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The theoretical and practical achievements in the field of the theory of strength and reliability of composite materials are discussed in a review conducted on the scientific research conducted on the effect of delamination on the reliability and quality of composites. The methodological aspects of the stability of the mechanical characteristics of composite materials under the combined action of cyclic and impact loads are examined, as are the manufacturing and processing technologies. The reasons for delamination, such as technological, manufacturing and application, free edge, joints and loads, are revealed. The influence of delamination on the bearing capacity of structural elements made of composite materials is analyzed. The mechanism of delamination growth is outlined, and the criteria and processes are defined, such as the growth of delamination cracks in a multidirectional laminated plate from a straight edge, edge delamination during plate bending, delamination in plates in the field of residual stresses, etc. The importance of taking into account the visco-plastic effect at the top of the edge crack of delamination of composite materials is emphasized. The concept of critical delamination behavior is characterized, and the issues of delamination stability are described. [ABSTRACT FROM AUTHOR]

This paper investigates the fracture mechanical properties of concrete, using crushed concrete aggregates (CCA) and granulated blast furnace slag (GGBS) for partial cement replacement. CCAs made from prefabricated concrete replace 100% of the fine and coarse fractions in concrete recipes with w/c ratios of 0.42 and 0.48. Two pre-treatment methods, mechanical pre-processing (MPCCA) and accelerated carbonation (CO2CCA), are investigated for quality improvements in CCA. The resulting aggregates show an increased density, contributing to an increase in the concrete's compressive strength. The novelty of this paper is the superposition of the effects of the composite parts of concrete, the aggregate and the cement mortar, and their contributions to concrete fracture. Investigations are directed toward the influence of fine aggregates on mortar samples and the influence of the combination of coarse and fine aggregates on concrete samples. The physical and mechanical properties of the aggregates are correlated with mortar and concrete fracture properties. The results show that CCA concrete achieves 70% of the fracture energy values of concrete containing natural aggregates, and this value increases to 80% for GGBS mixes. At lower w/c ratios, MPCCA and CO2CCA concretes show similar fracture energies. CO2CCA fine aggregates are the most effective at strengthening the mortar phase, showing ductile concrete behavior at a w/c ratio of 0.48. MPCCA aggregates contribute to higher compressive strengths for w/c ratios of 0.42 and 0.48. Thus, mechanical pre-processing can be improved to produce CCA, which contributes to more ductile concrete behavior. [ABSTRACT FROM AUTHOR]

The modeling of concrete has always been of great interest. In this work, a yield criterion with three surfaces is proposed to capture the plastic yielding behavior under different loading conditions, including uniaxial tension/compression, biaxial tension/compression and tension-compression. Material failure, characterized by the degradation and finally the complete loss of material integrity, can be modeled by the continuum damage approach. Within finite element methods, the nonlocal enhancement of integral-type or gradient-type is often required for the well-posedness of the partial differential equation system. In this work, a localizing gradient damage model has been adopted to obtain mesh-insensitive material responses, while ruling out the unphysical broadening of the damage zone often observed in constant length scale gradient damage models. Following a consistent derivation, the plasticity-damage coupled model has been implemented into an in-house finite element framework. Several representative and demonstrative examples serve to illustrate the capability of the proposed description in concrete modeling, followed by the conclusion, where some final insights are provided. [ABSTRACT FROM AUTHOR]

This paper presents the fabrication of a new class of high-porous composites made of polylactic acid (PLA) matrix reinforced with polyvinylidenfluorid (PVDF) long fibers. The composites were produced by printing the PLA matrix by using Fused Deposition Modeling (FDM) and adding continuous PVDF fibers by hand-layup. Materials properties were investigated, including the influence of two raster orientations during 3D printing ([0, 90] and [−45, +45]), by tensile and three-point bending tests. Experiments revealed that PVDF fibers within PLA matrices modified the composites behaviour providing higher ductility, whilst flexural strength decreased. Observation of the fracture surfaces showed that cracks followed the raster orientation lines avoiding a complete failure of the material structures and fibers (whereas unreinforced pure PLA samples fully broke). Composite configuration with [−45, +45] raster orientation exhibited higher strength and stiffness in comparison to the [0, 90] orientation, both in tensile and flexural conditions. Tensile failure was dominant in [−45, +45], and less influential in [0, 90]. Numerical models were also developed and compared. Two alternative approaches were considered in material modeling: Mori – Tanaka and 'two-material' models. No relevant differences emerged and a good agreement with experiments was achieved. [ABSTRACT FROM AUTHOR]

In a producing field, the fluid conduit nature of faults could initiate due to changes in the stress state within the reservoir. It has large impacts on different aspects (seal integrity, casing shear, mud loss during drilling, reservoir permeability, and induced earthquakes) of the field performance. In order to optimize the future drilling campaign and field management strategy, production induced faulting risk assessment was performed to evaluate the fault behavior in a depleted reservoir, where the reservoir pressure has decreased by 40% from its pre-production pressure. The risk assessment was done through (i) identifying the critically stressed fracture/fault in the pre-production stage, (ii) assesses the risk of production induced new fractures/faults generation & re-activation of existing faults, and (iii) risk ranking of faults within the reservoir. The assessment model was based on the relationship between failure mode and fluid flow with change in stresses on the fault planes due to reservoir pressure depletion. The model was built through integrating different data, such as, reservoir fault mapping, core data, drilling data, production data, and geomechanical modelling. The analysis revealed that all the faults at the pre-production stage were not critically stressed and therefore, were closed. Production induced new fractures/faults are neither generated nor existing faults re-activated till the current depleted reservoir condition in the study field. Risk ranking based on the slip and dilation tendency analysis suggests that because of the depletion fault slip tendency has not increased much and risk of fault slip is on the lower side. But, the dilation tendency has increased for all the faults which makes these faults more prone to extensional/tensile failure than shear failure. Furthermore, the analysis suggests that the severe mud loss events encountered in the post-production wells were due to re-activation of existing secondary fractures at the fault intersection depths during drilling. [ABSTRACT FROM AUTHOR]

Numerical models based on the discrete element method are used to study the fracturing process in brittle rock-like materials under direct and indirect tension. The results demonstrate the capacity of the model to capture the essential characteristics of fracture including the onset of crack propagation, stable and unstable crack growth, arrest and reinitiation of fracturing, and crack branching. Simulations of Brazilian indirect tension tests serve to calibrate the numerical model, relating macroscopic tensile strength of specimens to their micromechanical breakage parameters. A second suite of simulations reveals a linear relationship between the tensile strength of specimens and the loading stress for which mode I tensile crack propagation ensues. Based on these results, a crack initiation criterion for brittle materials is proposed, prescribing the stressing conditions required to induce tensile failure. Such a criterion, if broadly applicable, provides a practical means to rapidly assess the failure potential of brittle materials under tensile loads. [ABSTRACT FROM AUTHOR]

To precisely predict the failure performances of cratering and spallation in concrete slabs under the contact explosion, three modifications were made to the original K&C (Karagozian & Case) material model that is #72REL3 in LS-DYNA, i.e. the tensile damage accumulation, relationship between the yield scale factor and damage function, as well as the tensile DIF t . The mechanical properties of normal weight concrete simulated by the modified model were validated by a series of single finite element numerical tests, including unconfined uniaxial compression, triaxial tension and dynamic tension tests. Two field tests were then conducted to study the failure modes of concrete slabs under contact explosion, as well as to verify the modifications. A new erosion criterion referring to the tensile damage was proposed to improve the simulation of the tensile failure of the concrete slab. The modified model and erosion criterion were implemented in LS-DYNA with the user-defined material procedure. The corresponding predictions by the original K&C model were also presented to demonstrate the improved effects of the modifications. Sensitivity analysis of the proposed erosion criterion and tensile DIF t was carried out to investigate the stability of the modified model. [ABSTRACT FROM AUTHOR]

