Creep Characteristics Research Articles

Significance of reinforcement in Mg-based MMCs for various applications: A review

With the ability to replace conventional alloys in mass-saving applications while still providing greater stiffness and strength, magnesium is one of the lightest structural metals currently on the market. When compared to normal engineering materials, the incorporation of reinforcing elements into the metallic matrix significantly affects stiffness, specific strength, wear, and damping behavior, and creep characteristics. Magnesium metal matrix composites are suitable materials for a variety of applications due to their excellent physical and mechanical qualities and low density. This paper provides a summary of the effects of different reinforcements in magnesium and its alloys, highlighting both their advantages and disadvantages. It is carefully explored how reinforcing affects both the mechanical properties and the microstructure.

A nonlinear creep model for surrounding rocks of tunnels based on kinetic energy theorem

The initiating condition for the accelerated creep of rocks has caused difficulty in analyzing the whole creep process. Moreover, the existing Nishihara model has evident shortcomings in describing the accelerated creep characteristics of the viscoplastic stage from the perspective of internal energy to analyze the mechanism of rock creep failure and determine the threshold of accelerated creep initiation. Based on the kinetic energy theorem, Perzyna viscoplastic theory, and the Nishihara model, a unified creep constitutive model that can describe the whole process of decaying creep, stable creep, and accelerated creep is established. Results reveal that the energy consumption and creep damage in the process of creep loading mainly come from the internal energy changes of geotechnical materials. The established creep model can not only describe the viscoelastic–plastic creep characteristics of rock, but also reflect the relationship between rock energy and creep deformation change. In addition, the research results provide a new method for determining the critical point of creep deformation and a new idea for studying the creep model and creep mechanical properties.

Experimental study on mechanical properties and failure behaviours of new materials for modeling rock bridges

Rock bridges control the stability of rock slopes with intermittent joints. To better understand the evolution of water-induced damage in rock bridges, we developed new, water-sensitive similar materials and investigated their physical, mechanical, and hydraulic properties. The reasonable proportions of the material components required to simulate a rock bridge were systematically determined based on deformation failure modes, shear creep characteristics, deterioration degree, and the brittleness characteristics after water-rock interaction. Sensitivity analysis and contribution rates were used to characterize the effects of different influencing factors on the mechanical properties of similar materials. We found that one similar material specimen, SM-24, exhibited the analogous characteristics of natural rock bridge in shear creep behavior, crack propagation, and ruptured surfaces. Such similar material for modeling rock bridges has the following mechanical characteristics: reproducing the volume-expansion point, showing high deterioration degree and specific brittle failure characteristics after water-rock interaction, and reproducing the identifiable precursor of accelerated creep and stepped fracture morphology subjected to shear creep loading. The bone-binder ratio dominated mechanical properties of similar materials, while physical and hydraulic properties depended mainly on the bentonite content. These findings provide guidelines for reliably producing similar materials that meet the requirements of different geomechanical model tests in solving geotechnical engineering problems considering the water-rock interactions.

Forging Temperature Effects on Crack Tip Creep Behaviour of Hot Hammer Forged Ti-6Al-4V Alloy

The creep behaviour of hot hammer forged Ti-6Al-4V alloy under different forging temperatures has an essential influence on their high-temperature service performance. In this article, the high-temperature mechanical property and creep behaviour of Ti-6Al-4V alloy forging at different forging temperatures were first investigated experimentally. Then, in view of the critical effect of microcracks and other defects on the service life of components in forging, the creep characteristics at the tip of a 2 mm crack were characterized by the finite element method. The results show that when the Ti-6Al-4V alloy was forged at 1000°C, the high-temperature mechanical property was improved, and the steady-state creep rate and creep residual deformation were significantly reduced. In addition, the creep strain and creep rate at the crack tip were decreased apparently during creep, and the creep strain gradient in the crack tip region was also significantly reduced. This indicates that the creep deformation of Ti-6Al-4V alloy can be controlled by an appropriate forging temperature during hot hammer forging process and might delay the crack propagation to improve the service life of components.

