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Coal joints and cleats are geological discontinuities that are the most important factors that affect the mechanical responses of a coal mass under stress. The joint and coal mass interaction and the mode of failure dominate the mechanical behaviour of jointed coal masses, and therefore the stability of coal excavations. The shear or mixed shear/tensile failure changes to tensile failure by increasing the confining pressure, discontinuity length and angle. This paper extends a thermodynamic approach to constitutive modelling of the coal mass by developing local and non-local damage models based on the joint and cleat density and the dip angle. A consistent and rigorous statistical framework is constructed, which incorporates both local and non-local features into the constitutive modelling. This is an important consideration in developing damage constitutive models based on the trajectory of the failure surfaces in a coal mass. An equation is derived to calculate the fracture energy which is a function of the joint density either in a single direction or crossed conditions.

AbstractWhen the working face of a steeply dipping coal seam crosses a fault, the dynamic disturbance caused by the mining load induces fault activation, causing large deformation and failure of the roadway. In this study, a physical similarity simulation and numerical calculation method were used to determine the main factors that the deformation and failure of fault zone roadway and reveal the underlying mechanisms. The results show that when crosses a fault, the high static load formed by the coupling of mining stress and fault pillar stress induces mining stress-type fault activation, which leads to large deformation and damage to the roadway in the fault zone. The main controlling factors are the fault location and mining load intensity. In terms of fault location, the vertical stress concentration away from the roadway side of the fault surface was high, the principal stress deflection angle was large, and the surface deformation of the fault on the roadway left side was the greatest. In terms of mining load intensity, the deflection angle of the principal stress of the surrounding rock initially increased and then decreased, and tensile failure easily occurred near the fault surface. These results provide a reference for roadway support.

Research on stratified rock masses, which are common geological formations, has primarily focused on their mechanical properties, while studies on crack evolution and microscopic damage mechanisms remain limited. This study addresses this gap by investigating the combined effects of strength ratios and soft layer thicknesses on the microcrack evolution mechanism of stratified rocks using the discrete element method (DEM). Through FISH language programming in the particle flow code (PFC), this study reveals the acoustic emission (AE) characteristics, crack initiation and propagation, damage degree, and final failure characteristics. The key findings are: (1) Higher strength ratios between the hard and soft components of stratified rocks make specimens more sensitive to increases in soft layer thickness. (2) Three types of AE events were identified: continuous active, intermittent active, and silent. (3) Cracks initiate at the interface between components and propagate along the interface into the rock matrix. The strength ratios determine the crack propagation path and the damage extent of the components. (4) The failure of stratified rocks is primarily controlled by the soft component. Crack connections typically form vertical and sub-vertical tensile failure planes in the hard component, and a shear failure surface with a “V”-shaped intersection in the soft component.

The presence of random joints, cracks, and other defects significantly affects the meso-damage mechanism and macro-mechanical behavior of the rock. This study employed micro-CT scanning, digital image processing (DIP), and the rock failure process analysis system (RFPA3D) to reconstruct a genuine mesostructure, creating a three-dimensional (3D) numerical model of jointed sandstone. Under uniaxial stress, this model facilitated the meso-damage evolution process of prefabricated cracks in sandstone with varying dip angles. Additionally, the influence of jointed sandstone heterogeneity and prefabricated cracks with various dip angles on its failure mode and meso-damage mechanical properties were investigated. Utilizing the MATLAB platform, a 3D box dimension algorithm was developed to analyze the fractal characteristics of the mesodamage evolution in the sample. This algorithm enabled the quantitative characterization of the meso-damage evolution of sandstone. This study categorized three types of sandstone final failure modes: composite shear failure, shear failure along the joint surface, and tensile failure. Additionally, linear variations in the elastic modulus and compressive strength of the jointed sandstone were observed with increasing prefabricated fracture inclination, highlighting significant anisotropy. The presence of joints was found to induce and control the failure mode of sandstone. The meso-damage evolution process of sandstone was described in terms of the fractal dimension, indicating that more severe damage corresponded to a larger fractal dimension. This approach offers a novel statistical method for studying the progression of rock damage.

The full-grouted sleeve connection (FGSC) is widely used in prefabricated structural connections, but the complexity of the grouting process often leads to defects, primarily grouting voids. The mechanical properties of FGSCs with these defects under fire exposure are not well understood. This study examines 144 FGSCs with various grouting voids exposed to specific temperatures before cooling to room temperature for static tensile testing. Key parameters include defect type, volume defect rates, and temperature. The study identifies two primary failure modes: rebar tensile and pull-out failures, influenced by void size, distribution, and temperature. Below 300 °C, tensile failure occurs when the defect rate is ≤0.05; above this rate, pull-out failure dominates. Above 300 °C, all specimens exhibit pull-out failure. Specimens with concentrated voids show lower bearing capacities. The research provides a bond stress-slip constitutive model for FGSCs and a fitting formula for characteristic points. A neural network model predicts the ultimate bond stress based on defect type, volume defect rate, temperature, and grouting material strength, with sensitivity analysis indicating the volume defect rate as a critical factor. This work offers a comprehensive theoretical foundation for studying the impact of grouting defects on FGSCs' post-fire mechanical properties.

The fundamental influence of specimen size and micromechanical parameters on macroscopic structural behavior of natural building stone is investigated by particle flow numerical analysis. Laboratory tests of Dionysos marble cylindrical specimens under uniaxial compressive loading are simulated with a focus on the fracture development, failure mode and uniaxial compressive strength. Two series of simulations are performed with the PFC2D code, one to define the effects of different rates of deformation on the uniaxial compressive strength and fracturing of the specimens and the other to investigate the behavior of the specimens with the variation of five different parameters; platens velocity, specimen size, particle size distribution, standard deviation of randomized shear and normal strength as a micro-parameter and the ratio of shear to normal particle bonding strength. The specimen sizes also include rarely investigated smaller than NX dimensions. On selected specimens, the fracture development and the failure mode is depicted and discussed, and conclusions are drawn about the shear to tensile failure frequency and the crack patterns.

Underground energy storage facilities are subject to disturbances at varying strain rates during construction and operation, necessitating investigations into the effects of strain rate on the mechanical properties of rocks. This study employs a self-developed thermo-mechanical (TM) coupled experimental apparatus to conduct triaxial compression tests on salt rock and granite under real-time high-temperature conditions at different strain rates. Initially, the energy evolution during the rock loading process is described from an energy perspective, analyzing the changes in input energy density, elastic energy density, dissipation energy density, and the rate of energy change during the loading process. Subsequently, the impact of strain rate on the compressive strength, peak strain energy, and peak energy of salt rock and granite is explored. Finally, the fracture patterns of the rocks are described macroscopically, and the damage process is elucidated from an energy perspective. The results indicate: (1) A positive correlation between the compressive strength of granite and strain rate, whereas a negative correlation exists for salt rock. (2) Granite predominantly exhibits a shear failure mode, while salt rock, due to its strong viscoelastic rheological properties, primarily shows tensile failure. (3) The rate of energy change in salt rock is relatively steady throughout the loading process, in contrast, granite experiences more dramatic energy fluctuations, especially in the rockburst phase post-peak. (4) Differences in energy conversion between salt rock and granite during loading: approximately 50 % of the input energy in salt rock is converted to dissipated energy before reaching peak load, whereas about 90 % is stored as elastic energy in granite during this phase. The findings of this study can provide technical guidance for the construction of underground salt cavern storage facilities.

