Long-term Strength Characteristics Research Articles

Long-term Compressive Strength Properties of Concrete Incorporating Admixtures: Outdoor Exposure Testing in a Coastal Environment

In this study, concrete specimens were fabricated based on domestically manufactured materials, and long-term exposure tests were conducted in a domestic coastal environment. This study analyzes the long-term compressive strength characteristics of concrete mixed with admixtures. The mixed materials used were divided into blast furnace slag and fly ash. The blast furnace slag and fly ash were, respectively, produced by replacing ~ 30% and ~ 15% of the cement. The compressive strength was measured at 28 day, 1 year, and 10 years of age and compared with that of ordinary concrete. In addition, the long-term compressive strength results obtained in this study were compared with those of concrete mixed with admixtures reported in the literature. The strengths of the ordinary specimen at 1 year and 10 years of age increased by ~ 10 MPa and ~ 22 MPa compared with those at 28 day of age. However, concrete mixed with admixtures yielded compressive strength increases of ~ 5 MPa and ~ 26 MPa at 1 year and 10 years of age, respectively, compared with those at 28 day of age. A comparison of the compressive strengths of concrete mixed with admixtures reported in the literature (based on age) and those obtained in this study showed that there was an initial strength difference in the range of 10–25%. However, the compressive strength at 10 years of age was almost similar to those reported in the literature with differences of less than 5%. These findings confirmed that when using pozzolanic admixtures, the development rate of the initial strength may vary owing to various factors; however, the long-term strength converges within a certain range.

Experimental Study on Long-Term Strength and Creep Characteristics of Sandstone Under Different Water Content and Confining Pressure

Experimental Study on Long-Term Strength and Creep Characteristics of Sandstone Under Different Water Content and Confining Pressure

Experimental study on creep and long-term strength characteristics of expansive soil improved by the MICP method

Experimental study on creep and long-term strength characteristics of expansive soil improved by the MICP method

Anisotropic characteristics and creep model for thin-layered rock under true triaxial compression

The failure phenomenon of thin-layered rock tunnels not only exhibits asymmetric spatial characteristics, but also significant time-dependent characteristics under high in-situ stress, which is attributed to the time-dependent fracture of thin-layered rocks. This paper conducted a series of true triaxial creep compression tests on typical thin-layered rock siliceous slate with acoustic emission technique to reveal its anisotropic time-dependent fracture characteristics. The anisotropic long-term strength, creep fracturing process, and fracture orientation characteristics of thin-layered rocks under different loading angles (β, ω) and intermediate principal stress were summarized. A three-dimensional (3D) non-linear visco-plastic creep model for thin-layered rock was developed to simulate its anisotropic creep behavior. The time-dependent fracturing of rocks during true triaxial creep loading is reflected through the change of equivalent strain based on an improved Euler iteration method. By constructing the plastic potential function and overstress index related to loading angles and stress state, the anisotropic time-dependent fracturing process and propagation of thin-layered rocks under different loading angles and intermediate principal stress are expounded. The model was validated experimentally to show it can reflect the long-term strength and creep deformation characteristics of thin-layered rocks under true triaxial compression.

Experimental study on combined load-bearing instability of filling body and roadway surrounding rock in original roadway filling without coal pillar excavation

Under the influence of deep high ground stress and strong mining, stress of surrounding rock of gob-side entry increases. The original roadway filling without coal pillar excavation is proposed to cope with the difficult problem of surrounding rock control in deep roadway. In the technology, filling body and surrounding rock form a variety of combined structures, and these structures stability is closely related to roadway control. In this paper, theoretical analysis and mechanical test are used to analyze the strength characteristics of single lithology and bearing capacity deterioration characteristics of multi-lithology combination. The results show that the long-term strength of filling body is greatly affected by water-cement ratio, and the water-cement ratio of 2:1 has characteristics of high long-term residual strength and small residual strain. When analyzing the strength of the contact surface of the combined body, the D-P criterion is closer to test results than M-C. The strength of combinations is determined by the minimum value of the strength of the contact surface between filling body and other lithology and the strength of the non-standard sample of each lithology. And stress-strain curves of the combined body are elastic-plastic, and all belong to the shear tension composite failure. The UCS of combinations of two lithologies decreases with the increase of the volume ratio of filling body. When volume ratio of filling body is constant, the UCS of the three and four lithologic combinations increases with the increase of volume ratio of sandy mudstone to mudstone. The test results play a theoretical and reference role for the application of original roadway filling without coal pillar excavation in different geological conditions.

