtained threshold energy and crack growth rate provide insights for designing safe lives for bi-material interfaces in chip packaging under cyclic loads. These insights can guide future research directions, emphasizing the improvement of material properties and exploration of the influence of manufacturing parameters on delamination in multilayer semiconductors. [ABSTRACT FROM AUTHOR]

A method for detecting low-velocity impact damage in carbon fiber reinforced polymer (CFRP) is presented. It involves the use of the Impulse Excitation Technique (IET) and hysteresis loops to calculate the damping parameter of T700/NCT304-1 carbon/epoxy samples subjected to various low-velocity impact energies. The value of the coefficient of restitution (COR) is determined for each impact, ranging between 0.62 for the lowest impact energy to 0.48 for the highest one. The results reveal that a three-step increase in the damping parameter exists in all cases as the impact energy on the specimen increases. An abrupt jump in the damping parameter value is observed for impact energies exceeding ∼0.9 of the material's maximum capacity. Overall, at the highest impact energy equal to 3.65 J, the damping parameter increased by 43.3% compared to the pristine specimen. Additionally, two cases of cyclic tension-tension loading were applied to the specimens, with maximum stresses set at 150 MPa and 200 MPa. The measured values of plastic and elastic strain energy were used to determine the damping ratios. For both cases, the damping of the specimen subjected to the highest impact energy was ∼1.2 times greater than that of an intact specimen, with an increase pattern similar to the findings of the IET method. Optical microscope images of the specimens are provided to illustrate various damage modes observed in the composite materials. [ABSTRACT FROM AUTHOR]

In this study, adhesive bonding of aluminum (Al) to glass fiber reinforced polymer (GFRP) was investigated using finite element analysis to optimize bond strength. Mechanical surface preparation has a great influence on the chemical properties (increasing surface energy and creating a stronger bond) and mechanical properties (creating mechanical interlocking and increasing friction) of adhesive bonding. Hence, the response surface method was employed to examine the influence of groove number (1, 3, 5), groove angle (0, 45, 90°), groove shape (V-shape, square, concave), and joint type (metal–metal, metal–composite, composite–composite) on the tensile strength of the bond. To simulate the bond behavior of Al/GFRP under different parameter conditions, the cohesive zone model was used to consider the crack growth. Optimization results obtained by the desirability function method showed that the maximum bond strength was achieved with a groove number of 1, groove shape of square, groove angle of 0°, and metal–metal joint type. The optimization results predicted by the desirability function and finite element analysis were in good agreement with those obtained by experimental tests. [ABSTRACT FROM AUTHOR]

Fracture is one of the failure modes of a material or product which may occur during its manufacturing process or its usage. Fractography is the science and art of examination and analysis of the fractured surface. Texture and nature of the fractured surface provide valuable information on origin of a crack and crack propagation; thereby, it helps to understand the mechanism of fracture and facilitate to identify the likely causes of failure. This review covers an extensive study on various types of fractures of steel cord and bead wire which are primarily used in rubber composites viz tyre, conveyor belt etc. Different types of fractures studied include ductile, central burst, brittle, fatigue, torsion, corrosion, delamination and weld fractures. Fundamentals on specific characteristics of each type of fracture and their likely causes have been elaborately discussed Microscopic examination through scanning electron microscope (SEM) provides deeper insights of the fractured surface as well as microstructure of material. In addition to the fundamental aspects of fractures, examples of different types of fractured samples have been illustrated in this article. This focused review would be useful to the steel cord and bead wire manufactures as well as to tyre manufacturers for detailed failure investigation of material/product and thereby to take appropriate corrective measures. Graphical Abstract [ABSTRACT FROM AUTHOR]

