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Shales with inclined beddings are frequently encountered in formation reservoirs as burial depth increases. However, limited research has focused on the influence of wellbore orientation and stress magnitudes when hydraulically fracturing inclined shale reservoirs. To close this research gap, a series of hydraulic fracturing experiments were conducted to investigate the effects of different wellbore orientation angles (α = 0–90°) and axial stress magnitudes (σv = 0–20 MPa) on hydraulic fracture initiation and propagation in inclined bedding shales. After fracturing, a 3D laser scanner is employed to characterize the fracture surface roughness, and three independent parameters (SD, θs, Rs) are used to quantitatively evaluate the roughness of the fracture surfaces. The results show that increasing the wellbore orientation angles (α) results in a first decreasing and then increasing trend of the fracture initiation pressure (Pi), breakdown pressure (Pb) and roughness of fracture surfaces. The wellbore orientation angle (α) of 45° is beneficial to reducing the Pi, Pb and roughness of the fracture surface. However, relatively low critical fluid pressures will facilitate the reduction of surface roughness under the impact of wellbore orientation. Increasing axial stress in inclined bedding shale reduces the fracture initiation pressure (Pi) and breakdown pressure (Pb) while enhancing the roughness of the fracture surfaces. In field fracturing, adjusting the wellbore inclined to the bedding (45° is preferred) and in the burial depth with a stress difference greater than 10 MPa benefit the improvement of hydraulic fracturing performance. These findings offer an essential reference for designing hydraulic fracturing in shale reservoirs. Highlights: Fluid pressurization rate is used to identify fracturing stage during fluid injection. Low pressures facilitate reducing surface roughness under the impact of wellbore orientation. Increasing axial stress facilitates fracture initiation and shale breakdown. High stresses facilitate complex fractures and rough fracture surface formation. [ABSTRACT FROM AUTHOR]

The estimation of mechanical and physical properties of coal reservoirs is important for the successful exploration and development of coalbed methane (CBM). Unlike conventional sandstone reservoirs, coal reservoirs exhibit greater sensitivity to stress, resulting in distinct mechanical and physical behaviors. In this study, uniaxial compression tests were performed on both low-rank and high-rank coal samples under different moisture conditions to reveal the mechanical and physical property changes with stress. The results indicate that during axial stress loading (up to 2.8 MPa), axial strain initially increases rapidly and subsequently at a slower rate, with an axial strain of 0.13–0.25% observed at the maximum axial stress. The instantaneous Young's modulus increases linearly before stabilizing, ranging from 618.01 to 4861.10 MPa, while the Poisson's ratio remains relatively constant or increases linearly, ranging from 0.002 to 0.165. This results in a negative exponential decrease in both porosity and permeability, with maximum reductions of 1.77–4.21% and 5.38–12.25%, respectively. The mechanical properties of coal are influenced by both the cementation effect of water at low water saturation and the softening effect at high water saturation, which results in axial strain decreases and then increases as the water saturation increases. Concurrently, the elastic modulus initially increases and then decreases, while the Poisson's ratio exhibits a less pronounced change or tends to increase. Consequently, there is a trend in which porosity and permeability first increase and then decrease. In addition, during stress unloading, the influence of water in coal induces a notable strain hysteresis phenomenon in water-containing coal samples, and this phenomenon is more obvious in the low-rank coals. [ABSTRACT FROM AUTHOR]

Gob is one of the most serious areas of coal spontaneous combustion. With the continuous increase of mining depth, the residual coal in gob bears increased axial stress, the rise of ground temperature, and the degree of fragmentation of mining coal and rock continues to increase, the external water in the mining process changes the water content of broken residual coal, under the influence of multiple factors, the prevention and control situation of coal spontaneous combustion in gob is becoming more serious. Based on the actual accumulation environment of residual coal in gob, this paper carried out the oxidation and temperature rise experiment of broken coal under the condition of water bearing and pressure. The variation laws of oxidation characteristic parameters such as outlet oxygen concentration, oxidation derived gas, oxygen consumption rate and heat release intensity were obtained. It is found that with the increase of water content of coal sample, the water content first promotes and then inhibits the oxidation process of coal. It is found that after water molecules form a relatively stable hydrogen bond with oxygen-containing functional groups, the total energy of the two-phase materials decreases, and the reason for the decrease of wetting heat release is analyzed. The influence mechanism of axial stress loading on coal spontaneous combustion oxidation process is studied. It is found that the increase of axial stress first promotes and then inhibits the process of coal spontaneous combustion. [ABSTRACT FROM AUTHOR]

The axial compression performance of circular columns strengthened with carbon fiber-reinforced polymer (CFRP) was investigated using numerical simulation. The study's objective was to validate a finite element model to match results of experimental testing, to ensure consistent failure modes and load-displacement profiles. The investigation explored the impact of various parameters, including concrete strength, CFRP layer numbers, slenderness ratio, steel reinforcement ratio, and cross-sectional area, on CFRP column behavior. The analysis revealed valuable insights into stressstrain relationships and ultimate load-bearing capacity. This study provides vital information for structure engineering practices and design strategies in the industry, highlighting the significance to utilize CFRP technology to enhance structural performance, especially the consistent stress distribution on the concrete core. To understand the mechanical response of CFRP circular concrete columns, engineers can optimize design and construction techniques to create more efficient and durable structures elements, ultimately to improve public safety and to reduce maintenance processes. [ABSTRACT FROM AUTHOR]

