Stress State Conditions Research Articles

To justification of concrete strength criterion at biaxial compression

Introduction. Numerous experimental data from Russian and foreign researchers indicate that the classical plasticity hypotheses do not take into account the different resistance to uniaxial tension and compression, the influence of the spherical tensor. At the same time, experiments show that the ultimate resistance depends on the type of stress state, and hydrostatic pressure increases the strength and plasticity of solids. Aim. To establish the dependence of the influence of the second component of stresses during biaxial compression of concrete on the parameters of the complete diagrams of deformation of the material σbR and εbR it is necessary to describe these diagrams, and to construct a closed curve on the plane of the main stresses (concrete strength criterion). Materials and methods. Based on the experimental materials of foreign and domestic researchers, including the experiments of the authors of the article, methods of mechanics of deformed solids, limit curves and a closed curve on the plane of the main stresses in the form of a chain line forming the smallest area strength surface in the form of a catenoid are proposed. Results. The article provides an analysis of the known strength criteria from the point of view of their geometric interpretation in the stress space. It is shown that these studies relate mainly to metals and metal structures, and for the design of reinforced concrete and steel-concrete structures in a complex stress state, it is necessary to develop an appropriate criterion for the strength of concrete. Conclusions. As a result, the proposed limit curves and the surface (material strength criterion) on the plane of main stresses in the form of a catenoid accurately reflect the behavior of concrete under conditions of uniform and uneven flat stress state, and the equation of the surface in the form of a catenoid is a generalization of the equations of limit curves for each of the three types of flat stress state. At the same time, there is no overestimation of strength in the "compression – compression" area in this case.

Mechanical properties and constitutive model of artificial frozen sandy soils under true triaxial stress state conditions

Triaxial compression tests were conducted on frozen sandy soils under a constant minimum principal stress (σ3 = 1.6 MPa) and various intermediate principal stresses (σ2 = 1.6, 3.4, 5.2, 7.0, 8.8, 9.8 MPa). The purpose of the research was to investigate the influence of intermediate principal stress on mechanical properties of frozen soil, and to establish a constitutive model to predict the stress-strain relationship of frozen soil under complex stress state. The test results obtained demonstrated that the crack damage stress and failure stress initially increase and then decrease with an increase in the intermediate principal stress. However, the crack initiation stress exhibits an initial increase up to a specific value, after which it stabilizes. From the perspective of crack propagation, the influence mechanism of intermediate principal stress on the strength was discussed. With the variation of stress state, the difference between intermediate principal strain and minimum principal strain gradually increases. The deformation difference was revealed using the stress superposition principle and Poisson's effect. Finally, the constitutive model based on the Weibull distribution and Drucker-Prager strength criterion can accurately represent the stress-strain relationships of frozen sandy soils under true triaxial stress states.

Impacts and Countermeasures of Present-Day Stress State and Geological Conditions on Coal Reservoir Development in Shizhuang South Block, Qinshui Basin

Impacts and Countermeasures of Present-Day Stress State and Geological Conditions on Coal Reservoir Development in Shizhuang South Block, Qinshui Basin

Modelling dynamic fracture and fragmentation of rocks under multiaxial coupled static and dynamic loads with a parallelised 3D FDEM

In this study, a three-dimensional (3D) combined finite-discrete element method (FDEM) based simulator, which enables the robust simulation of full-scale triaxial Hopkinson bar (Tri-HB) testing system, including the capture of fracture and fragmentation processes of rock specimens as well as the detection and analysis of generated rock fragments, is developed for investigating the dynamic responses of rocks subjected to multiaxial coupled static and dynamic loads. An innovative two-step approach is proposed to first efficiently achieve the static stress state in the entire Tri-HB system and then apply dynamic loading to the system. The proposed approach with the schemes of mass scaling and local damping can significantly reduce the run time for static computation involving cohesive elements and complex contact mechanics in the framework of explicit time-integration. The developed program is well validated by achieving dynamic stress equilibrium in the Tri-HB system and comparing the simulated stress wave profiles with those from laboratory experiments. Then, the progressive dynamic fracture and fragmentation processes of rocks under different static stress conditions in a full-scale Tri-HB system are investigated, which vividly exhibit the confinement dependence of the dynamic behaviours of rocks in the testing system. It is found that, even with the same loading rate, separate input parameter sets for rock need to be established in FDEM for different confinement conditions to capture the actual rate dependent behaviour of rock. Further studies are also conducted to investigate the effect of interfacial friction on the fracture behaviour, and the results indicate that higher interfacial frictions significantly disturb the dynamic stress variations and the true rock behaviour in the system. Through high performance computing of 3D FDEM, this study is expected to contribute to the understanding of dynamic responses of rocks subjected to multiaxial coupled static and dynamic loads in deep underground engineering.