The failure mechanism of a deep hard rock tunnel under high geostress and high geothermalactivity is extremely complex. Uniaxial compression tests of granite at different temperatures were conducted. The complete stress-strain curves, mechanical parameters and macroscopic failure types of the rock were analyzed in detail. The brittleness index, which represents the possibility of a severe brittleness hazard, is proposed in this paperby comparing the peak stress and the expansion stress. The results show that the temperature range from 20 to 60°C is able to aggravate the brittle failure of hard rock based on the brittleness index. Tie closure of internal micro cracks by thermal stress can improve the strength of hard rock and the storage capacity of elastic strain energy. The failure mode ofthe samples changes from shear failure to tensile failure as the temperature increases. In conclusion, the brittle failure mechanism of hard rock under the action of thermal coupling is revealed, and the analysis result offers significant guidance for deep buried tunnels at high temperatures and under high geostress. [ABSTRACT FROM AUTHOR]

Fatigue failure of carbon fiber-reinforced plastics (CFRPs) under cyclic loadings has attracted the attention of researchers recently. In this study, the entropy-based failure criterion is proposed to investigate the fatigue lifetime of unidirectional CFRPs subjected to multiple-amplitude cyclic loadings. Due to the heterogeneity of CFRPs, a micro-finite element model considering matrix resin and fibers independently is developed, and the entropy-based damage criterion is implemented into a user-subroutine of Abaqus to model the progressive damage of matrix resin. The fatigue lifetime of CFRPs under typical loading sequences consisting of two stages, such as varying from low to high (L-H) or from high to low (H-L) loading sequence, is estimated with the proposed failure criterion. Numerical results show that the initial damage occurs near the area between two fibers, and a transverse crack propagates progressively under the cyclic loading. The difference in predicted lifetime to final failure in L-H and H-L stress levels is 6.3%. Thus, the effect of loading sequence on the fatigue lifetime can be revealed via the proposed entropy-based damage criterion. Comparisons with the conventional linear cumulative damage (LCD) and kinetic crack growth (KCG) theories are also conducted to demonstrate the validity of the proposed method. The entropy-based failure criterion is a promising method to predict the residual strength and fatigue lifetime of CFRP components. [ABSTRACT FROM AUTHOR]

To reach Earth's surface, magma must ascend from the hot, ductile asthenosphere through cold and brittle rock in the lithosphere. It does so via fluid-filled fractures called dykes. While the continuum mechanics of ductile asthenosphere is well established, there has been little theoretical work on the cold and brittle regime where dyking and faulting occurs. Geodynamic models use plasticity to model fault-like behaviour; plasticity also shows promise for modelling dykes. Here we build on an existing model to develop a poro-viscoelastic–viscoplastic theory for two-phase flow across the lithosphere. Our theory addresses the deficiencies of previous work by incorporating (i) a hyperbolic yield surface, (ii) a plastic potential with control of dilatancy and (iii) a viscous regularization of plastic failure. We use analytical and numerical solutions to investigate the behaviour of this theory. Through idealized models and a comparison to linear elastic fracture mechanics, we demonstrate that this behaviour includes a continuum representation of dyking. Finally, we consider a model scenario reminiscent of continental rifting and demonstrate the consequences of dyke injection into the cold, upper lithosphere: a sharp reduction in the force required to rift. [ABSTRACT FROM AUTHOR]

In recent years, interest in the effect of positive temperatures on rock faces has increased, as evidenced by the extensive literature on the subject. The subsurface rock masses of rock slopes are submitted to uncontrollable natural temperature cycles which cause deformation, displacement, opening and closing of discontinuities and can lead to surface failure or more important rockfalls. In this paper we offer a review of the publications on this theme focusing on in situ observations and measurements. Measurements are described and analysed considering conditions of temperature and insolation, displacements, strains, focusing on two French sites (Les Rochers de Valabres and La Roque-Gageac). Mechanical analysis is then performed. The impact of temperature variation is studied from basic assumptions to much more complex ones. Thermoelasticity is first assessed, and analytical computation is suggested, fracture mechanism and fatigue are also considered. Finally, we outline some challenges to be addressed in the coming years. Highlights: The article reviews the accumulated knowledge on the effect of positive temperature changes on slope stability. Excluding the effect of water freezing and thawing. The different types of measurements are reviewed considering surface and in-depth measurements of temperature, displacements, fracture opening, and strain. Simple thermo-elastic analysis, alongside solutions from fracture mechanics are proposed. A focus is proposed on 2 French sites to illustrate interpretations. The conclusion summarizes the points of consensus, the aspects that are still under discussion among authors or should be more debated and open to pending challenges for the next years. [ABSTRACT FROM AUTHOR]

Understanding the response mechanism of multiparameter rock fractures can improve the accuracy of crack diagnosis. In this study, three types of rock samples with notches were subjected to three-point bending tests. The fracture morphology was observed using scanning electron microscopy, and real-time resistivity and acoustic emission (AE) data were used to describe the crack propagation. The spatial distribution of the electric potential was simulated based on practical crack morphologies and can explain the correlation between rock fracture and rock resistivity or AE. The results reveal that the initiation and propagation of cracks reconstructs the electrical potential distribution characteristics of the rock samples and changed their overall resistivity. The crack growth rate was proportional to the rate of increase in the resistivity rate, and the resistivity increased with nonuniform crack growth. Crack geometry complexity affected circuit connectivity, and a higher resistivity change rate was typically caused by cracks with more uniform propagation. The resistivity variation had the same trend as the fracture toughness, whereas the AE energy exhibited a similar trend to the fracture energy evolution. The cumulative AE count of the granite fracture was the largest, and the peak AE count of coal was larger than that of sandstone. In the main frequency band of 100 ± 25 kHz, a relatively large AE event occurred during crack initiation, and the AE amplitude of granite was the largest. The primary fracture propagation increased the peak AE count, amplitude, energy, and fracture energy release efficiency. During rock fracture, the opening of a microscopic bedding plane and a matrix fracture results in time-varying resistivity and AE characteristics. The complementary electrical and acoustic parameters help describe the details of crack propagation behaviour. Highlights: Cracks restructure the electrical potential distributions in rock samples and change the resistivity. The acoustic emission (AE) energy exhibits a similar trend to the fracture energy evolution. The relationship between the resistivity and AE is mutually verified and complementary. [ABSTRACT FROM AUTHOR]

