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The existence of cracks is a key factor affecting the strength of concrete. However, traditional numerical methods still have some limitations in the simulation of crack growth in fissured concrete structures. Based on this background, the numerical treatment method of particle failure in smoothed particle hydrodynamics (SPH) is proposed, and the generation method for concrete meso-structures under the smoothed particle hydrodynamics (SPH) framework is developed. The concrete meso-models under different pre-existing micro-fissure inclinations and bridge angles (the inner tip line of the double pre-existing micro-fissure is defined as a bridge, and the angle between the bridge and the horizontal direction is defined as the bridge angle) were established, and numerical simulations of the crack propagation processes of concrete structures under tensile stress were carried out. The main findings were as follows: The concrete meso-structures and the pre-existing micro-fissures all have great impacts on the final failure modes of concrete. The stress–strain curve of the concrete model presents four typical stages. Finally, the crack initiation and propagation mechanisms of fissured concrete are discussed, and the application of smoothed particle hydrodynamics (SPH) in crack simulations of fissured concrete is prospected. [ABSTRACT FROM AUTHOR]

Complex coupled thermo-hydromechanical (THM) loading paths are expected to occur in clay rocks which serve as host formations for geological radioactive waste repositories. Exothermic waste packages heat the rock, causing thermal strains and temperature induced pore pressure build-up. The drifts are designed in such a way as to limit these effects. One has to anticipate failure and fracturing of the material, should pore pressures exceed the tensile resistance of the rock. To characterise the behaviour of the Callovo-Oxfordian claystone (COx) under effective tension and to quantify the tensile failure criterion, a laboratory program is carried out in this work. THM loading paths which correspond to the expected in situ conditions are recreated in the laboratory. To this end, a special triaxial system was developed, which allows the independent control of radial and axial stresses, as well as of pore pressure and temperature of rock specimens. More importantly, the device allows one to maintain axial effective tension on a specimen. Saturated cylindrical claystone specimens were tested in undrained conditions under constrained lateral deformation and under nearly constant axial stress. The specimens were heated until the induced pore pressures created effective tensile stresses and ultimately fractured the material. The failure happened at average axial effective tensile stresses around 3.0 MPa. Fracturing under different lateral total stresses allows one to describe the failure with a Hoek–Brown or Fairhurst's generalized Griffith criterion. Measured axial extension strains are analysed based on a transversely isotropic thermo-poroelastic constitutive model, which is able to satisfactorily reproduce the observed behaviour. Highlights: Thermal pressurization of clay rock in deep radioactive waste repositories can reduce the effective stresses, which can lead to damage or failure. Our novel laboratory triaxial device is able mimic in situ conditions: Constant vertical total stress, zero lateral deformation and thermal pressurization. Pore pressure increase, vertical extension strains and thermal pressurization failure were recorded in a series of tests on Callovo-Oxfordian claystone specimens. The effective tensile strength was reached at values around 3 MPa in tension and temperatures between 53 and 64 °C, creating sub-horizontal fractures. The experimental responses can be well reproduced using a thermo-poroelasticity model. Hoek–Brown and Fairhurst generalized Griffith criteria appear suitable to account for the rock's tensile resistance. [ABSTRACT FROM AUTHOR]

Purpose: The influence of the number and arrangement of bolts on the tensile properties of bolted composite laminates was studied in the present study. Design/methodology/approach: Based on the finite element model, the stiffness degradation method is used to simulate the damage evolution process for the failure of bolted composite laminates. Using ABAQUS finite element software combined with material failure criteria, the numerical calculation of the connection strength and failure mode of bolted composite laminates was carried out. Findings: The results of the study show that the tensile strength of the composite laminates connected by three bolts is higher than that of the laminates connected by two bolts. And the arrangement of different bolts has a great influence on the failure strength of bolted laminates. Originality/value: Bolted connection of composite laminates is a common problem in engineering practice. The effect of bolt arrangement and number on the strength of composite laminates is studied in this manuscript. [ABSTRACT FROM AUTHOR]