The excavation and maintenance of buried natural gas pipelines can lead to deformation and stress redistribution of the pipelines and even cause secondary damage to the pipes with issues. To clarify the impact of excavation unloading on buried pipelines, this study established a finite element three-dimensional pipe-soil model, investigated the mechanical response of pipelines under layered excavation and evaluated various parameters impacting the response. The parameters analyzed include the diameter-thickness ratio of the pipe, excavation length and width, thickness of top covering soil, elastic modulus of soil, specific weight of soil and initial displacement of the pipeline. The study results showed that the pipeline bulges upwards during excavation unloading, the pipe top in the middle is under tension, and the bottom of the pipe is under compression. Therefore, the axial stress and vertical displacement both increase first and then decrease, and they are distributed symmetrically along the pipeline axis; excavating the initially compressed pipeline leads to high strain areas in the pipeline and even local buckling. The response to slope excavation is more pronounced than that to straight trench excavation; the additional response of the pipeline increases with the increase of diameter-thickness ratio, excavation width, thickness of pipe top covering soil and specific weight of soil, but it decreases with the increasing soil elastic modulus. The additional response is closely related to excavation length and the initial displacement. The results of this study can provide a reference for pipeline construction, maintenance, and safety assessment. [ABSTRACT FROM AUTHOR]

This article studies the distribution law of axial stress in mountain pipelines under the action of lateral landslides. Firstly, a mechanical model is established for analysis, taking into account the geometric nonlinearity of the pipeline and the physical nonlinearity of the longitudinal resistance of the soil outside the landslide. The theoretical calculation method for the axial stress of the pipeline under two conditions of tension or pressure is obtained, and the calculation formula for the axial stress of mountain pipelines under the action of lateral landslides is summarized. The finite element method was used to simulate and calculate buried pipelines affected by lateral landslides based on nonlinear contact problems. The single variable method was used to analyze the influence of soil parameters, landslide body parameters, mountain body parameters on both sides, and some other parameters on the axial stress of mountainous pipelines. The axial stress distribution of buried pipelines under different influencing factors was analyzed, and the axial stress distribution in four different directions of pipelines 0, 3, 6, and 9 was obtained, Analyze the variation pattern of axial stress in the pipeline at the location of stress concentration, and the results indicate that stress concentration occurs in the pipeline at the middle and edge positions of landslide displacement; According to the relationship between the axial stress of the pipeline and various influencing factors, the relationship between the maximum axial stress of the pipeline and a single factor is fitted, and the axial stress distribution law of the mountain pipeline under the lateral landslide effect is finally obtained. [ABSTRACT FROM AUTHOR]

In response to the difficulty in effectively dealing with grouting defects in corrugated pipes within a suspension prestressed concrete arch bridge, a method for assessing the deterioration in the performance of prestressed concrete girders afflicted with grouting defects was established in the present study. Specifically, a time-varying model of steel strand corrosion within grouting defects was constructed by investigating the corrosion theory of steel strands. In addition, a full-scale numerical simulation model of the long-span prestressed concrete bridge was established based on a practical project. Through the described means, the long-term impact of steel strand corrosion at various locations, lengths, and quantities on the vertical displacement and axial stress of girders was elucidated. The results reveal that in the presence of corrosion affecting 16 steel strands located in the midspan bottom plate, a vertical displacement alteration of 17.55 mm was observed in the midpoint region of the girder over a 30-year period following the bridge's construction. Further, when considering the combined effects of concrete shrinkage, creep, and the corrosion of 16 steel strands in the midspan bottom plate, the axial compressive stress within the midpoint region of the girder decreased from an initial 6.30 MPa to 0.79 MPa over the same 30-year timeframe post-construction. It was observed that two indicators of vertical displacement and axial stress can be employed to evaluate the performance degradation of prestressed concrete bridge girders with grouting defects. The present findings may provide a reference for the operation and management of bridges with grouting defects. [ABSTRACT FROM AUTHOR]

Standard penetration test (SPT) has been widely used in offshore exploration because of its unique advantages. Unlike onshore exploration, offshore construction areas are characterized by high waves and water depths ranging from several meters to tens of meters. As a result, the reliability of offshore SPT is significantly reduced compared with onshore SPT. Currently, the probe rod length correction of SPT is not involved in geotechnical engineering investigation codes and related research, which greatly limits the application of this method in offshore exploration. Therefore, a series of SPTs were carried out in offshore environments with different water depths, with a maximum rod length of 65 m. The acceleration and axial stress at each test point of the rod were monitored by the dynamic signal data acquisition system, and the hammer impacting energy at each test point was obtained by Force–Velocity (F-V) method. The test results show that the correction of the rod length of the offshore SPT is different from that of the traditional SPT, and it needs to be further corrected for the water depth. In this paper, a modified method of rod length for offshore SPT is proposed, which can provide reference for the application of offshore SPT. [ABSTRACT FROM AUTHOR]

Current earth pressure calculation methods suffer from certain limitations because they do not consider the effect of retaining wall displacement. In this study, the soil behind the wall is assumed to be in a plane strain state, and drawing upon nonlinear elastic constitutive theory, an earth pressure calculation method is proposed, capable of considering both axial stress and wall displacement. To account for changes in soil modulus with confining pressure, the tangent modulus from the Duncan-Chang nonlinear model is introduced. Depending on the direction of the principal stress behind the retaining wall, the static earth pressure point, the major principal stress inflection point, and the minor principal stress second inflection point are determined. The conditions for the existence of the second inflection point are also given. These specific points, together with the limit earth pressure point, divide the earth pressures acting on the wall into six regions. The study provides earth pressure calculation formulas for T (translation) mode, RBT (rotation about a point below the base) mode, and RTT (rotation about a point above the top) mode based on the characteristics of wall displacement distribution in each mode. The proposed method exhibits good agreement with the test results, offering an effective approach for accurately calculating earth pressures related to displacement. [ABSTRACT FROM AUTHOR]