Ice, a quasi-brittle material with a complex crystal organization and found ubiquitously in nature, undergoes an impact fragmentation process that implies a rich physical mechanism, yet remains not thoroughly elucidated. We develop a highly robust and efficient meshless method for fluid–solid coupling, specifically designed to elucidate the mechanisms of crack propagation in S2 columnar ice subjected to high-speed water jet impacts. This method couples a low-dissipation Riemann smooth particle hydrodynamics approach with a non-ordinary state-based peridynamics model, 1 1 The computer program is available from: https://github.com/Luwen-Zhang/Icebreaking%5fPD-SPH. enabling detailed exploration of fracture process. Our theoretical advancements enhance numerical stability at the fluid–solid interface and establish a precise ice constitutive model by capturing the unique hydrostatic pressure-dependent and rate-dependent plasticity within the peridynamics framework, effectively addressing challenges in both fluid and solid phases. Combined with high-velocity water jet impact experiments, this study successfully delineates the initiation and expansion of circumferential and radial cracks in ice plates. We demonstrate that these cracks, both circumferential and radial, originate from tensile failure induced by circular elastic–plastic stress waves initiated by point source shocks. Specifically, circumferential cracks emerge and propagate from the upper to the lower surface driven by radial tensile stress, while radial cracks, motivated by circumferential tensile stress, develop from the lower to the upper surface. This investigation not only provides a foundational understanding of ice impact fracturing but also establishes a versatile theoretical framework applicable to a wide range of quasi-brittle materials. [ABSTRACT FROM AUTHOR]

To address the challenges of large-scale solid waste treatment and achieve the "dual carbon" goal, mixing modified magnesium-coal based solid waste with industrial CO2is an effective carbon-reduction technique. In this study, a novel modified magnesium-coal based solid waste CO2backfill material (CO2-MCSB) was produced. The mechanical characteristics, microstructure, and carbon sequestration impact of CO2-MCSB after CO2absorption were studied using uniaxial compressive strength, acoustic emission, X-ray diffraction, thermogravimetry, Scanning electron microscopy, and other tests. CO2-MCSB's mechanical properties improve greatly after absorbing CO2, with a 28-day strength increase of 0.514–3.103 MPa. Additionally, it has a high carbon sequestration effect (5.6–6.8 %). Under uniaxial compression, CO2-MCSB mainly exhibits tensile failure, and exhibits different acoustic emission characteristics under different stirring rates, ventilation rates, and stirring times, corresponding to the evolution process of its microstructure. The carbonation reaction of CO2-MCSB boosted its hydration reaction process, according to a thorough investigation of X-ray diffraction, thermogravimetry, scanning electron microscopy, and other techniques. Following aeration, the hydration of CO2-MCSB produced a significant number of needle-like AFt and C-S-H gel, as well as a large number of microscopic calcium carbonate crystals created following carbonization filled the pores, which was the main reason for the improvement of the strength of the backfill body. CO2-MCSB, a novel type of carbon sequestration backfill material, has huge applications in mining backfill.

The development of new engineering materials is in demand for different kinds of manufacturing sectors. In this context, metal matrix composites have been focused by many industrialists and recent researchers because of their various attractive features. In the present attempt, an aluminium-based hybrid metal matrix composites have been fabricated using stir casting process. AA2024 has been chosen as the base metal and tungsten carbide (WC) and graphene nanoparticles have been used as reinforcements. The size and shape of the reinforcement particles have been confirmed by means of SEM micrographs. By reinforcing the WC particles at different wt% such as 0, 1, 2 and 3, and reinforcing the graphene nanoparticles at 0, 0.15 and 0.3 wt%, 9 different composite samples have been prepared. The tensile, compression, hardness and impact tests have been performed on the prepared composite samples by following respective ASTM standards. The maximum tensile strength of 242.77 MPa and the maximum compressive strength of 398.05 MPa have been noted with the test samples. The tensile failure mechanism has been observed from the SEM micrographs and explained. The characterization study results revealed that there was a significant influence of adding WC and graphene nanoparticles on the mechanical properties of the proposed MMC.

Resistivity will have different response characteristics to the hydraulic fracture propagation process. In this work, a resistivity testing system for hydraulic fracturing specimens was established. Resistivity and acoustic emission (AE) information were jointly analysed to determine the dynamic response characteristics of resistivity during hydraulic fracture propagation. The results show that the water and fracture exert a competitive influence on the connection structure of the circuit, and there are two significant peak resistivity points in the curve, presenting a double peak therein. The peak resistivity data of the specimen with a larger fracture area are much different from the initial value. With the increase of the rate of injection, the range of variation of the highest value that can be reached with the specimen resistivity decreases. High resistivity rates or high resistivity fluctuations exhibit rapid a release of fracture energy. The fracture failure mode dominated by shear fractures makes the formation produce a “series+parallel” electrical connection structure; a calculation model of formation resistivity based on shear and tensile failure was proposed to characterize the proportion of different types of hydraulic fractures and elucidate the control effect of matrix resistivity on the electrical performance of the overall circuit structure.

In the construction procedure of infrastructure, the excavation disturbance significantly changes the initial stress state of rocks in deep geotechnical engineering, and a rock burst would occur accompanied by the ejection of rock block. In the present study, a true triaxial unloading test and its corresponding numerical analysis based on the three-dimensional particle flow code (PFC3D) are conducted on granite specimens to acquire characteristic information of static and dynamic failure during unloading and to explore the criteria for determining static and dynamic failure. The results indicate that the failure modes of granite under true triaxial unloading can be classified into three categories: ① no obvious coalesced crack, ② a main coalescence shear crack, and ③ a main coalescence shear crack with a V-shaped pit. The instantaneous kinetic energy in static failure is characterized by a sharp increase and sporadic sharp increases, while that in dynamic failure shows a sharp increase, a short quiet period, and a substantial increase. The critical stress combination for the transformation of static–dynamic failure of granite is proposed, which is consistent with the experimental results. The number of shear cracks gradually increases in static failure, while it suddenly increases in dynamic failure. During the transformation from static failure to dynamic failure, the ratio of a tensile crack number to that of shear crack number decreases. The proportion of shear cracks in particle bonding increases, while the trend of tensile failure weakens. However, tensile failure remains predominant in granite, accompanied by shear failure.

