Uniaxial Creep Tests Research Articles

An Experimental Investigation on the Creep Behavior of Deep Brittle Rock Materials.

The stability of deep rock engineering, especially during the excavation, is inextricably linked to the time-dependent mechanical properties of brittle rock. Therefore, the uniaxial creep test in a multilevel loading path is carried out, accompanying the real-time DIC (digital image correlation) and AE (acoustic emission) technologies. For the quartz sandstone, the lateral strain is more sensitive to increasing stress levels, and the lateral ductility is more significant during the creep process. The saturated quartz sandstone shows a certain bearing capacity before the volumetric dilation predominance. The softening effect of moisture causes a nearly invariable Poisson’s ratio during the middle stress stages, as well as the more notable increasing trend of a steady creep rate with an increasing stress level, reflected by the larger slope and the intercept in the fitting relations. The main shear pattern and the combination of the shear and splitting failures are separately shown by the dry and saturated quartz sandstone. For the granite, both compression and extension exist in the creep deformation, and the failure may first occur in the prominent deformation area with a cracking noise. The AE hits present a similar time-dependent behavior to the strain of rock, and the attenuation trend happens in both the AE amplitude and energy before the rock enters the unsteady phase. The incomplete specimen of granite exhibits a lower strength and a larger deformation, owing to the more remarkable damage accumulation reflected by the spatial distribution of the AE event points.

Study on creep characteristics and constitutive model of typical argillaceous salt rock in energy storage caverns in China

Salt rocks are internationally recognized as a suitable medium for energy storage because of their excellent properties, so the creep behavior of salt rocks has received a lot of attention from scholars at home and abroad. However, the creep behavior of Chinese salt rocks is different from that of high purity salt rocks because they are mostly interbedded, which poses a significant challenge to the safe operation of energy storage salt caverns in China. Therefore, this paper takes a typical muddy salt rock in China as the research object and conducts uniaxial creep tests, considering the accelerated creep phase as a coupled process of loading and creep. A new nonlinear creep damage constitutive model was derived, which was verified based on the creep test data and compared with the traditional classical element model. The model can describe the accelerated creep stage of the specimen well. The model's applicability is also examined, and it is found that the model has a good description for mudstone, high-purity salt rock and mudstone salt rock combination, which provides theoretical support for the safe operation of salt caverns containing interbedded energy storage.

High-precision molding simulation prediction of glass lens profile for a new lanthanide optical glass

Precision glass lens molding (PGLM) is a recently developed method for fabricating glass optical components with high precision in large volumes. Lanthanum optical glasses are extensively used as optical materials owing to their superior optical properties, such as high refractive index, low dispersion, and high transparency. However, the transformation temperature of currently available high refractive index glass is generally above 650 °C and poses a challenge in manufacturing ultra-hard molds, durable coatings, and high-temperature molding equipment using PGLM. In this study, a preparation method for obtaining high refractive index, low -melting -point lanthanide optical glass (B-ZLaT198) used in PGLM was developed to reduce the transformation temperature. The developed method also characterizes the glass refractive indices and thermal-mechanical properties. To achieve the high-precision prediction of a molding shape in a simulation, a viscoelastic constitutive model of glass was established based on a micro-deformation uniaxial compression creep test. Moreover, by solving the Tool-Narayanasway-Moynihan model parameters based on the specific heat capacity fitting of optical glass at different heating and cooling rates, the input parameters of the structural relaxation model (SRM) for simulation prediction of aspheric glass lens profile deviation in the annealing stage were obtained. Finally, the profile deviation of the aspheric lens was predicted using a finite element model simulation. The results showed that the simulation’s predicted profile of an aspheric lens using the SRM model was in good agreement with that of experimental molding profile. In addition, using the SRM provided a higher prediction accuracy than that of the thermal expansion model in the annealing stage. Adopting the SRM was necessary for the annealing simulations of molding pressing and also verified the accuracy of the proposed viscoelastic characterization method for calculating the thermomechanical parameters of optical glasses.

Experimental Study on the Mechanical and Permeability Properties of Lining Concrete Under Different Complex Stress Paths

A promising large-scale energy storage is underground compressed air energy storage (CAES) in lined rock caverns. To ensure the safety and stability of storage caverns because of the influence of periodic injection during production, it is crucial to understand the mechanical behavior of lining concrete under different complex stress paths. In this study, three types of uniaxial compressive fatigue test and uniaxial creep test were conducted on concrete. The following conclusions were obtained from the results. 1) The irreversible deformation after the interval was larger than that before the interval in the discontinuous multi-step cyclic loading (DMCL) test. 2) Loading velocity significantly influenced concrete fatigue, and the irreversible deformation in the cycle of low loading velocity was greater than that in the cycle of high loading velocity. 3) The residual strain increased with an increase in stress level. 4) The creep strain increased with an increase in stress level during the multi-step creep loading test; the fractional derivative results were more consistent with the experimental results. 5) The permeability of concrete increased rapidly under the influence of an external force when the stress level exceeded 0.73.