Tunneling, especially in urban areas, affects many structures on the ground, which directly influences the usability and stability of the structures. The settlement of and axial stress on the pile foundation are important factors that determine the behavioral characteristics of the pile foundation. Therefore, this study uses numerical analysis and machine learning to derive a prediction model of pile axial stress due to tunnel excavation adjacent to the piled raft. Numerical analysis data were utilized for machine learning purposes, and the effects of the input data on the prediction model were scrutinized. The numerical analysis revealed that the change in the pile axial stress resulting from tunnel excavation differed depending on the pile's location, with the greatest axial stress reduction occurring in the center of the piled raft. Furthermore, the rate of reduction was higher in soils with lower relative densities. Several algorithms were employed to derive the prediction model, with tree-based algorithms displaying notable performance in predicting pile axial stress. Additionally, preprocessing the data with appropriate feature engineering techniques exhibited superior predictive power, and incorporating settlement data aided in enhancing the prediction model's performance. [ABSTRACT FROM AUTHOR]

To study the shear behaviors of jointed rocks reinforced by basalt fiber-reinforced polymer (BFRP) bars and steel–FRP composite bars (SFCBs), we conduct laboratory tests and numerical simulations to analyze the shear strength, shear stiffness, energy dissipation, and bolt failure modes. Our results show that the shear stiffness of the BFRP bolted specimen is lower than that of the specimens bolted by steel bars and SFCBs, but the residual shear strength is higher. SFCB-reinforced jointed rock has the highest peak shear strength, and its residual strength is similar to that of the steel bar bolted specimen. The total energy absorbed by the BFRP bolted specimen is comparable to that absorbed by the steel bolted specimen. When the bolt inclination angle is 60°, the shear strength of the BFRP bar bolted specimens is higher than that of the steel reinforced one. The failure characteristics of BFRP bar bolted rocks can be categorized as resin matrix fracture, resin matrix and fiber shear, and fracture of resin matrix and rocks. The failure modes of the SFCB divided into surface FRP failure and steel bending. Based on numerical results, BFRP bars have larger axial force than conventional bolts, but lower shear stress. The axial stress of the BFRP bar increases as the bolt inclination angle decreases. Moreover, the BFRP bar is more likely to cause shear cracks at the interface between the rock and the bolt. Highlights: Experimental and numerical tests were conducted on the jointed specimens reinforced by Basalt Fiber Reinforced Polymer bar and Steel-FRP composite bar. In terms of shear stiffness and residual shear strength, the jointed specimen reinforced by Basalt Fiber Reinforced Polymer bar and Steel-FRP composite bar differ significantly from steel. The specimens reinforced by Basalt Fiber Reinforced Polymer bar are sensitive to the effect of the bolt inclination angles and have higher shear strengths than conventional bolts at the bolt inclination angle is 60°. The axial force and shear force variation laws of different types of bolts were discovered. [ABSTRACT FROM AUTHOR]

Piled raft foundation (PRF) is a cost effective and sustainable foundation option which can reduce the total and differential settlement, and improve the bearing capacity of the foundation in case of high-rise superstructures or marine structures. This study mainly focuses on a parametric analysis of PRF and also checks the impact of various design parameters on the structural and geotechnical aspects of PRF including the settlement of PRF and its components, raft-soil contact behaviour, bending moment, axial stress on pile, soil stress and settlement in different directions etc. To understand the contribution of piles in PRF, two factors such as relative improvement in the stiffness and settlement ratio are also evaluated. The results of the present study indicate that the differential and reference settlement can be controlled by the addition of piles under the raft, whereas the value of normalized differential settlement remains increasing with an increase in the pile spacing to diameter (s/d) ratio. In case of raft-soil contact behaviour, the contact pressure increases if the raft width is increased, but the rate of increment is prominent in case of a higher number of piles. The raft is showing both the sagging and hogging bending moment and as s/d ratio increases, bending moment also tends to increase. [ABSTRACT FROM AUTHOR]

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Strain-softening analyses were performed around a circular bore in a Mohr–Coulomb rock mass subjected to a hydrostatic stress field in cross section and out-of-plane stress along the axis of the bore. Numerical procedures that simplify the strain-softening process in a step manner were employed, and on the basis of the theoretical solutions of the elastic–brittle–plastic (EBP) medium, the strain-softening results of the displacements, stresses and the plastic zones around the circular bore were obtained. The numerical solution was validated based on the fact that the strain-softening process became EBP when the softening slope was very steep and elastic-perfectly plastic (EP) when the softening slope was near zero. The results illustrated that the stresses and displacements in the rock mass surrounding the bore were affected by axial stress and that a proper consideration of out-of-plane stress was necessary. Moreover, the presented results can be used for the verification of numerical codes. [ABSTRACT FROM AUTHOR]