In this study, AA6061-T651 Aluminium alloy was joined by gas metal arc welding (GMAW) and friction stir welding (FSW) processes. Both joints were compared and discussed in terms of their microstructure and fatigue properties. The microstructural evolution of the weldment was examined using optical microscopy (OM) and orientation image microscopy (OIM). FSW joints have finer grains than GMAW joints, which results in higher hardness. In GMAW and FSW joints, the tensile strength was 195 MPa and 230 MPa, respectively. This is 62% and 74% of the base metal. A fully reversible fatigue testing machine was used to investigate the fatigue properties of both joints. In comparison with GMAW joints, FSW joints have significantly better fatigue properties and reach infinite life at 90 MPa. Fractographic analysis shows that FSW joints had fewer crack initiation sites in stage I and better crack growth resistance in stage II than GMAW joints. [ABSTRACT FROM AUTHOR]

The grout pressure is one of the most significant parameters during grout injection into a jointed rock mass. Applying a low injection pressure reduces the penetration length and causes poor injection, while applying a high injection pressure leads to unwanted deformations and instability of joints during grouting operations. In this study a new analytical approach was developed and examined and is proposed to predict the critical grout pressure by assessing the stability of cross-joints with grout boreholes in an anisotropic rock mass. Joint stability was measured based on the maximum energy release rate (MERR) criterion. The critical injection pressure is the pressure at which the joint approaches the instability threshold. The study area for validation of the proposed method was the anisotropic rock mass of Sanandaj Azad Dam. To collect the required data, a series of laboratory tests were performed, including the uniaxial compression test to determine the mechanical properties of rock and the Brazilian disk test to determine the fracture toughness of rock on specimens with anisotropy directions of 0°, 30°, 45°, 60°, and 90° relative to the loading direction. The results show a good agreement between injection pressures predicted using the proposed method and data recorded during grouting operations at different depths. Moreover, to study the anisotropy impact on the predicted grout pressure, a horizontal joint was evaluated at the prescribed depths with different anisotropy angles relative to its direction. The results suggest that increasing the anisotropy angle is directly related to changes of the critical injection pressure. Accordingly, a higher anisotropy angle relative to the crack growth direction leads to a greater critical grout pressure. [ABSTRACT FROM AUTHOR]

The crack propagation law of pre-cracked rock has an essential influence on the overlying rock movement and the stability of roadway surrounding rock during mining. The surrounding rock stability of the mining field is severely affected by the mining pressure. However, the growth of pre-cracked rock cracks has a great influence on the mining pressure. So laboratory experiments and numerical simulations are used to study the crack propagation law and mechanism of pre-cracked rock with different pre-cracked rates. The results indicate that as the cutting rate gradually increases, the peak shear stress strength of the specimen decreases, and the time required for crack propagation from the beginning of loading decreases. Moreover, when the specimen contains a single-sided pre-crack, cracks initiate and propagate in both directions under shear stress, eventually forming a circular crack region. The size of the circular crack region gradually decreases with the increase of the cutting rate. This suggests that as the cutting rate increases, the influence of the fracture process on the integrity of the roadway roof gradually weakens. In the numerical simulation, microcracks develop in the specimen under the action of the shear stress, and the microcracks number decreases with the increase of the cutting rate. The numerical simulation results show good consistency with laboratory experimental results. The research results have important reference value for the stability control of gob-side entry and the popularization and application of non-pillar mining technology. [ABSTRACT FROM AUTHOR]