The mechanical creep property of shale for different loads and temperatures

The analysis of the mechanical properties and the failure mechanism of shale with the effect of temperature are of great significance to the shale gas exploitation. In this paper, triaxial creep mechanical tests and AE monitoring method for shale under step loading were carried out at different temperatures and confining pressures. The classic Poynting-Thomson creep model was improved and the modified creep model considering temperature and viscoelastic-plastic deformation were built. The test results showed that the strength of the shale was related to the confining pressures and temperatures. The creep mechanism of shale was analyzed by the acoustic emission parameters. The peak frequency of acoustic emission signals reflected the difference of the rock fracture mode. The Poynting-Thomson model was modified on the theory of nonlinear rheology, which is more suitable for creep deformation of shale. The correlation coefficient of the modified model was higher than that of the classical model. The modified model considering temperature fit the shale creep curve well and could explain the creep characteristics under the influence of temperature and stress. The new model can be widely used in the research of shale creep deformation, such as the deep exploitation of shale gas, wellbore stability and other aspects of engineering problem of shale.

Fractional creep model and experimental study of unsaturated silty clay in Fuyang

Due to the long-term overdraft of groundwater in Fuyang, silty clay in this area has been in an unsaturated state for a long time, which caused ground subsidence and threatened the safety of engineering construction. Creep is an important part of ground subsidence, but it is easily ignored in ground subsidence caused by groundwater overdraft. Therefore, in order to explore the creep behavior in this environment, a series of triaxial creep tests of unsaturated soil were conducted to research the effects of deviator stress, matrix suction and net confining pressure on creep. Then, based on analysis results of the geometric characteristics of the test creep curves, an improved fractional order Nishihara model of unsaturated soil under triaxial stress conditions was constructed by using fractional calculus theory. Finally, the effectiveness of improved fractional order Nishihara model was verified based on cooperation search algorithm and minimum mean square error principle. The simulation results show that the fitting curves of improved fractional order Nishihara model are in good agreement with the test curves, and it is feasible and effective to describe the creep characteristics of unsaturated silty clay in Fuyang.

One-dimensional non-linear rheological consolidation of clayey soils with Swartzendruber's flow law

Considering the non-linear creep characteristics of clays, the modified unified hardening (UH) relation is employed to reflect the visco-elastic-plastic deformation of soft soils. Simultaneously, Swartzendruber's flow in the form of a continuous function is captured to describe pore water flow through pores during consolidation. Thus, an upgraded one-dimensional (1-D) non-linear rheological consolidation (NRC) system of clays has been established, and its solutions are conducted using the finite volume method (FVM). Through comparisons against the theoretical solutions and laboratory test results, the effectiveness of the numerical algorithm and applicability of the modified UH relation is verified. Then, the effects of model parameters on non-linear rheological consolidation behavior were studied. The results indicate that ignoring the time effect of secondary consolidation coefficient overestimates the overall consolidation and settlement proceeding, and considering the viscous effect of the soils led to an unanticipated increase in excess pore-water pressure (EPP) at the initial loading period. Meanwhile, the rising EPP becomes more pronounced with the increase of the initial over-consolidated parameter, flow parameter, and soil thickness or the decrease of overlying load. Additionally, FVM and finite difference method (FDM) solutions, as well as the application scope of Darcy's law in the NRC system, are discussed.

Effects of temperature on the time-dependent compression and shear behaviour of a soft marine clayey soil

The thermal effects on geomaterials, especially on clayey soils are getting increasing concerns in many geotechnical applications. To study the effects of temperature on the stress-strain behaviour of Hong Kong marine deposits (HKMD), a series of temperature-controlled experiments were carried out. Oedometer and constant-rate-of-strain consolidation tests under temperatures from 10 °C to 60 °C were conducted on both intact and reconstituted HKMD considering different temperatures and stress paths. The effects of temperature history on the compression curves, thermally induced strain, and the characteristics of creep are revealed and discussed. The concept of virgin heating is proposed for interpreting the thermal plastic deformation. With increasing temperature, the creep coefficient is found to decrease while the creep strain rate increases. Consolidated undrained triaxial tests were performed on intact and reconstituted HKMD under different strain rates and temperature conditions. Under constant temperature, the undrained shear strength of HKMD is not significantly influenced by temperature. In triaxial tests subjected to step-changed temperature, the undrained heating causes a significant reduction of effective stress and rise of porewater pressure in HKMD. Finally, microscopic investigations with mercury intrusion porosimeter and scanning electron microscope are presented and discussed in this paper. It is found that the micropores of HKMD are evolutional with temperature.