Coal particles are broken to varying degrees under the axial stress, thereby affecting the particle size distribution, which has an essential impact on coal spontaneous combustion (CSC). This work investigated the evolution behavior of the characteristic parameters and limit parameters of the coal oxidation process under different breakage degrees. Results showed that as the axial stress increased, the breakage degrees of coal gradually increased, while the permeability gradually declined. The increase in the breakage degree of particle induced by axial stress would promote the CSC process and increase the risk of CSC. The promotion effect reached the most prominent at 12 MPa and began to decrease at 15 MPa. Particle breakage and increasing specific surface area caused by stress were responsible for the change in CSC characteristic parameters. The reduction in permeability caused by stress dominated the changes in CSC limit parameters. [Display omitted] • Studied the breakage and permeability evolution during coal low-temperature oxidation process. • Increasing breakage degree accelerated the process and risk of CSC. • Axial stress of 12 MPa significantly impacted the CSC process. • Breakage degree and permeability affected the characteristics parameters and limit parameters. [ABSTRACT FROM AUTHOR]

Stiffeners support lateral pressure and axial load and are one of the essential members of a ship structure composed of stiffened panels. Their scantling formulae are important to ensure adequate strength against lateral pressure and for the rapid and proper initial design of hull structures. However, the current rule scantling formulae are based on the elastic beam formulation, and the effect of the simultaneous axial stress is considered differently by the rule as a coefficient for reducing the allowable stress. In this study, based on the fully plastic moment under the action of axial stress, the stiffener bending strength corresponding to the plastic hinge formation criteria (initial hinge and plastic collapse) was determined using simple theoretical formulae considering the additional lateral force induced by the axial stress on the deflected stiffener. Subsequently, the structural behaviors were investigated comprehensively by theoretical parametric studies based on various stiffener scantlings and loading combinations, which were further compared with the results of finite element analysis (FEA) based on the residual deflection criterion, thereby verifying the validity of the theoretical proposals. Consequently, by combining the findings from the theoretical and numerical investigations, the effect of the axial stress on the stiffener bending strength was expressed as closed-form coefficients. These proposed axial stress coefficients were verified to govern the actual structural behaviors well and are expected to provide a rational basis and contribute to improving rule-scantling formulae. • Deriving a theoretical method to evaluate the stiffener bending strength under the axial load. • Considering fully plastic moment and additional lateral force affected by the axial load. • Comprehensive finite element analysis employing residual deflection criteria corresponding to the initial hinge formation. • Closed-from axial stress coefficient proposed based on the comprehensive theoretical and numerical results. [ABSTRACT FROM AUTHOR]

According to the industry standard, low permeability and tight reservoir are highly stress-sensitive in laboratory tests. However, this phenomenon has not been effectively confirmed in the field production test. For this reason, a method to measure stress sensitivity by axial loading is proposed. By loading fixed axial stress, the measurement method achieves the effect of equivalent preloading fluid pressure. Meanwhile, the equivalent theoretical curve can be obtained by translating the curve. Through this method, the stress sensitivity of the core with microfracture is studied. According to the results, the stress sensitivity of the cores can be divided into two stages: the first stage is the non-opening stage of microfractures, in which the stress sensitivity is weak; the second stage is the opening stage of microfractures, in which the stress sensitivity increases sharply. When the permeability changes significantly, the corresponding pressure is the microfracture's opening pressure. In addition, if the abscissa of the experimental results corresponds to the field production data, the field stress sensitivity curve of the field reservoir can be obtained. Conventional stress sensitivity test shows that the four cores in the study block exhibit medium to strong stress sensitivity. According to the stress sensitivity curve analysis after curve translation, the stress sensitivity of the reservoir is about 25% in the elastic production stage, which is classified as weak stress sensitivity. This conclusion can effectively support the phenomenon of the unobvious stress sensitivity in the field production of low permeability and tight reservoirs. [ABSTRACT FROM AUTHOR]

Temperature and stretching are important factors in the high-temperature treatment of carbon fiber. The axial stress during carbon-fiber high-temperature treatment affects its ability to stretch. The high-temperature axial stress evolution mechanism of polyacrylonitrile-based carbon fiber was studied through in situ tension tests, Raman spectroscopy, X-ray diffractometry, elemental analysis, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, thermal expansion coefficient tests, and density methods. The high-temperature axial stress evolution of polyacrylonitrile-based carbon fiber involved three stages: rapid increase, rapid decrease, and relaxation. The highest stress and relaxation temperatures of the polyacrylonitrile-based carbon fiber were 1600°C and 1950°C, respectively. The main factors that affected the fiber axial stress included carbon-structure rearrangement and the effect of thermal expansion and cold shrinkage on fiber length. During the first stage (T < 1600°C), carbon-structure rearrangement after nitrogen atom removal increased the fiber axial stress. In the second stage (1600 ⩽ T ⩽ 1950°C), the difference in the thermal expansion of fibers that entered the graphite furnace and the cold shrinkage of fibers that exited the graphite furnace increased gradually, which resulted in a decrease in fiber axial stress by up to 1950°C, where the fiber relaxed and the third stage (T > 1950°C) began. The difference between expansion and shrinkage increased significantly, which increased fiber relaxation. Carbon fibers with fewer nitrogen atoms and more regular structures had a lower axial stress during high-temperature treatment, but the trend and characteristic temperature remained unchanged. The corresponding fiber high-temperature maximum stretching ratio and axial stress showed opposite trends below 1950°C. The ability to stretch the carbon fiber increased above 1950°C, which differed from the axial stress relaxation. [ABSTRACT FROM AUTHOR]