Pumping-induced earth fissuring is a typical fluid–solid coupling problem involving fracture evolution. Despite recent advances in the numerical modeling of earth fissures, a holistic approach capable of predicting their evolution and understanding their behavior from fracture mechanisms is absent. This paper presents a coupled numerical method based on the framework of the material point method (MPM) and fracture mechanics for the process-oriented simulation of pumping-induced tensile earth fissures. In this method, fissure behavior is driven by the real-time stress field affected by soil consolidation. After an earth fissure initiates in the region of maximum tensile stress, it propagates in the direction of the maximum circumferential stress around the fissure tip. The great scale difference between a fissure tip and the entire model is addressed with an adaptive refinement strategy. Through the simulation of a physical model experiment reproducing pumping-induced earth fissures with the presence of a bedrock ridge, the complete fissuring process, including initiation, propagation, and arrest, is presented. The simulated fracture time (24 h after drainage), location (directly above the bedrock ridge), final length (165 mm), and morphology evolution (from straight to zigzag) of the main fissure basically were in agreement with the experimental results. The stress intensity factors (SIFs) around fissure tips, which are important fracture parameters that have not been discussed in existing earth fissure analyses, also are captured during the fissure growth. The proposed method quantifies the inhibition of geostatic stress fields in earth fissure development and correlate fissure morphology with the stress field at a fissure tip. [ABSTRACT FROM AUTHOR]

Fiber metal laminates (FMLs) are a kind of composite material prepared by alternately arranging fiber layers and metal sheets at a certain temperature and pressure. It has been widely used in aerospace and automobile transportation for its excellent combined mechanical properties. For the forming and failure processes of FMLs, the interfacial behavior and damage evolution of components are hard to be observed experimentally. Therefore, it is of great importance to simulate them accurately. In this article, the development and application of FMLs were first introduced. Then the comparison of constitutive models in FMLs simulation was given, especially the dynamic constitutive model applied to the metal layer. After that, the important aspects of damage evolution, interface behavior, and model optimization in the simulation on the forming and failure process of FMLs were analyzed, and the emphasis is on the nonlinear progressive damage model of different materials, the construction of cohesive zone model and superior meshing methods. Furthermore, the experimental verifications of FMLs simulation were given. It is shown that the deformation behavior and damage characteristics of various kinds of FMLs during forming and failure processing can be accurately predicted by reasonable numerical simulation. Eventually, the future outlooks for numerical simulation of FMLs was proposed. Through this review, scholars and engineers who are interested in FMLs can systematically understand the numerical simulation work of FMLs, which is helpful in improving the quality of research in this field. [ABSTRACT FROM AUTHOR]

Subcutaneous injection, which is a preferred delivery method for many drugs, causes deformation, damage, and fracture of the subcutaneous tissue. Yet, experimental data and constitutive modeling of these dissipation mechanisms in subcutaneous tissue remain limited. Here we show that subcutaneous tissue from the belly and breast anatomical regions in the swine show nonlinear stress–strain response with the characteristic J-shaped behavior of collagenous tissue. Additionally, subcutaneous tissue experiences damage, defined as a decrease in the strain energy capacity, as a function of the previously experienced maximum deformation. The elastic and damage response of the tissue are accurately described by a microstructure-driven constitutive model that relies on the convolution of a neo-Hookean material of individual fibers with a fiber orientation distribution and a fiber recruitment distribution. The model fit revealed that subcutaneous tissue can be treated as initially isotropic, and that changes in the fiber recruitment distribution with loading are enough to explain the dissipation of energy due to damage. When tested until failure, subcutaneous tissue that has undergone damage fails at the same peak stress as virgin samples, but at a much larger stretch, overall increasing the tissue toughness. Together with a finite element implementation, these data and constitutive model may enable improved drug delivery strategies and other applications for which subcutaneous tissue biomechanics are relevant. [ABSTRACT FROM AUTHOR]

Hybrid reinforced concrete (HRC) can be defined as a cementitious material in which the reinforcing secondary phase consists of a combination of continuous steel rebars and of short discontinuous fibers, randomly distributed within the concrete matrix. For these structural elements, experimental flexural tests highlight how the postcracking response of the composite is strongly affected by the amount of steel bars together with reinforcing fibers. In the present work, it is shown that the action of these two contributions can be clearly interpreted in the framework of Fracture Mechanics through the Updated Bridged Crack Model (UBCM). The model assumes nonlinear constitutive laws to describe the toughening action of the reinforcing secondary phases, which are related to the yielding of steel rebars and to the pull‐out of the short fibers. Under these assumptions, different postcracking regimes depending on three scale‐dependent dimensionless numbers can be predicted: the bar‐reinforcement brittleness number, NP, which is directly related to the steel bar area percentage, ρ; the fiber‐reinforcement brittleness number, NP,f, which is directly related to the fiber volume fraction, Vf; and the pull‐out brittleness number, Nw, which depends on the critical embedment length of the fiber‐reinforcement, wc. The couples of critical values of the two reinforcement brittleness numbers define the minimum reinforcement conditions of the HRC structural element—that is, the combination of ρmin and Vf,min required to guarantee a stable post‐cracking response—including its scale dependence. A parametrical analysis is presented together with the modeling of an experimental campaign reported in the literature in order to assess the effectiveness of the UBCM. [ABSTRACT FROM AUTHOR]

Injection of fluids during wastewater disposal and geologic carbon sequestration causes induced stress and changes in rock's elastic and failure properties, such as elastic moduli and fracture toughness. An accurate understanding of such changes in the response requires modeling and analysis of fluid-induced changes in rock's stress and deformation states for which core-scale mechanical loading tests are often employed. Using experiments and simulations of Single Edge Notched Beam (SENB) test on water-exposed and supercritical (sc) CO 2 -exposed Berea sandstones, we quantify the effect of pore fluid on tensile crack propagation, spatial distributions of shear and tensile stresses, and fracture toughness and J-integral properties. We use Digital Image Correlation (DIC) to monitor the changes in local stress and deformation fields during crack initiation and propagation. A representative numerical simulation model of dynamic crack propagation under SENB loading is built using the extended finite element method. Results are analyzed using the stress path analysis to identify shear and tensile stress concentration regions, which provide precursory information for failure events during the test. Stress profiles along and across the growing crack are analyzed and compared with Irwin's classical solutions to understand the impact of SENB loading. Evolution in stress intensity factor and crack length shows subcritical crack growth prior to Griffith-like failure at the structure scale. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing, which determines geothermal resource productivity, is one of the critical technical components in the construction of hot dry rock (HDR) reservoirs. Although the mechanical coupling between solid and fluid has been included in algorithms based on the discrete element method (DEM) generally employed to investigate hydraulic fracturing, crystalline rocks of reservoirs are mostly treated as homogeneous isotropic models without considering their petrographic texture. By combining a grain-based model (GBM) and a modified fluid–solid coupling algorithm, a novel hydro-GBM is constructed in this study to analyze the hydraulic fracturing response of polycrystalline rocks. Moreover, acoustic emission (AE) events during fracturing are extracted to describe the characteristics of hydraulic-fracturing-induced seismicity. Under in-situ conditions with high differential stress, the propagation direction of hydraulic fractures is mainly perpendicular to the direction of minimum in-situ stress, with little influence by material heterogeneity and fluid viscosity; under near-hydrostatic in-situ stress conditions, the microcracks along the mineral boundaries increase remarkably, and the fracture pattern tends to be complex, especially when a low-viscosity fluid is injected into the rock. From the Gutenberg-Richter type relationship between the AE event numbers and the moment magnitudes, it is found that large induced seismic events increase with in-situ stress and with fluid viscosity. In summary, the proposed hydro-GBM can well reproduce the propagation behavior of hydraulic fractures influenced by material heterogeneity, and the research results reveal the interactions between petrographic texture, in-situ stress, fluid viscosity, and hydraulic fracturing characteristics, which will provide a valuable reference for on-site reservoir stimulation. Highlights: A novel fluid-solid coupling grain-based model is proposed to simulate the hydraulic fracturing of crystalline rocks. The model well reproduces the propagation behavior of hydraulic fractures influenced by material heterogeneity. Effects of in-situ stress and fluid viscosity on breakdown pressure, fluid pressure field, and seismic event are analyzed to bridge the gaps in the existing studies. Injecting low-viscosity fluid contributes to complex fracture networks in brittle granitic reservoir because of inducing numerous mineral boundary cracks and reducing large seismic events. [ABSTRACT FROM AUTHOR]