Experimental study on creep failure characteristics of coal mass under multistage stress and construction of prediction model

The creep and failure mode of stratified coal is complex and challenging to predict. This is highly attributed to the internal stratified structure generated during coal forming. In this study, the inner stratified structure of coal during creep failure was explored. The evolution relationship between the key creep parameters and the inner stratified structure was evaluated through creep tests. The creep failure characteristics of coal under complex stress were determined. A neural network model was established to predict the creep failure modes, and the output results were subjected to smoothing and tracking to reduce the error. The results showed distinct creep characteristics of coal with various types of stratified structure. Key parameters such as creep variable, steady creep state and creep rate exhibited a nonlinear relationship with stress. The dual control neural network model constructed in the study showed high accuracy and stability. This model can be used as a guidance and reference for the study of creep failure mechanism and elucidation of creep behavior prediction of stratified coal.

Determination of mechanical and microstructure properties of paste backfills to avoid to overburden failure

Backfilling, a widely used technology for green and sustainable underground practice, is an environmentally sustainable engineering approach. This study comprehensively investigates the mechanical properties and microstructural characteristics of backfills maintained underground for three years to avoid overburden failure, considering the static and dynamic mechanical characteristics, creep characteristics, permeability, and microporous structures under dry, natural, and saturated states. The experimental results indicate that the backfill can maintain a high residual stress after peak stress with a very low permeability under triaxial compression. The strength of backfill worked three years under dry, saturated and natural states show a slight difference, indicating a slight water effect on the long-term backfill. The triaxial compression strength of the backfill is greater than the value of stresses in the goaf of panel, indicating that the backfill strength has been achieved an standard value that can support the surrounding rock mass and maintain the stability of themselves in a long term. The porous structure of the backfill show a micro basis for the macro mechanical properties. All the results show that backfills possess the low permeability, strong strength, stable microstructure characteristics under complex environment and indicating an improving function and strength in supporting the surrounding rock mass and maintaining the stability themselves. The findings indicate that the backfills can serve as a useful reference for the control mechanism, waterproof effect, and safety evaluation of the backfill on the deformation of the surrounding rock mass underground. Furthermore, the successful applications show a good verification, indicating a significant practical application in other coal mines with the similar engineering geological conditions, which also contribute to other underground practice.

Laboratory study of the factors affecting hydraulic fracturing effect for inter-salt oil shale layers, Qianjiang Depression, China

This study aims to investigate the potential factors affecting hydraulic fracturing of inter-salt oil shale reservoirs in the Qianjiang Depression, China. Using the inter-salt shale samples, the re-crystallization seepage tests, rock mechanical tests under high temperature and pressure, salt rock creep tests, and direct shear tests were conducted. The testing results suggest several major factors that affect hydraulic fracturing effects in the end. First, the seepage of reservoir and fracturing fluid through hydraulic fractures leads to salt dissolution and crystallization, reducing the effective seepage area of fractures. Second, the salt crystal may block the pore throats or micro fractures after brine invades the shale, decreasing the overall permeability. Third, the low strength and obvious plasticity of inter-salt shale and the strong creep characteristics of salt rock raise difficulties for proppant to effectively support fracture walls, thereby sharply narrowing the hydraulic fracture width. Lastly, the weak interfaces (bedding planes and lithology interfaces) in inter-salt oil shale reservoirs restrict the height of hydraulic fractures, resulting in the disconnection of seepage channels between multiple inter-salt shale reservoirs. Thus, several factors together reduce reservoir permeability, weaken the fluid flow capacity in the fracture, narrow the fracture width, and limit the effective stimulation volume, resulting in weaken the effect hydraulic fracturing.