Wellbore instability is one of the key factors limiting safe and efficient drilling in hot dry rock. A detailed understanding of the instability mechanism of hot dry rock wellbore is essential to achieve high efficiency in geothermal exploitation. Therefore, in this study, we established a numerical simulation model of wellbore stability that considers the heterogeneous properties of granite rock, and the coefficient of stability index for the wellbore of hot dry rock is proposed. The influence of heterogeneous characteristics of granite, in-situ stress, mud pressure, and temperature on the wellbore instability are analyzed. The results show that the granite heterogeneity leads to a significant disturbance of the near-wellbore stress. The degree of wellbore instability is more severe than that of the homogeneous rock formations. Radial tensile damage tends to occur at low mud pressures. High mud pressure and temperature difference between drilling mud and formation easily lead to tangential tensile failure. Shear damage occurs on the radial/tangential stress horizontal plane without temperature differences. However, a temperature difference dramatically increases the axial stress (the first principal stress), changing the direction of shear damage, which occurs mostly in the vertical plane or the wellbore's curved surface. Deeper wellbores are more severely unstable than shallow wellbores, and mud pressure can inhibit wellbore instability to a certain extent. However, increasing excess mud pressure can lead to tangential tensile failure. The temperature difference exacerbates the wellbore instability; especially after exceeding 260 °C, the wellbore shear thermal rupture is drastic. The study results are of great significance for the in-depth understanding of the mechanism of wellbore instability in deep hot dry rocks and for guiding the safe and efficient drilling of hot dry rocks.

This paper presents a systematic experimental study on the effect of shape memory alloy (SMA) fibres (0.25–1% by volume) on microstructure and engineering properties, in particular mechanical properties of polyvinyl alcohol (PVA) fibre (2% by volume) reinforced engineered cementitious composites (ECC). A series of tests were performed to characterise the flowability, compressive properties and flexural and uniaxial tensile behaviour in terms of strain/deflection-hardening and multiple microcracking performance and flexural and tensile strengths, as well as the 3D fractal crack analysis and microstructural evolution of ECC. Results indicate that the presence of more SMA fibres diminished the flowability of ECC, which can be ascribed to the higher surface roughness of SMA fibres as confirmed by the atomic force microscopy analysis but did not result in any change in the self-compacting characteristics of ECC. The highest compressive strength was achieved at the ECC mix with 0.5% SMA inclusion as 62.5 and 72.1 MPa for ambient curing of 28 and 56 days. Although the presence of 1% SMA fibre reduced the compressive strength of ECC due to the increased interfacial porosity and fibre cluster, the residual flexural crack width of ECC containing 1% SMA fibre was effectively reduced by about 54.3% as compared with the reference ECC mix, consistent with its best tensile performance. Accordingly, the failure patterns of ECC changed from the tensile failure with more uniformly distributed cracks and the highest dissipated fractal fracture energy (Ws/Gf=167.7) to the tensile failure along with less tortuous crack path and fractal fracture energy (Ws/Gf=80.2).

The effect of in situ stress on the coal-breaking characteristics of water jets remains unclear, prohibiting the deep coalbed methane (CBM) development. Water jet coal-breaking experiments under different mean three-dimensional (3D) stresses and horizontal stress differences were carried out with a self-developed in situ stress simulator. When the mean 3D stress increased, coal changed from shear and tensile failure to shear failure, and the volume of the erosion pit first decreased rapidly and then slowly. Upon increasing the mean 3D stress from 0 to 10 MPa, the volume of the erosion pit decreased by 79.7%, and the specific energy consumption increased nearly five times. With an increase in horizontal stress difference, coal transitioned from shear failure to shear and tensile failure, resulting in a shear crack farther from the erosion pit. At a horizontal stress difference of 15 MPa, the volume of the erosion pit and specific energy consumption had maximum and minimum values, respectively. Consequently, constructing a higher horizontal stress difference helps improve the coal-breaking efficiency of water jets. This study could lay the foundation for applying water jet technology in deep coal seams.

The application of high-strength and high-toughness titanium alloy in armour protection is increasing with its excellent properties. However, there is little research on titanium alloy in explosion protection, especially the dynamic response process under localised explosion load and finite element simulation of the explosion resistance process of titanium alloy sheet. In this paper, the deformation and fracture failure features of the 1.6 mm thick Ti6Al4V titanium alloy sheet during the blast loading process were studied by combining LS-DYNA finite element simulation and air explosion experiments. The error of the overpressure peak value of explosion shock wave between the value obtained by improved theoretical empirical formula calculation of cylindrical charge and the value obtained by numerical simulation is only 0.2%. By studying the influence of explosive shape and air domain mesh size on the overpressure value in the process of detonation wave propagation, the Structured Arbitrary Lagrangian-Eulerian (S-ALE) algorithm was selected for finite element simulation. When the air domain mesh size is 1 mm, the tensile failure criterion was introduced into the finite element simulation process. The results show that the numerical simulation results of the full-scale model and the test results were consistent. The destruction morphology of the target showed severe asymmetry. Under 50 g TNT blast load, the 1.6 mm thick Ti6Al4V titanium alloy target was damaged seriously, and the central area of the target was completely torn. The failure features were mainly petal-shaped warping tearing cracks and localised plastic deformation. The tensile stress is the main stress mode of the target failure. Therefore, dynamic tensile strength should be used as one of the selection criteria for explosion-resistant titanium alloy sheets. The target failure mode is dominated by adiabatic shear fracture.

The joint strength of self-piercing riveting is a key factor for evaluating riveting quality. Shot peening is a useful process to strengthen sheet metal. To improve the performance of self-piercing riveting joints, the shot peening process and self-piercing riveting process are combined in this work. The shot peening of AA5052-H32 aluminum alloy sheets, static and fatigue experiments of the self-piercing riveting joints are executed, the effects of shot peening on the mechanical performance of the joints are performed. The results show that: The shot peening process can induce residual compressive stress in the surface layer of aluminum sheets, which can improve the performance of the joints; In the shot peening process, the largest residual stress is around −187.3 ± 5.1 MPa; The shot peening process can improve the static strength of the joints by about 9.5%, and has little effects on the tensile failure forms of the joints; The fatigue strength of the shot-peened self-piercing riveting joints is increased by about 9.2%. The main fatigue failure type is the damage of the lower sheets at the contact zones with the rivet leg. The contact surfaces for the shot-peened self-piercing riveting joints are suffered less from the fretting damage than that of non-shot peened joints. The static strength, fatigue strength are increased, and the fatigue crack growth rate is decreased for shot-peened joints. The shot peening process can be innovatively combined with the self-piercing riveting process to improve self-piercing riveting joint performance. This work provides new ideas to improve the self-piercing riveting quality.

Drained triaxial tension tests were conducted to investigate the tensile and shear failure behaviors of cement-treated soils under effective confining pressures. In the tests, tensile force was applied on saturated cement-treated soil specimens at effective confining pressures. The experimental result for a cement-treated soil shows that tensile failure occurred at low effective confining pressures, while shear failure occurred at high effective confining pressures. Based on experimental evidence, a failure criterion for cement-treated soil is discussed. In this study, tensile failure is assumed to be dominated by the effective minor principal stress and the Mohr-Coulomb failure criterion is adopted for shear failure. The observed stress states at failure lie on the failure criterion consisting of the tensile and shear failure surfaces, indicating that the proposed failure criterion is suitable for cement-treated soils. The experimental results provide a good understanding of the failure behavior of cement-treated soils that exhibit tensile and shear failure modes.

