Tensile Stress Zone Research Articles

Dimensional change and springback of spherical shell in cryogenic forming

Cryogenic forming has been developed to manufacture thin-walled curved aluminum alloy components, whose final dimensions are affected by cryogenic shrinkage and springback. Therefore, dimensional changes of spherical shell in cryogenic forming were studied theoretically and experimentally. The cryogenic forming process was discussed to elucidate the factors affecting the dimensional change. The stress distribution was analyzed to qualitatively reveal the springback behavior. Cryogenic dimensional measurement devices were built to quantitatively evaluate the dimensional changes in the forming processes of punch cooling, springback, and specimen restoration to room temperature. The temperature dependencies of the elastic modulus and expansion coefficient were modeled to quantitatively calculate the effect of thermal expansion and contraction on the dimensions of specimen and punch. The theoretical analysis results indicate that depth reduction and opening expansion are produced by cryogenic springback, determined by the radial stress, hoop stress, and bending moment in different deformation regions. The cryogenic springback in the biaxial tensile stress zone was reduced by 37.8 % owing to the increasing radial deformation and decreasing bending deformation. In contrast, cryogenic springback in the tensile-compressive stress zone increased by 30.8 %. The punch cooling shrinkage and specimen warming expansion in depth direction can reduce for the dimensional deviation caused by springback but cause the opposite effect in hoop direction. Expansion and shrinkage were effectively predicted using the proposed model, with an error of less than 16 %. The deformation region of the biaxial tensile stress can be enlarged by the significantly improved hardening ability at cryogenic temperature, which benefits enhancing deformation uniformity and further reduces springback. Therefore, cryogenic forming offers considerable potential for the precision manufacturing of aluminum alloy deep-cavity thin-walled components.

High pre-tension reinforcing technology and design for ultra-shallow buried large-span urban tunnels

Due to the thin overlying formation of ultra-shallow buried large-span urban tunnels, the surrounding rock can be easily loosened and damaged after excavation. High prestressed anchoring support can improve the self-bearing capacity of the surrounding rock. However, traditional bolts often undergo necking fracture after the yield stage and strengthening stage, and the designed pre-tension is low, generally not exceeding 50 % of the yield strength of the bolt. To this end, our research group developed a new NPR bolt. After comparing the mechanical properties of the NPR bolt and the traditional bolt, the high strength, high elongation, and high prestressed properties of the NPR bolt are revealed. Subsequently, the constitutive relationship of the CABLE element in FLAC3D is modified, and the constitutive model of the NPR bolt is established, which can describe the whole process of bolt. Furthermore, a numerical comparison of prestressed control for ultra-shallow large-span tunnels is carried out. Compared with non-prestress, when the prestress of bolt reaches 100 kN, the area of tensile stress zone and plastic zone of surrounding rock are reduced by 79.6 % and 73.7 %, respectively. At the same time, the settlement of surface and roof is reduced by 66.7 % and 64.1 %, respectively. The control mechanism of the prestressed NPR bolt support is explored, and the design method for ultra-shallow large-span tunnels is proposed. The field application and monitoring results show that the surface and roof settlement of the tunnel supported by NPR bolts are 3.2 mm and 6.3 mm, respectively. The safe and effective stability control of the tunnel surrounding rock is achieved.

Refined cyclic R-curve determination through residual crack tip stress reduction by annealing

The accurate determination of the cyclic R-curve of the threshold of stress intensity factor range is a demanding experimental task. The main challenge concerns the introduction of a pre-crack that is open under tension loading and free of residual stresses. When pre-cracks are generated by cyclic compression-compression loading, a zone of tensile residual stresses up to the material yield strength is generated ahead of the pre-crack tip. These stresses may affect crack propagation at the beginning of fatigue crack growth and should consequently be removed especially for high-strength materials. However, subsequent stress-relief annealing is impractical for numerous metallic material classes due to undesired microstructural alterations at the temperatures required for stress relief. In the present contribution, the above-described issue is addressed for a high speed steel using single-edge notched bending specimens pre-cracked at various stages of a typical high speed steel heat treatment process. It is shown that stress-relieving via a heat treatment route is a significant improvement compared to the conventional compression pre-cracking procedures to measure material-specific cyclic R-curves.