In deep tunnelling, mining and subsurface energy recovery, the reliable estimate of rough fracture (or fault) strength and the potential for reactivation are of vital importance for the assessment of dynamic geo-hazard, such as fault slip rock-burst and induced earthquakes. In this study, true triaxial loading tests were conducted on pre-fractured granite with different orientations to the maximum principal stress, and the fracture slip process was studied with the aid of acoustic emission and deformation monitoring. The reactivation strength was also compared with the theoretical predictions based on the analytical model. Results show that the critical fracture angle for the rock matrix failure and original macro-fracture reactivation is ~ 51°, above which fault slip occurs along the original macro-fracture at a gradually smaller differential stress. The microscopic analysis indicates that new faulting develops traversing the original fracture which becomes more compacted and closed with insignificant damage when the fracture angle is below ~ 44°. The differential stress required for rough fracture reactivation is well predicted by the single plane of weakness theory. In addition, variation of acoustic emission signals (especially AE energy) and deformation along σ3 direction with loading time are very consistent, which can be used to analyse the preparatory and evolutionary process of the fracture reactivation. The stress rate decreases whilst deformation rate increases both from a steady state value to a very large value as the fracture is gradually reactivated, accompanied by the b value decreasing from 1.4–2 to 0.4–0.6, which can be used as precursors for the dynamic fault slip. The findings in the present study will provide new insights into the mechanics of rough fracture reactivation in deep tunnelling, mining and underground resource extraction. Highlights: True triaxial loading tests are conducted on pre-fractured granite with different fracture orientations to the maximum principal stress. Three different failure modes are revealed for pre-fractured granite specimens with increasing fracture angles from 39° to 57°. The original macro rough fracture is reactivated when the fracture angle is above ~50°. The obtained fracture reactivation strengths are very consistent with the theoretical prediction by the single plane of weakness theory. Acoustic emission hits (counts) and deformation along σ3 are effective indicators to show the preparatory process before fracture reactivation. [ABSTRACT FROM AUTHOR]

In many engineering applications, it is essential to have information about rocks that inherently contain preexisting flaws under dynamic loading conditions. Dynamic impact tests are conducted on samples with varying flaw angles using the split Hopkinson pressure bar (SHPB) test system and the Digital Image Correlation system (DIC). The characteristics of the samples after dynamic loading, including dynamic strength, energy dissipation, and fractal fracture, are compared and analyzed. As the flaw angle increases, the peak stress and strain exhibit a typical V-shaped pattern, reaching the minimum value at 30°, and the initial initiation position shifts from the flaw tips to the middle of the flaw. Failure modes can be divided into three modes depending on the flaw angle. The progressive failure process, taking into account the heterogeneity of the rock, is demonstrated by developing an elastic damage constitutive model that uses dynamic compression and tensile tests to parameterize it. As the flaw angle increases, the initial damage zone also moves from the flaw tips to the middle of the flaw. Failures around the hole with redistributed stress are observed, and the failure mechanisms can be explained with the aid of strain energy density (SED). Using fracture mechanics, the analytical solution of stress around the flaw is provided, and the variation of crack initiation angle, stress distribution, and energy dissipation under different flaw angles is theoretically explained, which is in good agreement with the experimental and simulated results. [ABSTRACT FROM AUTHOR]

In the present study, the effects of angle and brittleness of interlayers on the shear failure behavior of notched bedding rock have been scrutinized using experimental shear tests and particle flow code (PFC) simulation. Notched bedding models with dimensions of 20 cm × 24 cm × 5 cm containing soft interlayer and hard interlayer were prepared. The ratio of compressive strength to tensile strength in soft gypsum and hard gypsum are 12 and 7.8, respectively. The layer angel changed from 0° to 90° with an increment of 15°. The lengths of notches in each model are similar and were equal to 20 mm, 40 mm, and 60 mm. Models were tested by Punch shear by displacement loading rates of 0.05 mm/min. Results showed that pure tensile fracture was developed from the tip of the notch, and propagated in the direction of the shear loading axis till coalescence with the model boundary. Whereas soft brittle gypsum has less deformability in comparison to hard ductile gypsum therefore the continuity of shear displacement associated with crack growth in soft interlayer was less than that in hard interlayer. Also, soft brittle gypsum has less shear strength in comparison to hard ductile gypsum therefore the shear strength of bedding rock has maximum value when hard ductile gypsum was occupied more percentage of shear surfaces. The failure mechanism was alike in both the numerical simulation and the experimental test. [ABSTRACT FROM AUTHOR]