The brittle failure of rock material is closely related to crack propagation and coalescence. Wing crack is the most common manifestation of crack propagation subject to compression. Previous studies proved that under uniaxial compression, 2D wing cracks were able to propagate extensively and split the rock specimen. In contrast, the propagation of 3D wing cracks was somehow restricted under uniaxial compression. However, under biaxial compression, even a small intermediate principal stress could trigger extensive 3D wing propagation. It remains elusive why extensive 3D wing propagation was so sensitive to intermediate principal stress. In this study, we develop a numerical model to simulate the propagation of 3D wing cracks under compression. The morphology of 3D wing cracks is correlated with axial compressive stress, which sheds light on the triggering mechanism of extensive 3D wing propagation. According to our interpretation, the reasons that extensive 3D wing propagation is sensitive to intermediate principal stress are twofold. (1) Intermediate principal stress is very effective to suppress lateral spalling, thus the specimen failure is solely attributed to the wing propagation. (2) Upon specimen failure under uniaxial compression, 3D wing cracks are in a critical state transitioning from stable to unstable propagation, and intermediate principal stress facilitates such transition. This study provides an in-depth understanding of the extensive 3D wing propagation under compression, which reconciles all elusive experimental phenomena in previous studies. Article Highlights: A numerical model is developed to simulate the propagation of 3D wing cracks. Compression–morphology relationship is obtained during extensive 3D wing propagation. A critical state exists for 3D wing cracks transitioning from stable to unstable propagation. Axial loading is the first-order mechanism for extensive 3D wing propagation. Intermediate principal stress can suppress lateral spalling to facilitate extensive 3D wing propagation. [ABSTRACT FROM AUTHOR]

There has been little research on the impact resistance of mortar–rock slope protection structures. To ensure that the mortar–rock interface has good adhesion properties under the action of impact loading, in this paper, based on fracture mechanics theory, a theoretical impact model was established for mortar–rock binary material. Dynamic fracture tests were carried out on mortar–rock interfaces using the split-Hopkinson pressure bar (SHPB) system. The Brazilian disc (CSTBD) specimen was prepared with one half in granite and the other half in mortar. The specimen used for the dynamic impact test was 48 mm in diameter and 25 mm thick. The effects caused by the change in interface inclination and interface shape on the dynamic fracture mode were discussed. The dynamic model parameters were obtained for different inclination angles and interfaces. The results show that both the interface inclination and interface shape have significant effects on the dynamic mechanical properties of the mortar–rock binary material. The fracture modes of the mortar–rock specimens can be classified into three types. When the interface inclination is 0°, the specimen shows shear damage with an interface fracture; when the interface inclination is in the range of 0–90°, the dynamic splitting strength of the mortar–rock material increases with increasing interface inclination, and the interface undergoes composite fracture; and when the interface inclination is 90°, the dynamic splitting strength of the specimen reaches its peak, and the interface undergoes tensile fracture. The mortar–rock interface damage follows the M-C criterion. The roughness of the interface shape has a large influence on the dynamic splitting strength of the specimens. The rougher the interface shape, the higher the interface cleavage strength and the higher the peak load that causes the material to damage. The results of this study can provide a reference for the design of mortar–rubble structures to meet the demand for impact resistance and have strong engineering application value. [ABSTRACT FROM AUTHOR]

The failure behavior of field-scale rock masses has long been studied indirectly through laboratory compression tests on rock specimens with preexisting flaws. However, little to no attention has been paid to size effects on these cracking processes that may be governed by the relative size between the fracture process zone and the rock structure. Here, we investigate such size effects on the compressive strength and cracking behavior of flawed rocks through high-fidelity simulations of mixed-mode fracture in quasi-brittle materials. We perform a series of numerical uniaxial compression tests on geometrically similar gypsum specimens with single and double flaws, across a wide range of sizes from 0.25 times a standard laboratory specimen size to 16 times. The results suggest strong size effects on both the uniaxial compressive strength and cracking patterns. The size effect on the compressive strength appears qualitatively similar to Bažant's size effect law derived for the tensile strength of notched structures. However, the quantitative changes in the strength deviate from the existing size effect law which does not account for mixed-mode fracture. As for the cracking behavior, three types of cracking patterns per flaw configuration are identified as the specimen size changes. Remarkably, the cracking patterns that emerge at the field scale, where the size of the fracture process zone is negligible, are analogous to those observed from laboratory experiments on highly brittle materials. The findings of this work provide important insights into how to bridge observations on rock fracturing processes across scales. [ABSTRACT FROM AUTHOR]

The progressive brittle failure of high-stress hard surrounding rock masses caused by layered excavation severely threatens the stability of large underground caverns. We summarize the stress path in the potential failure zone of the Shuangjiangkou underground powerhouse during layered excavation, and then design the multistep unloading confining pressure test and the cyclic loading–unloading test with multistep unloading confining pressure. The deformation, strength, acoustic emission, and failure characteristics of granite, as well as the evolution of critical stress thresholds and mechanical parameters are analyzed based on experimental results under different stress paths. The peak strength of the rock significantly depends on the stress path and the amount of unloading confining pressure. The rock failure process under multistep unloading confining pressure is divided into stable and unstable crack growth stages, with the failure mode shifting from tensile cracks to mixed tensile–shear cracks. The evolution modes of critical stress thresholds, cohesion, and internal friction angle with amount of unloading confining pressure differ significantly from that with the equal plastic strain. The former is nearly linear, while the latter is exponentially nonlinear. The evolution of the deformation modulus, cohesion, and internal friction angle confirms the rationality of the cohesion-weakening–friction-strengthening model with degraded deformation modulus for the stability analysis of surrounding rock masses under layered excavation. The experimental methods for obtaining the evolution of rock mechanical parameters using a single specimen under unloading confining pressure are also discussed. The results imply that the layered excavation-induced nonlinear deterioration of the deformation modulus and strength parameters of high-stress hard surrounding rock masses should be considered in the stability analysis of large underground caverns. Further, the timely compensation of the excavation-induced confining pressure loss is critical to ensure the stability of high-stress hard surrounding rock masses under layered excavation. Highlights: A stress path was designed to reflect the stress evolution characteristics for rock masses of the large underground cavern under layered excavation; The deformation, strength, acoustic emission, and failure characteristics of granite under the layered excavation stress path were analyzed; The evolution of rock mechanical parameters with unloading confining pressure and the equivalent plastic strain were obtained; An experimental method for obtaining the evolution of rock mechanical parameters using a single specimen under unloading confining pressure was discussed. [ABSTRACT FROM AUTHOR]