A case study on early-age cracking of high-strength concrete construction by coupled thermal-mechanical analysis and field monitoring

The use of high-strength concrete (HSC) in constructing high-rise reinforced concrete buildings and their deep foundations is becoming more popular because of the significant reduction in member size and, thus, self-weight in structural members. However, the higher cement content in HSC may result in increased heat of hydration and autogenous shrinkage, growing concerns about forming early-age cracks in HSC structural members. This case study assessed the early-age cracking of HSC in bored pile construction of high-rise buildings. A nonlinear transient coupled thermal-mechanical response analysis was performed to evaluate the potential of early-age cracking caused by the heat of hydration generated in the HSC using ABAQUS and two subprograms, USDFLD and UEXPAN. A 3-D finite element model was developed considering thermal effects, time-dependent evolution of early-age strength and stiffness of HSC, crack strain development, autogenous shrinkage, and stress-relaxation in the analysis. Field measurements were carried out on a 3-m diameter bored pile constructed using Grade C50/60 HSC (fck,cube = 60 MPa) for a high-rise building project in Hong Kong to evaluate the validity of the numerical model. Optical fiber sensors were used to monitor the temperature changes of the pile for approximately 380 hours after casting. The temporal and spatial temperature distributions measured in the field measurements agree well with the simulations. Large thermal gradients and high tensile stress zones were observed across the horizontal cross-sections of the pile, resulting in the formation of vertical annular cracks. Based on the results of the analysis, the maximum crack widths were predicted and compared with the durability crack width limits.

Initial factors impacting vertical displacement, stresses, and propagation of cracks in three-layered steel fiber concrete beams

The main aim of the study is to examine the input parameters of three-layered steel fiber concrete beams. Bending RC beams is studied by many different scientists. Multilayer RC beams using SF concrete are one way. In high compressive and tensile stress zones, this method improves beam load strength and reduces cracks. The main goal is to decrease beam stress. Repairing broken three-layered SF-reinforced concrete beams by adding more concrete on top or below the current layer is a benefit. Modifying input parameters during three-layered beam design greatly affects beam efficacy. In this paper, the authors conducted a study using ANSYS simulation and nonlinear material analysis. The objective was to investigate the behavior of three-layered bending RC beams subjected to two concentrated loads. Specifically, the study examined the impact of varying the content of SF in the RC, as well as the effects of stirrup at the ends of the beams. Furthermore, the study explored the influence of changes in the quantity and size of steel bars in the region of tensile strength, along with the effect of varying the SF concrete layer's thickness. The survey results on three-layered beams were used to create diagrams that depict the relationship between load and vertical displacement and load and stress area. These diagrams also help determine the initiation and progression of cracks in three-layered beams, starting from the application of load until the beams become damaged. Ultimately, this information allowed us to identify the specific load level that caused the beams to crack and become damaged.

Optimization of Prestressed Anchor Cable Layout for Left Pier of Jinchuan Hydropower Station

With the increase of the flood discharge capacity of the flood discharge structure, the size of the flood discharge orifice is widened, which leads to the increasing thrust of the radial gate. A large tensile stress zone will be formed at the connection between the anchor block and the pier, which may lead to concrete cracking. The optimization design of the prestressed anchor cable of the pier has always been a concern of the engineering community. In this paper, the left pier of Jinchuan Hydropower Station is taken as the research object, and the finite element simulation model is established. The layout position of prestressed anchor cable is optimized, and different layout schemes of prestressed anchor cable on the left pier are designed and calculated. The results show that when the amount of main anchor cable is the same, it should be arranged outside the pier as far as possible; under the condition that the number of main anchor cables on the inner side of the pier is constant, when the direction of the main anchor cable force is consistent with the direction of the radial gate thrust, the main anchor cable prestress plays the greatest role; the arrangement of balance cable has no obvious effect on the tension of concrete at pier neck. It can provide reference for structural design.