• Put forward to the evaluation index of durability of brittle rock. • Explain the phenomenon of tensile failure of brittle rock under compression. • Establish a new rock durability prediction expression with more explicit physical meaning. • Analyse the rule between the durability index and rock types. When subjected to long-term static loading, with the accumulation of damage and fracture, brittle rock would fails when the static loading is lower than the compressive strength of the rock, and the increasing of the static loading can generally shorten the time required for the brittle rock failure. To explore the relationship between the durability of the brittle rock and the applied static loading, in this paper, the propagation modes of the microcracks in brittle rock under the static loading in the subcritical crack growth stage are uniformly attributed to the tensile mode, and the total time required for microcrack propagation in the subcritical crack growth stage is chosen as the criterion for judging the durability index of brittle rock. Based on the Maxwell model, the local effective tensile stress that drives crack propagation is derived. The phenomenon of brittle rock exhibiting tensile failure under the compressive stress is preliminarily explained, and the corresponding requirements for this phenomenon to occur are also presented. A new expression of rock durability prediction with a clear physical meaning is established. The key parameter, i.e., the durability index " n " in the newly derived expression is obtained through fitting, and the relationship between the durability index and the rock type is preliminarily analysed. This study may provide an approach for preliminarily estimating the durability of brittle rock. [ABSTRACT FROM AUTHOR]

The precise prediction of surface subsidence under mining influence is significant for ecological environment and personal safety. This paper presents a numerical approach for surface subsidence analysis based on a case study conducted at Gucheng mine, China. Considering the rock mechanical characteristics of fractured zone and caved zone, the tension-weakening model and the double-yield model with back-analyzed properties are implemented into FLAC3D to simulate the mining-induced fracture weakening and the goaf compaction process, respectively. The reasonable goaf material and the relationship between the intensity of fractures and the residual stiffness of rock mass are built in the numerical model. An analysis of tension-weakening and double-yield models is carried out and the simulated results are compared to strain-softening model. It is shown that the combination of tension-weakening and double-yield models provides a more realistic simulation of surface subsidence than strain-softening model. The numerical approach can be utilized for control and prediction of surface subsidence under similar geotechnical conditions. [ABSTRACT FROM AUTHOR]

Highlights • An alternative vibration test is employed to investigate acoustic fatigue damage. • A nonlinear form of step-change downward of the fundamental frequency is confirmed. • The proposed method develops a new failure criteria for the C/SiC composite material. • Typical tensile failure mode is confirmed for specimen excited in its first cantilever mode. Abstract As an alternative method of acoustic experiment, vibration tests of C/SiC thin-wall plates were conducted to study the random fatigue behavior. The vibration specimen was clamped with cantilever boundary and subjected to limited-bandwidth vibration excitation. The strain history and fundamental frequency of the specimens were monitored during the vibration test. It was found that the measured strain value and strain amplitude value accorded with Gauss and Rayleigh distribution, respectively. The fundamental frequency decreased greatly when the vibration specimen failed, which could be recognized as failure criterion for the specimen under random loading. The microscopic fracture morphologies showed that the majority failure mode of fibers in the longitudinal and transverse directions were fiber pull-out and fiber splitting, respectively. The tensile force induced by reversed bending load is the main driving force for damage in specimen. [ABSTRACT FROM AUTHOR]