• Efficient strain predictions are achieved with DeepLabv3+, enhancing the speed and accuracy of structural analysis. • Rapid and precise failure mode determinations are made possible by VGG-16, employing focal loss for efficient classification. • Accurate interpretability is ensured through Grad-CAM, aligning neural focus areas with fracture mechanics theory. • The method significantly improves efficiency, providing quick and confident results without the need for labor-intensive re-modeling during the FEA process. The joint is a critical component that facilitates force transmission across precast bridge slabs while maintaining structural integrity. Finite Element Analysis (FEA) is commonly used to examine the mechanical properties and load-bearing capacity of joints with different configurations. However, the repetitive process of "modeling-meshing-computing" consumes substantial manpower, time, and expertise. This study proposes an approach consisting of three key steps: (1) predicting strain distributions, (2) determining failure modes, and (3) analyzing interpretability of predicted results. Specifically, DeepLabv3+ is used to map the Finite Element (FE) model to strain distributions. A binary classification model based on VGG-16 predicts the failure state of the specimen. Interpretability analysis, using Gradient-weighted Class Activation Mapping (Grad-CAM), provides insights into the neural network's decision-making process. The optimized models achieve a Mean Intersection over Union (MIoU) of 0.5691 for strain distribution prediction on the cross-sectional surface and 0.6424 on the front surface. The failure detection model achieves an F1 score of 0.992 in determining failure states. The interpretability results align well with fracture mechanics theory, enhancing confidence in the model's predictions. This method serves as a surrogate to reduce the workload of finite element analyses, facilitating strain analysis and failure detection. [ABSTRACT FROM AUTHOR]

The article discusses the additive manufacturing, or three-dimensional (3-D) printing, of stainless steel, with a particular focus on the visualisation of the tensile failure during additive manufacturing. An overview of the fracture behavior of the stainless steel during Kolsky bar loading is provided.

A common problem encountered in rock engineering involves the specification of strength anisotropy in transversely isotropic rocks such as slate and schist. The strength anisotropy under compression has been relatively well established, whereas the formulation of the tensile strength anisotropy has received less attention due to the difficulty of experimental validation. In this paper, a recently developed 3-D tensile failure function for transversely isotropic rocks is further examined. The failure condition incorporates three strength parameters and employs a 2nd order tensor, whose principal directions are those of material fabric, to describe the spatial variation of the tensile strength. It is shown that both the single plane of weakness theory of tensile failure and Nova and Zaninetti's failure condition are two special cases of the 3-D criterion in the context of the spatial distribution of the tensile strength. A procedure for the identification of the strength parameters is outlined. The direct tensile tests on the transversely isotropic samples with various orientations are simulated by invoking the critical plane approach with the 3-D tensile failure function. The loading conditions considered in the simulation include uniaxial tension, triaxial tension, triaxial extension and true triaxial extension. The ultimate value of the axial tensile stress at failure and the associated orientation of the failure plane are discussed. [ABSTRACT FROM AUTHOR]

This paper presents a novel approach for modeling the tensile failure of quasi-brittle materials by incorporating a multivariate random field to represent material parameters in the phase field model. The aim is to characterize and propagate uncertainties in the behaviors of materials, taking into account the spatial variability and probabilistic dependence. Copula theory is employed to investigate the probability distributions and dependence configuration of the parameters. The proposed framework combines the generation approach for the multivariate random field with the phase field model, resulting in a complete numerical analysis methodology. The governing equations of the deterministic phase field model for quasi-brittle solids are outlined, followed by a description of the multivariate random field for quasi-brittle media and its uncertainty characterization procedure using copula theory. A numerical approach for generating samples of the multivariate random field is introduced. The numerical analysis procedure for the tensile fracture of concrete is presented, and the effects of material uncertainties on failure patterns and macroscopic responses are discussed. The results demonstrate that the developed methodology effectively captures the random damage and fracture processes in concrete specimens. The interaction between the correlation length of the random field and the characteristic length scale of the phase field significantly influences the probabilistic characteristics of the random responses. The research findings emphasize the importance of determining the correlation length values accurately. This study contributes to the field of structural reliability analysis by providing a comprehensive framework for modeling the stochastic behavior of quasi-brittle materials. The integration of a multivariate random field and copula theory allows for the realistic representation of material uncertainties and their probabilistic dependence. The proposed methodology, combined with the probability density evolution method (PDEM), offers a powerful tool for analyzing and predicting the probabilistic evolution of material behaviors under tensile loading conditions. [ABSTRACT FROM AUTHOR]

• For some materials and conditions failure mechanism can change from shear in Low Cycle Fatigue to tensile in High Cycle Fatigue. • A modified form of the FS parameter recently proposed by Gates and Fatemi is able to capture the failure mechanism transition. • A wide range of materials from the literature with failure mechanism change are evaluated based on the modified parameter. Depending on several factors, the failure mechanism can change with life for a particular material, from shear in Low Cycle Fatigue (LCF) to tensile in High Cycle Fatigue (HCF). Critical plane-based approaches to multiaxial fatigue have the capability of reflecting the damage mechanisms of the material and can, therefore, not only provide fatigue life estimation under different stress states, but also predict the highest damage (i.e., critical) plane orientation. While Fatemi-Socie critical plane parameter has been used extensively in the literature for many materials with shear failure mechanism, a modified form of the parameter recently proposed by Gates and Fatemi is shown to also be able to capture the failure mechanism transition from shear in LCF to tensile in HCF. The mechanisms by which this parameter accounts for the transition from shear to tensile is discussed in this work. A wide range of materials from the literature with failure mechanism change from shear in LCF to tensile in HCF are evaluated based on this parameter. [ABSTRACT FROM AUTHOR]