Mechanical properties and failure mechanism of rock-like specimens with arc-shaped flaws under freeze–thaw cycles

To investigate the failure characteristics of jointed rock masses in cold regions after freeze–thaw (F-T) cycles, uniaxial compressive tests are conducted on rock-like specimens (a type of cement mortar) with prefabricated arc-shape flaws subjected to 0, 25, 50 and 100F-T cycles to determine their mechanical properties. Test results reveal that F-T cycles greatly affect the uniaxial compressive strengths (UCS) of the specimens, and the relationship between the peak strength and the number of freeze–thaw cycles is exponentially fitted. Notably, the initial 50 cycles have a more pronounced effect on UCS reduction compared to the subsequent 50 cycles, indicating that early F-T cycles are more detrimental to UCS. To further investigate the failure mechanism of the specimens, two-dimensional particle flow code (PFC2D) is utilized to simulate the F-T cycle process of the specimens. The mechanical properties and failure mechanism of specimens can be obtained after F-T cycles under uniaxial compression through numerical simulations. The findings indicate that F-T cycles can obviously deteriorate the ductility of the specimens, and to some extent, they also affect crack propagation during specimen failures. Through crack classification statistics and stress contour analysis of the simulated specimens, it reveals that different prefabricated flaw morphologies lead to distinct stress distributions, subsequently affecting crack propagation and specimen failure. It is also worth mentioning that the large curvature and inclination angle of the flaws may generate additional tensile and shear stress zones at the top of the flaw vault, which in turn may contribute to the development of crack propagation. The relevant experimental and numerical results are useful for investigating mechanical properties and failure mechanisms of rock masses in cold regions subjected to F-T cycles.

Mechanism of surface modification on monocrystalline silicon during diamond polishing at nanometric scale

The demand for polished silicon wafers has increased significantly in recent years to cater to the development of the semiconductor industry. For example, polished silicon wafer has direct applications in integrated circuits, radio frequency amplifiers, micro-processors, micro-electromechanical systems, etc. To carry out mechanical polishing, lapping, grinding, or single-point diamond turning of silicon, diamond abrasives were extensively used before the implementation of chemo-mechanical polishing. During the diamond-based polishing, a few problems have already been identified, such as the formation of an amorphous phase, heat-affected zones, low material removal, etc. Some research work has also reported that nano-structured abrasives lead to a thin layer of the amorphous phase and a better material removal rate. In the same direction, a molecular dynamics simulation is carried out in this paper to investigate the mechanism of material removal from monocrystalline silicon during the diamond-abrasive-based polishing process. The present work is mainly focused on the dynamics of material removal phenomena near the abrasive particles at the nanometric scale by considering stress, lattice, cohesive energy, etc. This reveals that a higher value of indentation force results in surface buckling, which creates a zone of both compressive and tensile stresses, which increases the coordination number and forms β-silicon just ahead of the abrasive particle. This mechanism happens by developing a β-silicon phase on the surface with a thickness beyond a certain value of indentation force on the zone of compression. Buckling on this phase happens due to stress localisation in compression, as the flow stress of this phase is less than that of diamond cubic lattices. To avoid the mechanism of surface buckling and process silicon material on the surface, the indentation force needs to be maintained below a critical value. In the present case, it was found that the indentation force of less than or equal to 190 nN for the abrasive size of ϕ8 nm does the material removal by surface processing only without surface buckling. It was also found that surface processing helps to reduce the depth of the amorphous layer significantly without compromising the material removal rate or the generation of a wavy surface. Thus, the present mechanism will help in the polishing of silicon with minimum defects and reduce processing time for the final stage of polishing towards manufacturing ultra-smooth and planer surfaces.