Abstract Gas sorption in coal, in particular CO 2 , is known to cause swelling and internal structural change or damage. For instance, CO 2 adsorption on coal causes swelling which in turn can close the fractures or cleats whereas its desorption can open pre-existing fractures as well as create new fractures, thus enhancing gas flow paths in coal seams. Gas release and the associated induced damage are relevant for several applications such as coal burst in coal mining operations and for geological sequestration of CO 2 into coal seams to reduce both the carbon footprint and increase the efficiency of energy extraction. However, the sorption-induced internal structural damage in coal, in particular the link between the micro-scale damage phenomena to macro-scale processes in confined and unconfined stress states, is not yet fully understood. We carried out a series of experiments to study the potential sorption-induced damage in both confined and unconfined stress states at micro (μm) and macro (cm) scale. 3D images, obtained by X-ray microcomputed tomography (micro-CT) technique, showed that in the unconfined stress state, adsorption of CO 2 closes some pre-existing fractures while new fractures form in the specimen of coal. This is well supported by high pressure CO 2 adsorption analyses that were independently conducted. After CO 2 release, the overall fracture intensity, defined as the area of fractures per volume of rock mass, was considerably increased by opening both pre-existing fractures together with new fractures. In the confined condition, the gas adsorption was significantly lower confirming the closure of the initial fractures by swelling without creation of induced fractures. However, the gas release under shearing stress was significantly higher showing that the shearing stress causes further damage assisted by gas release and its induced micro-structural damage. Highlights • Multi-scale sorption-induced damage in coal in confined and unconfined stress states was studied. • Micro-CT was able to map the changes in the fracture system due to CO 2 sorption. • Gas release is unlikely to cause coal failure but it can induce micro-fractures. • Gas release can cause coal failure if the sample is under shearing stress. • Macroscopic tensile failure is dominant when sample is failed by gas release. [ABSTRACT FROM AUTHOR]

Characterizing a coal from an engineering perspective for design of mining excavations is critical in order to prevent fatalities, as underground coal mines are often developed in highly stressed ground conditions. Coal pillar bursts involve the sudden expulsion of coal and rock into the mine opening. These events occur when relatively high stresses in a coal pillar, left for support in underground workings, exceed the pillar's load capacity causing the pillar to rupture without warning. This process may be influenced by cleating, which is a type of joint system that can be found in coal rock masses. As such, it is important to consider the anisotropy of coal mechanical behavior. Additionally, if coal is expected to fail in a brittle manner, then behavior changes, such as the transition from extensional to shear failure, have to be considered and reflected in the adopted failure criteria. It must be anticipated that a different failure mechanism occurs as the confinement level increases and conditions for tensile failure are prevented or strongly diminished. The anisotropy and confinement dependency of coal behavior previously mentioned merit extensive investigation. In this study, a total of 84 samples obtained from a Utah coal mine were investigated by conducting both unconfined and triaxial compressive tests. The results showed that the confining pressure dictated not only the peak compressive strength but also the brittleness as a function of the major to the minor principal stress ratio. Additionally, an s-shaped brittle failure criterion was fitted to the results, showing the development of confinement-dependent strength. Moreover, these mechanical characteristics were found to be strongly anisotropic, which was associated with the orientation of the cleats relative to the loading direction. [ABSTRACT FROM AUTHOR]

A quantitative damage model of ceramic coating systems was developed based on their load-displacement curves obtained from three-point bending tests. According to the energy mechanism of damage, the normalized damage rate of such systems can be simply expressed using the load and the tangent slope of their load-displacement curves. The experimental results demonstrated the thickness dependence of fracture and damage. In thin coating systems, tensile failure was found to be predominant and multiple transverse cracks appeared in the coatings. In contrast, thick coating systems showed a predominance of interface shear failure and the occurrence of interface delamination. These observations are consistent with previous experimental results. The damage of the systems displayed catastrophic characteristics when the load tended to reach the failure point, i.e., the damage increased rapidly, and the damage rate displayed a power-law singularity at the failure point. These results are consistent with the damage characteristics predicted using the mathematic model. The damage evolution in the case of interface delamination in the thick coating systems was faster than that for transverse cracking in the thin coatings because of the difference in the degree of damage localization. The present model provides an effective method to elucidate the damage behavior of brittle ceramic coating systems, and hence, it is expected to greatly aid the coating design. [ABSTRACT FROM AUTHOR]

