Twin Shear Unified Strength Theory Research Articles

Failure criterion and damage evolution of high-strength geopolymer concrete under compression-shear composite loading after high temperatures

Geopolymer concrete (GPC) holds significant potential for building fire safety design due to its green, low-carbon, and high-temperature resistant properties. In practical applications, concrete structures often face multiaxial composite loads. Therefore, investigating the composite stress performance of high-strength geopolymer concrete (HGPC) after exposure to high temperatures is crucial for assessing structural damage and enhancing fire prevention measures. This paper examines the compressive shear performance of HGPC at various temperatures T (20°C, 200°C, 400°C, 600°C) and stress ratios k (0.15, 0.3, 0.45, 0.6). The study analyzes failure patterns, shear load-displacement curves, shear strengths and their components, and peak shear deformation. Failure criteria are proposed to describe the damage patterns of HGPC under compressive shear loading after high temperatures. Additionally, the damage evolution process is evaluated using the energy method. The results show that HGPC damage surfaces penetrate the coarse aggregate at room temperature under compression-shear composite loading but shift to the aggregate-mortar interface at 600°C. Shear stiffness, peak shear strength, and residual strength increase with higher stress ratios but decrease with rising temperatures. Temperature affects peak shear strength by over 95 %, significantly more than the stress ratio. The shear strength decreases more slowly than compressive strength up to 400°C but more rapidly beyond 400°C. The cohesive strength of HGPC decreases with increasing stress ratio under different temperatures, while contact friction strength exceeds 50 % of peak shear strength. Shear dilation strength generally increases, then decreases, peaking at k = 0.45. The failure criterion based on the modified twin shear unified strength theory aligns best with experimental results. Furthermore, an increased stress ratio retarded the development of damage evolution, which was first accelerated and then slowed with rising temperature.

Active Earth Pressure of Finite Soil Based on Twin-Shear Unified Strength Theory

Active Earth Pressure of Finite Soil Based on Twin-Shear Unified Strength Theory

Damage softening constitutive model of HTPB propellant for biaxial loading

In order to establish the damage constitutive model of composite solid propellant, it is assumed that the micro element strength in HTPB propellants follows Weibull distribution. Using the twin-shear unified strength theory, a damage distribution variable is established, and a constitutive model characterizing the damage deformation process of HTPB propellants is established. Based on the biaxial loading experimental data of HTPB propellant at room temperature (298.15 K) and three loading rates (0.40 s−1, 4.00 s−1, and 14.29 s−1), the model parameters were fitted, and the relationship between the parameters in the damage constitutive model and strain rate was given. The model was validated using experimental results at room temperature (298.15 K) and three other loading rates (1.00 s−1, 2.00 s −1, and 8.00 s−1). The results show that the damage constitutive model can well describe the stress strain relationship of HTPB propellant under biaxial loading, although the model prediction effect of the nonlinear transition region of the stress strain curves is poor. Finally, it is concluded that the constitutive model established in this paper reflects the softening characteristics of HTPB propellants and the phenomenon that the strength of HTPB propellants is affected by the stress state. The damage constitutive model can provide a reference for the damage analysis of composite solid propellants under complex loading conditions.

Novel Method for Characterizing the Mechanical Properties of the Cement Sheath Based on Hollow-Cylinder Specimen and Multiaxial Load Tests

Summary Mechanical properties such as Young's modulus, Poisson's ratio, and strength are important parameters for evaluating the cement sheath integrity. However, the current triaxial testing methods for the mechanical properties of intact-cylinder cement specimens ignore the “ring” shape of the wellbore cement sheath and its nonuniform 3D stress distribution, which may lead to deviations from the actual mechanical properties of the wellbore cement sheath that can affect the theoretical analysis results. In this study, we designed a hollow-cylinder cement specimen to address the aforementioned conditions and conducted uni, tri, and multiaxial compression tests to analyze the differences in the deformation, damage, and mechanical failure of the hollow- and intact-cylinder cement specimens. It was found that under the same confining pressure, the Young’s modulus of the hollow-cylinder cement specimen with hole pressure was approximately 1.2 times that of the intact-cylinder cement specimen; however, the difference in Poisson’s ratio was not significant. The uniformity of the radial and circumferential stress distributions in the hollow-cylinder cement specimens was the main factor affecting the volumetric strain curves and the damage threshold. Under tri and multiaxial compression conditions, the deviatoric stress of the hollow-cylinder cement specimens was higher than that of the intact-cylinder cement specimens. Compared with the Mohr-Coulomb criterion, the twin-shear unified strength theory can more accurately reflect the relationships between the radial, circumferential, and axial stresses and the strength of the hollow-cylinder cement specimens. Through a failure test of the cement sheath based on a self-developed wellbore simulation device, it was verified that the mechanical properties of the hollow-cylinder cement specimens can better reflect the failure law of the wellbore cement sheath compared with intact-cylinder cement specimens. The findings of this study can contribute to the understanding of the mechanical behavior of wellbore cement sheaths.