Creep behavior and long-term strength characteristics of pre-peak damaged sandstone under conventional triaxial compression

A series of creep tests were carried out on sandstone specimens with different pre-peak instantaneous damage characteristics under different confining pressures. The results revealed that the creep stress was the key factor affecting the occurrence of the three stages of creep, and the steady-state creep rate increased exponentially with increasing creep stress. Under the same confining pressure, the larger the instantaneous damage of the rock specimen was, the more quickly creep failure occurred and the lower the creep failure stress was. For the pre-peak damaged rock specimens, the strain threshold for accelerating creep was the same for a given confining pressure. The strain threshold increased with increasing confining pressure. In addition, the long-term strength was determined using the isochronous stress–strain curve and the variation in the creep contribution factor. The results revealed that the long-term strength decreased gradually with increasing pre-peak instantaneous damage under lower confining pressures. However, the instantaneous damage had little effect on the long-term strength under higher confining pressures. Finally, the macro–micro-failure modes of the sandstone were analyzed according to the fracture morphology observed via scanning electron microscopy. It was found that the macroscale creep failure patterns of the sandstone specimens could be divided into a shear-dominated failure mode under high confining pressures and a mixed shear-tensile failure mode under low confining pressures. At the microscale, as the confining pressure increased, the micro-fracture mode of the sandstone changed gradually from a single brittle fracture to a mixed brittle and ductile fracture mode.

Study on strength and deformation characteristics of cemented gangue backfill body under the coupling action of load and salt erosion

The durability of backfill body is very important to maintain the stability of goaf. To explore the long-term strength and deformation characteristics of backfill body under the coupling action of load and salt erosion, the cemented gangue backfill body (CGBB) under different compressive stress levels was eroded in MgSO4 solution for 210 days. The ultrasonic and resistivity changes of CGBB at different ages were monitored and the microstructure of CGBB was analyzed by scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). The results showed that the compressive strength of CGBB increased with the rise of compressive stress level, and the compressive strength of the eroded specimens was less than that of the not eroded specimens. The deformation of CGBB increased with the increase of time and compressive stress level, and there was still a certain increase in the later deformation of the eroded CGBB. The ultrasonic pulse velocity (UPV) of eroded CGBB can be divided into three stages: rapid growth, slow growth and gradual decrease. The resistivity of the eroded CGBB decreased rapidly in the early stage, and then increases slowly. The greater the compressive stress is, the smaller the growth rate is. By analyzing microstructure, the formation of expansion products changed the compactness of CGBB in different erosion stages, and the compressive stress within 50 % stress-strength ratio inhibited the erosion of CGBB. The research results can provide reference for the design of backfill body.

Experimental Investigation on Long-Term Strength and Acoustic Emission Characteristics of Coal Pillar under Inclined Compression Loading

Pillar rheological failure is one of the important reasons to induce earthquake, surface collapse, and water inrush disaster in underground mining engineering. Pillar is generally present under an inclined state and is significantly influenced by combining compression and shear loading. However, many scholars regard the long-term strength of coal or rock mass under pure uniaxial compression loading as the main evolution index of pillar strength, which is not consistent with engineering practice. In this paper, a new inclined uniaxial compression strength (IUCS) test system was developed and then used to carry out the IUCS test and creep test of coal specimens combined with an acoustic emission (AE) technology at various inclination angles (0°, 5°, 10°, 15°, and 20°). The variation of time-dependent deformation, peak strength, long-term strength (LTS), creep fracture model, and AE behavior of the coal with the inclination angels were discussed in detail. The results indicated that the peak strength and LTS of the coal nonlinearly decreased with the inclination angle increasing. The proportion of the peak strength to the LTS had remained constant between 58.5% and 62.9%, which can be considered as the inherent properties of coal rock. The creep failure model of the coal was transformed from tension-splitting failure at the inclination angle of 0° and 5° to tension-shear failure at the inclination angle of 10°-20°, which revealed that the inclination angles were favorable to the initiation and propagation of shear cracks. No matter any inclination angles, AE events can be divided into quiet period, low amplitude rising period, and high amplitude rising period with the periodic mutation of multistage loading points. Moreover, the cumulative AE energy gradually decreases with the increase of the inclination angles, which indicated that the shear stress caused by the specimen inclination can make crack initiation and propagation with less energy absorption. The research results will assist in the long-term strength design and time-dependent stability assessment of the coal pillar.