Comparing the efficiency of strengthening timber beams reinforced with carbon composite rods and plates

This study reports the results of experiments comparing the effect of structural reinforcement for timber beams strengthened with different carbon composite materials. Samples of one series were strengthened by gluing a carbon fiber plate from the outside to the face of the beam. The samples of the other series were strengthened by laterally gluing two carbon fiber rods inside the beam. According to the bending research program, the ultimate loads and deflections were determined for both series. As a result of the analysis of the results and comparison with unreinforced samples from the control series, the effect of beam reinforcement and the model of their destruction were determined. Studies have shown that the effect of external reinforcement with a plate was 86.7 % according to the criterion of full failure load, 20.5 % according to the criterion of ultimate load, 13.4 % according to the criterion of ultimate deflection. The effect of rod reinforcement was 48.6 %, 18.6 %, and 4.1 %, respectively. The theoretical analysis of the results showed a convergence of up to 8.2 % with the experimental results. External reinforcement with a plate compared to lateral gluing of rods showed better results due to the placement of the plate in the zone of maximum tensile stresses. This arrangement more effectively limited the spread of ultimate stresses and the development of cracks. The reinforcement parameters of the samples (materials, placement, percentage of reinforcement) were selected under the condition of the same theoretically predicted bearing capacity after reinforcement. However, comparative experimental studies have revealed differences in the processes of deformation and destruction of reinforced beams. The results will contribute to making rational project decisions and for choosing a relevant technique of strengthening timber beams with carbon composite materials

The Wetting Deformation Model of Rockfill and Its Two Methods for Simulating Rockfill Dam Collapse Settlement.

The wetting deformation of the upstream dam shell material during the impoundment of the core wall rockfill dam seriously affects the safety of the dam. Based on the proposed Ew - νw wetting model, this paper proposes its corresponding two methods to simulate the collapse settlement of the rockfill dam: the initial strain method and the initial stress method. By simulating the collapse settlement of the Guanyinyan core wall rockfill dam, it is found that the simulated result using the initial stress method is in good agreement with the field monitoring data, while the displacement simulated using the initial strain method is larger. The distribution of displacement contours simulated using the initial strain method is obviously inconsistent in the area where the wetting deformation occurs, and the simulation results of the initial stress method are more reasonable. With the rise in the water level, the wetting deformation of the upstream dam shell material causes the tensile stress zone at the top of the dam. Therefore, the wetting deformation is the direct cause of the crack at the top of the dam, and the initial stress method should be preferred in the simulation of the wetting deformation of rockfill materials.

Quantitative Study of the Weakening Effect of Drilling on the Physical and Mechanical Properties of Coal-Rock Materials.

Coal seam drilling is a simple, economical, and effective measure commonly used to prevent and control rock burst. Following rock burst, coal exhibits significant dynamic characteristics under high strain-rate loading. Our purpose was to determine the physical processes associated with impact damage to drilled coal rock, and its mitigation mechanism. An impact test was carried out on prefabricated borehole coal specimens, and the impulse signals of the incident and transmission rods were monitored. The crack initiation, expansion, and penetration of coal specimens were video-recorded to determine the mechanical properties, crack expansion, damage modes, fragmentation, and energy dissipation characteristics of coal specimens containing different boreholes. The dynamic compressive strength of the coal specimens was significantly weakened by boreholes under high strain-rate loading; the dynamic compressive strength and the dynamic modulus of elasticity of coal rock showed a decreasing trend, with increasing numbers of boreholes and a rising and decreasing trend with increasing borehole spacing; the number and spacing of boreholes appeared to be design parameters that could weaken coal-rock material under high strain-rate loading; during the loading of coal and rock, initial cracks appeared and expanded in the tensile stress zone of the borehole side, while secondary cracks, which appeared perpendicular to the main crack, expanded and connected, destroying the specimen. As the number of boreholes increased, the fractal dimension (D) and transmission energy decreased, while the reflection energy increased. As the borehole spacing was increased, D decreased while the reflective energy ratio decreased and increased, and the transmissive energy ratio increased and decreased. Drilling under high strain modifies the mechanical properties of impact damaged coal rock.