We present numerical models of faults propagating by work optimization in a homogeneous medium. These simulations allow quantification and comparison of the energy budgets of fault growth by shear versus tensile failure. The energy consumed by growth of a fault, Wgrow, propagating by in‐line shearing is 76% of the total energy associated with that growth, while 24% is spent on frictional work during propagation. Wgrowfor a fault propagating into intact rock by tensile failure, at an angle to the parent fault, consumes 60% of the work budget, while only 6% is consumed by frictional work associated with propagation. Following the conservation of energy, this leaves 34% of the energy budget available for other activities and suggests that out‐of‐plane propagation of faults in Earth's crust may release energy for other processes, such as permanent damage zone formation or rupture acceleration. Comparison of these estimates of Wgrowwith estimates of the critical energy release rate and earthquake fracture energy at several scales underscores their theoretical similarities and their dependence on stress drop. Work associated with breaking intact rock is the largest component of energy budget of fault growthWork optimization, a global growth criterion, does not require failure mode to be selected beforehand; failure can be in tension or shearTensile failure at an angle to the parent fault releases energy for additional processes

We combine geophysical and experimental observations to interpret preeruptive unrest at Volcán de Colima in 1998. 17,893 volcanic earthquakes were detected between 1 October and 31 December 1998, including 504 clusters. Using seismic ambient noise interferometry, we observe a drop in velocity prior to the eruption linked to damage accumulation during magma ascent. This is supported by experimental observations where static stress causes a velocity decrease prior to failure. Furthermore, we observe acoustic emission clusters during the experiments, with lower porosity samples producing higher numbers of repeaters. This behavior introduces tensile failure as an additional viable mechanism for clusters during magma ascent. The findings suggest that preeruptive magma ascent may be monitored to variable degrees of accuracy via descriptions of damage accumulation and associated seismic velocity changes. Multidisciplinary seismic and experimental investigation of the 1998 eruption of Volcán de ColimaRelative velocity decrease seen in seismic ambient noise and during experimental Brazil tests prior to eruption/failureEvidence for tensile failure in the volcanic edifice as a source of repeating earthquakes during magma ascent

Hydrofracturing is an important technique in petroleum industry to stimulate well production. Yet the mechanism of induced fracture growth is still not fully understood, which results in some unsatisfactory wells even with hydrofracturing treatments. In this work we establish a more accurate numerical framework for hydromechanical coupling, where the solid deformation and fracturing are modeled by discrete element method and the fluid flow is simulated directly by lattice Boltzmann method at pore scale. After validations, hydrofracturing is simulated with consideration on the strength heterogeneity effects on fracture geometry and microfailure mechanism. A modified topological index is proposed to quantify the complexity of fracture geometry. The results show that strength heterogeneity has a significant influence on hydrofracturing. In heterogeneous samples, the fracturing behavior is crack nucleation around the tip of fracture and connection of it to the main fracture, which is usually accompanied by shear failure. However, in homogeneous ones the fracture growth is achieved by the continuous expansion of the crack, where the tensile failure often dominates. It is the fracturing behavior that makes the fracture geometry in heterogeneous samples much more complex than that in homogeneous ones. In addition, higher pore pressure leads to more shear failure events for both heterogeneous and homogeneous samples. Hydrofracturing is an important technique in petroleum industry to stimulate well production. Yet the mechanism of induced fracture growth is still not fully understood, which results in some unsatisfactory wells even with hydrofracturing treatments. This problem may not be solved in continuum scale so that we establish a pore-scale numerical framework to reproduce and simulate this process. The results show that the failure patterns in hydrofracturing are quite different from those in normal fracturing by pressure. The shear failure plays a very important role besides the tensile failure. The strength heterogeneity has a significant influence on hydrofracturing. A quantitative characterization of heterogeneity and fracture is proposed in this work. Predictions by this method agree well with existing experimental data for several cases. The results will improve understanding of mechanism of hydrofracturing mechanics and therefore help to optimize the hydrofracturing process in applications. Pore-scale modeling for hydraulic fracture mechanism of heterogeneous rocksFracture growth patterns are clarified by modelingQuantitative description of strength heterogeneity and fracture network is proposed

According to the news editors, the research concluded: "The microstructure results revealed the bridging effect of fibrous mask particles in improving the load transfer in polymer matrix, and thereby reducing the brittle tensile failure in bricks. Keywords: Materials Research; Technology EN Materials Research Technology 810 810 1 08/07/23 20230807 NES 230807 2023 AUG 13 (NewsRx) -- By a News Reporter-Staff News Editor at Respiratory Therapeutics Week -- New research on materials research is the subject of a new report. [Extracted from the article]

ABSTRACTAnchor cables often break in various forms. Tensile failure can be resisted by using high-strength materials, while there is no good solution for shear failure. A new supporting structure called Anchor Cable and C-shaped tube that can bear the transverse shear force was invented and a preliminary introduction to the mechanism of the structure is given. Field investigations and in-situ stress tests were carried out on the panel 3210 in the Nanguan Mine. The failure modes of the surrounding rock and cables and the in-situ stress test results show that the working face is obviously affected by the horizontal stress. Thus, a new full-section anchor cable and C-shaped tube support scheme were proposed. The results of FLAC3D show that the new scheme is excellent in controlling the surface convergence and plastic zone extension. The rationality of the scheme was verified by two-and-a-half months of monitoring. Compared with the original scheme, the amount of surface displacement was at least 20% smaller, and the time for the displacement to reach equilibrium was at least 27.4% shorter. The surface of the roadway is flat without supporting material damaging. The new scheme is worth popularizing in similar working conditions.

Reducing solid waste emission is the development direction of cleaner production in the future. This paper proposes to use the agricultural solid waste rice husk ash (RHA) as an alternative cementitious material for the underground filling of mines. The pore structure and mechanical properties of cemented paste backfill with different RHA dosages (RCPB) are analyzed. The results reveal that: (1) The pore structure of RCPB is divided into three types: micropores (<0.1 μm), secondary pores (0.1–100 μm), and main pores (>100 μm). The total number of micropores accounts for the largest proportion, followed by secondary pores and main pores. The total volume of secondary pores accounts for the largest proportion, followed by micropores and main pores. (2) The porosity decreases and the UCS increases with the increase of RHA dosage and curing time. The UCS decreases linearly with the increase of RHA dosage. The UCS has a linear negative correlation with porosity. (3) The cracks of RCPB initiated at the yield point, gradually expanded at the peak point, and finally penetrated at the post-peak point. The failure mode of RCPB is mainly a tensile failure, accompanied by a small number of secondary tensile cracks. (4) The hydration products of RCPB are mainly C–S–H and AFt gels. The greater the RHA dosage and CT, the more hydration products, the fewer pores, the greater the strength, and the greater the average gray value. The outcome of this work will afford a new ideas and methods for using solid waste (Rice husk) as backfilling.

AbstractLaboratory tests and numerical simulation are conducted to study mechanical behavior of rock containing two pre-existing holes in this paper. First of all, rock-like specimens containing two pre-existing holes were prepared to investigate mechanical and acoustic emission characteristics in the uniaxial compression. Different geometries were obtained by changing the angle of the center line of holes with respect to the direction of axial loading. Experimental results show that the stress-strain curves present obvious fluctuation, and evolution process of acoustic emission signal can be divided into four stages. There are three kinds of failure modes: only shear failure, mainly shear and auxiliary tensile failure, mainly tensile and auxiliary shear failure. Then, parallel bonded mode (PBM) based on the particle flow code in two dimensions (PFC2D) were established to study strength and failure modes in the biaxial compression. Finally, process of cracks initiation, propagation and coalescence, and fracture mechanism were further discussed. The most important conclusion reached is that the deformation and failure process of rock mass containing holes defects can be divided into two stages: rock bridge penetration and overall instability, which are sequential.