This paper proposes a three-dimensional failure criterion model for unidirectional fiber-reinforced composites. In contrast to previous models that only accounted for the effect of fiber misalignment in the presence of longitudinal compressive stress, the proposed failure criterion model comprehensively considers the effect of localized misaligned regions on the failure under any stress state. Another key contribution of this study is the introduction of the effective misalignment angle. Considering effective misalignment angles, the proposed failure criterion model can reasonably reveal the effect of localized misaligned regions on the failure behavior under different stress states. The agreement between the predicted results and the experimental data proves that the proposed model has good applicability. Furthermore, the influence of initial misalignment angles on failure is analyzed under varying stress conditions. The results indicate that even under longitudinal tensile stress, the initial misalignment angle still plays an important role in the failure behavior of materials. [Display omitted] • A three-dimensional failure model considering fiber misalignment is proposed. • The traditional method for determining the misalignment angle is corrected. • Fiber misalignment has an effect on material failure under longitudinal tensile stress. • The influence of the initial misalignment angle on failure is analyzed. [ABSTRACT FROM AUTHOR]

An efficient computational model to simulate tensile failure of both hybrid and non-hybrid composite materials is proposed. This model is based on the spring element model, which is extended to a random 2D fibre packing. The proposed model is used to study the local stress fields around a broken fibre as well as the failure process in composite materials. The influence of fibre strength distributions and matrix properties on this process is also analysed. A detailed analysis of the fracture process and cluster development is performed and the results are compared with experimental results from the literature. [ABSTRACT FROM AUTHOR]

In this work, a cohesive zone model is applied to model fracture behavior of polycrystalline cylindrical samples under uniaxial loading conditions. The model is based on the implicit finite element method that is combined with Park-Paulino-Roesler formulation for cohesive potential. We implement an adaptive time stepping scheme that takes into account the rate of damage and failure of cohesive zones. Material properties and model parameters have been calibrated using available experimental data for laboratory-made freshwater ice samples. Simulations are performed for samples with different grain sizes, and the resulting stress-strain and damage accumulation curves are recorded. Investigation of the dependency between the grain size and fracture strength shows a strengthening effect that is consistent with experimental results. Fracture patterns observed in simulated failure events are also consistent with observations from experiments. [ABSTRACT FROM AUTHOR]

This paper investigates the influence of plastic anisotropy on the tensile ductility of a high-strength aluminium alloy. To this end, finite element simulations of smooth and notched tension tests in different material directions are performed with an anisotropic plasticity model. The stress and strain histories from these simulations are then applied in localization analyses with the imperfection band approach, using the anisotropic plasticity model outside the band and an anisotropic version of the Gurson model inside the band. The imperfection within the band is represented by a volume fraction of void nucleating particles. The high-strength aluminium alloy AA7075-T651 is considered in this study. The results show that the directional dependency of the tensile ductility of the alloy found experimentally is predicted with good accuracy using the adopted approach. The numerical study indicates that plastic anisotropy plays an important role in determining the anisotropic tensile ductility of this high-strength aluminium alloy. [ABSTRACT FROM AUTHOR]

A modified split Hopkinson pressure bar (SHPB) system is utilized to load Brazilian disc (BD) samples statically, and then exert dynamic load to the sample generated by impact. The pulse shaper technique is used to generate a slowly rising stress wave to facilitate the dynamic force balance in dynamic tests. Five groups of Laurentian granite BD samples (with static tensile strength of 12.8 MPa) under the pre-tension of 0 MPa, 2 MPa, 4 MPa, 8 MPa, and 10 MPa were tested under different loading rates. The results show that the rock dynamic tensile strength decreases with the increase of the pre-tension. It is also observed that under the same pre-tension stress, the dynamic tensile strength increases with the loading rate. However, the total tensile strength of the rock is roughly independent of the pre-tension. The failure patterns of the samples also reveal the rate dependence of the dynamic tensile strength of rocks. [ABSTRACT FROM AUTHOR]