The influence of microtexture tools on the geometric morphology of serrated chips based on stress triaxiality

PurposeThis study aims to investigate the effect of microtextured tools on the geometric morphology of serrated chips, and further improve the cutting performance of polycrystalline cubic boron nitride (PCBN) tool and extend the tool life and the surface quality of the machined surface.Design/methodology/approachA three-dimensional finite element cutting model of hardened steel AISI D2 with microtextured PCBN tools were established using the finite element software Abaqus, and cutting tests were carried out. Furthermore, the stress distribution in the primary deformation zone was investigated based on the triaxiality of stress, and the influence of microtexture on the geometric morphology of serrated chips and crack development was researched.FindingsThe results show that compared with nontexture tools, elliptical pits and wavy grooves microtexture tools have lower serrated degree Gs, higher serrated frequency f per unit length and more miniature serrated step Pc. The serrated phenomenon is intensified because the tensile stress zone of chips generated by nontextured tools is longer than that of elliptic pits and wavy grooves microtexture tools. Simultaneously, the maximum value of triaxiality in the tensile stress zone achieved by nontexture tools is larger than that of the two microtexture tools, and chips obtained by nontextured tools are more susceptible to propagation fractures.Originality/valueThis paper mainly studies the effect of microtexture on chip microgeometry, which is relatively little studied at present. At the same time, this paper has a certain engineering significance for PCBN tool turning hardening steel.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-05-2023-0149/

Research on the Elastoplastic Theory and Evolution Law of Plastic Zone Contours of Horizontal Frozen Walls under Nonuniform Loads

The study of the changes in stress and deformation of frozen walls during excavation has always been a hot topic in underground freezing engineering, and the size of the plastic zone is an important basis for evaluating the stability of frozen walls. In response to the shortcomings in the current design of horizontal frozen walls, based on the internal excavation unloading conditions of the frozen wall in artificial ground freezing, an elastoplastic mechanical model for the interaction between a circular horizontal freezing wall and unfrozen soil mass is established under nonuniform loads to obtain the corresponding solutions for stress and displacement in the system. In this study, considering the shear stress of the plastic zone, the method for solving the traditional plastic zone contour equation is modified; consequently, the modified solution of the contour equation of the plastic zone for the frozen wall is obtained. Using this theoretical solution, the influence of the external load p0 and the lateral pressure coefficient λ on the contour line of plastic zone and tensile stress zone are analyzed by combining the project case, the calculation results show that: the λ=0.485 is the critical point where the inner edge of the frozen wall just happens to have tensile stress. When λ<0.485, the tensile stress zone is inevitable in the inner edge of the frozen wall vertical direction, and its range is only related to λ and increases with the decrease of λ. The p0 only affects the magnitude of tensile stress in the region, but does not affect its range. At this time, the frozen wall compression plastic zone contour evolves from crescent shaped to ear shaped with the increase of p0. When 0.485<λ<0.61, there will be no tensile stress zone, the frozen wall compression plastic zone contour also evolves from crescent shaped to ear shaped with the increase of p0. When λ>0.61, there will be also no tensile stress zone, with the increase of p0, the compression plastic zone contour evolves from the crescent shaped in the horizontal direction to the elliptical shaped, and there is no ear-shaped plastic zone in the whole evolution process. Based on our results, we show that our method can be used to provide a theoretical basis for the stability evaluation and parameter (thickness) design calculation of horizontal frozen walls under nonuniform loads.

Modeling and fracture behavior of mesoscale concrete considering actual aggregate shapes and placement domain shapes

The establishment of meso-concrete models under complex boundary conditions still faces great challenges at present. In this study, a numerical framework for constructing concrete meso-structures in irregularly placement domains is developed based on the dot matrix idea. In addition, a solution to quickly obtain numerous 3D aggregates with realistic morphology as well as 2D cross-section aggregates is proposed. On this basis, wedge splitting (WST) tests with different coarse aggregate contents are conducted and the numerical results agree well with experimental results. The fracture mechanism of the wedge splitting specimen was then investigated by analyzing the contact force chain and crack evolution. Finally, parametric analysis was used to investigate the impact of aggregate and mortar strength on the fracture characteristics of WST specimens. The results show that the numerical framework has good robustness. Tensile stress is the driving force for crack extension in WST specimens, the area of the pure tensile stress zone and the distance between the front end of the pure tensile stress zone and the crack tip decreases as the crack extension length increases. With the increase of coarse aggregate content, the deflection effect of the crack is intensified, the crack path becomes more tortuous, and the fracture zone range also increases. The mechanical properties of concrete and the crack extension path are significantly influenced by the strength of the mortar and the aggregate, but the aggregate does not contribute as much to the overall strength of the concrete structure as the mortar matrix does.