Nonlinear creep model of salt rock used for displacement prediction of salt cavern gas storage

To explore the creep response and unloading rebound effect of salt rock, uniaxial compression creep and unloading rebound tests are performed at different axial stresses on salt rock specimens. The test results indicate that the instantaneous strain generated by salt rock mainly contributes by unrecoverable instantaneous plastic strain under a higher axial stress, and the creep strain mainly consists of unrecoverable visco-plastic creep strain. The ratio of visco-plastic creep strain to the total creep strain gradually increases with increases in time and axial stress. A hardening viscous body and a damage viscous body are proposed to describe the hardening effect and damage effect of rock during the creep process. Subsequently, a new five-component nonlinear creep model is established by connecting the two viscous bodies with the conventional elastic body and Kelvin model in series. The creep and unloading rebound test data of salt rock are utilized to testify the reasonability of the nonlinear creep model. The results indicate that the theoretical curves of the nonlinear creep model exhibit good coherence to the test data in the creep and unloading stages. Based on the secondary development platform of the FLAC3D software, the nonlinear creep model is redeveloped to obtain the computer application program. Thus, the creep and unloading rebound test of salt rock is simulated using the secondary development program of the nonlinear creep model. In general, the results from the numerical simulation and creep and unloading rebound test of salt rock are in good agreement, and this proves the accuracy of the secondary development program of the nonlinear creep model. Finally, the computer application program of the model is utilized to predict the surrounding rock deformation of a salt cavern gas storage during operation.

Rapid estimation of creep constitutive equation using digital image correlation and FEM

Estimation of the creep constitutive equation requires uniaxial creep tests under various stress conditions, which is costly and time consuming. In this study, a rapid estimation method of a creep constitutive equation is investigated based on an inverse analysis using digital image correlation (DIC) and FEM. First, a steady-state creep strain field that develops non-uniformly near the stress raiser was measured by DIC. Next, an inverse analysis was performed to minimize the difference between the strain measured by DIC and the strain calculated by FEM, and the creep constitutive equation in the steady creep region was estimated. The strain rate calculated from the estimated creep constitutive equation were consistent with the results obtained from uniaxial creep tests at high stress side, though the strain rates were overestimated at lower stress condition, where the duration of transient creep is relatively longer.

Small punch creep behavior and microstructure of A5052 alloy at around 0.7Tm

Recently, the small punch creep (SPC) test, one of the small sample methods, has attracted much attention. However, since the SPC test is a new test, there is a great demand for the accumulation of data to compare with the uniaxial creep test. In this study, SPC tests of A5052 alloy at around 0.7Tm were performed under various loads, and load dependence of displacement velocity were evaluated. The microstructures before and after fracture were observed by EBSD to investigate the microstructural changes and deformation behavior during the tests. The results showed that the displacement velocity and load followed a power law relationship and behaved similarly to the uniaxial creep test.

The Creep Test Study and Macro‐Detail Analysis of Argillaceous Red Sandstone in Different Water‐Containing States

In order to study the creep characteristics and failure mechanism of argillous red sandstone in a water‐bearing state, uniaxial creep tests of argillous red sandstone under dry and saturated states are carried out under fractional loading. Burgers model is used to fit the test results and identify the parameters, and scanning electron microscopy is carried out on the cross section of the damaged sample. The mechanism of creep difference of argillaceous red sandstone with different water content is studied from the microscopic point of view. The results show that the uniaxial compressive strength of the saturated clay red sandstone is 46% of that of the dry clay red sandstone. The deformation level of the saturated clay red sandstone is lower than that of the dry clay red sandstone. The limit deformation of the creep failure of the water‐bearing clay red sandstone is much smaller than that of the dry clay red sandstone. The long‐term strength of the muddy red sandstone is 46.4 MPa in the dry state and only 14.1 MPa in the saturated state, as calculated at the steady‐state creep rate. It is found that the Burgers model can describe the attenuation and stable creep of muddy red sandstone well. Through macro‐ and microanalysis, it is found that the compression failure form of argillaceous red sandstone belongs to x‐shaped conjugate inclined plane shear failure. After saturation, the internal pores of argillaceous red sandstone become larger, and the cementation ability decreases, resulting in the decrease of rock sample strength.