Long-Term Shear Strength Characteristics of Biopolymer-Stabilized Municipal Solid Waste Fines and Statistical Analysis

Long-Term Shear Strength Characteristics of Biopolymer-Stabilized Municipal Solid Waste Fines and Statistical Analysis

GENERAL ANALYSIS OF STRESS AND DEFORMATION OF PARTS MOVING WITH HIGH VELOCITY IN A LIQUID OR GAS (REVIEW ARTICLE)

The article provides an analytical review and analysis of stresses and deformations of parts moving at high speeds in a liquid or gas. The working conditions of materials and parts of turbines (blades, rotor and casing) operating at high temperatures and loads are analyzed. The main ways of solving the problem of ensuring the strength of such parts are presented. The main ways to solve the problem of reliability of parts or the product as a whole are given: mathematical modeling (calculated determination of strength, durability and reliability); physical modeling (model testing); testing of full-scale products in reproducible real or operational conditions.It is impossible to speak about the strength of a part only from the calculation of deformations and stresses, even taking into account their change over time, so it is necessary to have strength criteria that establish the relationship between the strength parameters. It is emphasized that in the general case, the criterion of strength should answer the question: will the part collapse or not with the known laws of change in time of stresses, strains and temperatures It is shown that the considered standard characteristics of creep and long-term strength can be directly used in calculations only for those parts in which the uniaxial stress state at constant stresses and temperature is realized, when the working conditions of the material fully meet the test conditions of materials.An analytical view of deformation diagrams is considered as the main means of carrying out practical calculations of material strength. It is shown that in order to determine the stresses and strains in parts that move at high speeds in a liquid or gas, it is necessary to take into account the model of parts exploitation, the processes of creep and thermal fatigue of the material, and the unsteadiness of load processes. Keywords: stresses and deformations of turbine parts; deformation diagram; operating model of turbine parts; creep and thermal fatigue; nonstationarity of loading processes

An experimental and theoretical investigation of time-dependent cracking and creep behavior of rocks under triaxial hydro-mechanical coupling

Subcritical crack propagation and rock creep deformation are the main factors affecting the long-term stability and strength characteristics of rocks under hydro-mechanical coupling. Triaxial creep tests were performed on rock-like mortar specimens containing a pre-existing 3-D crack to investigate the time-dependent mechanical behavior of the cracked rock. A modified theoretical model for the irreversible deformation of rocks based on subcritical crack propagation was used to analyze the creep mechanism of the specimens and the effects of water pressure. The specimens with creep damage mainly exhibited a tensile-shear failure mode, and the macrofracture presented more tensile characteristics with increasing water pressure. The water pressure promoted the prefabricated crack propagation, inhibited the formation of microcracks, increased the creep rate, and accelerated specimen failure, which had a negative effect on the long-term stability of the specimens. The modified model accurately predicted the variation in irreversible axial strain and time required for specimen failure, especially under higher water pressures. The creep deformation of the specimens was much larger than the instantaneous deformation at the same crack propagation length, indicating that subcritical crack propagation is the main factor leading to the failure of rocks.

ИССЛЕДОВАНИЕ ХАРАКТЕРИСТИК ПРОЧНОСТИ ДИСПЕРСНОУПРОЧНЕННОГО МЕТАЛЛИЧЕСКОГО КОМПОЗИЦИОННОГО МАТЕРИАЛА МАРКИ ВКМ22

In many industries, such as mechanical engineering and aerospace, traditional alloys are being replaced by metal matrix composites (MCM). Compared to unreinforced alloys, MCM is characterized by increased strength and rigidity combined with low density. To calculate the resource of nodes for the safe and reliable operation of new equipment, it is necessary to have a set of calculated values of the MCM strength characteristics, including the characteristics of short-term and long-term strength, low-cycle (LCF) and high-cycle (HCF) fatigue. The work is devoted to the study of the characteristics of short-term and long-term strength, LCF and MCF fatigue of dispersion-strengthened MKM grade VKM22.

