Elastic-plastic Deformation Stage Research Articles

Study on the Self–Bearing Mechanism and Mechanical Properties of Gangue Slurry under Overburden Loading

The mining of coal resources is accompanied by a large amount of solid waste such as gangue, which seriously affects the ecological environment. The gangue grouting backfilling technique can achieve the dual goals of gangue disposal and surface deformation control by injecting gangue slurry into the underground. The bearing mechanical characteristics of gangue slurry directly affect the surface deformation control effect of the grouting backfilling technique. In this study, a loading simulation system of grouting backfilling materials was designed, uniaxial confined compression tests were conducted, and the self–bearing mechanism of large particle–sized gangue slurry with different fluidities under instantaneous and creep loading modes was investigated. Additionally, the mechanical characteristics of the compacted body (i.e., the gangue slurry after creep loading) were analyzed. The results indicate that the self–bearing process of gangue slurry can be divided into three stages: the rapid compression and drainage stage, the pore compaction and water bleeding stage, and the particle crushing and elastic–plastic deformation stage. The uniaxial compressive stress–strain curve of a compacted body can be classified into four stages: elastic stage, yield stage, reinforcement stage, and crushing stage, and the strength of the compacted body is affected by the loading time and fluidity of the slurry. When the slurry with a fluidity of 240 mm is subjected to constant pressure for 3 h, the compressive strength of the slurry reaches the maximum value of 4.98 MPa, and 13.1% stress damage occurs when the constant pressure reaches 4 h. This research provides a theoretical basis for the improvement of the proportion and bearing characteristics of gangue grouting materials.

Theoretical prediction of cross-sectional deformation of circular thin-walled tube in large elastic–plastic deformation stage under lateral compression

The present paper focusing on the most important energy absorbing system of the circular thin-walled tubes or shells under lateral compression between two rigid plates, which is also a basic mechanical issue, studies the cross-sectional deformation in terms of the compressive force applying to the tube in the large elastic–plastic deformation stage during the lateral compressive process by both theoretical as well as finite element analysis (FEA) methods. Firstly a FEA simulation for the thin-walled tube’s lateral compressive process between two parallel plates is carried out to analyze the mechanical response and deformation characteristics, and then it is validated by tube’s lateral compressive experiment using digital image processing technology. Following the simulation results, some assumptions are proposed for theoretical derivation. Secondly focusing on the large elastic–plastic deformation stage (Stage II, in this paper), the compressive deformation of thin-walled tubes is divided into elastic and plastic regions according to the von Mises yield criterion. In elastic region the linear strain–displacement relations of shell theory and Hooke’s stress–strain relations are employed, and in plastic region the nonlinear large strain–displacement relations of doubly-curved shell theory and the power hardening rule are employed. Then considering both geometric and material nonlinearities, the equilibrium equation, which is a complicate integral equation with the variable upper limit, is derived using the principle of virtual work. It is solved by expanding the integrand as a set of polynomials power series. Then the predicted cross-sectional deformation in terms of the compressive force can be obtained by the iterative method. Finally some lateral compressive simulations involving various tubes with different geometric parameters are carried out, and the theoretical model is validated by comparison of the cross-sectional profiles predicted by theoretical model with those obtained by the simulations as well as other methods. At the same time, the applicable conditions of this model are discussed. This paper will play a positive role in studying the cross-sectional deformation for various shells under lateral compression in different application areas.

Effect of Salt Solution on the Mechanical Behaviours of Geopolymer Concrete under Dry-Wet Cycles

Seawater splashing and frequent rainfall in the coastal area will often cause the concrete members to be affected by dry-wet cycles. In order to investigate the effect of salt solution on the mechanical behaviours of geopolymer concrete under dry-wet cycles, a series of tests on erosion of geopolymer concrete by three different salt solutions were carried out by using the mass loss rate, compressive strength, erosion resistance coefficient, and elastic modulus. The results show that, with the increase of dry-wet cycles, the mass of the specimen increases slightly at 20 dry-wet cycles and then decreases gradually. Then, the stress-strain curve of geopolymer concrete can be divided into three stages: linear growth stage, deceleration growth stage, and slow decline stage, which correspond to the elastic deformation stage, elastic-plastic deformation stage, and crushing stage, respectively. The overall trend of the stress-strain curve is similar under different dry-wet cycles, and the peak stress decreases with the increase of dry-cycle. The relationship between stress and strain of geopolymer concrete samples with different salt solutions is similar, while their strength characteristics have changed obviously. Furthermore, with the increase of dry-wet cycles, the compressive strength and the relative elastic modulus of geopolymer concrete gradually decrease, which reflects that sulfate erosion will lead to the strength loss of geopolymer concrete, causing its internal damage, thus having adverse effects on it. Different salt solutions have the great influence on the mass loss rate, compressive strength, erosion resistance coefficient, and elastic modulus of geopolymer concrete. In detail, the mixed salt solution erosion results in the greatest damage to geopolymer concrete and accelerates the damage. Chloride salt solution erosion causes middle damage to geopolymer concrete. Compared with the other two salt solutions, sulfate solution erosion leads to the least damage to geopolymer concrete and then sulfate has a certain inhibitory effect on the damage of geopolymer concrete. In addition, based on the test results, the constitutive model considering dry-wet cycles damage of salt solution is proposed, and the correctness is verified. This study has a good guiding value for geopolymer concrete engineering in coastal areas.