Fatigue delamination damage is one of the most important fatigue failure modes for laminated composite structures. However, there are still many challenging problems in the development of the theoretical framework, mathematical/physical models, and numerical simulation of fatigue delamination. What is more, it is essential to establish a systematic classification of these methods and models. This article reviews the experimental phenomena of delamination onset and propagation under fatigue loading. The authors reviewed the commonly used phenomenological models for laminated composite structures. The research methods, general modeling formulas, and development prospects of phenomenological models were presented in detail. Based on the analysis of finite element models (FEMs) for laminated composite structures, several simulation methods for fatigue delamination damage models (FDDMs) were carefully classified. Then, the whole procedure, range of applications, capability assessment, and advantages and limitations of the models, which were based on four types of theoretical frameworks, were also discussed in detail. The theoretical frameworks include the strength theory model (SM), fracture mechanics model (FM), damage mechanics model (DM), and hybrid model (HM). To the best of the authors' knowledge, the FDDM based on the modified Paris law within the framework of hybrid fracture and damage mechanics is the most effective method so far. However, it is difficult for the traditional FDDM to solve the problem of the spatial delamination of complex structures. In addition, the balance between the cost of acquiring the model and the computational efficiency of the model is also critical. Therefore, several potential research directions, such as the extended finite element method (XFEM), isogeometric analysis (IGA), phase-field model (PFM), artificial intelligence algorithm, and higher-order deformation theory (HODT), have been presented in the conclusions. Through validation by investigators, these research directions have the ability to overcome the challenging technical issues in the fatigue delamination prediction of laminated composite structures. [ABSTRACT FROM AUTHOR]

In this work, we present a peridynamic-based simulation method for modeling quasi-static fracture propagation in isotropic and anisotropic rock within the framework of peridynamic least square minimization (PDLSM). The original isotropic elastic PDLSM is further extended to investigate fracture propagation in anisotropic materials in this study. The proposed AN-PDLSM model integrates an anisotropic model in fracture mechanics to analyze the failure process of anisotropic rocks. An important advancement in this research lies in the incorporation of the maximum energy release rate criterion (MERR) into the PDLSM framework for the first time. This enhancement enables accurately determining crack propagation and the associated crack angles. The proposed model utilizes the energy release rate calculated through the J-integral method to assess bond breakage, and it employs a mesh-independent, piecewise linear fracture model to describe crack propagation. The proposed method fully combines the merits of traditional fracture mechanics with the unique capabilities of peridynamics. To demonstrate the effectiveness of the proposed model, a simulation of fracture evolution in isotropic plates subjected to semi-circular bending tests is presented and compared with experimental results. It is shown that the proposed model accurately replicates fracture trajectories in isotropic specimens. In the context of anisotropic rock, the effect of a weak coefficient on crack morphology is discussed in order to obtain a suitable value. Additionally, the impact of bedding angles on fracture paths through our proposed model is also explored, revealing excellent agreement with experimental results. The findings in this research demonstrate that the proposed AN-PDLSM model is exceptionally proficient at capturing the intricate, oscillating crack paths observed in anisotropic rock materials. [ABSTRACT FROM AUTHOR]

Failure of many brittle materials and structures can be modeled using interface‐oriented finite elements combined with intrinsic cohesive zone models. The discontinuous Galerkin (DG) finite element method provides an innovative framework for modeling brittle crack propagation with zero‐thickness interface elements, which can accommodate extrinsic cohesive laws to avoid the artificial compliance required in intrinsic cohesive models. However, robust formulations and implementations of DG methods are critical in alleviating the well‐known convergence issues for both crack nucleation and propagation with reduced instability. This paper presents a robust interface element formulation by modifying the incomplete interior penalty Galerkin (IIPG) method, which successfully avoids the initial element interface penetration across elements that occurs prior to crack nucleation, and thereby greatly reduces the instability issue as cracks open. We further verified and validated our implementation by using a bar tension test and a beam fracturing benchmark. The robustness of our proposed interface element method was demonstrated by a micromechanics fiber/matrix debonding problem with 64 fibers embedded in a bulk matrix. [ABSTRACT FROM AUTHOR]