The paper presents the experimental and numerical study of the failure behaviour of Fused Filament Fabricated (FFF) Polylactic Acid (PLA) samples subjected to tensile load. The examined samples are printed in flat orientation with 0°, 45° and 90° raster angles. During the experiments the deformation of the specimens is continuously scanned with the 3D Aramis measuring system utilizing the digital imaging correlation technique, enabling the determination of strain and stress distribution. In the modelling, it is assumed that each printed layer is a homogeneous transversely isotropic medium with the raster direction treated as the favoured one. The finite element models are developed in the Abaqus-Standard package. A two-dimensional equivalent single-layer approach is utilized to describe the deformation and stress state of the samples. The failure progress of the material is simulated by making use of the Hashin damage algorithm with energy-based softening, whereas the non-linear in-plane shear behaviour is included. • Experimental and numerical failure study of 3D printed PLA sheets subjected to tensile load is performed. • The strain field of the specimens is scanned with the Aramis system with digital imaging correlation technique. • The simulations are performed in the Abaqus program by taking the advantage of the formalisms of the laminates modelling. • Advantages and limitations of the approach are discussed. [ABSTRACT FROM AUTHOR]

• A new corrected strain-rate enhancement model is proposed by introducing a time scale. • The new model captures delayed stress response of concrete under variations of strain-rate. • The multiple cracks and crack branching in concrete are well predicted using this new model. • The limitations of frequently-used instantaneous strain-rate enhancement model are addressed. Dynamic tensile failure (e.g. spall) is frequently observed when concrete structures are subjected to blast and impact loadings, which is accompanied by the increase of tensile strength with increase of loading rate. Accurate prediction of dynamic tensile failure in concrete material is a challenging problem, in which the strain-rate effect should be properly considered. In the present study, a new corrected strain-rate enhancement model is proposed by introducing a time scale to control the evolution of strain-rate, as to capture the delayed stress response under variations of strain-rate. This corrected strain-rate enhancement model is then combined with the damage-based nonlocal model recently proposed by Kong et al. [18] to investigate the influence of strain-rate effect on numerical predictions of dynamic tensile failure. Numerical examples of a 1D spalling test demonstrate that the introduction of a time scale in the corrected strain-rate captures the delayed stress response and has a weak regularization effect. Three sets of experiments, namely, the Split-Hopkinson Tensile Bar (SHTB) test, the Modified Split-Hopkinson bar (spalling) test and the Compact Tension test, are numerically simulated using the proposed corrected strain-rate enhancement model and the frequently-used instantaneous strain-rate enhancement model. It is found that, considering the instantaneous strain-rate enhancement in the material model, the multiple cracks and crack branching cannot be well predicted, and the capability of energy absorption of concrete structures is overestimated, which may lead to an unsafe design. Promising predictions of dynamic tensile failures in concrete, including multiple cracks, crack branching and the structural resistance are obtained by the damage-based nonlocal model incorporating with the corrected strain-rate enhancement model. [ABSTRACT FROM AUTHOR]

• A new damage-based nonlocal model is proposed. • The new model resolves all the limitations of the original nonlocal model. • The new damage-based nonlocal model is validated against two sets of dynamic tensile tests. • It is demonstrated that the new model can well predict the dynamic tensile failure of concrete. When concrete structures subjected to the blasts and high velocity impact loadings, dynamic tensile failure (e.g. spall) is frequently observed. It is known accurate prediction of dynamic tensile failure is a challenging problem. In the present study, a new damage-based nonlocal model is proposed to predict the dynamic tensile failure of concrete material, which is based on the local Kong-Fang concrete material model recently proposed (Int J Impact Eng 2018, 120: 60–78). In this nonlocal model, the interaction domain decreases with the increase of local damage, which leads to a desirable description of the localization at total damaged regions and free surfaces, i.e., non-localities vanish at these regions. Numerical examples of a 1D spalling test and a 2D dynamic tension test of a concrete plate demonstrate that the damage-based nonlocal model can resolve all the limitations of original nonlocal models. The damage based nonlocal model is then validated against two experiments, namely, the Split Hopkinson Tensile Bar (SHTB) test and the Modified Split-Hopkinson bar (spalling) test. Numerical simulations show that, the proposed nonlocal model can reasonably predict the dynamic tensile failure location. But the local model cannot correctly predict the dynamic tensile failure, and the original nonlocal model cannot predict the multiple cracks under intense loading rate. [ABSTRACT FROM AUTHOR]