Behaviour of CFRP confined square concrete-filled stainless steel tubular columns under uni-axial compression

To study the behaviour of the carbon fibre-reinforced polymer (CFRP) confined square concrete-filled stainless steel tubular (CFSST) stub column, three square CFSST stub columns and nine CFRP confined square CFSST stub columns were tested under axial compression. The test parameters included the number of CFRP sheet layers and the thickness of the stainless steel tube. The test results showed that failure modes of all CFRP confined square CFSST stub columns were the fracture of CFRP sheets and the outward local buckling of the stainless steel tube at the chamfering. The typical load–displacement curves of the tested columns exhibited four stages, i.e., elastic stage, elastoplastic stage, descending stage and residual stage. CFRP sheets could restrain the local buckling of CFSST, and improve the ultimate bearing capacity and the ductility index of the columns. With the increase in the number of CFRP sheet layers and the wall thickness of the stainless steel tube, the bearing capacity of the column increased linearly. Based on the twin-shear unified strength theory, a calculation method of the ultimate bearing capacity of the square CFSST stub column was proposed. Besides, through the superposition method and regression analysis, two simplified methods to predict the ultimate bearing capacity of CFRP confined square CFSST stub column under axial compression were obtained.

Axial compression behavior of CFRP-confined square concrete-filled double skin tube stub columns with stainless steel outer tube

Concrete-filled double skin tube (CFDST) stub columns with a stainless steel outer tube is a new kind of composite columns and potential to be widely utilized in ocean engineering owing to the excellent performance characterized by high corrosion resistance. To improve the ultimate bearing capacity and delay the local buckling of stainless steel tube, confining materials such as carbon fiber reinforced polymer (CFRP) can be adopted in these composite columns. Thus, this study conducted an experimental investigation on the axial compression behavior of CFRP-confined square CFDST stub columns with a stainless steel outer tube. The main variables were the width-to-thickness ratio of stainless tube and the number of CFRP layers. The typical failure modes, load-shortening curves, and load-strain curves of the CFDST and CFRP-confined CFDST columns were obtained. The test results indicated that the local buckling of CFDST columns can be well suppressed with CFRP wrapping, prior to CFRP rupturing. The CFRP can provide an effective confinement on the sandwich concrete and the ultimate bearing capacity enhancement increased with increasing the number of CFRP layers. Based on theoretical analysis a design model for the inner steel tube has been proposed to prevent the premature failure of the inner steel tube which can leads serious ultimate bearing capacity lose. Additionally, new methods for predicting the ultimate bearing capacity of the square CFDST and CFRP-confined CFDST are proposed based on the limit equilibrium method and twin shear unified strength theory. The predicted results are in good agreement with the experimental results and collected test data.

Twin Shear Unified Strength Solution of Shale Gas Reservoir Collapse Deformation in the Process of Shale Gas Exploitation

The collapse deformation of shale has a significant influence on the exploitation process. Experimental analysis has indicated a correlation coefficient range from 0.9814 to 0.9981 and the established sample regression formula could be used to express the relationship between the dynamic elastic modulus and static elastic modulus of shale specimens. Based on the twin shear unified-strength theory, where coefficient b was considered to express the effect of intermediate principal stress, with the deduced regression formula, the unified solution of major principal strains describing a critical collapse of the shale shaft wall was derived. The results showed that the intermediate principal stress had a significant influence on the major principal strain, describing the critical collapse of the shale shaft wall. At the same depth, the critical collapse major principal strain increased with the increase in the b values. With the change in b value from 0 to 1, the calculated difference in critical collapse major principal strain with the same wellbore depth would change from 22.1% to 45.5%. With the change in b value from 0 to 1, the calculated difference in critical collapse major principal strain with the same wellbore temperature would change from 22.1% to 45.6%. The unified solution formula of the major principal strain, describing the critical collapse of the shale shaft wall expressed by the dynamic elastic modulus, could adjust the contribution of intermediate principal stress by changing the values of b, while considering the influence of temperature and confining pressure. The twin shear unified-strength solution of the shale gas reservoir collapse deformation could be used to effectively evaluate the shale gas reservoir stability during shale gas exploitation.