Modeling of Creep and Long-Term Strength of Rods at Alternating Stresses

The results of testing of three rods made of different alloys for creep and long-term strength under normal or tangential stresses, which change once or repeatedly in time, are presented. The kinetic theory of Yu. N. Rabotnov was used as a basis for modeling the obtained data. As a result, a good agreement was obtained between the theoretical characteristics of creep and long-term strength and experimental values.

Long-term mechanical and statistical characteristics of binary- and ternary-blended concrete containing rice husk ash, metakaolin and silica fume

An experimental study has been executed to examine long-term strength and durability characteristics of concrete mixes incorporating three types of supplementary cementitious materials (SCMs), namely rice husk ash (RHA); metakaolin (MK); and silica fume (SF) as substitution to cement content. These SCMs were mixed in binary- and/or ternary-blended form in the cement-bound composites having percentages of replacement in range between 5 and 10%. Seven different types of mixes were prepared and assessed for workability, compressive strength, splitting tensile strength and water absorption. For evaluating short- and long-term strengths, the concrete specimens were tested at curing age of 3, 7, 28, 56 and 180 days, whereas 28, 56 and 180 days of curing age were chosen to assess the water absorption value. Results revealed that considered SCMs decrease the strength of matrix up to 28 days of curing but aids in restoring it in later ages. For all mixes, experimental results indicate that the substitution of RHA and MK, or RHA and SF while altering cement content increases the composite potential in resisting higher loads in compression as well in tension, while on other side, degrades the water absorption capacity. A multivariable nonlinear regression model having higher order of accuracy has also been proposed for estimating compression strength, tension strengths and the water absorption capacity along with analysing cost and economic benefits of utilising SCM concrete.

A long-term strength criterion for frozen clay under complex stress states

To study long-term strength characteristics under complex stress states, a series of hollow cylinder creep tests were conducted on frozen clay under different stress levels at a temperature of −6 °C. Experimental results showed that the creep characteristics of frozen clay were significantly affected by the mean principal stress and the intermediate principal stress coefficient. Most generalized shear creep curves of frozen clay presented typical non-attenuation creep under axial load and torque. A long-term strength criterion for frozen soil, exhibiting the strength characteristics of multiple stress components under complex stress states, was obtained based on activation energy theory. In addition, a modified elliptical function and a generalized nonlinear strength theory (GNST) were introduced to describe the evolution of the strength envelope with time. Finally, a new strength criterion for frozen clay in three-dimensional principal stress space, considering the effect of time, was established by combining the modified elliptical equation and the GNST shape function. The proposed strength criterion has great significance for predicting the strength characteristics of frozen clay under complex stress states.

Oxide-Dispersion Strengthened Radiation-Resistant Steels

The better characteristics of the long-term strength of the EP-450-ODS (oxide dispersion strengthened) steel are determined compared to the well-known oxygen-free reactor steels. Using the Mossbauer analysis of the deformed powder mixture 56Fe3O4 + 57Fe, the possibility of the deformation-induced dissolution of iron oxides in the steel matrix, which makes it possible to obtain high quality ODS steels in the process of subsequent high-temperature annealing (sintering), is shown. The enhanced strength characteristics are obtained in recrystallized samples of ODS iron (YS0.2 ~ 570 MPa, UTS ~ 632 MPa) in the absence of any other alloying elements except for oxygen of air.