Coal-like materials have been widely used to simulate low-rank coalbed methane (CBM) reservoirs to develop fracturing stimulation technology since the transportation and preparation of low-rank coal produce easily secondary damages, causing large dispersion of test results. However, the coal particle sizes have a significant effect on the performance of coal-like materials among many factors. Hence, to accurately simulate low-rank CBM reservoirs for promoting the development of reservoir stimulation technologies, cement, gypsum, pulverized coal, and sand were selected as similar materials to make coal-like samples with different coal particle sizes to study the influence law of coal particle size on the mechanics, pore structure, and permeability of samples. The ultrasonic wave velocity, dynamic parameters, uniaxial compressive strength, elastic modulus, and uniaxial tensile strength of samples increased with increasing coal particle size in the range of 0–2 mm. However, these parameters suddenly decreased when the coal particle size increased to 2–3 mm. The porosity and permeability of the samples exhibited an opposite trend compared to the above parameters. The fracture occurred in cementing materials, and the crack extension direction was affected by the coal particle sizes. The fracture complexity of the coal-like samples decreased with increasing particle size under uniaxial compressive testing. The failure mode was mainly a tensile failure, supplemented by shear failure. The coal-like materials can well meet the performance requirements of low-rank CBM reservoirs.

Changes in reservoir water level alter the hydraulic environment of bank slopes and negatively impact their stability by accelerating the process of water–rock interaction. This study, therefore, studied the failure characteristics of a red bed slope near the bank of a hydropower reservoir in Southwest China. Bank material specimens with various initial fracture angles were numerically simulated under different pore water pressure environments and a slope stability analysis was conducted to determine the resulting slope deformation characteristics and stability. The results show that: (1) Pore water pressure mainly affected the peak compressive stress and post-peak behavior of the rock. Under the same pore water pressure, the peak stress first increased and then decreased with increasing initial fracture angle. (2) The initial fracture angle mainly affected the distribution of microscopic tensile failures in the specimen, which in turn affected its compressive strength. The main factor controlling factor rock mass deformation and failure was the pore water pressure. (3) When the long-term water level fell to a given level, the safety factor of the bank slope was lower than that when the long-term water level rose to the same level. The main failure mode of the slope was a shear failure, while tensile failure exhibited a strong correlation with the safety factor. (4) The bank slope risked local instability when the long-term water level rose and risked sectional instability when the long-term water level fell, and there was instability and penetration of the section, indicating the possibility of landslides. (5) When the long-term water level fell, there was a considerable difference between the deformation of the shallow rock mass and the deformation of the deep rock mass, and the traction and deformation effect of the shallow rock mass on the deep rock mass was more obvious.

This work investigates the influence of porosity reduction by silica precipitation on pore pressure development, heat transfer and reactive flow in a geothermal system. Porosity reduction due to silica precipitation can generate excess pore pressure, thus affecting the stability of the rock mass. In addition, porosity reduction increases the thermal diffusivity of the rock, affecting the thermal front, while silica precipitation generates enthalpy. Consequently, the effective stresses associated with solute deposition must be considered for accurate studies of rock failure. We use COMSOL multiphysics to solve the governing equations and explore these complex and interacting processes. We investigate a single-phase, reactive flow model coupled to hydrodynamic flow and heat transfer. Model results show that porosity reduction rate depends on the initial porosity and can lead to overpressure in the matrix, thus reversing the Darcy velocity. This would result in silica deposits on the walls of the well and heat transfer from the reservoir to the well, thus accelerating the cooling front. Consequently, the system observes a drastic decrease in the effective stresses and an increased likelihood of tensile failure of the rock. These results have important implications regarding the long-term thermo-hydraulic properties of geothermal reservoirs.

At present, magnesium alloys are extensively used in numerous applications due to their light weight and better characteristics. Welding of magnesium alloys is regarded as one of the most complex phenomena in various industries. The friction stir welding of magnesium alloys has encouraged abundant scientific and industrial interest as it has the potency to form a good quality joint. Post welding heat treatment is an appropriate process to further improve the properties or performance of FSW joints. Therefore, the present work aims to join AZ31 Mg alloy plates by friction stir welding, and microstructural and mechanical properties of the joint have been examined. Furthermore, the consequence of post welding heat treatment on the microstructure and mechanical properties of FSW joint has been evaluated. Tensile strength and elongation of FSW joint were about 145.4 ± 4.9 MPa and 9.5 ± 0.9%, respectively. It was found that post welding heat treatment was beneficial in homogenizing grains and to enhance mechanical properties. Tensile strength and elongation of the joint were improved by 4.74% and 15.78% respectively after PWHT. The highest microhardness of stir zone decreased about 6.84% (73 Hv to 68 Hv) after heat treatment and hardness pattern of weld became relatively smooth. Toughness of PWHT joint was 4.5 ± 0.17 Joules. The mode of tensile failure of as-welded and PWHT joint was ductile.

In material extrusion (MEX) based 3D printing, inter-filament voids are intrinsic to printing process. The void orientation, volume and shape are affected by multiple factors including the nozzle shape, stacking sequence and printing direction. In this study, the adverse effects of the inter-filament voids on tensile properties and damage modes were investigated numerically on 3D printed acrylonitrile butadiene styrene-carbon fiber (ABS/CF) continuous fiber-reinforced composites (CFRCs). Uniaxial tensile simulations were performed considering various nozzle geometries (circular, square), fiber orientations (θ=0o,30o,45o,60o,90o) relative to loading direction, and a regular stacking sequence of extrudates. The extrudate cross-section was modeled either using elliptical or superelliptical extrudates deposited from a circular or square nozzle, respectively. Excellent agreement was seen when the simulated results were benchmarked against several published experimental and numerical work. Simulated results showed that changing the nozzle shape from circular to square improved the mechanical properties across all fiber angles by lowering the void content by 7−8%and increasing the ultimate tensile strength (σT) by 11−18%, tensile stiffness (ET) by 6−8%, and the tensile failure strains (ϵT,fail) by 1−11%. For superelliptical extrudates the number of observed damage modes also reduced, and this is due to a 37.2% and 58.2% improvement in the inter-filament and inter-layer bond lengths, respectively. Also, when fiber angle became increasingly off-axis to tensile load direction, the strengths, moduli, and failure strains reduced for both circular and square nozzles. The significance of using microstructure geometries and explicitly modeling inter-filament voids for simulating MEX printed CFRCs was highlighted by comparing these results with both analytical calculations and simulated results of homogeneous solid CFRC blocks without voids.