Summary: The accuracy of multiscale modeling approaches for the analysis of heterogeneous materials hinges on the representativeness of the micromodel. One of the issues that affects this representativeness is the application of appropriate boundary conditions. Periodic boundary conditions are the most common choice. However, when localization takes place, periodic boundary conditions tend to overconstrain the microscopic problem. Weakly periodic boundary conditions have been proposed to overcome this effect. In this study, the effectiveness of weakly periodic boundary conditions in restoring transverse isotropy of representative volume elements (RVE) for a fiber‐reinforced composite with elastoplastic matrix is investigated. The formulation of weakly periodic boundary conditions is extended to allow for force‐controlled simulations where a uniaxial stress can be applied. A series of simulations is performed where the orientation of applied stress is gradually varied and the influence of this orientation on the averaged response is examined. An original method is presented to test the correlation between the ultimate principal stress and average localization angle of shear bands within an RVE. It is concluded that weakly periodic boundary conditions alleviate anisotropy in the RVE response but do not remove it. [ABSTRACT FROM AUTHOR]

Buried gas pipelines are vital assets of natural gas transportation system for every country. Therefore, investigation of their potential failure's roots is prominent. There are different causes for their failure among which imposed loads are the majority. In this study, considering soil as an elastoplastic material and deriving its constitution equation, a steel gas pipeline was modeled in its critical cross section where it passes under a road. Studying the behavior of the axial stress as maximum stress component and pipeline displacement under live and dead loads, a safe depth of 2 m was found so as to decrease and increase failure probability and safety factors, respectively. This height not only negates live loads, it alleviates the effect of dead loads. [ABSTRACT FROM AUTHOR]

The axial stress of high strength bolts for bridges affects the service life directly, it is necessary to measure it regularly. Based on the theory of acoustic elasticity and combination of longitudinal and shear waves, a new theoretical analysis method is proposed. The stressed and non-stressed areas of bolt are separated, and the derived formula only use the flight time and difference of acoustoelastic coefficients to get axial stress. Further, the preload experiments with high strength bolts of grade 8.8S and 10.9S is designed, the experimental results show that the proposed method is applicable to the axial stress of the bridge bolts with any specifications, material and different screwing depth used in this paper, the effect of elastic deformation on the flight time is eliminated, and the measuring error is within 5%. [ABSTRACT FROM AUTHOR]

Superior structural, physical, and electronic properties make ferroelectric nanocrystals essential in enabling a range of next‐generation devices. Ferroelectric responses are determined by crystal structure and domain morphology. The ability to reversibly displace, create, and annihilate elastic domains is critical to device applications. Although electric‐field control has been demonstrated for ferroelectric 180° surface domain walls and vortices, similar control of ferroelastic domains and domain boundaries within individual nanocrystals remains challenging. Using controlled external compressive and tensile axial stress, deterministic and reversible control of highly mobile ferroelastic domains and axial polarization in three dimensions is demonstrated. While many studies exist on ferroelastic domains in thin films and bulk, little is known about ferroelastic interactions at the single nanoparticle level, especially involving domain boundaries. Through combining Bragg coherent X‐ray diffractive imaging and Landau theory, strain gradients in individual BaTiO3 nanocrystals are shown to stabilize needle‐like ferroelastic twin domains. These domains are highly labile under applied axial stress, producing a locally enhanced electric polarization mediated by a ferroelectric phase transition. The efficacy of Bragg coherent X‐ray diffractive imaging in studying in operando domains in three dimensions is demonstrated, while synergy with theory provides a paradigm for domain boundary engineering and potential for nanoscale functional devices. [ABSTRACT FROM AUTHOR]

Temperature and stretching are important factors in the high-temperature treatment of carbon fiber. The axial stress during carbon-fiber high-temperature treatment affects its ability to stretch. The high-temperature axial stress evolution mechanism of polyacrylonitrile-based carbon fiber was studied through in situ tension tests, Raman spectroscopy, X-ray diffractometry, elemental analysis, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, thermal expansion coefficient tests, and density methods. The high-temperature axial stress evolution of polyacrylonitrile-based carbon fiber involved three stages: rapid increase, rapid decrease, and relaxation. The highest stress and relaxation temperatures of the polyacrylonitrile-based carbon fiber were 1600°C and 1950°C, respectively. The main factors that affected the fiber axial stress included carbon-structure rearrangement and the effect of thermal expansion and cold shrinkage on fiber length. During the first stage (T < 1600°C), carbon-structure rearrangement after nitrogen atom removal increased the fiber axial stress. In the second stage (1600 ≤ T ≤ 1950°C), the difference in the thermal expansion of fibers that entered the graphite furnace and the cold shrinkage of fibers that exited the graphite furnace increased gradually, which resulted in a decrease in fiber axial stress by up to 1950°C, where the fiber relaxed and the third stage (T > 1950°C) began. The difference between expansion and shrinkage increased significantly, which increased fiber relaxation. Carbon fibers with fewer nitrogen atoms and more regular structures had a lower axial stress during high-temperature treatment, but the trend and characteristic temperature remained unchanged. The corresponding fiber high-temperature maximum stretching ratio and axial stress showed opposite trends below 1950°C. The ability to stretch the carbon fiber increased above 1950°C, which differed from the axial stress relaxation. [ABSTRACT FROM AUTHOR]