Micro-mechanism of brittle creep in saturated sandstone and its mechanical behavior after creep damage

Understanding the micro-mechanism of brittle creep in saturated rocks and studying their mechanical behaviors after creep damage are of great significance to the design of rock engineering and the prediction of the long-term evolution of the Earth's crust. In this study, we first performed a series of uniaxial creep tests with different creep stage cut-offs on saturated intact sandstones. The brittle creep mechanism of the specimens was explored microscopically by analyzing P-wave velocity, acoustic emission (AE), and nuclear magnetic resonance (NMR) measurements. Then, specimens with different degrees of creep damage were subjected to a secondary short-term loading test or creep test, and their short- and long-term mechanical behaviors, together with AE monitoring, were systematically investigated. The experimental results show that (1) the anisotropy of the specimens increases significantly during creep, with the axial and lateral P-wave velocities exhibiting a difference of 30% at the onset of tertiary creep. (2) The tensile cracks dominate the transient creep stage and the early part of the steady creep stage, while the shear cracks dominate thereafter. (3) The porosities of the specimens increase with the loading stages, and the porosity increments corresponding to small pores are greater than those corresponding to large pores, for all creep stages. (4) Specimens with different degrees of creep damage show greater deformability, and their short-term and creep mechanical parameters, except for uniaxial strength, change dramatically. (5) The AE characteristics of creep-damaged specimens differ significantly from those of intact specimens, and the Kaiser stress memory effect was observed during reloading, with the memory stress being the previous creep stress.

Experimental Study on the Short-Term Uniaxial Creep Characteristics of Sandstone-Coal Composite Samples

In this investigation, the uniaxial short-term creep tests with multi-step loading were conducted on the sandstone-coal composite samples, and the characteristics of creep strength, creep deformation, acoustic emission (AE), and creep failure of composite samples were studied, respectively. The creep strength of the composite sample decreased with the stress-level duration, which was mainly determined by the coal and influenced by the interactions with the sandstone. The creep deformation and damage of sandstone weakened the deformation and damage accumulation within the coal, resulting in the larger strength for the composite sample compared with the pure coal sample. The axial creep strain of composite sample generally increased with the stress-level or the stress-level duration under same conditions. The AE characteristics of composite sample were related to the creep strain rate, the stress level, the stress level duration, and the local failure or fracture during creep loading. The micro or macro failure and fracture within the composite sample caused the rise in the axial creep strains and the frequency and intensity of AE signals, especially the macro failure and fracture. The creep failures of composite samples mainly occurred within the coal with the splitting ejection failure accompanied by the local shear failure, and no obvious failures were found within the sandstone. The coal in the composite sample became more broken with the stress-level duration.

Assessment of Creep Properties Using Small Punch Test for a 9%Cr-Mo-Co-B Power Plant Steel

The present study provides a feasible method to evaluate creep properties for a 9%Cr-Mo-Co-B power plant steel by comparing two sets of data obtained from small punch tests and conventional uniaxial creep tests. The method includes three steps: firstly, conduct a series of small punch tests and conventional creep tests in different load and temperature conditions; secondly, convert the load and central deflection data obtained from the small punch test to stress and strain data; thirdly, determinate the best fit correlation factor by comparing the two sets of data in selected creep models. It is found that two sets of data show a similar trend in stress–rupture time relation, stress–minimum strain rate relation and LMP–stress relation. The correlation factor, ksp, can effectively bridge the gap between the load in small punch test and the stress in conventional creep test. For a high-Cr martensitic heat-resistant steel named as CB2, the ksp value 1.4 can make a good prediction for rupture time, while for minimum creep rate and the Larson–Miller parameter, the ksp value 1.4 will lead a conservative prediction in the low-stress range.

Improved Maxwell Model Describing the Whole Creep Process of Salt Rock and its Programming

Based on the creep test results of salt rock, three stages of changes in the viscosity coefficient with time during the creep process of salt rock were delimited from the perspective of creep rate, namely, gradually increased stage, basically unchanged stage and gradually decreased stage. According to this variation, a nonlinear dashpot was proposed, and its viscosity coefficient was assumed as a function of creep time. Then, the dashpot in the Maxwell model was replaced by this nonlinear dashpot to establish an improved Maxwell model that can describe the whole creep process of rock. The rationality of the improved Maxwell model was verified based on the uniaxial compression creep test results of salt rock and the triaxial compression creep test results of coal. Comparisons of the theoretical curves of the model and test results show good agreement between them, and the model can describe the three stages of the whole creep process of rock with a unified expression, thereby overcoming the shortcomings of the traditional mechanical model that requires segmentation processing. Based on the secondary development platform of the FLAC[Formula: see text] software, the secondary development of the improved Maxwell model was achieved by using VC++ 6.0 development environment, and the computer application program of the model was obtained. Then, the secondary development program of the model was used to simulate the uniaxial compression creep test of salt rock and the triaxial compression creep test of coal. In general, the numerical simulation results are in good agreement with the test results, proving the correctness of the secondary development calculation program of the model.