In this paper, failure mechanism of 20 carbon steel and QT400-15 specimens is in situ investigated by a microscope under combined torsion–tension loading (CTTL) at room temperature. Moreover, to obtain the effect of pre-torsional loading on the failure mechanism of materials under CTTL, a finite element analysis model is conducted to predict the specimen stress distribution. Results of theoretical calculation, finite element analysis, and experimental investigation indicate that the pre-torsional strain of materials is increased with an increase in the pre-torsional loading angle. The pre-torsional strain refined the material grains under CTTL, which in turn slowed down the crack growth. Consequently, material tensile strength is improved under CTTL. The in situ images effectively characterized grain refinement of materials under pre-torsional loading. Furthermore, the failure mechanism of materials under CTTL is different from the mechanism under pure tension loading, which was characterized by intergranular material fracture. However, the material fracture under CTTL was characterized by both transgranular and intergranular fractures. [ABSTRACT FROM AUTHOR]

Featured Application: The present work is conducive to focused use of the Brazilian test to obtain the tensile strength and other information of a rock specimen. The fracture behavior of the disc specimens in the Brazilian test is closely related to the reliability and accuracy of the experimental results. To comprehensively investigate the effect of various loading methods and rock material types on the failure mechanism of the Brazilian discs, five different rock types tested with three typical loading methods were employed in this work. The digital image correlation (DIC) method was applied to record and analyze the strain and displacement field of the specimens during the loading process. Experimental results indicate that the peak load and deformation characteristics of the Brazilian discs are strongly affected by the loading types. The Brazilian test with the Chinese standard is evidently not suitable for measuring the tensile strength of rocks, and the other two testing methods may lead to an invalid failure mode for rock materials with high stiffness and tensile to compressive strength ratio. Furthermore, it revealed that the maximum equivalent stress point of a disc specimen is co-controlled by the material stiffness and its tensile–compression ratio. The present work shows that it is necessary to select a suitable loading configuration for each rock type in the Brazilian test. [ABSTRACT FROM AUTHOR]

• Direct methods for the simulation of tensile failure in large composite fibre bundles. • Predicting size effects (decrease in strength with size) in large composite bundles. • Investigation on the role of dynamic stress concentrations and fracture mechanics. • Fracture Mechanics shown to be important factor for size effects on bungle strength. This paper investigates the role of dynamic stress concentrations, and of fracture mechanics-driven growth of critical clusters of fibres, on the longitudinal tensile failure of fibre-reinforced composites. For this purpose, we developed a semi-analytical fibre bundle model to simulate the longitudinal tensile failure of large composite bundles of continuous fibres. The model uses shear-lag to calculate the stress recovery along broken fibres, and an efficient field superposition method to calculate the stress concentration on the intact fibres, which has been validated against analytical and Finite Element (FE) results from the literature. The baseline version of the model uses static equilibrium stress states, and considers fibre failure driven by strength of materials (stress overload) as the only damage theory which can drive bundle failure. Like other models in the literature, the baseline model fails to capture the correct size effect (decreasing composite strength with bundle size) shown by experimental results. Two model variants have been developed which include dynamics stress concentrations and a fracture mechanics failure criterion respectively. To the knowledge of the authors, it is the first attempt in the literature to investigate these two effects in a fibre bundle model by direct simulation of large composite bundles. It is shown that, although the dynamic stress concentration significantly decreases the predicted bundle strength, it does not allow to predict the correct trend of the size effect. Finally, the results suggest that fracture mechanics may be the physical mechanism which is necessary to include to correctly predict the decreasing composite strength with bundle size shown by experimental results. [ABSTRACT FROM AUTHOR]

In this paper, the failure behavior of SiC fiber-reinforced Ti-6Al-4V matrix composite (SiCf/Ti) under longitudinal loads was studied experimentally and theoretically. Experimental results show that the interface of SiCf/Ti composite is continuous and integral. Through SEM observations, it was found that transverse matrix cracks initiated from the debonding region of the interface. A three-dimensional representative volume element was developed to simulate the failure process of SiCf/Ti-6Al-4V composite. A continuum cohesive zone model is employed to describe the debonding behavior at the interface, compared with a discrete one. More accurate simulated results were obtained by a continuum cohesive zone model, corresponding to experimental results. The transverse cracking behavior of the matrix was simulated. The stress-strain curves predicted by the continuum cohesive zone model show a good agreement with the experimental results. The fiber volume fraction, interface strength and interface toughness are investigated to exhibit their influences upon the longitudinal mechanical characteristics of SiCf/Ti-6Al-4V composite. [ABSTRACT FROM AUTHOR]