The present paper is concerned with the failure analysis of the wall loss defect in pipelines reinforced with a polymer-based composite repair system. The main goal is to propose a methodology that accounts for the pipe extremities (axial stress) in an analysis to predict an accurate failure pressure. The proposed methodology defines a simple expression for a real test specimen condition (closed-cap cylinder), which allows to estimate the failure pressure using the elastic properties of the materials and test specimen geometry. Hydrostatic tests performed in different laboratories are used to validate the proposed methodology. The results show a good agreement between the model prediction and the experimental failure pressure results in all cases. However, a careful selection of the remaining strength factor is needed, as it impacts on the accuracy and conservative level of the failure pressure. In addition to the axial stress, there is a possibility to refine the theoretical prediction of the failure pressure value by accounting for the plastic deformation far from the defect region as well as the radial stress in the failure analysis. [ABSTRACT FROM AUTHOR]

The present research deals with the study of transversely isotropic thermoelastic beam in the context of Green-Naghdi (GN) theory of thermoelasticity of Type-II and Type-III. The mathematical model is prepared for the thin beam in a closed form with the application of Euler Bernoulli beam theory. The Laplace Transform technique has been used to find the expressions for displacement component, lateral thermal moment, deflection and axial stress in transformed domain. The general algorithm of the inverse Laplace Transform is developed to compute the results numerically in physical domain. The effect of two theories of thermoelasticity Green- Naghdi-II and Green-Naghdi-III has been depicted on the various quantities. Some particular cases have also been deduced. [ABSTRACT FROM AUTHOR]

The permeability characteristics of gas‐containing coal under different radial stress, different axial stress, and gas pressure were studied by orthogonal experiments, using a self‐developed three‐axis servo fluid infiltration system with a gas‐solid coupling of gas‐containing coal, on the basis of a single‐factor influence on the permeability of gas‐containing coal. By considering the effective stress, three kinds of relationships between permeability and radial stress, permeability and axial stress, as well as permeability and gas pressure were established. The results show that radial stress, axial stress, and gas pressure have a great influence on the permeability characteristics of gas‐containing coal: (a) The influence of radial stress on gas permeability is significant, whereas the influence of axial stress is negligible. The degree of influence of radial stress, gas pressure, and axial stress on the permeability decreases in turn; (b) The permeability decreases following a power function with the increase of the radial stress; (c) The permeability gradually increases with the increase of axial stress. With the increase of axial stress, the permeability increases following a power function; (d) The increase in gas pressure will reduce the effective stress on the coal, and the number of pores and cracks inside the coal body will decrease. This will increase the effective seepage flow and gas flow rate of the gas and eventually lead to the increase of coal permeability. There is a quadratic function relationship between the permeability and gas pressure. The influence of radial stress on gas permeability of gas‐containing coal is significant, whereas the influence of axial stress is negligible. The permeability of coal sample increases with the radial stress and it decreases following a power function. With the increase of axial stress, the permeability of coal samples increases following a power function. There is a quadratic function relationship between the permeability of gas‐containing coal and gas pressure. [ABSTRACT FROM AUTHOR]

The paper presents new and original results of investigation on magnetoelastic effect in Ni-Zn ferrite formed into ring core and subjected to the axial stress up to 20 MPa. The core was magnetized with relatively low magnetizing field, corresponding to the so-called Rayleigh region. The obtained results indicate, that there is a significant correlation between applied stress and magnetic properties of the material and the Ni-Zn ferrite can be utilized in development of magnetoelastic sensor of force and stress. [ABSTRACT FROM AUTHOR]

Abstract The reliable prediction of excavation responses for squeezing rock remains a challenging issue in rock engineering. This paper presents a numerical procedure for a strain-softening rock mass around highly deformed circular opening based on the finite strain theory, so the overall elasto-plastic analysis is a Lagrangian method which focuses on the motion of each material point and is well suited for the problems involving large deformation. The influence of axial in-situ stress is considered by introducing the initial axial stress into the elastic constitutive equations, and the axial plastic flow is realized by redefining the plastic internal variable and considering different stress distributions in the plastic zone. Thus, the numerical procedure enables prediction of the rock deformation and squeezing potential in a more rational and rigorous manner. The capability of the method for estimating the small strain and squeezing deformation is verified by being compared with the existing analytical results and the field measuring data. Negligence of the axial in-situ stress or the axial plastic flow underestimates the rock deformation, and the induced deviation essentially depends on the post-peak constitutive relation and the rock heterogeneity after rock yields. The higher level of the material homogeneity in the plastic zone, the lower effects of the axial in-situ stress on the rock responses. The stress distribution and strain-softening behavior of surrounding rock can be significantly affected by the axial in-situ stress and the critical plastic internal variable η *. Tunnel instability is independent of large deformation for the high η * but is closely associated with the squeezing problem when η * is small. As η * decreases, both of the plastic radius and wall convergence increase initially and remain constant afterwards. Lastly, the paper investigates the rock responses and ground-support interaction when the small strain theory is applied on the squeezing rock. [ABSTRACT FROM AUTHOR]

The phenomenon of core disking is commonly seen in deep drilling of highly stressed regions in the Earth’s crust. Given its close relationship with the in situ stress state, the presence and features of core disking can be used to interpret the stresses when traditional in situ stress measuring techniques are not available. The core disking process was simulated in this paper using the three-dimensional discrete element method software PFC3D (particle flow code). In particular, PFC3D is used to examine the evolution of fracture initiation, propagation and coalescence associated with core disking under various stress states. In this paper, four unresolved problems concerning core disking are investigated with a series of numerical simulations. These simulations also provide some verification of existing results by other researchers: (1) Core disking occurs when the maximum principal stress is about 6.5 times the tensile strength. (2) For most stress situations, core disking occurs from the outer surface, except for the thrust faulting stress regime, where the fractures were found to initiate from the inner part. (3) The anisotropy of the two horizontal principal stresses has an effect on the core disking morphology. (4) The thickness of core disk has a positive relationship with radial stress and a negative relationship with axial stresses. [ABSTRACT FROM AUTHOR]