Ice exhibits good mechanical performance at low temperatures, and when it replaces the weakest pores in cement-based materials, it can noticeably enhance the overall mechanical properties of the material. However, the specific micro-mechanisms that contribute to the improvement of tensile mechanical properties in gel pores at low temperatures are still not fully understood. Based on the results of molecular dynamics simulations, significant roles have been attributed to pore size and temperature in the mechanical performance of gel pores at low temperatures. A thorough investigation was conducted on the process of tensile failure in saturated gel pores, as well as the changes in bonding interactions between calcium silicate hydrate (CSH)-ice and the unfrozen water film (UWF) at different pore sizes and temperatures. The results indicate that increasing pore size and decreasing temperature can improve the mechanical properties of saturated gel pores. With the decrease in temperature, the mechanical properties of the three gel pore systems with heights of 100 Å, 68 Å, and 54 Å were significantly enhanced. Under an engineering strain rate of 8×10−4/ps, the tensile strength and elastic modulus of gel pores increased from 0.57 GPa and 17.81 GPa at 250 K, to 0.97 GPa and 20.87 GPa at 150 K, respectively. To explain this phenomenon, further studies were conducted on the radial distribution function and dynamic hydrogen bonding behavior. The critical contribution of the hydrogen bond network to the mechanical performance of gel pores was discovered. The hydrogen bond structure which experienced the earliest damage, was investigated. As the strain increased, the hydrogen bonds continuously broke and reformed. This research provides a theoretical understanding of the mechanical properties of gel pores under low-temperature conditions.

In deep rock engineering, natural cracks in rock mass subjected to thermo-hydro-mechanical-chemical (THMC) loads have tendency to initiate and propagate, and result in potential safety hazards. Current literature on cracking behaviors and mechanism of pre-cracked rock under multi-field loads is focused on single (M), two (TM, HM) or three fields (THM, HMC), less on THMC fields. In this study, a series of self-designed triaxial compressive tests were performed to study different THMC fields influencing on mechanical behaviors and failure patterns of pre-cracked red sandstone specimens. A new THMC fracture criterion, recently proposed by our group team, is used to further analyze the crack initiation mechanism of red sandstone under THMC loads. Research results suggest that the stress-strain curve is greatly influenced by the temperature, confining pressure and acidity and alkalinity of chemical solution, but relatively less by the hydraulic pressure. As the temperature and hydraulic pressure increase, the confining pressure decreases, and the acidity or the alkalinity of chemical solutions becomes strong, both the elastic modulus, crack initiation stress and peak strength of specimen are decreased, and the failure pattern is changed from shear failure to tensile failure. The crack initiation stress and peak strength usually have the largest, middle, smallest values in the shear, tension-shear and tensile failures, respectively, while the peak strain has largest value in the mixed tensile-shear failure. The acid solution has greater weakening effect than the alkaline solution with the same concentration. Theoretical results indicate that the temperature and chemical fields influencing on crack initiation is mainly attributable to its weakening of rock fracture toughness, while seepage and stress fields on crack initiation to its change of the stress state at crack-tips. The THMC mechanism of crack initiation can be well revealed by the new THMC fracture criterion.

Extensive research has explored cracking in asphalt mixtures under pure mode-I conditions; however, heavy traffic loads often lead to crack formation under mixed-mode fracture conditions in asphalt pavements. This paper presents a phase field damage model to simulate mixed-mode I-II fracture phenomena observed in asphalt concrete. In contrast to the conventional approach employing cohesive zone models, which restrict crack propagation exclusively to the peripheries of interface elements within the computational mesh, the model presented herein can simulate crack evolution throughout the material domain. Crack inception and material degradation were modeled using a unified failure criterion and a modified G-criterion. Mode I and mixed-mode I-II asphalt fracture experiments reported in the literature are replicated using the phase field model. The results show that the model can effectively simulate the fracture response with sufficient accuracy. The effect of the offset ratio on the peak load, dissipation energy, mode-mixity, and mode of failure is studied. In addition, the fracture response under different failure criteria is simulated using the parameter χ. It is concluded that with an increase in notch offset, the peak load, dissipated energy, and mode-mixity increase. Furthermore, it is demonstrated that in the case of the provided notched beam with a mere 23 % mode II contribution, the modified and tensile failure criterion-based PFMs are equivalent for χ≥ 0.707.

This work addresses the problem of describing ductile fracture behavior and predicting ductile failure initiation in dented pipelines under tensile loading based upon a 3-D computational cell approach coupled with a stress-modified, critical strain (SMCS) criterion for void coalescence. A series of tension tests conducted on notched tensile specimens with different notch radius for a carbon steel pipe provides the stress–strain response of the tested structural steel from which the SMCS criterion is calibrated. Full scale cyclic bend tests also performed on a 165 mm O.D tubular specimen with 11 mm wall thickness enable verification of the proposed approach in assessing ductile cracking behavior in damaged pipelines. These exploratory analyses predict the tensile failure load for the pipe specimen associated with ductile crack initiation in the highly damaged area inside the denting and buckling zone which are in good agreement with experimental measurements.

In order to study the effect of temperature-confining pressure coupling on the creep behavior of rock-like specimen with precast internal fissures, the creep tests under different temperatures and confining pressures are carried out. The test results show that the rock presents primary creep stage and steady creep stage at low stress levels. Under the condition of high stress, the creep process of rock shows an obvious accelerated creep stage. The curve of accelerated creep stage of rock gradually smoothing with the increase of confining pressure, and the corresponding creep failure time gradually increases. The failure under low confining pressure is mainly tensile failure. The high confining pressure is the shear failure, or even the mixture of shear and plastic failure.

The shear failure mechanism of polycrystalline gas hydrates is critical for understanding marine geohazards related to gas hydrates under a changing climate and for safe gas recovery from gas hydrate reservoirs. Since current experimental techniques cannot resolve the mechanism on a spatial and temporal nanoscale, molecular simulations can assist with proposing and substantiating nanoscale failure mechanisms. Here, we report the shear failure of polycrystalline methane hydrates using direct molecular dynamics simulations. Based on these simulations, we suggest two modes of shear behavior, depending on the grain sizes, d, in the polycrystal: grain-size-strengthening behavior with a d1/3grain size dependence for small grain sizes and grain-size-weakening behavior for large grain sizes. Through the crossover from strengthening to weakening behavior, the failure mode changes from shear failure with a failure plane parallel to the applied shear to tensile failure with a failure plane lying at an angle with the applied shear, spanning a network of grain boundaries. The existence of such a change in mechanism suggests that the Hall–Petch breakdown in methane hydrates is due to a change from grain boundary sliding to tensile opening being the most important failure mechanism when the grain size increases.

The strength and rigidity of proppant and coal are quite different. Proppant is easily embedded in coal, resulting in the decrease of fracture aperture and conductivity. The mechanism of proppant embedded in the coal fracture surface needs further study. In this paper, through theoretical analysis and experimental research of single proppant embedded, the proppant embedded law is analyzed from macro and micro aspects. The damage factor of coal surface is introduced, the damage model of the whole process of single proppant pressure embedded is established, and the mechanism of coal fracture surface embedded pit and cracks is revealed. The results show that, from the center of the contact surface to the outside, the surface failure forms of coal fracture are pressure failure, tensile-shear failure, and tensile failure. The proppant is embedded in the coal fracture surface to form four areas: central compaction area, layered fracture area, fracture development area, and original state area. During the energy release of proppant embedded in coal, the energy of proppant is released first and the energy of coal is released later. Greater than 3 times the contact radius, the influence of single proppant on the coal fracture surface can be ignored.