For solving the computationally intensive problem encountered by the discrete element method (DEM) in simulating large-scale engineering problems, it is essential to establish a numerical model that can effectively simulate large-scale rocks. In this study, the coarse-graining effect of a linear-Mindlin with bonding model was studied in the unconfined compression strength (UCS) and Brazilian tensile strength (BTS) tests. We found that the main reason for the coarse-graining effect of the BTS tests is that the type I fracture toughness is positively correlated with the size of the particles. Based on the results analysis and fracture mechanics, the coarse-grained (CG) modeling theory was combined with a bonded particle model (BPM) for the first time and a coarse-grained bonded particle model (CG-BPM) was developed, which can be effectively used to model the tensile strength of large-scale rocks with different particle sizes. The excavation damage zone (EDZ) in an underground research laboratory (URL) was selected as an application case, which shows that the coarse-grained bonding model presented in this paper is more accurate and reliable than the traditional one in large-scale rock simulation, at least in the scenario where tensile failure is dominant. [ABSTRACT FROM AUTHOR]

Underground rock mass conditions are often complex, crack propagation and coalescence results in a reduction in rock strength and can cause brittle failure. We study the influence of the fluid pressure and the lateral pressure on the propagation process of parallel flaws, the failure mechanism of rock bridges and the rock failure modes, respectively. A lateral pressure loading device and a water injection system are designed for experimental analysis. A fracture-damage model is introduced in the continuous discontinuous discrete element method to describe the crack propagation process. The following conclusions are obtained: compared with the specimen without fluid pressure in the flaws, the fluid pressures (0.5 MPa, 1 MPa and 1.5 MPa) affect the rock bridge coalescence modes, strength characteristics and failure forms under uniaxial compression. With the increase of fluid pressure, the failure form of the rock bridge changes from tensile shear failure to tensile failure. Without fluid pressure in the flaws, tensile shear failure occurs in the rock bridge under uniaxial compression. With the increase in the lateral pressure coefficient (30%), wing crack growth is restrained, and shear cracks appear in rock bridges, causing the failure mode of the rock bridge changes from tensile shear failure to shear failure. Compared with the specimen without fluid pressure in the flaws, with the increase of fluid pressure, the tensile stress at the flaw tips increases, and the shear stress at the rock bridges decreases , which promotes the growth of tensile cracks and decreases the peak strength of the specimen. Especially, high-pressure fluid promotes the coalescence of rock bridges and cause tensile failure occurs in the specimen, resulting in more obvious brittle failure characteristics of the specimen than the specimen with low fluid pressure and without fluid pressure. Highlights: The propagation process of parallel flaws and the failure mechanism of rock bridges under different fluid pressures were investigated. The rock brittle failure characteristics and the initiation stress of the parallel flaws under the fluid pressure were analyzed. The improved fracture-damage model in the CDEM can well simulate crack propagation process under hydromechanical coupling. [ABSTRACT FROM AUTHOR]

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Studying the penetration characteristics of blast‐induced cracks during double water jet slotted boreholes blasting is theoretically and practically significant since it acts both by the "superposition effect" of explosion‐induced stress waves and the "guiding effect" of water jet slots. A combination of experimental testing and numerical simulation was adopted to investigate the transmission law of explosion‐induced stress waves and the propagation law of blast‐induced cracks first. Then the penetration characteristics of blast‐induced cracks was analyzed according to rock blasting theory and rock fracture mechanics theory. The results indicate that the rock between the two boreholes is mainly subjected to a tensile failure mode due to the combined effect of the "superposition effect" of explosion‐induced stress wave and the "guiding effect" of water jet slot during double water jet slotted boreholes blasting. The hole spacing significantly affects the penetration mode of blast‐induced cracks during double water jet slotted boreholes blasting, and unless it reaches a certain value, a single flat through‐crack will form between two slotted boreholes. In addition, when the blast‐induced cracks successfully penetrate each other, the hole spacing of double water jet slotted boreholes increases by 33.3% compared with that of double circular boreholes under the same conditions. The research results can provide guidance for the application of directional controlled blasting technology with water jet assistance. [ABSTRACT FROM AUTHOR]