A computational and experimental assessment of the pipeline stress state under bending load and internal pressure

Main pipelines are subjected to a complex of loads during operation. Monitoring of the stress state of the pipeline wall is necessary for performing strength calculations and evaluating the pipeline reliability.The article is devoted to the method of computational and experimental study of the stress state of a pipe under a bending load and combined action of a bending load and internal pressure.The experiments have been carried out on a laboratory bench. The object of the study is a pipe that has the following characteristics: an outer diameter of 325 mm, a wall thickness of 8.5 mm and steel grade of "14XGS". Electrical resistance strain gages were used to measure the strain of the pipe wall. Formulas for calculating the stress state components of the pipe wall in the elastic-plastic deformation stage are proposed. It is given formulas for calculating the stress state components of the pipe wall in the elastic-plastic deformation stage. Plots of hoop and longitudinal stresses as well as von Mises stress are obtained for the case of bending load on the pipe and the case of combined loading under bending and internal pressure. The areas of maximum values of von Mises stress where the transition to the limiting state is most likely have been determined.When only the bending load is applied, the maximum von Mises stress zone is observed on the lower area of the pipe in its central region. When combined loading under bending and internal pressure, the maximum von Mises stress zone is observed on the lateral area of the pipe in its central region.

In Situ Shear Test for Revealing the Mechanical Properties of the Gravelly Slip Zone Soil.

Slip zone soil is usually composed of clay or silty clay; in some special geological environments, it contains gravels, which make the properties of the slip zone soil more complex. Unfortunately, in many indoor shear tests, gravels are removed to meet the demands of apparatus size, and the in situ mechanical property of the gravelly slip zone soil is rarely studied. In this study, the shear mechanical property of the gravelly slip zone soil of Huangtupo landslide in the Three Gorges Reservoir area of China was investigated by the in situ shear test. The test results show that the shear deformation process of the gravelly slip zone soil includes an elastic deformation stage, elastic–plastic deformation stage, and plastic deformation stage. Four functions were introduced to express the shear constitutive model of the gravelly slip zone soil, and the asymmetric sigmoid function was demonstrated to be the optimum one to describe the relationship of the shear stress and shear displacement with a correlation coefficient of 0.986. The comparison between the in situ test and indoor direct shear test indicates that gravels increase the strength of the slip zone soil. Therefore, the shear strength parameters of the gravelly slip zone soil obtained by the in situ test are more preferable for evaluating the stability of the landslide and designing the anti-slide structures.

Experimental Study on Flexural Properties of Side-Pressure Laminated Bamboo Beams

In this paper, two factors of width-to-height ratio and span-to-height ratio are considered and 10 side-pressure laminated bamboo beams are prepared and tested for flexural capacity to study the flexural performance when they are used as structural members. The failure mode, load-displacement curves, and the change rule of cross-section strain with height are obtained through the test. The results demonstrate that the side-pressure laminated bamboo beams have experienced elastic deformation stage, elastic-plastic deformation stage, and failure stage; the midspan deflection of beams with considerable span-to-height ratio is too large to be applied to the ultimate load, and these specimens are regarded at the ultimate limit state; stiffness or deflection should be considered as the control index in the engineering application of the side-pressure laminated bamboo beams; the bearing capacity increases proportionately with the beam width; the average strain of the section basically conforms to the plane section assumption. The experiment proposes a method for calculating the flexural capacity of the side-pressure laminated bamboo beams with nonlinear deformation, and also, the comparison between the test value and the calculated value shows that they agree well.

Experimental study on characteristics of surface potential and current induced by stress on coal mine sandstone roof

Rocks generate voltage and current when subjected to load. Hence, the study of their characteristics is critical to the prevention and management of mine dynamic disasters and their secondary impacts. To investigate the characteristics of surface potential and current induced by stress on coal mine sandstone roof, an experimental scheme for determining the mechanical–electrical characteristics of rocks is constructed. The aforementioned system tests the evolution rules of current and voltage generated by the coal mine sandstone roof at different loading rates; the formation mechanism and coupling rules with mechanical properties of sandstone are also analysed. Experimental results show that although loading rates may differ, the variations of pressure stimulated current (PSC) and pressure stimulated voltage (PSV) curves are similar. Throughout the loading process, the PSC is divided into three stages: rapid growth, slow growth, and peak; on the other hand, the PSV curve exhibits a ‘double peak’ characteristic. The loading rate changes the current and voltage growth rates and peak values. When combined with mechanical properties, it can be observed that the PSC is related to the effective elastic modulus, whereas the PSV is related to micro-crack and macro-crack events in rocks. The current and voltage generated by the coal mine sandstone roof subjected to load are consistent with stress. The mechanical–electrical properties of sandstone indicate that the change processes in the PSC and PSV result from the coupling effects of various mechanisms. In the compaction stage, these are mainly caused by the piezoelectric effect of sandstone. In the elastic–plastic deformation stage, they are primarily generated by the electric dipoles formed by micro-cracks. In the sandstone fracture deformation stage, the change processes are mainly caused by the crack tip discharge. This study provide an experimental basis and practical results that reflect the microscale process and electrical precursors of sandstone fracturing.