During the diagenetic processes of compaction and cementation, shale forms multiple beddings, significantly effecting the rock anisotropy. Therefore, it is important to study the effect of bedding on the mechanical properties of shale to guide fracturing engineering. To study the failure mode and fracture morphology of shale with bedding planes under different bedding dip angles, Brazilian disc splitting tests and finite element numerical simulations are performed for shale samples and the results are compared. (1) The bedding angle has a significant effect on the failure mode and tensile strength of shale, and the tensile strength tends to increase with increasing bedding angle. (2) The failure modes of shale under different bedding dip angles can be divided into three types: tensile failure along the bedding plane; comprehensive shear and tensile failure of the matrix and the bedding plane; and matrix tensile failure. (3) The direction of secondary cracks is mostly perpendicular to the bedding plane, and a smaller angle between the load application direction and the bedding direction results in a larger number of generated cracks with more complex shapes. (4) When the loading angle of the bedding is 30°, the tensile strength is low and the matrix and the bedding plane are comprehensively destroyed by shear and tension, resulting in a more complex joint mesh; conversely, when the bedding angle is greater than 60°, the tensile strength increases and the complexity of the seam mesh decreases. These findings can provide guidance for the selection of the bedding angle and the design of fracturing schemes in fracturing engineering.

This paper deals with surface wave propagation (WP) effects on buried segmented pipelines. Both simplified analytical model and finite element (FE) model are developed for estimating the axial joint pullout movement of jointed concrete cylinder pipelines (JCCPs) of which the joints have a brittle tensile failure mode under the surface WP effects. The models account for the effects of peak ground velocity (PGV), WP velocity, predominant period of seismic excitation, shear transfer between soil and pipelines, axial stiffness of pipelines, joint characteristics, and cracking strain of concrete mortar. FE simulation of the JCCP interaction with surface waves recorded during the 1985 Michoacan earthquake results in joint pullout movement, which is consistent with the field observations. The models are expanded to estimate the joint axial pullout movement of cast iron (CI) pipelines of which the joints have a ductile tensile failure mode. Simplified analytical equation and FE model are developed for estimating the joint pullout movement of CI pipelines. The joint pullout movement of the CI pipelines is mainly affected by the variability of the joint tensile capacity and accumulates at local weak joints in the pipeline.

Following the previous analytical studies performed with ATENA software for a series of RC moment resisting frame models, it were used in the pre-processing stage the stress-strain relation laws for concrete and steel reinforcement. These mathematical and graphical relations represent a necessity in the current conditions of numerical analysis and imply a correct knowledge of the deformation mode of the „reinforced concrete” which is a composite material. Thus, it is desired through this research paper the theoretical exposition of: equivalent uniaxial law for concrete, biaxial compressive failure and tensile failure consideration laws for concrete, bilinear with hardening law for steel reinforcement, cycling steel reinforcement model and steel reinforcement bond model. Finally, it will be possible to validate the correctness of the analytical RC frame systems through the experimental results of the optimal RC frame model after seismic platform testing.

AbstractThe failure mechanism of surrounding rock is an important issue in the field of underground engineering. To quantitatively understand the failure process and failure mechanism of the surrounding rock mass in deep underground engineering, the underground powerhouse of the Houziyan hydropower station was selected as a case study. The study area was determined by analysing the spatiotemporal distribution of microseismic (MS) events and site investigations. After noise reduction, P-wave arrival time extraction and positioning of MS signals, the moment tensor inversion method was used to determine the failure mechanism and fracture direction of the surrounding rock mass. The results showed that (1) the failure processes and mechanism of the surrounding rock mass can be effectively analysed through MS monitoring with moment tensor analysis and (2) the failure type of the surrounding rock mass downstream of the main powerhouse was characterized by shear failure. The stress state in this region was a strike-slip stress state. (3) The failure type of the surrounding rock mass at the arch roof of the main transformer chamber was characterized by tensile failure. The tensile stress of the intermediate stress axis was obvious, and the tectonic stress presented a state of near uniaxial compression-biaxial tension.

In this paper, we performed uniaxial compression tests on fine sandstone–coal, coarse sandstone–coal, and oil shale–coal bi-material samples to investigate the lithology effects on their strength, macro-failure initiation (MFI), energy evolution, and failure characteristics, respectively. Uniaxial stress–strain characteristics of the samples primarily depended on the coal and were affected by coal–rock interactions. These interactions involved the following aspects: (1) The differences in the mechanical properties of rock and coal affected their stress states near the interface. They were in triaxial compression state with strength increasing or in triaxial compression-tension state with strength decreasing. (2) During the pre-peak stage, the rock deformation affected the values of input energy, elastic energy, and dissipated energy. Damage and energy accumulation in coal were also limited by rock deformation, resulting in higher strengths for the bi-material samples compared to pure coal samples. (3) During the post-peak stage, the rebound deformation of rock was accompanied by the elastic energy release, which promoted the failure of coal. MFI of the samples occurred within the coal. MFI models mainly included the tensile fracture of coal near the interface, the initiation and propagation of macro-cracks, and the development and evolution of micro-defects, accompanied by varying degrees of surface spalling or ejection failure. The coal showed the splitting ejection failure. The oil shale and coarse sandstone exhibited tensile failure or tensile–shear composite failure due to crack propagation in coal. Failure was not evident in the fine sandstone. The coal in the oil shale-coal bi-material samples became more broken due to the most serious rebound deformation of oil shale.

Poly(ethylene terephthalate) (PET) and poly(ethylene 2,6-naphthalate) (PEN) homopolyesters were synthesized and then copolymerized at different molar feed ratios into PET-co-PEN (TN) block copolyesters. The sequence distribution of the TN copolyesters was analyzed through proton nuclear magnetic response spectroscopy. The results demonstrated that the block segments of PET and PEN are highly miscible and tend to be randomly or nearly randomly distributed in the synthesized TN molecular chains. For TN block copolyesters with PET as the main component and number-averaged sequence length of PET > 3, wide-angle X-ray diffraction (WAXD) profiles indicated that all TN copolyester crystals were related to PET. TN block fibers were produced from a TN block copolyester through melt spinning at a 90:10 PET-to-PEN molar feed ratio and then subjected to different heat treatments. The relaxation of the oriented amorphous chain at elevated temperatures reduced the birefringence of the TN fiber. When the relaxation heat treatment time or stretching was longer, the oriented amorphous molecular chains were reoriented and crystallized, which increased the birefringence of the TN fiber. The differences in the segmental mobility of PET and PEN led to a multinecking and a mushroom-head morphology with transverse striations along the fiber axis because of the snapback effect after tensile failure.