Deformation Control Technology of Gob-Side Entry Retaining with Large Volume CFST Roadway Side Support in Top-Coal Caving Longwall and Stability Analysis: A Case Study

Considering the factors affecting the surrounding rock stability of gob-side entry retaining, the applicability of a large-diameter, concrete-filled steel tube roadside support body in a top-coal caving fully mechanized face is discussed, and a new approach to gob-side entry retaining is proposed in this study. The mechanical model of the surrounding rock structure of gob-side entry retaining in a top-coal caving fully mechanized face was established, the critical state of column–roof contact shear slip instability was clarified through Prandtl foundation failure theory, and the deformation mechanism of the surrounding rock of the retained roadway was analyzed through numerical simulation. The results indicated that the range of the tensile stress zone and extreme tensile stress of the roof between columns are closely related to the spacing of columns, which is the key factor influencing the deformation of the retained roadway. In addition, besides uncontrollable factors, the stability of the contact interface between the roof and columns is directly related to the area of the contact interface between the concrete-filled steel tubes and the roof, and the size of the critical contact area is directly related to the properties of top-coal mass. Finally, a field test was carried out in 91–101 working panels in the Wang-Zhuang Coal Mine; the maximum convergence of the roof and floor was 510 mm, and the area of the retained roadway section reached 12.9 m2, which is within a reasonable range.

Performance of Rock-like Materials Containing Filled Kinked Fissures under Uniaxial Tension at Failure

To investigate the tensile failure phenomenon of rock masses with filled natural defects, the mechanical behavior of 3D printed rock-like materials containing filled kinked fissures under uniaxial tension was identified. Different inclination angles of branch fissure were considered, and cement and gypsum slurry were selected as the infilling materials. It was found that filling materials enhanced the strength of the specimen, which presented to be cement filling > gypsum filling > no filling. The change of inclination angle of branch fissure had significant influence on the strength and failure mode of the specimen and the strength order was −135° > −45° > 90° and +45° > 90° > +135°. Two types of failure pattern, namely tip failure and non-tip failure, were observed in the experiment. Three types of newborn cracks were identified using a high-speed camera. The result achieved by using the digital image correlation technology indicated that different filling conditions led to the position change of high-strain zone. The displacement field of the filled specimen developed from being continuous to being discontinuous. Furthermore, the extended finite element method was adopted to simulate the filled and unfilled specimens, consistent with the experimental results. This study can provide a reliable reference for studying the tensile stress zones in rock exploration.

Study on large deformation and failure mechanism of deep buried stratified slate tunnel and control strategy of high constant resistance anchor cable

Aiming at the significant deformation problem of deep buried stratified soft rock tunnel, taking the No.3 inclined shaft of Muzhailing tunnel as background, the deformation and failure mechanism of the tunnel and the control strategy of high constant resistance anchor cable are studied. The deformation and failure mechanism of stratified tunnel surrounding rock is studied by physical model test, including the tunnel's temperature field, strain field, and displacement field from excavation to failure. Lateral stress significantly influences the deformation of the surrounding rock of the horizontal stratified tunnel. The surrounding rock of the tunnel forms a dumbbell-shaped tensile stress zone under the action of stress. The high tensile stress of the roof and floor leads to dislocation and layer separation, which causes floor heave failure and temperature change. The indoor tensile test verified the super strength characteristics and large deformation of the high constant resistance anchor cable. Based on the excavation compensation method and the coupling control mechanism of high constant resistance long and short anchor cables, a new high prestress constant resistance coupling control strategy is proposed. The numerical simulation and field test studies according to the new design are presented, which show that the high constant resistance anchor cable coupling support significantly reduces the tensile stress zone, effectively controls the deformation of tunnel surrounding rock, and avoids the damage of supporting structure and the cracking and deformation of the lining. The research results can provide a reference for constructing and supporting deep-buried, stratified soft rock tunnels.