Unravelling the microstructure – indentation creep resistance relationships for friction stir welded modified 9Cr-1Mo steel and LN-type 316 stainless-steel dissimilar joints

In this article, the indentation creep characteristics and the microstructures are assessed for the friction stir welded modified 9Cr-1Mo steel and LN-type 316 SS dissimilar joint. Microstructures are characterized for the post-weld heat treated and indentation crept samples at different conditions with optical microscopy, scanning electron microscope and energy dispersive X-ray spectroscopy. Distinct heat-affected zones were revealed with the refined and reformed martensite (α′) and austenite (γ) structures that formed in view of the distinct thermal conductivity property and the differences in lower critical(Ac1) and upper critical(Ac3) transformation temperatures. The size of prior austenite grain sizes was found to be varied significantly at the different regions. Indentation creep data were acquired in distinct weld zones at 873, 898 and 923 K. Zone-wise indentation depth, steady-state creep behaviour (SSCR) and activation energy ( Q) were evaluated and correlated with the microstructures. The formation of small prior austenitic grain (PAG) structures and the grain boundary vacancies has led to maximum creep deformation (86.83%) at the heat affected zone of the modified 9Cr-1Mo steel. Furthermore, the reduction in lath structures and the formation of tiny α′ with soft substructures have lowered the hardness values to around 210 ± 21 HV, which is 20 HV less than the post weld heat treated sample. In the stir zone, the presence of dynamically refined and rich γ along with fine α′ layers has improved the steady-state creep rate of 9.112 E-7, which is less than the modified 9Cr-1Mo steel and the heat affected zone at modified 9Cr-1Mo side. The coarse austenite structures that existed in the LN-type 316 base metal have resisted more creep damage and exhibited high activation energy (i.e. 490 kJ mol−1) than the stir zone and heat affected zone of the LN-type 316 SS.

水环境下岩石蠕变特性及本构模型

水环境下岩石蠕变特性及本构模型

Огляд впливу повзучості бетону на роботу попередньо-напружених сталезалізобетонних конструкцій споруд цивільного захисту

The rheological properties of concrete are mainly determined by its creep properties, which affect the stress-strain state of the structure over time. In the case of pre-stressing a steel-reinforced concrete structure, forces are redistributed in its section between highly loaded and lightly loaded elements, namely between steel reinforcement and concrete. It should be noted that the supporting frame of buildings and structures (in addition to reliability, safe operation and economic feasibility) must guarantee functional suitability. It is impossible to determine the functional suitability of the structure without a correct prediction of the redistribution of stresses in time between the concrete and the reinforcement, which occurs as a result of the creep of concrete. According to the current norms of DBN B.2.6-98:2009 and EN (Eurocode 2), the criterion for the appearance of the limit state of reinforced concrete structures is the achievement of limit values by the deformations of compressed concrete. Therefore, a detailed study of the rheological properties of prestressed reinforced concrete is undoubtedly an urgent issue. The paper analyzes general information about the creep of concrete and its effect on losses during prestressing, including with the use of modern finite element modeling programs, which allow not only physically nonlinear characteristics of concrete to be specified when creating models of reinforced concrete structures, but also take into account the geometric nonlinearity of the work of composite reinforced concrete structure, but also to specify the rheological properties of concrete, in particular, creep characteristics. Conducted studies of the influence of concrete creep on the work of bent pre-stressed steel-reinforced concrete structures prove that the main reason for the increase in deflections of such structures under long-term load action is the creep of concrete in the compressed cross-sectional area. Taking into account during the numerical modeling of the bent pre-stressed combined structure the influences of the second order, namely the creep in time of the stressed concrete, leads to a decrease in the stresses in the compressed zone of the concrete by 6.5%, but to an increase in the stresses in the stretched reinforcement by 0.6% and , which is most significant, to increase the deflections of the structure by 23%.

Modeling of dynamic and spectral viscoelastic characteristics of composite materials

When designing products made of composite materials intended for use in difficult conditions of inhomogeneous deformations and temperatures, it is important to take into account viscoelastic, including spectral and dynamic, properties of the binder and fillers. The article considers dynamic characteristics (complex modulus, complex malleability, their real and imaginary parts, loss angle tangent) and spectral characteristics of relaxation and creep and their dependence on each other. The abovementioned characteristics were found for all known types of creep kernel and relaxation kernel. To find the spectral characteristics, one of the numerical methods of reversing the Laplace transformation was used - the method of quadrature formulas. Algorithms and computer programs have been compiled to implement this method.