A method for estimating coefficient of lateral earth pressure based on cone penetration tests

The coefficient of lateral earth pressure at rest (K0) is a key state soil variable for the design of foundations and underground structures, characterizes in-situ stress state and soil condition. In this study, a method for the in-situ estimation of K0 using the cone penetration test (CPT) is proposed considering vertical and inclined cone resistances (qc). For this purpose, a series of laboratory CPTs in a soil chamber were conducted to obtain and characterize vertical and inclined qc values at various inclination angles (θ) and relative densities (DR). It was observed that the values of qc increased as θ increased, which was more pronounced at higher DR. Coupled Eulerian-Lagrangian (CEL) finite element analyses were performed to quantify the values of inclined qc at various cone penetration and soil conditions. Based on results from laboratory CPTs and CEL analyses, a CPT-based K0 correlation model was established, which was given as a function of vertical and inclined qc values. The model parameter for the proposed method was evaluated and quantified. The validity of the proposed method was confirmed from the comparison with case examples.

Deformation state of a graphene sheet within the framework of the continuum moment-membrane theory of elasticity

The paper proposes an approach to finding the stress-strain state (SSS) of structures containing graphene, a novel nanomaterial that has currently found а wide range of practical applications in nanoelectromechanical systems. Graphene is a 2D basic building block for other carbon structures such as membranes, sheets, nanotubes, etc. To describe the SSS of a graphene sheet, the phenomenological continuum moment-membrane theory of plates is used, from which, due to the fact that graphene is an ultrathin material, the concept of thickness is excluded. The physical elasticity relationships of a graphene sheet are expressed through its rigidity characteristics, determined using the harmonic potential of interatomic interactions in carbon. A differential formulation and the corresponding variational formulation are given for the problem of static deformation and determination of the natural frequencies and modes of vibration of a graphene sheet. The variational formulation is based on the Lagrange principle and is implemented numerically using the finite element method. Finite element relations are constructed taking into account moment effects of the behavior of a graphene sheet. For approximation, a 4-node rectangular finite element is used. Numerical solutions to several problems of static deformation of a graphene sheet under conditions of a plane stress state and transverse bending are presented, and the analysis of its natural vibrations is also performed. Good convergence of numerical simulation results in all considered problems is demonstrated. The obtained numerical solutions are essential in designing and calculating resonators in which ultrathin nanostructures are used. The establishment of the fact that a graphene sheet has a high intrinsic frequency falling in the GHz region (for example, quartz resonators are characterized by megahertz frequencies) opens up new prospects for using graphene itself as an ultrasensitive nanomechanical resonator for detecting small masses and ultrasmall displacements.

Feasibility of using building-related construction and demolition waste-derived geopolymer for subgrade soil stabilization

A high-value utilization approach is essential to improve the utilization rate of building-related construction and demolition waste (brCDW). This study developed a novel approach by synthesizing a brCDW-derived geopolymer to stabilize high liquid limit subgrade soil. The unconfined compressive strength (UCS) test, shear strength test, resilient modulus test, and permanent strain test were conducted to investigate the effects of brCDW-derived geopolymer dosage, curing time, stress state, and moisture condition on the engineering properties of geopolymer stabilized soil. These test results demonstrated that increasing the geopolymer dosage effectively improved the UCS, shear strength and resilient modulus of stabilized soil and reduced the permanent strain of stabilized soil. The 8% brCDW-derived geopolymer provided adequate UCS, shear strength, resilient modulus, and resistance to permanent strain for stabilized soil, which showed the most economical improvement. Mechanistic-empirical models were employed to accurately estimate the stress-dependent resilient modulus and permanent strain of geopolymer stabilized soil at any given stress state. The Scanning Electron Microscope (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) test were performed to investigate the strengthening mechanism of brCDW-derived geopolymer stabilization of subgrade soil. The SEM test results indicated that the porosity of stabilized soil was significantly decreased when the geopolymer dosage increased to 8%. The EDS test results demonstrated that the predominant gel types generated from the geopolymer stabilization might be Calcium–Aluminum-Silicate-Hydrate (C-A-S-H), Calcium-Silicate-Hydrate (C–S–H), and Calcium-Aluminate-Hydrate (C-A-H) gels. There was also a small amount of Sodium-Alumino-Silicate-Hydrate (N-A-S-H) gel detected in the geopolymer stabilized soil. Finally, the sustainability of brCDW-derived geopolymer stabilization approach was assessed in terms of material production cost, carbon dioxide emission, and energy consumption. The brCDW-derived geopolymer was proven as a sustainable stabilizer for subgrade soil.