Experimental and Numerical Investigation of the Tensile Performance and Failure Process of a Modified Portland Cement

A better understanding of the tensile performance and tensile failure mechanism of cement paste is significant in preventing rock reinforcement failure. Therefore, this paper aims to reveal the tensile performance and failure mechanism of a modified Portland cement: Stratabinder HS cement. To achieve this objective, the split tensile test was conducted on specimens followed by simulating the failure mechanism numerically. The results indicated that the water–cement rate significantly influenced the tensile performance of the cement paste. When the water–cement rate increased from 0.35 to 0.42, the tensile strength declined from 1.9 MPa to 1.5 MPa. It was also observed that vertical tensile failure constantly occurred regardless of the water–cement rate. During the testing process, tensile cracks and shear cracks occurred. The increasing rate in the number of specimen cracks was dependent on the tensile stress state. Before the tensile stress reached the peak, the crack quantity increased slightly. After the peak, the crack quantity increased dramatically. During the vertical loading process, horizontal tensile stress occurred in the specimen. This horizontal tensile stress zone showed a diamond shape. The higher the tensile stress is, the larger the area of the horizontal tensile stress zone. When the tensile strength was reached, horizontal tensile stress mainly concentrated at the vertical centre of the specimen. This finally led to tensile failure of the specimen. This paper indicated that the water–cement rate was the key factor in evaluating the tensile strength of the Stratabinder HS cement.

Failure modes and excavation stability of large-scale columnar jointed rock masses containing interlayer shear weakness zones

The mechanical properties of large-scale columnar jointed rock masses (CJRMs) after excavation would be reduced due to the unloading relaxation, in addition to its influence on the stability of underground caverns . Moreover, the cutting of the interlayer shear weakness zones through the cavern complicates the overall stability of CJRM, and brings challenges to the stability control of project. In view of such problems encountered during the construction of underground caverns, we reconstructed the large-scale CJRMs containing interlayer shear weakness zones to conduct the 3D rock failure process analysis. The numerical models of surge tank caverns during excavation were established and the mechanical properties, failure modes and acoustic emission (AE) characteristics were analyzed. The simulation results indicate that the large-scale columnar joints and C2 interlayer shear weakness zones influence the mechanical properties and failure modes significantly. Three types of damage modes in numerical models containing crush-shear failure along columnar joints, tensile-slip failure along C2 interlayer shear weakness zone and crush failure inside and around caverns were summarized. The concentration zones of tensile stress were found around each cavern and gradually shrunk from sector shape to linear shape. The caverns were divided into two parts along C2 shear weakness zone in the process of X stress loading and two sector damage zones occurring on both horizontal sides of each cavern. In the process of Y stress loading, some crack networks consisting of large-scale columnar joints, intralayer joints and C2 shear weakness zone were formed which led to the complete failure of the numerical models. A new method based on BP neural network was proposed to evaluate the stability of rock mass with large-scale columnar joints and interlayer shear weakness zones. The training results show that the method can distinguish the stability of rock mass well, while its accuracy reaches 97.6%.

Finite element analysis of TGO thickness on stress distribution and evolution of 8YSZ thermal barrier coatings

AbstractAccording to the experimental research results of the thermally grown oxide (TGO) layered growth during the pre‐oxidation process of 8 wt.% yttria‐stabilized zirconia thermal barrier coating (TBC), a two‐dimensional sinusoidal TC/bonding coat (BC) curve interface model of the longitudinal section of TBCs based on finite element simulation was constructed; the thickness and composition of the TGO layer relative to the TC/BC curve interfacial stress distribution and its evolution during the thermal cycling process were studied. The results show that when the TGO layer uses α‐Al2O3 as the main oxide (black TGO), the thicker the black TGO layer, the more uniform the stress distribution of the TC/BC interface. When the TGO layer is dominated by spinel‐structured Co and Cr oxides (gray TGO), the stress “band” of the TC/BC interface is destroyed; it shows the alternating phenomenon of tensile stress zone and compressive stress zone, and after the rapid random growth of TGO, the concentrated tensile stress increased by a large jump. Affected by the thickness of the prefabricated black TGO layer, there is a limit peak in the thickness of the black TGO layer, the normal stress at the TC/BC boundary is minimized, and the magnitude of the stress change is also minimized.