A Research on the Air Permeability of High Polymer Materials Used to Produce Sports Clothing Fabrics

Composite fabrics based on Polytetrafluoroethylene (PTFE) polymer displays several notable properties. They are waterproof, windproof, permeable to moisture and thermally insulating at the same time. In the present study, PTFE fibers are used as raw material to make fiber membranes. The film is formed by crisscrossing interconnected fiber filaments and the related air permeability: tensile creep characteristics and other properties are tested. The results show that the pore size, thickness, and porosity of the film itself can affect the moisture permeability of the film. The water pressure resistance of the selected fabric is 8.5 kPa, and the moisture permeability is 7038 g/(m2·24 h).

Study on Shear Creep Characteristics of the Discontinuities with Different 3D Morphologies

The rheological phenomenon of rock mass affects the long-term safety of rock mass engineering. In this study, gneiss samples with different 3D morphologies are prepared by splitting tests and are tested through multi-step creep tests. The long-term strength of rock discontinuities is determined by using several methods. The test results show that as the 3D morphological parameter increases, the creep deformation, creep rate, and the duration of failure all decrease. The long-term strength of rock discontinuities is linearly related to the 3D morphological parameter. Based on the principle of damage mechanics for rock mass, a damage variable is introduced in the creep model, and an improved non-linear Burgers model is established. Research results are of great theoretical significance and practical value for the design, construction, and long-term safety of rock mass engineering.

Time‐Dependent Creep Damage Model of Frozen Soil with Unsteady Fractional Order under Temperature and Stress Coupling

In order to study the creep characteristics of frozen soil under different confining pressures, triaxial creep tests of frozen soil under different temperatures are carried out and the creep deformation law of frozen soil under different confining pressures and different freezing temperatures is analyzed. In this paper, the unsteady Abel dashpot is constructed by the unsteady viscosity coefficient. On this basis, the fractional order of the viscosity dashpot is unsteady. Then, a time‐dependent creep constitutive model for frozen soil based on the unsteady fractional order is established. The model can better describe the law of accelerated creep deformation of rock. Through the Levenberg–Marquardt algorithm, the parameters of the triaxial creep model are identified to verify the correctness of the creep model. The results show that the model can describe the creep deformation of frozen soil well and overcome the shortcoming that the traditional model cannot describe the law of accelerated deformation. At the same time, the agreement between the model curve and the test curve is also much greater than that between the traditional Nishihara model curve and the test curve. This lays the foundation for the later use of the creep model to simulate the settlement and horizontal displacement of subgrade soil under long‐term loads and also provides a theoretical basis for the construction of other roadbed projects.

The Effect of Temperature on the Creep Ageing Characteristics of Al-Li Alloys

Abstract: In this paper, the evolution of creep strain and mechanical properties of 2195 Al-Li alloy is investigated at different temperatures (160/170/180°C), with the aim of discovering the mechanism of temperature influence on creep properties and providing a reference for improving the creep properties of the target material by adjusting the temperature in the future. This study demonstrates that the creep curve is strongly influenced by temperature, with a plateau and a transition period occurring at low temperatures. As the temperature increases, the creep curve gradually reverts to the typical two-stage creep characteristics due to the interaction of dislocation density proliferation and dislocation reversion within the material at high temperatures. In addition, by means of tests such as TEM, it was confirmed that the decrease in mechanical properties of the material caused by increasing the creep temperature is mainly due to the coarsening of precipitates and the appearance of precipitation-free zones at grain boundaries.

Study of Energy Dissipation in a Two-Phase Soil System

The dependence of energy dissipation coefficient on filtration properties and on creep characteristics of soil skeleton is examined. The study was carried out on the basis of the general solution of the joint task of creep and consolidation theory for a two-phase soil system obtained on the basis of the generalized model of bulk forces, taking into account the interaction of soil phases, changes over time in the general stress state at any point of the soil and additional pressures in pore water, and incomplete transfer of external pressure to the pore water. Interactions between phases are taken into account in the solution of the one-dimensional joint task of the theory of creep and consolidation, and the creep kernel is taken in the form of an exponential function. The formula for the deformation of two-phase soils is presented as the sum of two syllables due to primary and secondary consolidation of the soil. The strain formula uses experimentally obtained values of filtration characteristics and creep parameters of the soil skeleton. Using the formulas obtained, hysteresis loops under sinusoidal stress changes can be plotted and the energy expended per deformation cycle and the dissipation coefficient for two-phase soil can be obtained.