Effect of stress-hold and strain-hold during Creep-Fatigue Interaction of alloy 617 M

Alloy 617 M is a Nickel based superalloy compatible for utilization in structural components of Very High Temperature Reactors (VHTR). In these complicated structural components, loading scenarios include both static and cyclic stresses at elevated temperatures. In order to understand the effect of both static and cyclic stress state on Alloy 617 M at high temperatures, Creep-Fatigue Interaction (CFI) studies with a strain amplitude of ± 0.6% and a hold/ dwell time of 60 seconds have been conducted at 600 °C, 650 °C, 700 °C, and 750 °C. At the peak tensile strain of the applied cyclic loads during CFI, the specimen was subjected to either a strain-hold (represented as εCFI) or a stress-hold (represented as σCFI) to understand their influence on mechanical behavior at elevated temperatures. This kind of testing is imminent for understanding the damage caused by both the loading scenarios. In either case of loading, with the increase in test temperature, the number of cycles to failure is reduced. However, for a given test temperature, σCFI showed lower cycles to failure compared to εCFI. Fractography studies revealed that σCFI undergoes predominant creep damage in comparison with εCFI.

DEVELOPMENT OF LOADING MODELS AND ANALYSIS OF THE STRESS-STRAIN STATE OF THE MAIN GAS PIPELINE WITH COMPOSITE LININGS

The operation of sections of main gas pipelines that are on the verge of developing a design resource is associated with a number of problems in ensuring industrial safety. One of the most acute problems is the occurrence of local defects. The main material for the manufacture of main gas pipelines is steel. During the operation of the pipeline, local defects occur due to metal corrosion or mechanical damage to the gas pipe. Such concentrators with high internal pressure in the gas pipeline can lead to the destruction of the entire structure. In this case, the situation of destruction is especially critical, since it can cause a man-made disaster. To increase the shelf life of damaged sections of main pipelines in places where defects are located, banding with composite linings is used. The article contains part of a comprehensive study, which consisted in the development of methods for modeling the main gas pipeline for oil and petroleum products under seismic loads, dynamic stress-strain state under the influence of seismic loads and stress-strain state of a damaged gas pipeline reinforced with composite linings under the action of internal pressure. The technology of manufacturing linings from composite material is also described in detail. At the same time, a geometric model of the section of the main gas pipeline with composite linings was developed and an analysis of the loading of the section of the main gas pipeline in ANSYS was carried out, provided that the lining is mounted without tension on the pipeline and provided that the lining is mounted with tension, where a comparative analysis showed that the installation condition of composite linings does not significantly affect the static stress state.

Optical strain measurements during pulsed laser beam welding to improve the understanding of hot crack formation of EN AW-6082 aluminum alloy

E-mobility turned in the last years to be an emerging market and one solution to fossil fuel free mobility for the future. E-mobility requires, compared to fossil-fueled mobility concepts, a huge amount of welding tasks, which have to guarantee different functionality, as for example high strength, ductility, but also low resistivity and tightness. Last is especially for housing or cases of aluminum in automotive challenging, as pores and cracks can occur. Pulsed laser welding presents, due to the adaptable heat input and the temporal modification of stress state and solidification conditions advantages for this type of applications. EN AW-6xxx group of aluminum alloys is mainly used for such components due to their favorable mechanical properties. However, these alloys are susceptible to hot cracking during solidification from the molten phase. This article aims to present a methodology for demonstrating the resulting strain during pulsed laser beam welding of hot crack susceptible aluminum alloys. It will highlight the influence of factors such as pulse shape, shielding gas, and flow rate on strain and strain rate.