Strength Criterion of HTPB Composite Solid Propellant under Tension‐Shear Loading at Low Temperature

AbstractThe strength limits of composite solid propellants are different at different temperatures and loading states. To study the ultimate bearing capacity of Hydroxyl‐terminated polybutadiene (HTPB) propellant, the tensile‐shear loading fixture and propellant specimens were designed, and the tensile‐shear tests of HTPB propellant at different temperatures (223–298 K) and strain rates (0.01–1.00 s−1) were carried out. The variation of ultimate load of HTPB propellant with loading conditions was analysed. Based on the twin shear unified strength theory, the strength criterion of HTPB propellant under low temperature tension‐shear load was established, and the variation of mechanical parameters and stress limit surface with temperature and strain rate were analysed. The results show that the twin shear unified strength theory can be well applied to the strength analysis of propellants. The ultimate bearing capacity of HTPB propellant is the smallest near the loading angle of 45°. The shape of strength limit surface is an equilateral semi‐infinite hexahedron. With the decrease of temperature and the increase of shear rate, the strength limit surface of HTPB propellant expands outward.

Active Earth Pressure of Finite Width Soil Considering Intermediate Principal Stress and Soil Arching Effects

Traditional earth pressure theories are based on the assumption of semi-infinite space. The existence of intermediate principal stress and soil arching effects is ignored, which will cause significant errors in the application of finite width soil. This study introduced the intermediate principal stress based on the twin-shear unified strength theory; the stress deflection caused by soil arching and the uniform surcharge on the retained soil surface were also considered. An improved calculation method for cohesive soil’s lateral earth pressure coefficient, an analytical solution for active earth pressure of finite width soil, the resultant force, and its action point were proposed. The lateral earth pressure distribution of finite width cohesive soil was studied by calculation examples. In addition, relevant parameters were also analyzed. The results indicate that due to the influence of the intermediate principal stress and soil arching effects, the resultant active earth pressure is lower than the traditional method. The lateral earth pressure coefficient gradually increases with the depth, but it is always lower than the traditional one. As the soil width increases, the resultant force action point presents a nonlinear trend that first decreases, then rises, and stabilizes. The proposed method was compared with the previous studies and got better results; it can provide a new idea for estimating the active earth pressure of finite width soil.

Experiment on axial compression performance of glass fiber reinforced polymer-walled concrete-filled steel tube columns

In order to study the axial compression performance of glass fiber reinforced polymer (GFRP)-walled concrete-filled steel tube columns, the static axial compression tests were conducted on a group of non-GFRP constrained specimens and two groups of GFRP constrained specimens. According to the test results, a strong and weak constraint model of the wall column was proposed. Based on the twin-shear unified strength theory, the formula was established to calculate the axial compression bearing capacity of the GFRP-walled concrete-filled steel tube columns. Finally, the theoretical results, obtained through the establishment of the finite element model, were compared with the experimental results. And a parametric analysis was used to study the effect of the yield strength of steel and the strength of concrete on the axial compression performance of the new type wall-filled concrete-filled steel tube columns. The results show that the wall-filled concrete-filled steel tube columns eventually fail due to crushing of the concrete, buckling of the steel tube and excessive deformation of the specimens. At the beginning of the second linear section of the specimen, the steel begins to yield and its strength is fully exerted, which demonstrates that GFRP can effectively improve the peak load of the member, however, the ductility has decreased. The results of the theoretical formula are in agreement with the experimental results. The bearing capacity can be improved by increasing the strength of concrete and the yield strength of the steel. Compared with the strength of concrete, the yield strength of steel bars has a greater influence on the bearing capacity.

The mechanical behavior of RPC under combined shear and compressive loads

In this study, the shear behavior of reactive powder concrete (RPC) in terms of shear strength and shear slip was studied by direct shear tests on cubic specimens and the influences of water-binder ratio, steel fiber volume ratio and normal compressive stress ratio were investigated. The test results indicated that using a low water-binder ratio and a reasonable fiber content could enhance the shear strength of RPC specimens. The RPC specimen containing steel fiber will not be severed into two individual parts due to the connection of steel fiber. Moreover, the normal stress was the key contribution to the shear strength, and the residual shear capacity was mainly provided by the friction of the shear surface. The cracking of the specimens became more severe but the shear strength in the cyclic horizontal test did not decrease obviously compared with that in the monotonic horizontal test. Applying the twin-shear unified strength theory to the test data, it was indicated that the elliptic curve showed a good agreement. Therefore, the unified twin shear strength theory could predict the proper RPC strength envelope curve under combined compressive-shear stress states.