This study investigated the fracture characteristics of plain concrete and polypropylene fiber-reinforced concrete (PFRC) using pre-notched three-point bending beam tests with the digital speckle correlation method (DSCM). Then, the fracture instability behavior of the two types of beams was simulated in finite elements based on the plastic damage model and the cohesion model, for which the applicability was assessed. Furthermore, the stability of the Big Gang Mountain Dam made from plain concrete or PFRC subjected to the earth-quake loading was simulated with the plastic damage model. The results show that the limiting length of the non-local deformation zone can be used as an indicator of instability damage in a concrete structure. The simulation results of the plastic damage model agreed well with the local deformation in the pre-notched three-point bending beam test obtained from the DSCM. The plastic damage model was found to be capable of describing the residual strength phenomenon, which the cohesive model was not capable of. The damage evolution regions of the PFRC dam are strictly constrained in some regions without the occurrence of the local deformation band across the dam, and PFRC can dramatically reduce the failure risk under earthquake loading. The numerical solution proves that PFRC is an advisable material for avoiding failure in concrete dams. [ABSTRACT FROM AUTHOR]

This paper discusses the challenges in using natural fibers for the development of textile-reinforced mortar (TRM) composites with pseudo-strain-hardening and multiple cracking behavior. The particular characteristics of natural vegetal fibers are analyzed with reference to data from the literature. It is concluded that the efficient use of these fibers as composite reinforcement requires the development of treatment or impregnation protocols for overcoming durability issues, eliminating crimping effects in tensile response and imparting dimensional stability. Relevant experimental research on the synthesis and performance of natural TRMs is reviewed, showing that the fabrication of such systems is, at present, largely based on empirical rather than engineering design. In order to set a framework regarding the properties that the constituents of natural TRM must meet, a comparative analysis is performed against inorganic matrix composites comprising synthetic, mineral and metallic reinforcement. This highlights the need for selecting matrix materials compatible with natural fibers in terms of stiffness and strength. Furthermore, a rational methodology for the theoretical design of natural TRM composites is proposed. First-order analysis tools based on rule-of-mixtures and fracture mechanics concepts are considered. Based on the findings of this study, paths for future research are discussed. [ABSTRACT FROM AUTHOR]

The radial (minimum) stress of the near-boundary rockmass in deep excavations plays a key role in inducing rock failures such as spalling and strainbursts. In this study, a strainburst is treated as a structural failure of the near-boundary rockmass. The influence of the radial stress acting asymmetrically on one boundary surface of the burst volume on strainbursts is numerically investigated using a 3D bonded block distinct element method (DEM). The 3D DEM model for simulating the overall process of strainbursts is introduced and its effectiveness is verified by comparing with the analogous true triaxial experiments. Then based on the simulation results, the influence of radial stress on strainbursts is studied from the perspectives of deformation and strength characteristics, fracturing process, crack development, failure mode and energy balance. In addition, discussions of the mechanisms of the influence of radial stress on strainbursts are presented from the perspectives of fracture mechanics and a graphical explanation. Results show that, as the radial stress increases, the strength of the strainburst sample exhibits a quadratic law that increases first and then decreases, the number of shear cracks increases remarkably, the degree of accumulated rock damage precedes burst is getting higher and higher, the failure mode of strainbursts transforms gradually from a tensile-type to a shear-type, and the kinetic energy is getting higher and higher. The fundamental influence of radial stress on strainbursts is concluded as its suppression effect on the tensile failure mechanism of brittle rocks. - Influence of radial stress on strainbursts is numerically investigated using a 3D bonded block distinct element method. - The influence of radial stress on strainbursts is discussed from the perspectives of fracture mechanics and a graphical explanation. - As the radial stress increases, the failure form of strainbursts transforms gradually from a tensile-type to a shear-type, and the released kinetic energy of a shear-type strainburst is larger than that of a tensile-type strainburst. - The fundamental influence of radial stress on strainbursts lies in its suppression effect on the tensile failure mechanism of brittle rocks. [ABSTRACT FROM AUTHOR]

In this study, the tensile and compressive performance of adhesively bonded single-lap joint of green composites (bamboo fiber/Polylactic acid (PLA)) has been experimentally investigated. The green composites have been fabricated by film stacking method in compression molding. The important factors affecting the performance of the adhesively bonded joint such as overlapping length of the specimen (20, 40, and 60 mm) and width of the specimen (20, 26, and 32 mm) have been studied for epoxy, polyurethane, and parent polymer (PLA) used as bonding materials for joining of developed green composites. From the experimental analysis, it has been found that the higher levels of width and overlapping length of the joint specimen offer better tensile and compressive strength of the joint specimen. The failure loads of the joint specimens have been found to be more for epoxy as compared to PLA and polyurethane used for the purpose of joining. The relative significance of the chosen input parameters has also been found by performing analysis of variance (ANOVA). The most significant parameter has been found to be the width of the specimen for tensile failure load and type of bonding material for compressive failure load. The failure mode analysis has revealed that adhesive and cohesive failures are the major mode of failure of the adhesively bonded green composites under tensile and compressive loadings. [ABSTRACT FROM AUTHOR]

• Dynamic peak load, strain field distribution, fracture process of sandstone containing parallel pre-cracks were investigated by dynamic Brazilian splitting tests. • Six fracture modes of sandstone containing parallel pre-cracks under impact load were identified. • The damage and stress distribution in the fracture process of sandstone containing parallel pre-cracks were numerically studied by ANSYS/LS-DYNA. Parallel cracks have a significant influence on the dynamic tensile failure behavior of rock. The systematic investigation of dynamic tensile mechanical properties and fracture evolution characteristics of fractured rocks are significant for guiding the safe excavation and stability sustaining of rock engineering. Two groups of dynamic Brazilian splitting tests were conducted for the sandstone specimens containing parallel pre-cracks. The influences of crack inclination angle (Group A) and rock bridge length (Group B) on the tensile mechanical properties and fracture evolution characteristics were analyzed. Results showed: the dynamic peak load of pre-cracked specimens is significantly lower than that of intact specimens. The peak load presents a V-shaped evolution trend with the growth of crack inclination angle. Differently, the dynamic peak load increased monotonically with the increasing of rock bridge length. In addition, the crack initiation points move from crack tips to crack walls when the crack inclination angle is increasing. When rock bridge length is large enough, the crack propagation paths are observed near the central axis of specimens. By utilizing the DIC method, six different failure modes are identified to explain the crack initiation and propagation mechanisms under dynamic loading. The tensile-shear fracture modes are mainly presented for specimens with different crack inclinations. Differently, the ultimate failure modes change from tensile-shear mode to tensile mode when the rock bridge length is increasing. In addition, numerical results (ANSYS/LS-DYNA) are in good agreement with the experimental results as a whole, which can verify and explain the experimental results. This study is expected to provide guidance for controlling fractured rock masses. [ABSTRACT FROM AUTHOR]

In this paper, a two-dimensional numerical approach named plastic-bond-based smoothed particle hydrodynamics (PB-SPH) is proposed to simulate the fracture of rock with pre-existing flaws. Under the PB-SPH framework, interaction particles are interconnected through bonds. The Drucker–Prager criterion of a tensile zone cut-off coupled maximum tensile stress criterion is applied to judge shear failure and tensile failure. To verify the capability of the PB-SPH method in simulating rock fracture behavior, two-dimensional numerical simulations of a single flaw, two coplanar flaws, and multiple flaws of uniaxial compression are carried out. Compared with previous experimental and numerical results, the present numerical results show good agreements on crack initiation positions, crack morphology and crack patterns. The numerical simulation of the progressive failure process of rock slope with multiple flaws is carried out, which shows that the PB-SPH method can be well applied to rock mechanics engineering. The present results show that the PB-SPH method has a certain reference in the study of rock fracture mechanics and the application of rock mechanics engineering. [ABSTRACT FROM AUTHOR]