To reveal the failure mechanism of fractured sandstone under the influence of the infilling material strength, uniaxial compression tests, and PFC3D numerical simulation analyses were conducted on fractured sandstone with different infilling material strengths, and the influence of the infilling material strength on the basic mechanical properties of the fracture‐filled sandstone was studied. The results showed that, with increasing infilling material strength, the peak strength and elastic modulus of the specimens increased nonlinearly and eventually stabilized. The failure pattern of the specimens changed from shear to tensile. When the infilling material strength was low, a wing crack or anti‐wing crack was generated; however, when the infilling material strength was high, a vertical tensile crack was generated. From the perspective of fracture mechanics, the filling of sandstone cracks can reduce or even suppress the stress concentration effect at the prefabricated crack tip and improve the bearing capacity of the fractured sandstone. Highlights: The basic mechanical properties of the fracture‐filled sandstone specimens were obtained.PFC3D numerical simulation analyses were conducted on fractured sandstone.The influence mechanism of filling materials on the fractured rock was revealed. [ABSTRACT FROM AUTHOR]

To investigate the influence of geometric position distribution and interaction mechanisms of internal defects of the loaded coal body on its mechanical behavior, this study conducted a uniaxial loading test on double-fractured coal samples with different inclination angles of rock bridges. The compression failure process was scanned in stages using computed tomography equipment, and the dynamic evolution law and deformation damage characteristics of the internal fractures of the loaded coal samples were qualitatively analyzed by image processing technology, three-dimensional reconstruction technology, and digital volume correlation methods. Meanwhile, the PFC3D simulation program was used to realistically restore the stress field distribution characteristics of different stages of the loading process. Based on the experimental method and linear elastic fracture mechanics theory, the fracture characteristics of fractured coal in a three-dimensional state were discussed. The results show that: (1) With the increase of the inclination angle of the rock bridge, the macroscopic intensity of the coal sample gradually decreased, and the law was obvious. (2) Under uniaxial compression conditions, the internal microcrack development of coal samples with double fractures was mainly tensile cracks, supplemented by shear cracks. The number of tensile crack growth angles parallel to the loading direction is relatively large, and the development direction of shear microcracks is mainly concentrated in the direction parallel to the prefabricated fracture plane and the rock bridge dip angle. (3) In the early stage of loading coal samples containing fractures, the force form from the edge of the sample to the central area is first reduced and then increased. The force at the edge of the sample was the largest, and the stress distribution was more uneven, with a greater difference. When the sample reached the failure stage, the force form transformed into the middle area with the smallest force and the transition zone with the largest force. (4) The X, Z-direction average strain ratio of coal samples with different rock bridge inclination angles gradually increased with the increase of the square area, whereas the Y-direction strain ratio of coal samples with rock bridge inclination angles of 0° and 90° increased first and then stabilized, and those with rock bridge inclination angles of 30° and 60° showed a trend of first decreasing and then stabilizing. (5) The type I stress intensity factor of each crack tip was always greater than the type II and type III stress intensity factors, with a significant difference. With the increase of the inclination angle of the rock bridge, the type I stress intensity factor first decreased and then increased, while the type II stress intensity factor showed a trend of first increasing and then decreasing. Compared with the inner crack tip, the outer crack tip of coal samples with the same rock bridge inclination angle was more likely to crack. Highlights: From a mesoscopic perspective, it further revealed the form and mechanism of mutual influence between fractures under different rock bridge inclinations. The study investigated the internal strain evolution law and stress propagation path of a fracture-bearing coal body at a three-dimensional scale. Make up for the current research shortfall in accurately solving the three-dimensional fracture tip stress intensity factor through experimental methods. [ABSTRACT FROM AUTHOR]