Study on the minimum transverse impact toughness required to resist cracking of non-API high grade steel casing

Abstract In Tarim oilfield of China, the non-API high grade steel casing (HGSC) is frequently used as the intermediate and production casing in ultra-deep well with true vertical depth (TVD) more than 6000 m. So far, no internationally accepted standard has been established for non-API high grade steel casing (HGSC) presently, although some obvious shortcomings exist, such as sensitivity to crack defects and service environment, so that the fracture failure accidents of non-API HGSC happens frequently in Tarim oilfield. Here, firstly, this paper deduces the unified algorithm of burst pressure of non-API HGSC based on twin shear unified strength theory and bilinear hardening model, which takes strain-hardening, Bauschinger effect of material and effect of intermediate principal stress into consideration. Secondly, the burst pressure formula of non-API HGSC has been presented based on weighting twin shear stress yield criterion. Thirdly, the calculation method which can calculate the minimum transverse impact toughness (TITmin) required to resist cracking of non-API HGSC, has been established based on the presented burst pressure formula and API/ISO standard, and the effects of four kinds of defects, factor of effective thickness and non-destructive examination (NDE) level on the TITmin required to resist cracking of pipe body under diffenrent internal pressures have been analyzed. Finally, the full-scale ductile burst test of non-API 140 casing have been conducted, by which the accuracy and reliability of burst pressure formula and calculation method of TITmin have been validated. Research results can provide an important reference for non-API HGSC design in ultra-deep well.

Axial capacity of reactive powder concrete filled steel tube columns with two load conditions

Reactive powder concrete (RPC) is a type of ultra-high strength concrete that has a relatively high brittleness. However, its ductility can be improved by confinement, and the use of RPC in composite RPC filled steel tube columns has become an important subject of research in recent years. This paper aims to present an experimental study of axial capacity calculation of RPC filled circular steel tube columns. Twenty short columns under axial compression were tested and information on their failure patterns, deformation performance, confinement mechanism and load capacity were presented. The effects of load conditions, diameter-thickness ratio and compressive strength of RPC on the axial behavior were further discussed. The experimental results show that: (1) specimens display drum-shaped failure or shear failure respectively with different confinement coefficients, and the load capacity of most specimens increases after the peak load; (2) the steel tube only provides lateral confinement in the elastic-plastic stage for fully loaded specimens, while the confinement effect from steel tube initials at the set of loading for partially loaded specimens; (3) confinement increases the load capacity of specimens by 3% to 38%, and this increase is more pronounced as the confinement coefficient becomes larger; (4) the residual capacity-to-ultimate capacity ratio is larger than 0.75 for test specimens, thus identifying the composite columns have good ductility. The working mechanism and force model of the composite columns were analyzed, and based on the twin-shear unified strength theory, calculation methods of axial capacity for columns with two load conditions were established.

Elastic-plastic collapse limit analysis of coiled tubing under complex stress state

Collapse failure of coiled tubing (CT) due to complex stresses is an important issue during the operations in the oil and gas wells. This paper presents new analytical models to define the collapse limit of CT and solve the problem of CT failures. Elastic-plastic collapse limit models of CT are established based on the twin shear unified strength theory. The effects of axial force, buckling, and internal pressure on the elastic-plastic collapse limit are analyzed. Moreover, the comparisons among the new model, the existing model and API specification are further discussed. The results indicate that the elastic-plastic collapse limit models determined using the twin shear unified strength theory can describe the mechanical behaviors of CT under complex stress state. Elastic-plastic collapse limits of CT decrease with increasing axial tensile force and increase with increasing axial compressive force. The effect of buckling and bending stress on the elastic-plastic collapse limit cannot be ignored in the analysis of CT collapse. With increasing external pressure, the plastic region keeps extending to the outer surface of CT. Plastic collapse limit is the value of external pressure for which the entire wall thickness of CT becomes plastic. The proposed approach can be used to set the elastic-plastic collapse limits for other oil tubulars.

Elastic–Plastic Limit Load of Coiled Tubing Under Complex Stress State

Coiled tubing (CT) endures complex stresses during the operations. The loading imposed on the CT can push it close to its performance limits. This paper presents new analytical models to define the limit load of CT and solve the problem of CT failures. Elastic–plastic limit load models of CT are established based on the twin shear unified strength theory. Moreover, the effects of residual bending stress, buckling, axial force, and external pressure on the elastic–plastic limit load are discussed. The results indicate that the elastic–plastic limit load models determined using the twin shear unified strength theory can describe the mechanical behaviors of CT under complex stress state. The initial yield elastic limit load of CT decreases with increasing axial compressive force. The effect of buckling and residual bending stress on the initial yield elastic limit load cannot be ignored in the analysis of CT strength. With increasing internal pressure, the plastic region keeps extending to the outer surface of CT. Plastic limit load is the value of internal pressure for which the entire wall thickness of CT becomes plastic. The proposed approach can be used to set operation limits for other oil tubulars.