This study aimed to analyze the outcomes of stress intensity factor (KI) and new bond strength tests of oil well cement (class H) with a water-to-cement ratio (w/c) of 0.38. Mechanical properties of the cement paste, such as the compressive and flexural strengths, were tested and qualified at 1, 7, and 28 days of curing. The relationship between the elastic modulus and axial strain using the differential of the Vipulanandan p-q model for the cement paste was obtained. The stress intensity factor of the cement paste was between 0.3 and 0.6 MPa.m, and the crack tip opening displacement (CTOD) was between 2.798 and 6.254 µm at three different ratios between the initial notch height (a) and the thickness of the beam (d) (a/d = 0.3, 0.4, and 0.5). The nonlinear Vipulanandan p-q model was used to model the compressive and flexural stress–strain behavior of the cement at three curing times. The bonding strength between the cement and steel tube representing the casing in the borehole was 0.75, 1.89, and 2.59 MPa at 1, 7, and 28 days respectively. [ABSTRACT FROM AUTHOR]

Hydrogen transport by blending hydrogen into natural gas transmission pipelines and by pure-hydrogen pipelines is a prospective mode of energy transmission during the transition to renewables. The risk of hydrogen embrittlement (HE) in pipeline steels must first be quantified to ensure safe pipeline operation. This review provides an overview of HE in pipeline steels. Most pipeline steels have reduced ductility when exposed to hydrogen partial pressures of 100 bar and above. Higher-strength pipeline steels (X80 and X100) have been found to undergo HE at ∼50 bar hydrogen. Hydrogen-induced subcritical crack growth in pipeline steels has not been reported in the literature. There are few articles on HE in pipeline welds, with some indications that the weld is more susceptible to HE, and some indications that it is less. The relationship between hydrogen pressure and absorbed hydrogen concentration has not been evaluated. Gaps in knowledge are identified in the conclusions. [ABSTRACT FROM AUTHOR]

The numerical approach is a vital means for evaluating the stability of flawed rocks. In this paper, the extended non‐ordinary state‐based peridynamics (NOSB‐PD) theory is employed to simulate the fracture process of rocks containing two pre‐existing flaws. Two stress criteria, including the maximum tensile stress criterion and the Mohr–Coulomb criterion, are implemented into the NOSB‐PD numerical method to respectively judge the tensile and shear failure of rock materials. The effects of inclination angles and ligament angles on the crack initiation and coalescence modes for flawed rocks are investigated based on the numerical results. The numerical results are in good agreement with the previous experimental results. The fracture mechanism of flawed rocks is revealed based on the evolutionary distribution characteristics of the maximum principal stress field and shear stress field obtained by the extended NOSB‐PD theory. Highlights: The extended non‐ordinary state‐based peridynamics (NOSB‐PD) theory is employed.The fracture characteristics of flawed rocks are investigated by the extended NOSB‐PD.The fracture failure mechanism of flawed rocks is revealed by the extended NOSB‐PD. [ABSTRACT FROM AUTHOR]

In the present work, stress-dependent matrix crack opening behavior of SiC/SiC composites was analyzed considering stochastic fragmentation in fibers. The stochastic matrix multiple fragmentation model was adopted to determine the fragmentation lengths of three different types, the accompanying interface debonding process was assessed by the fracture mechanics theory, and stochastic fiber fragmentation was determined by the global load sharing (GLS) criterion. Considering the aforementioned multiple micro-damage mechanisms, matrix crack opening displacements (CODs) in different matrix fragmentation states were determined using a micromechanical technique. Relationships between CODs and the constituent properties of SiC/SiC composites with fiber fragmentation were established. The crack opening behavior of SiC/SiC composites was also analytically predicted. Effects of fiber stochastic fragmentation on crack opening-related damage parameters were examined. [ABSTRACT FROM AUTHOR]

Copyright of Acta Materiae Compositae Sinica is the property of Acta Materiea Compositae Sinica Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Bolted spherical joints, due to their prominent merits in installation, have been widely used in modern spatial structures. Despite significant research, there is a lack of understanding of their flexural fracture behaviour, which is important for the catastrophe prevention of the whole structure. Given the recent development to fill this knowledge gap, it is the objective of this paper to experimentally investigate the flexural bending capacity of the overall fracture section featured by a heightened neutral axis and fracture behaviour related to variable crack depth in screw threads. Accordingly, two full-scale bolted spherical joints with different bolt diameters were evaluated under three-point bending. The fracture behaviour of bolted spherical joints is first revealed with respect to typical stress distribution and fracture mode. A new theoretical flexural bending capacity expression for the fracture section with a heightened neutral axis is proposed and validated. A numerical model is then developed to estimate the stress amplification and stress intensity factors related to the crack opening (mode-I) fracture for the screw threads of these joints. The model is validated against the theoretical solutions of the thread-tooth-root model. The maximum stress of the screw thread is shown to take place at the same location as the test bolted sphere, while its magnitude can be greatly reduced with an increased thread root radius and flank angle. Finally, different design variants related to threads that have influences on the SIFs are compared, and the moderate steepness of the flank thread has been found to be efficient in reducing the joint fracture. The research findings could thus be beneficial for further improving the fracture resistance of bolted spherical joints. [ABSTRACT FROM AUTHOR]

The alternate load path method for progressive collapse analysis ignores the cause and mechanism of member failure and its influence on the dynamic effect of the remaining structures. It must assume the location of member failure, which is unsuitable for grid structures with many members. In this paper, by using the double broken line follow-up strengthening model and the proposed VUMAT material constitutive model, taking the member instability and material failure as the member failure criteria and considering the geometric nonlinearity and material nonlinearity, the instantaneous member removal method for deleting the failure member in real time was proposed. On this basis, nonlinear dynamic analysis for a three-layer space truss was used to compare the effects of different member failure criteria on the structural responses, such as the member failure sequence and the occurrence time of the continuous structural collapse. The results show that when considering the instability of members, the initial member failure and the final progressive collapse occur earlier than when using the material failure criterion, which is more similar to the actual situation. Furthermore, the proposed VUMAT material constitutive model can apply to study the progressive collapse behavior of space truss structures. [ABSTRACT FROM AUTHOR]

To explore mode‐I fracture process of hard brittle rocks, semi‐circular bending (SCB) experiments were performed with the concurrent monitoring of acoustic emission (AE) and digital image correlation (DIC). Experimental results revealed that all specimens exhibited ductile fracture characteristic and the fracture planes deviated from the ligament plane to different degrees due to interference from mineral crystals or pre‐existing weak surfaces within the intergranular boundaries. The whole cracking process can be divided into four stages: Microcrack initiation preparation, microcrack initiation and stable propagation, macrocrack formation and unstable propagation, and macrocrack coalescence. The onset of the microcrack initiation occurred at about pre‐80% load. The fracture process zone (FPZ) maintained a constant length as the crack propagated and the length of the traction free zone (TFZ) increased. The identified crack lengths based on the AE locations and energy statistical analysis consisted well with the crack lengths obtained from the DIC. Highlights: The fracture behaviors were explored by three‐point bending tests on granite specimens.Four crack development stages were recognized by the CTOD and AE parameters.The crack process and fracture characteristics for granite were determined by DIC and AE. [ABSTRACT FROM AUTHOR]