An electromagnetic method for determining axial stress in ferromagnetic bars is proposed. In this method, the tested bar is under the static magnetization provided by permanent magnets. The tested bar do not have to be magnetized up to the technical saturation because reciprocal amplitude of initial differential susceptibility (RAIDS) is adopted as the feature parameter. RAIDS is calculated from the radial magnetic flux density Br Lo = 0.5 at the Lift-off Lo =  0.5 mm, radial magnetic flux density Br Lo = 1 at the Lift-off Lo =  1 mm and axial magnetic flux density Bz Lo = 1 at the Lift-off Lo =  1 mm from the surface of the tested bar. Firstly, the theoretical derivation of RAIDS is carried out according to Gauss’ law for magnetism, Ampere’s Law and the Rayleigh relation in Rayleigh region. Secondly, the experimental system is set up for a 2-meter length and 20 mm diameter steel bar. Thirdly, an experiment is carried out on the steel bar to analyze the relationship between the obtained RAIDS and the axial stress. Experimental results show that the obtained RAIDS decreases almost linearly with the increment of the axial stress inside the steel bar in the initial elastic region. The proposed method has the potential to determine tensile axial stress in the slender cylindrical ferromagnetic bar. [ABSTRACT FROM AUTHOR]

Woody biomass is densified in the form of pellets in order to improve its physical and mechanical properties during handling and storage. However, limited research work has been conducted on the mechanical properties of pellets made from agricultural and wood biomass blends. Two commonly available forestry biomass, spruce (S) and pine (P), and three agricultural biomasses, reed canary grass (RCG), timothy hay (H) and switchgrass (SW), were used to form pellets. The mechanical properties were evaluated for three different particle sizes (150-300, 300-425 and 425-600 μm). An Instron attached with an in-house built single unit pelletizer and temperature controlled die was employed to produce a pellet. The aim of this study is to investigate the effect of particle size and blending (agricultural and woody biomass) on the mechanical properties (density and intrinsic yield stress). For all biomasses, pellets made from lower particle size (150-300 μm) exhibited higher density (950-1178 kg/m 3 for spruce and pine; 668-800 kg/m 3 for RCG, H and SW; 900-970 kg/m 3 for blended biomass). The intrinsic yield stress exhibited differences in values for individual forestry (40 MPa) and agricultural biomass (27-48 MPa), however after blending the values converged closest to that value for forestry biomass. In conclusion, blending low cost and abundant available agricultural biomass with woody biomass could not only result in better mechanical properties but would also help to meet the pellet market demand in future. [ABSTRACT FROM AUTHOR]

The residual stress field of fused silica induced by continuous wave CO2 laser irradiation is investigated with specific photoelastic methods. Both hoop stress and axial stress in the irradiated zone are measured quantitatively. For the hoop stress along the radial direction, the maximum phase retardance of 30 nm appears at the boundary of the laser distorted zone (680-µm distance to center), and the phase retardance decreases rapidly and linearly inward, and decreases slowly and exponentially outward. For the axial stress, tensile stress lies in a thin surface layer (<280 µm) and compressive stress lies just below the tensile region. Both tensile and compressive stresses increase first and then decrease along the depth direction. The maximum phase retardance induced by axial tensile stress is 150 nm, and the maximum phase retardance caused by axial compression stress is about 75 nm. In addition, the relationship between the maximum axial stress and the deformation height of the laser irradiated zone is also discussed. [ABSTRACT FROM AUTHOR]

Modeling of soil elastic and permanent plastic volumetric strains (compaction) caused by loading from machinery vehicles using the modified Cam-clay soil constitutive model requires understanding the behaviors of compression and rebound parameters under unsaturated soil conditions. Oedometer tests were conducted on a sandy loam, a loam, and a clay loam soil, all tropical soils, at three initial soil moisture contents and five maximum stress levels (50, 100, 200, 300 and 400 kPa). The objectives were to investigate the effects of soil moisture content and maximum applied stress on the modified compression index (λ*) and modified rebound index (κ*) parameters of a modified Cam-clay soil model on the three soils and predict the compressibility indices using the shape-restricted modeling technique. The clay loam soil showed higher compressibility at lower maximum stress levels and wet moisture conditions (-10 kPa soil moisture potential) but as the maximum applied stress increased (> 200 kPa), the modified compression index (λ*) variations with soil moisture content were insignificant (p > 0.05). A loam soil exhibited similar compression characteristics to a clay loam soil at 26.12% d.b. and 23.67% d.b., respectively. For a sandy loam soil, both critical state parameters were less sensitive to the variations in soil moisture content. The loam soil, which had an organic matter content of 6.33%, rebounded more than clay loam and sandy loam soils especially at higher applied stress values. On average, the modified compression index (λ*) was about 23 to 36 times the modified rebound index (κ*). Shape-restricted and quadratic model fittings are presented to explain the relationship between the critical state parameters and maximum applied stresses for each soil moisture content. The model fitting results indicated that shape-restricted regression predicted the modified Cam-clay model parameters as a function of maximum applied stress (or pre-compression stress) at very low Average Squared Error Loss (ASEL) and did so better than parametric quadratic equations. [ABSTRACT FROM AUTHOR]