Microwave energy and in situ stress cooperatively influence the permeability of coal reservoirs in microwave-assisted coalbed methane (CBM) extraction. In this study, the combined influence of microwave energy and load on the failure characteristics of coal reservoirs was investigated using acoustic emission (AE) technology. The failure characteristics of anthracite derived from microwave irradiation during uniaxial compression was analyzed from aspects such as the correlation dimension (D) of the AE time series, b-value, and AF–RA correlation. Then the combined mechanism of microwave energy and in situ stress was explored. The results indicated that microwave energy significantly affected the AE activity of anthracite under loading. Moreover, microwave irradiation lessened the damage intensity and increased the crush degree during loading. After low-energy microwave (≤ 90 kJ) irradiation, the AE signals of anthracite were active only in the unstable crack growth stage, with less damage under load, and mainly characterized by the rapid expansion of large fractures. In contrast, after high-energy microwave (> 90 kJ) irradiation, AE signals penetrate the entire loading process, and the required energy for damage decreased, mostly manifested as small-scale microcracks. Additionally, microwave irradiation changed the failure mode of anthracite under loading from shear failure to tensile failure. In summary, microwave thermal effect reduced the stress threshold for fracture propagation by improving the physical structure of coal reservoirs. Cracks initiated by microwave irradiation could cause in situ stress redistribution, forming stress reduction areas and stress concentration areas, thereby accelerating the expansion of fracture networks. Therefore, the results of this study have great significance for optimizing the scheme of microwave-assisted CBM extraction and improving the CBM production rate through effective utilization of in situ stress. Highlights: Microwave irradiation enhances the AE activity of anthracite under load. Microwave irradiation decreases the failure intensity and increases the damage degree of anthracite. Microwave irradiation changes the failure mode of anthracite from shear failure to tensile failure. The cooperative mechanism of microwave energy and in situ stress during microwave-assisted CBM extraction is discussed. [ABSTRACT FROM AUTHOR]

The symmetrical fissures located within the surrounding rock of the roadway (borehole) in tunnel engineering activities can easily induce damage and instability of the surrounding rock. Therefore, studying the impact of perforated symmetrical fissures on the mechanical properties of rock with a hole has significant practical significance. Based on indoor experimental results, conventional triaxial compression simulations were performed on symmetrical fissure-hole sandstone using PFC2D. The impact of the dip angle and length of symmetric fissures on the mechanical properties of the hole-containing sandstone was analyzed. Furthermore, the relationship between crack propagation and the macroscopic mechanical properties of the specimen was discussed. The results show that: (1) The deterioration effect of symmetric fissures on hole-containing sandstone can be controlled by increasing the fissure dip angle, suppressing the stress drop phenomenon. However, increasing the fissure length exacerbates the deterioration effect. (2) The effect of symmetrical fissure dip angle on the displacement field near the hole decreases with increasing dip angle while increasing fissure length exacerbates the effect of fissures on the displacement field. (3) As the angle between the fissure and the vertical principal stress increases, the degree of tensile failure weakens while the degree of shear failure increases. (4) During the crack development phase, the extension of the stress concentration zone drives rapid crack growth. It exhibits a stress drop in the macroscopic mechanical properties, followed by the evolution of the stress field with loading, allowing rapid expansion of the microcracks and eventually leading to rock destabilization damage. [ABSTRACT FROM AUTHOR]

2.5D woven structures are of interest in new thermal protection systems for their excellent interlayer properties and structural stability. In this paper, dense (CDP) and lightweight (CLP) carbon/phenolic 2.5D woven composites were prepared. Quasi‐static tensile experiments were performed on the specimens before ablation (TCDP‐0, TCLP‐0) and after ablation for 30 s (TCDP‐30, TCLP‐30). A quarter macro–meso hybrid model that takes into account the degradation of material properties after ablation and ablative crack pores was innovatively proposed. The predicted tensile elastic modulus is within 9% error from the experimental results. The results show that TCDP‐30 and TCLP‐30 have rapid damage expansion and lethal injury at smaller displacements than the pre‐ablation specimens. The displacement of the maximum load point of TCDP‐30 is 38.7% lower than that of TCLP‐30. The forms of damage in TCDP‐30 are dominated by matrix fragmentation and matrix cracks. The damage forms of TCLP‐30 are dominated by debonding and delamination. Highlights: Micro‐CT was used to collect matrix fabrication defects and ablative crack pores defect.A quarter macro–meso hybrid model that takes into account the degradation of material properties after ablation and ablative crack pore was innovatively proposed.The tensile failure mechanism of the material after ablation was revealed by the combination of experiment and numerical method. [ABSTRACT FROM AUTHOR]

Take the metamorphic sandstone as the reference object, by making rock like samples with fractures, the conventional triaxial loading and unloading test and PFC numerical simulation of rock like sample with single fracture were conducted, and the effects of the loading path, inclined angle of fracture, axial stress level during unloading, initial confining pressure during unloading on the compressive strength, peak strain and crack propagation evolution of the samples were considered. The compressive strength of the specimen under triaxial unloading is smaller than that under triaxial loading. The peak strain of the specimen under triaxial unloading is also smaller than that under triaxial loading. The specimen is more prone to brittle failure. When the axial stress level is the same during unloading, with the increase of the initial confining pressure during unloading, the difference of the compressive strength of the specimens with different inclined angles of fracture gradually decreases. Under the condition of uniaxial compression and triaxial compression, the failure of all specimens is tensile failure, and the shear failure is the main one during unloading. [ABSTRACT FROM AUTHOR]