THE RESULTS OF THEORETICAL STUDIES FOR DETERMINING OPERATIONAL LOADS ON GRAVITY-TYPE BERTHING STRUCTURES

The current waterfront of Ukrainian ports includes structures that have been developed in the past and have over 50 years of experience. Open-piled quay walls and sheet-pile quay walls are the most common types of quay walls used for berths in Ukrainian ports. However, there are gravity-type quay walls. The share in the total the waterfront is not large. They were built in the past and require modernization and reconstruction. Most of these berthing structures have defects in concrete and reinforcement, which reduce the durability and bearing capacity. Furthermore, the development of freight and passenger maritime traffic and the construction of modern ships led to the need to increase the depths at existing berths and define the operational loads meeting modern requirements. Thus, the issue of reconstruction of gravity-type quay walls is relevant for many ports of Ukraine. The choice of the reconstruction method depends on the correct estimation of the actual technical condition of an existing structure. Gravity-type structures are those that rely primarily on their weight and grip on the foundations to resist any possible adverse load combinations. The requirements for such structures lead to the solution of one of the main tasks ‒ the determination of the reactive capacity of the soil base. The reactive capacity calculation of the soil bases for considered structures is essential. The purpose of the calculation is to provide both strength and stability of soil bases. An improved method for determining the reactive capacity of the soil base of gravity-type quay walls has been developed. This method allows determining the reactive capacity of the soil base in conditions of the mixed stress state (limit and sublimit stress state zones in the soil base are considered). This paper reviews some results of applying the proposed method for the reactive capacity estimation of the soil base of gravity-type quay walls. The obtained results have been used to analyze the preliminary reconstruction options for the mentioned structures and determine operational loads.

Failure criterion for concrete under volumetric stress state conditions

Based on the experiments conducted by the authors, a six-parameter failure criterion for concrete has been developed, which makes it possible to take into account the volumetric stress state in strength calculations of massive concrete and reinforced concrete structures. The developed strength criterion is adapted to a spatial eight-node finite element (solid type) and implemented in the PRINS software. To verify the developed criterion, the work provides a com-parison with both experimental data and calculation results that meet other strength criteria widely used for concrete. Thus, the compression and tension meridians of the developed fracture criterion were compared with experimental data, as well as with the Willam & Warnke criterion and the modified Drucker & Prager criterion with Mohr & Coulomb constants. A comparison of compression meridians shows that in the mode of low hydrostatic stresses , these criteria converge with each other and with experimental data. In the mode of average hydrostatic stresses , the criterion proposed by the authors and the Willam & Warnke criterion show similar results, while the modified Drucker & Prager criterion shows on 20% overestimation of the failure value. In the mode of high hydrostatic stresses , the Willam & Warnke criterion in com-parison with the proposed criterion and experimental data, gives an underestimated value of concrete failure.

Finite Element for the Analysis of Massive Reinforced Concrete Structures with Cracking

The solid finite element has been developed for the calculation of massive reinforced concrete structures with cracks. When constructing a finite element in the compression-compression-compression mode, the Willam Warnke failure criterion was used. The tensile behavior of concrete was assumed to be linear before the crack initiation. Modern building codes require non-linear calculations of concrete and reinforced concrete structures, taking into account the real properties of concrete and reinforcement. In this regard, a technique has been developed and a solid finite element has been built, adapted to the PRINS software, which makes it possible to perform calculations of massive reinforced concrete structures, taking into account their actual work. Development of a method for calculating reinforced concrete structures under conditions of a three-dimensional stress state, taking into account the brittle fracture of compressed concrete and cracking in tensile concrete. Based on this technique, the implementation of a solid finite element in the PRINS software. To verify the developed finite element, a series of test calculations of a beam under three-point bending conditions was carried out. Comparison of the calculation results with the data of experiments by the authors confirmed the high accuracy and reliability of the results obtained. The developed solid finite element included in the PRINS software can be effectively used by engineers of design and scientific organizations to solve a wide class of engineering problems related to the calculations of massive reinforced concrete structures.