• The oxidation experiment of coal under axial stress of 0–15 MPa was conducted. • The coal with axial stress of 9 MPa has the strongest chemical activity. • The effect of the axial stress on the spontaneous combustion of coal was investigated. • The spontaneous combustion tendency of broken coal under stress conditions was analyzed. As mines continue to extend deeper, the stress and ground temperature of the residual coal in the goaf are increased, and the degree of coal rock fragmentation also increases, making the spontaneous combustion of coal more prominent. In this paper, a low-temperature oxidation experiment of coal under axial stress loading conditions was conducted. By analysing the outlet gas flow rate, oxidizing gas derivatives, oxygen consumption rate, heat release intensity, apparent activation energy and other parameters, we studied the effect of axial stress variations on the spontaneous combustion of coal. The results showed that, in an experiment of heating oxidation with the same axial stress, the flow rate of gas generated by the oxidation in the coal sample room gradually decreased with an increasing temperature; at the same temperature, the flow rate of the outlet gas decreased as the axial stress increased. At a low temperature, as the axial stress increased, the rate of oxygen consumption and the amount of derived gas in the reaction system both increased, and the activation energy decreased. When the temperature reached the critical temperature and continuously increased, with an increase in the axial stress, the oxygen consumption rate, heat release intensity and the amount of derived gas in the reaction system first increased and then decreased. Moreover, the temperature point of ethylene gas produced by oxidation was advanced. The effect of the axial stress on the spontaneous combustion of coal was obtained, and the increase in the axial stress first promoted and then suppressed the spontaneous combustion of broken coal. [ABSTRACT FROM AUTHOR]

The present paper addresses the stresses in dynamic steel tube umbilicals at the platform hang-off exposed to large motions corresponding to an extreme storm condition. Experimental tests were carried out by mounting fibre-optic Braggs inside steel tubes of a 9 m long umbilical specimen. Then the specimen was exposed to constant tension and dynamic curvature by means of a bellmouth and the stresses were measured. Realizing that the geometry of the umbilical cross section was too complex to apply full 3D modeling, a beam and penalty contact modeling procedure was applied to describe the extreme stress behavior. Then a simplified method was proposed to determine the contact stiffness which was further validated by full scale umbilical axial stiffness testing. As hoop contact interfaces between steel tubes and separation fillers were observed in the test specimen, a tailor-made contact element was formulated to facilitate hoop contact. Different modeling alternatives were used to investigate the hoop contact effect on the extreme axial stress and Coulomb friction stress range, which were further validated against measured dynamic axial stress. Good correlation was found by the combination of describing all contact interfaces and constraints due to the grooves in the inner sheath. • Nonlinear axial stress was measured in a full scale testing of umbilical. • Friction stress prediction is sufficiently accurate to determine the axial stress. • Hoop contact was formulated to improve the axial stress behavior. • Numerical model was validated against the full scale testing. [ABSTRACT FROM AUTHOR]

The successful pilot testing of depressurization production for natural gas hydrate reservoir was conducted at Shenhu Area in the South China Sea. However, pressure changes due to depressurization production cause shrinkage of creeping pore and throat space for fluid flow, ultimately resulting in permeability damage in this reservoir. To optimize gas recovery for natural gas hydrate reservoir, it is important to understand the substantial variations of the pore structure and physical properties under different pressure conditions. In this study, by integrating computed tomography imaging with water flow experiments on Clay-silt sediment sample, we analyze how pore scale structural parameters (such as average pore and throat radius, pore and throat median radius, maximum pore and throat radius, and fractal dimension) change under different axial stresses by using fractal geometry approach. It is found that there is a negative relationship between axial stress and structural parameters. Meanwhile, with the axial stress increases, the range of pore and throat radius distribution located on the right of center distribution decreases. On the basis of fractal geometry theory, a fractal model is then proposed to explain the effects of axial stress and creeping microstructure on permeability for natural gas hydrate reservoir. Results show that the model provides good match with experimental data when the axial stress is larger, while poor agreements with experimental results at low pressure. This study helps us understand fundamental mechanism for permeability changes during the depressurization of gas hydrate reservoir. • The creeping microstructural images of gas hydrate reservoir sediment are obtained by micro-CT. • The quantitive microstructure parameters under different axial stresss are simulated. • A creeping permeability fractal model is proposed and tested by experiments. [ABSTRACT FROM AUTHOR]

When underground openings are excavated in weak rocks under a high overburden, large deformation and squeezing problems may take place. This paper generalizes the finite strain theoretical solution of Vrakas and Anagnostou (2014) for the ground squeezing response to elastic-brittle-plastic deformed rocks subjected to both the in-plane hydrostatic stress and the arbitrary axial in-situ stress. The ground behavior is analyzed based on the motion of material points in deformed configuration, thus the geometric non-linearity of rock mass is considered. Three different types of the plastic region can develop which depends on the magnitude of the axial in-situ stress, and the effects of axial in-situ stress and axial plastic flow on the squeezing response of rock mass are clarified in the case of large deformation. Comparisons among the large strain formula, the small strain theory and the numerical simulation results are made by using some examples, and the applicability of small strain method and some simplified models neglecting the axial in-situ stress is investigated. Finally, an engineering application in the severe squeezing Yacambú-Quibor tunnel in Venezuela is introduced to show the validity of the present large strain method in practical engineering. It is therefore very helpful from a practical standpoint for the preliminary design of cavities in squeezing rock mass, where the large deformation may exist and the classical small strain elasto-plastic analyses are inappropriate. [ABSTRACT FROM AUTHOR]