Crack initiation and propagation in rocks exposed to injected fluids must be understood better for sustainable sequestration and geo-energy applications. One specific challenge is the characterization of the fluid-induced changes in the stress state and mechanical properties when multiple cracks grow and interact with each other. We conduct crack propagation tests on brine-exposed and supercritical CO 2 -exposed Indiana limestone samples under uniaxial compression conditions. By analyzing the displacement and strain fields using Digital Image Correlation and the first arrival time and amplitude of the transmitted shear waves, we characterize the effect of crack propagation and interaction events on stress and toughness. We build 3-D simulation models of crack propagation under uniaxial compression using the extended finite element method to analyze the 3-D stress and deformation fields in high resolution. We quantify the evolution of crack-tip's J-integral value with the crack length for wing cracks and shear cracks in samples exposed to mixtures of brine and supercritical CO 2 at subsurface conditions. Experimental and simulation results are synthesized to gain insights into crack propagation, shear-to-tensile failure transition, and interaction of cracks in fluid-exposed rocks. Crack length and J-integral analysis quantifies the impact of fluid exposure on the fracture toughness. [ABSTRACT FROM AUTHOR]

Geological dynamic hazards during coal mining can be induced by the structural instability of a composite structure of roof rock, coal and floor rock layers, whereas the joint in coal plays a vital role in the corresponding structural instability. In this paper, the effects of coal persistent joint on the uniaxial compression failure of a roof rock-coal-floor rock composite sample were analyzed using PFC2D software. The results show that with an increase of included angle α between the loading direction and coal persistent joint plane, the uniaxial compressive stress (UCS) and peak strain of the composite sample decreased firstly and then increased. The change of elastic modulus was not obvious with α. The UCS and peak strain at α of 45° were the smallest. Additionally, the coal bodies in composite samples were mainly destructed as a shear failure. The roof or floor rock presented a tensile failure or no damage. And four failure patterns were observed for composite samples after failure, i.e., inverted V-shaped shear failure in coal and tensile failure in roof or floor rock, M-shaped shear failure in coal and tensile failure in floor rock, shear failure along the joint plane in coal and tensile failure in roof and floor rocks, shear failure along partial joint plane without obvious damage for roof or floor rock. [ABSTRACT FROM AUTHOR]

Composite materials consist of a matrix and fibers, and their failure modes differ significantly from those of isotropic materials. In undamaged composite materials, under increasing loads, matrix damage typically occurs first. At this stage, the damaged area experiences a stiffness reduction, redistributing stress until failure occurs elsewhere. As matrix damage progresses, debonding between fibers and the matrix begins, leading to delamination failure. With further load increase, the fibers eventually fracture. Finite element simulation with stiffness degradation methods can be employed to study composite material damage. This involves gradually degrading damaged elements, ultimately revealing the final failure mechanism of the composite material. Advances in finite element software now allow for precise engineering simulations, widely applied in the field. Consequently, this method can be used to simulate and analyze the failure of composite material flywheel rotors, identifying the factors contributing to their failure. This study uses progressive damage theory to simulate the failure process, establishing rational failure criteria for composite materials and utilizing a finite element stiffness degradation model. Greater interference fit results in higher compressive stress and initial stress, making the compressive stress and initial stress distribution of composite material flywheel rotors dependent on the interference fit. The study also analyzes the critical speeds of flywheels with single‐ring and multiring rotors under different interference fits, revealing the influence of interference fit on the maximum speed of the flywheel. [ABSTRACT FROM AUTHOR]

The mechanical behavior of fractured rock in tensile stress environment is a hot topic in underground mining engineering. Here, real surrounding rock of coal mine roadway was simulated by using rock-like materials and the tensile failure behavior of Brazilian discs with intermittent double fissures was investigated experimentally. The deformation response, fracture evolution, and failure mode of rock were analyzed. The fissured disc specimen's discrete element model was proposed in particle flow code (PFC2D). The microforce field, crack, and energy evolution processes of model specimens were discussed. The results showed that the load-displacement curves exhibit single-peak and double-peak types, corresponding to the splitting penetration and wing crack penetration damage modes of the specimen. The fissure angle or rock bridge angle showed a great influence on the evolution of main cracks and secondary cracks. The double-fissured Brazilian disc failed due to the initiation and transfer of microcracks in the stress concentration zone, combined with the continuous propagation and convergence of those microcracks. The splitting failure of the Brazilian disc is a continuous process of strain energy accumulation from the early stage of loading and instantaneous release of strain energy after obtaining the peak strength as the dissipative energy sharply rises. [ABSTRACT FROM AUTHOR]