Parametric Methods for Determining the Characteristics of Long-Term Metal Strength

A large number of parametric methods were proposed to calculate the characteristics of the long-term strength of metals. All of them are based on the fact that temperature and time are mutually compensating factors in the processes of metal degradation at high temperature under the action of a constant stress. The analysis of the well-known Larson-Miller, Dorn-Shcherby, Menson-Haferd, Graham-Wallace, and Trunin parametric equations is performed. The widely used Larson-Miller parameter was subjected to a detailed analysis. The application of this parameter to the calculation of ultimate long-term strength for steels and alloys is substantiated provided that the laws of exponential dependence on temperature and power dependence on strength for the heat resistance are observed. It is established that the coefficient C in the Larson- Miller equation is a characteristic of the heat resistance and is different for each material. Therefore, the use of a universal constant C = 20 in parametric calculations, as well as an a priori presetting of numerical C values for each individual group of materials, is unacceptable. It is shown in what manner it is possible to determine an exact value of coefficient C for any material of interest as well as to obtain coefficient C depending on stress in case such a dependence is manifested. At present, the calculation of long-term strength characteristics can be performed to a sufficient accuracy using Larson-Miller’s parameter and its refinements described therein as well as on the condition that a linear law in logσ–Р dependence is observed and calculations in the interpolation range is performed. The use of the presented recommendations makes it possible to obtain a linear parametric logσ–Р dependence, which makes it possible to determine to a sufficient accuracy the values of ultimate long-term strength for different materials.

30年経過した深層混合処理改良体の強度特性

30年経過した深層混合処理改良体の強度特性

МОДЕЛИРОВАНИЕ ПОВЕДЕНИЯ МАТЕРИАЛОВ И ЭЛЕМЕНТОВ КОНСТРУКЦИЙ, НАХОДЯЩИХСЯ ПОД ВОЗДЕЙСТВИЕМ АГРЕССИВНЫХ СРЕД (ОБЗОР)

The review of a number of scientific works in the field of modeling processes of interaction of an aggressive medium with materials and structural elements and their behavior under the action of loads is offered. The investigated phenomena are described using theoretical-experimental methods for the further prediction of short-term and long-term strength characteristics of materials and structural elements subject to the influence of an aggressive medium. In view of the complexity of the phenomena under consideration, an interdisciplinary scientific approach is used that takes into account both the physics-chemical interaction of the aggressive medium with materials and the systematization of mechanical-mathematical (phenomenological) models. The main attention is paid to the study and modeling of high-temperature corrosion damage of materials and structural elements. It is taken into account both the diffusion penetration of the aggressive medium and the chemical interaction of its elements with the material. The effect of various factors, such as liquidmetal media and radiation exposure; oxide films formed on corroding surfaces; cracking of stressed stainless steels in the conditions of hydrogen embrittlement, on the corrosion process is presented. The dependence of corrosion rate on the stressed state with elastoplastic deformations also is represented. Publications with various versions of phenomenological models of corrosion wear processes, loss of plasticity and embrittlement in aggressive media are given, as well as publications using the probabilistic-statistic approach to modeling corrosive failure. An extensive list of wellknown literature sources, that reflect scientific research taking into account all aspects of interaction of the material with an aggressive environment is given. Keywords: modeling, physics-chemical models, mechanical-mathematical models, aggressive environment, liquid metal environment, strength, corrosion, corrosion layer, corrosion failure, diffusion, chemical reaction, ionizing radiation, experiment.

Creep Behavior and Long-Term Strength Characteristics of Greenschist Under Different Confining Pressures

Abstract Creep and long-term strength are important time-dependent behaviors of rock. To study the time-dependent behavior of hard rock under different confining pressures, conventional triaxial tests and multistage creep tests were conducted on the greenschist in the Jinping II Hydropower Station at different confining pressures. Based on the test results, the characteristics of axial, lateral, and volumetric creep were analyzed, and the creep properties under different confining pressures were also studied in detail and evaluated quantitatively by a simple model. Meanwhile, a new method is proposed by which the long-term strength can be determined using the inflection point in the isostrain-rate curve. The test results revealed that the creep was weakened and the ratio of long-term strength to instantaneous strength decreased with increasing confining pressure, during which the deformation of the rock sample transitioned from brittle to ductile. It was found that the duration of the transient creep stage decreased and the lateral strain was more sensitive to the creep stress than the axial strain when the confining pressure increased. It is also observed that the long-term strength determined by the inflection point method is consistent with those estimated by the creep rate in multistage creep tests, and the accuracy of the method proposed in this research is verified.