The traditional approach of strength of materials applied to failure of unidirectional fiber reinforced composites is examined in view of the failure mechanisms observed. While the elastic limit of isotropic materials such as metals is given by the measured yield strength and reflects the manifestation of the underlying crystalline slip processes that is not the case for unidirectional composites that are orthotropic and have different independent mechanisms operative under axial tension, axial compression, transverse tension, transverse compression, and in-plane shear. The paper discusses why the generalization of yield theories of isotropic metals to textured metals of orthotropic symmetry cannot be used for describing failure of unidirectional composites. Furthermore, the failure criteria proposed in the literature to describe interactions under combined loading are scrutinized to clarify their inadequacy to treat those interactions. The use of tractions on an assumed failure plane to formulate failure criteria is also discussed as without basis in the failure mechanisms observed. Finally, a way forward for physical modeling of failure in unidirectional composites is proposed. [ABSTRACT FROM AUTHOR]

Antidip bedding rock slopes always contain a developed set of joints that dip into the free surface; however, sometimes they have some gently inclined cross joints, and the persistence ratio (k) of these cross joints exerts important influences on the stability and failure modes. In this work, a limit-equilibrium model combined with a fracture mechanics (FM) method was established to predict the stability of antidip bedding rock slopes with very low-persistent cross joints. Rock layers that might undergo toppling failure were analyzed with the theories of Mode I fracture in FM; layers with the potential for shear–sliding failure were studied by solid mechanics and theories for Mode II fracture from FM. Then, a practical slope and a model test (reported in previous studies) were used to verify the feasibility of the proposed method. The calculated results were consistent with the actual state of the slope and the experimental result, which indicated that a limit-equilibrium model combined with an FM method could be used to predict the stability of such slopes. To further approve the rationality of the method, a contrastive analysis was conducted that used two other methods. The results showed that the predicted factors of safety (FS) of the proposed method were conservative and on the safe side. The proposed method could be used by engineers to evaluate and design such rock slopes. [ABSTRACT FROM AUTHOR]

Copyright of Acta Materiae Compositae Sinica is the property of Acta Materiea Compositae Sinica Editorial Department and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

It is well known that the installation loads during offshore pipeline installation are usually higher than the operation loads. Fibre-reinforced plastic (FRP) materials for offshore pipeline applications seem to be very promising due to their high strength, light weight and excellent corrosion and fatigue resistance. However, during S-lay installation, high bending stresses that may cause material failure or buckling take place. Due to the anisotropic nature of multi-layered FRP pipelines, the permissible bending curvature is difficult to estimate. Few published works have proposed a framework for bending stress calculations and those that have provide complex equations that are not convenient for use. In this study, an open numerical code in Mathematica is offered for the first time. Numerical results for S-lay installation of representative pipelines are provided and commended. [ABSTRACT FROM AUTHOR]

In recent decades, the number of applications of fiber-reinforced polymer (FRP) reinforcement for concrete has significantly expanded. This fact is mainly due to the FRP reinforcement cost competitiveness, the increasing availability of design provisions, and due to the advantageous physical-mechanical and chemical properties of this composite material, which open up new possibilities for the design and implementation of extremely durable elements. Currently, the most commonly used types of reinforcement bars are those containing glass FRP (GFRP) or basalt FRP fibers, which is mainly the result of their favorable price and high resistance to aggressive environments. The use of composites in construction is wide-ranging: it is possible to apply FRP bars not only in the design and strengthening of concrete structures but also in the installation of shear dowels in concrete pavement, as rock bolts or as anchors for overhanging facade components. In the case of the design of load-bearing elements subjected to a combination of tensile and shear force, it is necessary to quantify their shear resistance and concisely describe the effect of the interaction of the tensile and shear force on the load-bearing capacity of the reinforcing element. This is a broadly addressed area as regards composite laminates and fabrics, but when it comes to FRP bars, there are very few available experimental results. For that reason, this research deals with the experimental testing and quantification of the influence of the interaction of normal and shear force. The behavior of GFRP bars from two different manufacturers, with three different diameters, different surface treatments, and mechanical characteristics, was experimentally verified. The findings are presented, a material failure curve is compiled, and the results are compared with those predicted according to the available relationship for composite laminates and von Mises theory. The presented research was driven foremost by industry demand concerning prestressed fiber-reinforced polymer (FRP) rock bolts. In situations where such bolts are used, the slippage and displacement of parts of rocks can be expected, and consequently, the creation of a shear plane in the prestressed anchor occurs. However, there are no existing methods for determining the load-bearing capacity of such loaded FRP bars. This paper aims to determine the short-term behavior of specimens under the combined action of tensile and shear forces (failure envelope) and to compare their load-bearing capacity with available relationships for steel and composite laminates. [ABSTRACT FROM AUTHOR]

The prediction of damage and failure to fiber-reinforced polymer composites in extreme environments, particularly when subjected to impact loading, is a crucial issue for the application and design of protective structures. In this paper, based on the prototype microelastic brittle (PMB) model and the LaRC05 composite materials failure model, we proposed a bond-based peridynamic (BB-PD) model with the introduction of plastic hardening of the resin matrix for fiber-reinforced polymer composites. The PD constitutive relationships of the matrix bond and interlayer bond under compressive loading are considered to include two stages of linear elasticity and plastic hardening, according to the stress–strain relationship of the resin matrix in the LaRC05 failure model. The proposed PD model is employed to simulate the damage behaviors of laminated composites subjected to impact loading. The corresponding ballistic impact tests of composite laminates were carried out to observe their damage behaviors. The PD prediction results are in good agreement with the ballistic experimental results, which can verify the correctness and accuracy of the PD model developed in this study in describing the impact damage behaviors of composite materials. In addition, the characteristics and degree of damage in composite laminates are analyzed and discussed based on this PD model. The difference in the impact resistance of composite laminates with different stacking sequences is also studied using the numerical simulation results. [ABSTRACT FROM AUTHOR]

Fissures and holes widely exist in rock mechanics engineering, and, at present, their failure mechanisms under complex compress and shear stress states have not been well recognized. In our work, a fracture mark, ξ, is introduced, and the kernel function of the smoothed-particle hydrodynamics (SPH) is then re-written, thus realizing the fracture modelling of the rock media. Then, the numerical models containing the fissures and holes are established, and their progressive failure processes under the compress and shear stress states are simulated, with the results showing that: (1) the improved SPH method can reflect the dynamic crack propagation processes of the rock masses, and the numerical results are in good agreement with the previous experimental results. Meanwhile, the improved SPH method can get rid of the traditional mesh re-division problems, which can be well-applied to rock failure modeling; (2) the hole shapes, fissure angles, fissure lengths, fissure numbers, and confining pressure all have great impacts on the final failure modes and peak strengths of the model; and (3) in practical engineering, the rock masses are in the 3D stress state, therefore, developing a high performance 3D SPH program and applying it to engineering in practice will be of great significance. [ABSTRACT FROM AUTHOR]