Dynamic mechanical characteristics of deep Jinping marble in complex stress environments

To reveal the dynamic mechanical characteristics of deep rocks, a series of impact tests under triaxial static stress states corresponding to depths of 300–2400 m were conducted. The results showed that both the strain rates and the stress environments in depth significantly affect the mechanical characteristics of rocks. The sensitivity of strain rate to the dynamic strength and deformation modulus shows a negative correlation with depth, indicating that producing penetrative cracks in deep environments is more difficult when damage occurs. The dynamic strength shows a tendency to decrease and then increase slightly, but decreases sharply finally. Transmissivity demonstrates a similar trend as that of strength, whereas reflectivity indicates the opposite trend. Furthermore, two critical depths with high dynamically induced hazard possibilities based on the China Jinping Underground Laboratory (CJPL) were proposed for deep engineering. The first critical depth is 600–900 m, beyond which the sensitivity of rock dynamic characteristics to the strain rate and restraint of circumferential stress decrease, causing instability of surrounding rocks under axial stress condition. The second one lies at 1500–1800 m, where the wave impedance and dynamic strength of deep surrounding rocks drop sharply, and the dissipation energy presents a negative value. It suggests that the dynamic instability of deep surrounding rocks can be divided into dynamic load dominant and dynamic load induced types, depending on the second critical depth.

Influence of microstructural refinement on cold deformation behavior of nanocrystalline Al–Zn–Mg alloy processed by solid-state mechanical milling and hot pressing

This research work was aimed at studying the workability behavior of Al–Zn–Mg alloys during a cold upsetting process. Nanocrystalline Al 7017 alloys were synthesized by mechanical milling (MM) at different milling times and consolidated by hot-pressing. The nanocrystalline Al 7017 alloy samples were incrementally deformed with a load step of 0.5 ton in a universal testing machine. Artificial image capturing and comparison techniques were introduced in this study and deformed images were captured using a high-resolution camera. For each incremental load (without removing the sample from the platens), different parameters such as the height of the deformed samples, and the corresponding contact diameters (top and bottom) were measured and analyzed using Matlab software (image processing). Cold upsetting was performed until the first crack formed. The influence of milling time on cold workability and strain hardening behavior was investigated under triaxial stress state conditions. The results revealed that the Al 7017-10 h nanocrystallite alloy exhibited improved deformation properties in terms of various tri-axial stress, strain hardening, formability stress index, and stress ratio parameters owing to the fine grain size, lower amount of dislocation pile-up, and fewer migrated atoms near the grain boundaries compared to other samples (after10 h). The microstructure and phase evolution were studied using an X-ray diffractometer and various electron microscopes. Several strengthening mechanisms are also discussed among which the dislocation strengthening mechanism plays a major role in the deformation behavior. Finite element analysis (FEA) was performed to examine the equivalent stress distribution during cold-upset forging.

Stress Monitoring of Segment Structure during the Construction of the Small-Diameter Shield Tunnel.

Segmental stress during the construction process plays a pivotal role in assessing the safety and quality of shield tunnels. Fiber Bragg grating (FBG) sensing technology has been proposed for tunnel segment stress monitoring. A laboratory test was conducted to validate the reliable strain measurement of FBG sensors. The field in situ monitoring of a sewerage shield tunnel was carried out to examine the longitudinal and circumferential stresses experienced by the segments throughout the construction phase. The cyclic fluctuations in stress were found to be synchronized with the variations in shield thrust. A comparison was made between the longitudinal and circumferential stress variations observed during the shield-driving and segment-assembly processes. Additionally, the time required for the grouting to reach its full curing strength was estimated, revealing its impact on the stress levels and range of the pipe segment. The findings of this study offer an enhanced understanding of the stress state and health condition of small-diameter shield tunnels, which can help in optimizing the design and construction process of tunnel segments, as well.