Viscoelastic–plastic creep constitutive model of frozen soil

In accordance with the twin-shear unified strength theory and the improved Nishihara model, a viscoelastic–plastic creep constitutive function of frozen soil has been established based on the associated flow rules, and the viscoelastic–plastic soft matrix is further derived from numerical calculations. The results show that the viscoelastic–plastic creep constitutive function has considerable influence on the non-linearity and the intermediate principal stress of materials, and it can describe the creep properties of frozen soil as a unified solution. The proposed model herein is applicable to the creep study of frozen soil.

A constitutive model for confined concrete in composite structures

The constitutive relation is an important factor in analysis of confined concrete in composite structures. In order to propose a constitutive model for nonlinear analysis of confined concrete, lateral restraint mechanism of confined concrete is firstly analyze to study the generalities. As the foundation of the constitutive model, peak stress and peak strain is the first step in research. According to the generalities and the Twin Shear Unified Strength Theory, a novel unified equation for peak stress and peak strain are established. It is well coincident with experimental results. Based on the general constitutive relations and the unified equation for peak stress and peak strain, we propose a unified and convenient constitutive model for confined concrete with fewer material parameters. Two examples involved with steel tube confined concrete and hoop-confined concrete are considered. The proposed constitutive model coincides well with the experimental results. This constitutive model can also be extended for nonlinear analysis to other types of confined concrete.

A New High Collapse OCTG Collapse Strength Model Based on Twin Shear Unified Strength Theory

Drilling through and completing wells through deep and acid environment regions are technically challenging and costly. Based on twin shear unified strength theory (TSUST), a new high collapse (HC) oil country tubular goods (OCTG) collapse strength model, involving the manufacturing imperfections and significant anisotropy of the material, was developed in this paper. Comparisons of numerical calculations with full-scale test collapse data show that the new HC OCTG collapse strength model gets higher calculation accuracy for predicating HC OCTG collapse strength than both American Petroleum Institute (API) Bulletin 5C3 and ISO/TR 10400. Thus, the new HC OCTG collapse strength model will provide a more scientific method and exciting possibility for deep and acid environment wells design and construction.

Theoretical Study on Working Mechanics of Smith Expansion Cone

The deformation failures of many casings are in urgent need of one reliable repair technology recently. Smith expansion cone (SEC) repair technology is high efficient to repair deformed casing. However, the reshaping force is a very important parameter for designing and optimizing the SEC and construction parameters. Hence, the mechanical mechanism of SEC repair technology is studied, and one mechanical model of SEC used to repair deformed casing is proposed based on twin shear unified strength theory in this paper. In this model, the effects of material hardening and the ratio of yield strength to tensile strength on casing repair were taken into full account. The mechanical model can calculate reshaping force that is used to repair the deformed casing under any confining pressure, and there is a good agreement between calculated results and experimental data. Based on this model, the effects of expansion amount every time, friction efficient, cone angle and length of equal diameter section on the reshaping force were analyzed in detail, by which the correlations between the reshaping force and the expansion amount every time, friction efficient, cone angle and length of equal diameter section were obtained. Research results can provide theoretical guidance for design and optimization of the structure and construction parameters of SEC.

Burst strength of tubing and casing based on twin shear unified strength theory.

The internal pressure strength of tubing and casing often cannot satisfy the design requirements in high pressure, high temperature and high H2S gas wells. Also, the practical safety coefficient of some wells is lower than the design standard according to the current API 5C3 standard, which brings some perplexity to the design. The ISO 10400: 2007 provides the model which can calculate the burst strength of tubing and casing better than API 5C3 standard, but the calculation accuracy is not desirable because about 50 percent predictive values are remarkably higher than real burst values. So, for the sake of improving strength design of tubing and casing, this paper deduces the plastic limit pressure of tubing and casing under internal pressure by applying the twin shear unified strength theory. According to the research of the influence rule of yield-to-tensile strength ratio and mechanical properties on the burst strength of tubing and casing, the more precise calculation model of tubing-casing's burst strength has been established with material hardening and intermediate principal stress. Numerical and experimental comparisons show that the new burst strength model is much closer to the real burst values than that of other models. The research results provide an important reference to optimize the tubing and casing design of deep and ultra-deep wells.