In order to study the influence of sudden change of loading rate on crack propagation and failure mode of rock bridge of fractured rock mass, the specimens used in this paper were rock-like materials containing two prefabricated fractures. The mechanical properties and the failure mode of the specimens under different constant loading rates and sudden changes in loading rate were tested. The photographic monitoring and acoustic emission were carried out at the same time. The results show that: (1) In the process of sudden change loading, the specimen shows the characteristics of approximate elastic deformation, and the stress-strain curve after sudden loading is similar to the corresponding stress-strain curve at the corresponding constant loading rate. (2) Combined with acoustic emission detection, it is found that when the loading rate is not much different before the mutation, the loading rates is the same after the mutation and the stress-strain curve of the specimen is similar. In the low-speed loading stage, the acoustic emission count is generally low, while in the high-speed loading stage, the acoustic emission count is generally high. The sudden change from low-speed loading to high-speed loading easily induces stress drop, resulting in crack generation and increase of acoustic emission count. (3) The rate of the specimen during the crack development period plays a decisive role in the failure mode of the specimen. Before the sudden change of loading rate, the low-speed loading within a certain range has little effect on the specimen. When the high-speed loading is carried out when the stress is low in the early stage of the mutation, the acoustic emission count of the specimen is high, which will cause some damage inside the specimen. As a result, even if the rate is the same after the mutation, the final peak stress and failure mode may be different. [ABSTRACT FROM AUTHOR]

Strength anisotropy is one of the most distinct features of layered formations, but anisotropic tensile strength has received scarce attention in the prediction of fracture pressure. Therefore, the characteristics of anisotropic tensile strength of layered rocks were investigated, main anisotropic tensile failure criteria were reviewed and compared, and the fracture pressure models were deduced for three different anisotropic failure criteria. The results demonstrate that the tensile strength of layered rocks is strongly anisotropic. The Nova–Zaninetti criterion is recommended for the characterization of the anisotropic tensile strength. The variation of the fracture pressure caused by anisotropic tensile strength is close to 10% (it even exceeds 10%), and the anisotropy has a greater influence on the fracture pressure with increasing anisotropic degree. The higher the degree of anisotropy is, the lower the fracture pressure is; the higher the horizontal stress ratio and pore pressure are, the higher the impact of anisotropy on the fracture pressure is. A low dip angle of the bedding plane is beneficial for the fracture of the vertical wellbore, and it is the worst when the minimum horizontal stress is perpendicular to the bedding planes. [ABSTRACT FROM AUTHOR]

In this paper, molecular dynamic simulations are utilized to investigate the effect of temperature and topological defects on the mechanical response and tensile strength of grain boundaries in single-layer hexagonal boron-nitride (h-BN) nanosheets. Six different cases of grain boundaries are examined including four symmetric and two asymmetric models. We study various types of misorientation angles and defects at different temperatures. The tensile tests of all models are also performed at different strain rates at room temperature. The results clearly show that increasing the misorientation angle of the grain boundary leads to a decreasing tensile strength, elastic modules and strain-at-failure of h-BN nanosheets. The mechanical response of high-angle misorientation grain boundaries were observed to be quite unique under uniaxial tension, since a chain of atoms between two fractured parts was observed even after tensile failure. Increasing the temperature also leads to a decreasing fracture strength and strain-at-failure for all six models. The low-angle misorientation of grain boundaries are more sensitive to variations in the temperature than grain boundaries with high-angle misorientation. The stress-strain curve shows a large drop for low-angles grain boundaries when the temperature elevates from 100 K to 300 K. In addition, small fluctuations could be observed for all models at high temperature, which does not depend on the misorientation angle of the grain boundary. [ABSTRACT FROM AUTHOR]

Acoustic emissions (AE) and stress–strain curve analysis are accepted methods of analyzing crack propagation and monitoring various failure stages, including crack closure and the crack initiation level, during rock failure under brittle rock loading. This study designed a series of three-point bending tests and shear fracture tests to investigate the features of AE signals for different modes. The Extended Finite Element Method (XFEM) is also adopted to simulate these experiments processes. The obtained results indicate that the maximum load for the shear fracture mode is approximately three times larger than that for the tensile fracture mode and that the displacement in the shear fracture mode is also larger than that in the tensile fracture mode. AE signals had higher AF (average frequency) and lower Rise Angle (RA) (ratio of the rise time to the peak amplitude) values when tensile failure occurred, while they exhibited lower AF and higher RA values when shear failure occurred. Accumulative AE energy for the shear mode is significantly larger than that for type I tensile fracture. The influence of inhomogeneity is also evident, as signals acquired at different distances exhibit distinct characteristics. The results show that AE leads to the characterization of the dominant fracture mode using only two AE descriptors. Note that we also should reasonably arrange AE sensors to monitor large-scale projects in the field. [ABSTRACT FROM AUTHOR]

The accuracy of the multi-parameter approximation of the stress fields in a cracked body is studied in the paper. This approximation, which uses the Williams power series expansion (WE), is intended to be used to estimate the extent of the nonlinear zone, which plays a significant role within tensile failure and the fatigue assessment of non-brittle materials. The characteristics of this zone could be potentially incorporated into methods of determining the true values of fracture parameters and the fatigue behaviour descriptors of materials exhibiting nonlinear failure. Considering the fact that in the case of elastic–plastic and especially quasi-brittle materials the size of this zone is substantial in comparison to specimen dimensions, it is necessary to consider a large region around the crack tip for this task. An automatic routine implemented as a Java application was created to determine the values of coefficients of the higher order terms of the WE that describe crack-tip fields. These values are calculated using the Over-Deterministic Method (ODM), which is applied to the results of the finite element (FE) analysis of an arbitrary mode I test geometry. Furthermore, another Java application developed by the authors provides an analytical reconstruction of the crack-tip stress field by means of the truncated WE, and enables detailed analysis of the crack-tip stress field approximation. The developed procedures simplify the analysis of the description of mechanical fields at a greater distance from the crack tip considerably. The presented research is focused on the optimisation of the selection of FE nodal results entering the ODM procedure used to determine the values of coefficients of the higher order terms of the WE. The aim is to improve the accuracy of the approximation. [ABSTRACT FROM AUTHOR]

• Mode II dynamic fracture properties of UHSS are studied by a new experimental method. • The mode II fracture toughness is positively correlated with the loading rate. • The materials show a gradual transition from tensile fracture to adiabatic shearing failure mode. • The failure of the materials under mode II dynamic fracture is dominated by different mechanism. • The rate sensitivity parameter n is proposed to reflect the loading rate effect on DFT. 40Cr and 30CrMnSiNi2A are both ultra-high-strength steel (UHSS) that is usually used in engineering structures such as aircraft landing gear, wing girder, and fastening bolts. Under dynamic loading, fracture properties of such materials are very important for structural design. A novel mode II dynamic fracture testing technique is adopted to study the mode II dynamic fracture characteristics of these two materials under high loading rates. The mode II dynamic fracture specimen is designed for the SHPB equipment, and the stress intensity factor curve at the crack tip is determined by an experimental-numerical method. The crack initiation time of the specimen is determined by the strain gage method. In the end, the mode II dynamic fracture toughness (K IId) of the two materials is obtained, and the loading rate effects are compared and analyzed in detail. The results show that within the loading rate range of this research (1.08 ∼ 7.73 TPa·m1/2/s), the K IId of the two materials has a positive correlation to the loading rate. Both of the materials exhibit a gradual transition process from tensile failure to adiabatic shearing failure mode. According to the fracture morphologies of the two materials, the failure mechanism of the two materials is analyzed for different failure modes. Failure mode transition (FMT) is specially investigated with the increase of the loading rate for both of the materials. [ABSTRACT FROM AUTHOR]