Models of nonlinear deformation of concrete in a triaxial stress state and their implementation in the PRINS computational complex

Modern construction standards and regulations prescribe to carry out calculations of concrete and reinforced concrete structures in a nonlinear formulation with account of the real properties of concrete and reinforcement. However, the most of finite-element program complexes cannot perform such calculations in a nonlinear formulation with account of plastic deformations of concrete and reinforcement. To solve this problem, a methodology has been developed and a solid finite element adapted to the PRINS computing complex has been created, which made it possible to perform calculations of reinforced concrete structures considering their actual work. The aim of the study - development and implementation of a method for calculating reinforced concrete structures under conditions of a three-dimensional stress state, considering both brittle fracture and elastic-plastic deformation of concrete. A finite-element methodology, algorithm, and program for calculation of massive reinforced concrete structures with account of plastic deformations of concrete have been presented. The methodology is based on the modified Willam and Warnke strength criterion supplemented with the flow criterion. Two models of volumetric deformation of concrete have been regarded: the elastic model at brittle failure and the ideal elastoplastic model. An eight-node finite element with linear approximating functions of displacements implementing the mentioned deformation models is created. Verification calculations of a massive concrete structure in three-axial compression testify to the accuracy and convergence of the developed finite elements. The PRINS can be effectively used by engineers of designing and scientific organizations to solve a wide class of engineering problems related to calculations of building structures.

Study on dynamic strength and liquefaction mechanism of silt soil in Castor earthquake prone areas under different consolidation ratios

Under the Castor earthquake, there is a risk of liquefaction instability of saturated tailings, and the evolution of dynamic pore pressure can indirectly reflect its instability process. Before applying dynamic loads, the static stress state of soil is one of the main factors affecting the development of soil dynamic strength and dynamic pore pressure, and there are significant differences in soil dynamic strength under different consolidation ratios. This paper conducted dynamic triaxial tests on saturated tailings silt with different consolidation ratios, and analyzed the dynamic strength variation and liquefaction mechanism of the samples using the discrete element method (PFC3D). The results showed that 1) as the Kc′ gradually increased, and there was a critical consolidation ratio Kc′ during the development of the dynamic strength of the sample. The specific value of Kc′ was related to the properties and stress state of saturated sand. The Kc′ in this research was about 1.9. When Kc < 1.9, dynamic strength was increased with the increase in Kc; when Kc > 1.9, dynamic strength was decreased with the Kc. 2) Under the impact of cyclic load, when samples were normally consolidated (Kc =1), the pore water pressure would tend to be equal to the confining pressure to cause soil liquefaction. In the case of eccentric consolidation (Kc > 1), the pore water pressure would be less than the confining pressure, thus, the soil liquefaction would not be induced, and the pore pressure value would decrease with the increase of consolidation ratio. This paper provides engineering guidance value for the study of dynamic strength and liquefaction mechanism of tailings sand and silt in Castor earthquake prone areas under different consolidation ratios.

A DEM Investigation on the Influence of Cyclic and Static Stress Ratios and State Variables on the Pore Water Pressure Generation in Granular Materials

A granular soil may undergo liquefaction failure due to excess pore water pressure (Δu) generation when subjected to cyclic loading such as an earthquake. Several past studies indicated Δu generation may be independent of cyclic stress ratio (CSR), whereas others contradicted this observation. A soil element in its natural state often remains in K0 consolidation state, i.e., subjected to nonzero static shear stress ratio (SSR). Using the discrete element method (DEM), this study performs a large number of cyclic triaxial tests and evaluates various factors influencing the magnitude and rate of Δu generation under symmetric and nonsymmetric cyclic loading conditions. The study also evaluates the capabilities of several existing Δu generation models to predict the development of Δu under different failure scenarios. It was observed that the generation of Δu can be influenced by CSR for specimens subjected to symmetrical cyclic loading and by both CSR and SSR for nonsymmetric cyclic loading conditions. A new prediction model for Δu generation was developed along with an objective framework to estimate its input parameters. The model showed good agreement with the DEM simulation results.

Influence of the initial static stress state on the accumulation behaviour of a coarse-grained soil under long-term cyclic loading

A series of cyclic triaxial tests were carried out to explore the accumulation behaviour of coarse-grained soil (CGS) with different initial static stress states. The test results show that more accumulated axial strain can be detected for the specimen with a higher initial stress ratio or a lower confining stress under identical cyclic loading. The plastic shakedown limits are then identified, which indicates that a higher initial stress ratio or a lower confining pressure yields a lower plastic shakedown limit. Furthermore, the stress states at the plastic shakedown limit for the soils with various initial static stress states are presented in the plane of p−q, and all data trended towards converging on a straight line, which is approximately parallel to the strength envelope. A united plastic shakedown criterion is then proposed for soils under different initial stress states, which provides a simple approach to predicting the plastic shakedown limit.