Role of layered structure in ductility improvement of layered Ti-Al metal composite

Layered Ti-Al metal composite (LMC) was designed and fabricated by hot-rolling and annealing of pure Ti and Al sheets. The as-prepared composite exhibits high tensile ductility, being superior to any individual Ti or Al sheets. The stress/strain evolution and fracture behavior of the LMC were analyzed by in-situ observations during the tensile deformation. Three deformation stages of LMC were clearly observed by neutron diffraction: elastic stage, elastic-plastic stage and plastic stage. It is found that stress partitioning at the elastic-plastic deformation stage improves the strain balance of LMC, but leads to an internal stress accumulated at the interface. Additionally, a strain-transfer from Ti to adjacent Al layers relieves the strain localization of Ti layers in LMC, which improves the ductility of Ti. Both stress partitioning and strain localization of Ti layers facilitate the nucleation of cracks at a low macro strain. However, the crack propagation is constrained by layered structure. In terms of the Al layers, the constrained micro-cracks relieve the stress concentration in Al layer and improve the ductility of Al layers, so that cracking indirectly affects the plastic deformation behavior of LMC, then improving its entire ductility. This work provides a new structural strategy towards simultaneously improving strength and ductility to develop high performance LMC by structural design.

Load-deflection model for T-section rail press straightening process under lateral loads

Straightening is an important process in the production route of a T-section rail. The three point reverse bending method has been utilized extensively for rail straightening process due to its apparent simplicity and good flexibility. Therefore, this paper studies the load-deflection model in the three point bending and its application for T-section rail press straightening process under lateral loads, considering the work-hardening materials. An elastic-plastic power-law hardening model of the material is adopted. The analytical formulas of the relationship between the loading force and the bending deflection is given in the elastic deformation stage and the elastic-plastic deformation stage during the bending process. The corresponding loading deflections for straightening certain initial deflections of a rail are also calculated according to the simple unloading law. The theoretical results are then compared with the experimental data of rails with different initial bending deflections. It is found that the load-deflection model and straightening model established in this study could achieve an allowable straightness error. The results of this subject can be provided as an analysis reference for the automatic control strategy of a rail straightening process.

Experimental Investigation on Wellbore Strengthening Based on a Hydraulic Fracturing Apparatus

Lost circulation is a serious problem which always exists in the petroleum industry. Wellbore strengthening by lost circulation materials (LCMs) is a commonly applied method for mitigating lost circulation. This paper presents a hydraulic fracturing apparatus to investigate the effect of material type, concentration, and particle size distribution (PSD) of LCMs on wellbore strengthening behavior. In addition, the characteristics of pressure curves in the fracturing process are analyzed in detail. The results showed that the fracture pressure of the artificial core can be increased by LCMs, and there exists an optimum concentration of LCMs for the maximum wellbore strengthening effect. The LCMs with wide PSD can significantly increase the fracture pressure. However, some LCMs cannot increase or even decrease the fracture pressure; this is resulting from the LCMs with relatively single PSD that makes the quality of mud cake worse. The representative pressure curve in the fracturing process by drilling fluids with LCMs was divided into five parts: the initial cake formation stage, elastic plastic deformation stage, crack stability development stage, crack instability development stage, and unstable plugging stage. The actual fracturing curves were divided into four typical types due to missing of some stages compared with the representative pressure curve. In order to strengthen the wellbore in effective, good LCMs should be chosen to improve the maximum pressure in the elastic plastic deformation stage, extend the stable time of pressure bearing in the crack stability development stage, and control the crack instability development stage.

Laminated Ti-Al composites: Processing, structure and strength

Laminated Ti-Al composite sheets with different layer thickness ratios have been fabricated through hot pressing followed by multi-pass hot rolling at 500°C.The laminated sheets show strong bonding with intermetallic interface layers of nanoscale thickness between the layers of Ti and Al. The mechanical properties of the composites with different volume fractions of Al from 10% to 67% show a good combination of strength and ductility. A constraint strain in the hot-rolled laminated structure between the hard and soft phases introduces an elastic-plastic deformation stage, which becomes more pronounced as the volume fraction of Al decreases. Moreover, the thin intermetallic interface layer may also contribute to the strength of the composites, and this effect increases with increasing volume fraction of the interface layer.