Coral reef limestone (CRL) is a marine sedimentary rock dominated by biogenesis, and its composition as well as internal structure characteristics are different from traditional terrigenous rocks. This article used CT scanning and petrographic thin section observations to visually and quantitatively analyze crack morphological characteristics after triaxial shear failure at a microscopic scale. The analysis revealed the mechanism of crack initiation and propagation from the perspectives of geological genesis and fracture mechanics. Results show that the crack propagation mechanism in the bio-cemented framework includes propagation along crystal boundaries and across crystals. Cracks can be classified as intergranular cracks, transgranular cracks, and bifurcation cracks based on crack morphology. The destruction mode of the specimen is controlled by the pore structure and cemented framework. Based on the pore distribution, the destruction mode can be divided into local tension failure and monoclinic shear failure. Understanding CRL crack propagation mechanisms has important scientific significance and practical engineering value for enriching rock damage theory and long-term stability analysis of island reef engineering foundations. [ABSTRACT FROM AUTHOR]

In order to obtain the characteristics of the effects of cyclic impact loading on the damage of coal-rock in the presence of a local static load constraint, the evolution of the damage factor and the fracture rate during the process and incremental cyclic impact on raw coal and briquettes has been studied. Experimental results show that the presence of local static load restraint improves the impact resistance of the coal-rock, and the damage factor of the coal-rock shows obvious zoning characteristics. When the coal-rock is in an elastic state, the partition with a larger static load restraint area has stronger impact resistance, when the coal-rock is in a plastic state, the partition with a larger static load restraint area has a weaker impact resistance. Increasing impulsive cyclic impacts have a higher damage efficiency to coal-rock than constant impulsive cyclic impacts. The difference in rock breaking efficiency between the two cyclic impact methods is mainly reflected in the partition with the largest constrained area. The crack propagation on the coal-rock surface is more consistent with the partition characteristics of the damage factor. When the static load constrained zone is in an elastic state, the static load has an inhibitory effect on the crack growth. When the static load confinement zone is in a plastic state, the cracks mainly propagate in the static load confinement zone, and the constrained zone mainly consists of tensile cracks that grow in the vertical direction, while the cracks in the non-constrained zone mainly grow in an oblique direction. Finally, fracture mechanics was applied to analyze the failure type of the sample. [ABSTRACT FROM AUTHOR]

In order to explore the secondary bond anchorage performance between prestressed tendons and concrete after the fracture of steel strands in post-tensioned, prestressed concrete (PPC) beams, a total of seven post-tensioned, prestressed concrete specimens with a size of 3 × 7ϕ15.2 mm were constructed firstly, and the steel strands at the anchorage end were subjected to corrosion fracture. Then, the pull-out test of the specimens was conducted to explore the secondary anchorage bond mechanism of the residual stress of prestressed tendons experiencing local fracture. Moreover, the influences of factors such as the embedded length, release-tensioning speed, concrete strength, and stirrup configuration on anchorage bond performance were analyzed. Finally, the test results were further verified via finite element analysis. The results show that the failure of pull-out specimens under different parameters can be divided into two types: bond anchorage failure induced by the entire pull-out of steel strands and material failure triggered by the rupture of steel strands. The bond anchorage failure mechanism between steel strands and the concrete was revealed by combining the failure characteristics and pull-out load–slippage relation curves. The bond strength between prestressed steel strands and concrete can be enhanced by increasing the embedded length of steel strands, elevating the concrete strength grade, and enlarging the diameter of stirrups so that the specimens are turned from bond anchorage failure into material failure. [ABSTRACT FROM AUTHOR]

Discrete element models are being increasingly applied to model rock failure processes. Bonded-particle models, based on circular or spherical particle systems, have been successfully used for two decades. More recently, bonded-block models, using polygonal or polyhedral elements, have proven to be a powerful alternative. This paper describes the basis of the application of these models in the numerical simulation of failure in rock materials. The critical governing parameters are identified, and their influence is discussed. The model calibration procedure based on the analysis of laboratory tests is discussed. An application example of an underground excavation problem is presented using a simple bonded-block model employing rigid blocks and a bilinear softening contact model. The results show the capability of this approach to reproduce observed failure modes involving block fractures. [ABSTRACT FROM AUTHOR]