Although the application of linear elastic fracture mechanics (LEFM) is one of the most prevalent theories used in hydraulic fracturing, when the damaged area around the fracture tip exceeds the limit, LEFM is not applicable. In this paper, the singularity-dominated and fracture process zones are defined to examine the applicability of using LEFM under constant fracture pressure in simulations of hydraulic fracturing. The sizes of the singularity-dominated and fracture process zones are evaluated at the hydraulic fracture tip under different faulting conditions. Furthermore, this study indicated that the fracture process zone dominated the singularity-dominated zone around the fracture tip under the various conditions investigated. However, when there is tensile failure in rocks the use of the LEFM is not appropriate. The sensitive analyses demonstrated that fracture pressure and minimal principal stress are the dictating factors regarding the relevance and applicability of LEFM. The results presented in this investigation can be used to define the damage process around the fracture tip under specific conditions while modeling hydraulic fracture treatment. The primary outcome of this study was to improve the understanding of LEFM and its impact on modeling of hydraulic fracture treatments. In addition, the results of this study also demonstrated the capacity of the model in relation to capturing the essential characteristics of the crack tip, including the onset of unconfined compressive strength, tensile strength, reservoir pressure, and various stress conditions. Moreover, the results of this study can also contribute to the understanding of the damage mechanism around a fracture tip under similar conditions, which can lead to improved modeling of hydraulic fracturing.

The mechanical properties and failure mechanism of composite geomembrane air expansion deformation occurring in plain reservoirs or artificial lakes are investigated. Composite geomembranes are formed by the heat combination of a core geomembrane and the geotextiles on two sides. The deformation is simplified as a spherical bulging deformation under ring-restrained conditions. The experiment is performed by a self-developed apparatus. Rules of the air expansion deformation and failure are summarized, and the following conclusions are drawn. Air expansion deformation and failure belong to tensile failure, which begins at the crown top, and a typical failure mode is a crack distributed along the crown (great circle). The composite geomembrane produces synergistic deformation during the air expansion process, and the outer geotextile bears a large tensile force. The geotextile and geomembrane are broken simultaneously and are not separated during the failure process. A 34-Hz frequency is selected as the basic loading rate for the air expansion tests of the composite geomembrane; at this frequency, the bursting pressure is 3.24 MPa, and the bursting height is 26.1 mm. The tensile stress and strain curve of a composite geomembrane can be divided into four stages: elastic stage, yield stage, strengthening phase, and breaking stage. The air expansion deformation degree is inhomogeneous, at which the maximum is at the crown top and the minimum is at the ring constraint. The air expansion elongation at break is 17.3 %, which is far less than the uniaxial tensile elongation at break of 51.7 %.

The subject of the article is the introduction of the results of the first series of tests of precast reinforced concrete pillars. These are the pillars simulating parts of walls that can, for example, form in the precast wall of residential buildings after an opening has been carved. The pillars are variously reinforced: from the simple reinforcement with wire mesh, to the reinforcement with standard reinforcement bars. Depending on the type of reinforcement and the type of loading, the deformation and tension of the individual pillars are studied. All of the pillars failed due to local compression of the concrete, a transversal tensile failure occurred in all cases.

ABSTRACTAn in situ hot press bonding technology has been developed to clad aluminium on magnesium. Followed by regular hot rolling, magnesium sheets, covered by ductile and corrosion-resistant aluminium without detectable oxides in the interface, are produced. The new technology requires no welding, vacuum, protective atmosphere or barrier layer, and it makes good interfacial strength and rollability. Aluminium–magnesium intermetallic phases are formed along the clad–core interface at elevated temperatures. They are not detrimental under compression but may cause clad-core delamination in tensile strain. However, the tensile failure is more dependent on the formability of magnesium core than on the strength of interface.

The location and geometry of large-scale asperity present at the foundation of concrete gravity dams and buttress dams affect the shear resistance of the concrete-rock interface. However, the parameters describing the frictional resistance of the interface usually do not account for these asperities. This could result in an underestimate of the peak shear strength, which leads to significantly conservative design for new dams or unnecessary stability enhancing measures for existing ones. The aim of this work was to investigate the effect of the location of first-order asperity on the peak shear strength of a concrete-rock interface under eccentric load and the model discrepancy associated with the commonly used rigid body methods for calculating the factor of safety (FS) against sliding. For this, a series of direct and eccentric shear tests under constant normal load (CNL) was carried out on concrete-rock samples. The peak shear strengths measured in the tests were compared in terms of asperity location and with the predicted values from analytical rigid body methods. The results showed that the large-scale asperity under eccentric load significantly affected the peak shear strength. Furthermore, unlike the conventional assumption of sliding or shear failure of an asperity in direct shear, under the effect of eccentric shear load, a tensile failure in the rock or in the concrete could occur, resulting in a lower shear strength compared with that of direct shear tests. These results could have important implications for assessment of the FS against sliding failure in the concrete-rock interface.

Most of drilling hole problems are attributed to wellbore stability issues which adversely cause excessive lost time and cost millions of dollars. The past drilling experiences in Kupal oilfield showed excessive mud losses, kick flows, tight holes and pipe stuck leading to repeated reaming, fishing and sidetracking. Most of the drilling-associated problems in this field occurred during drilling the 12 ¼-in. hole, which is across the non-reservoir Gachsaran formation (consisting of anhydrite, gypsum and marl with thin limestone layers). Mainly due to the lack of required formation evaluation data, no geomechanical studies of this formation have been conducted to date. In this work, first, we constructed a geomechanical model to investigate the root of the problems. This is a pioneer wellbore stability work for such a complex lithology formation which included finding the equations best-matching with core data and field observations. Finally, to overcome the field challenges and hole problems, the study proposes some field remedial actions. The results of the geomechanical modeling show that the pore pressure, shear and tensile failure gradients are greatly variable with the safe mud weight window becoming excessively narrow at some intervals. This accounts for the encountered wellbore stability issues as managing the mud weight in these situations requires several casing strings. To mitigate the extent of the problem, this study proposes the application of innovative drilling technologies including casing while drilling to eliminate the casing running time with potential reduction in drilling time, and continuous circulation system to prevent cuttings settling and kick flows during connections. These technologies are capable of elimination of the geomechanical part of the drilling delay (30% of the average 77 drilling days) per well.

The effects of high-performance polypropylene fiber (HPPF) content, shear span to depth ratio, and stirrup ratio on the shear behavior of lightweight aggregate concrete (LWAC) beams were studied according to shear tests on eleven high-performance polypropylene fiber lightweight aggregate concrete (HPPLWC) beams and three LWAC beams under symmetrical concentrated load. The failure phenomenon, stirrup strain, shear capacity, and deflection of each specimen were analyzed, and it was determined that the failure patterns of the beams were classified as diagonal tensile failure, diagonal compression failure, shear compression failure, and bending shear failure, in which the beams without stirrups having the HPPF content of 0 kg/m3and 3 kg/m3were a diagonal tensile failure, the beam with the shear span to depth ratio of 1 was diagonal compression failure, the beams with the stirrup ratio of 0.38 % and the HPPF content of 6 kg/m3and 9 kg/m3were bending shear failure, and the rest of the specimens were shear compression failure. Adding HPPF to concrete could effectively improve its tensile strength, limit the growth of cracks, and further improve its initial cracking strength. By increasing the HPPF content and stirrup ratio, the shear capacity and ductility of the beams could be improved. Increasing the shear span to depth ratio of the beams could improve their deformability of the beams while simultaneously decreasing their shear capacity. Based on the experimental study, the contribution of each influencing factor to the shear capacity was analyzed, and the shear capacity model of the diagonal section of the HPPLWC beam was established. The model has great prediction accuracy as shown by error analysis.