Study on the Creep Damage Characteristics of Shale under the Combined Action of Osmotic Pressure and Disturbance Load

In the in situ modified fluidized mining engineering, the surrounding rock of the shaft wall is prone to creep instability damage under the action of disturbance and seepage water pressure, which seriously affects the stability of the surrounding rock of the deep in situ modified fluidized mining. In order to study the nonlinear creep damage and fracture characteristics of deep rocks under the combined action of seepage water pressure and disturbance load, a self-developed rock perturbation creep test rig under the action of seepage water pressure was used, and shale was used as the rock sample. In the method of staged loading, the rock uniaxial compression perturbation creep test under static axial pressure, different perturbation frequencies, and different seepage water pressures was carried out, and the creep characteristics of shale under the combined action of perturbation and seepage were studied. The results show that with the increase of seepage water pressure, the creep failure time of the rock decreases, and the ultimate strain value increases; with the increase of the disturbance frequency, the creep failure mode of the rock gradually transitions from shear failure to tension failure. When water pressure and disturbance load exist at the same time, rock creep is more sensitive to seepage water pressure; based on experimental results, a shale perturbation creep damage model considering the influence of seepage water pressure and disturbance frequency is established, and the model is verified. The research results have important theoretical significance for guiding the wellbore stability control of in situ modified fluidized mining engineering.

Energy evolution mechanism of the mechanical and creep properties of layered phyllite under uniaxial compression and creep tests

Energy evolution mechanism of the mechanical and creep properties of layered phyllite under uniaxial compression and creep tests

Creep damage behaviors of red sandstone subjected to uniaxial compression after high‐temperature heat treatment using acoustic emission technology

AbstractThis work discusses the results from tests conducted to investigate the uniaxial compression and creep behavior of red sandstone. An original untreated sample and an 800°C treated sample were selected to carry out the experiments. High temperature had an obvious influence on the mechanical properties of the red sandstone. The relationship between creep strain and instantaneous strain, as well as the instantaneous deformation modulus and creep viscosity coefficient, was analyzed. High temperature reduced the ability of the red sandstone to resist instantaneous deformation and creep deformation. Acoustic emission (AE) technology was also used in the loading process of uniaxial compression and creep tests, providing a powerful means for damage evolutionary analysis of the red sandstone.

Viscoelastic–Plastic Constitutive Model Considering the Damage Characteristics of Concrete in the Process of Nonlinear Creep

Under long-term load, the creep deformation of concrete materials has a serious impact on the structural safety of hydraulic structures, especially under the action of ultra-high stress levels, the concrete materials will undergo nonlinear creep, which is extremely easy to cause structural damage. In this study, the uniaxial nonlinear creep test of concrete specimens was used to establish the damage index based on the wave velocity value of ultrasonic flaw detection, and the creep and damage degree curve of the concrete specimen were obtained. The ideal elastic element, the Kelvin body, and the nonlinear viscoplastic element are connected in series, and a new viscoelastic–plastic model considering the creep characteristics of concrete is proposed. Based on the principle of least squares, the Levenberg–Marquardt (LM) algorithm is used to inverse the parameters of the nonlinear creep test. In addition, the model is verified by the measured data of linear creep. At the same time, the sensitivity of each model parameter is analyzed. The research shows that the LM algorithm can give the fitting parameters of the model better and faster, and the fitting values of the model are similar to the experimental results. The sensitivity analysis of the parameters shows that the proposed model has good stability and good adaptability. The model has a more accurate description of the various stages of creep, and may be conveniently applied to concrete creep calculations in actual projects.

A subcritical microcrack informed creep-damage model for quasi-brittle materials

A subcritical microcrack informed creep-damage model is proposed to describe the creep-failure behavior of quasi-brittle materials where the subcritical microcracks propagation is characterized by the subcritical damage. Total free energy is decomposed into the time-dependent component and time-independent component where the time-dependent Helmholtz free energy is associated with time-dependent behavior. Driven by the time-dependent free energy the subcritical damage growth is originated from the nano-scale fracture kinetics. Within the continuum damage framework, a modified stochastic damage model is introduced to account for instantaneous behavior and subcritical damage evolution. The viscoelastic behavior is characterized by the Burgers model. The proposed model is validated through several numerical simulations including uniaxial loading tests, uniaxial creep tests and bending creep test. The results demonstrate that the present model is promising for the long-term analysis of underground structures.

Experimental study of creep Poisson ratio of hydraulic concrete under multi-age loading-unloading conditions

Elastic creep theory, which is widely used in concrete dam engineering, assumes that the elastic Poisson ratio and the creep Poisson ratio equal the effective creep Poisson ratio. However, differences in creep test conditions and stress state make this assumption controversial. To accurately clarify the Poisson effect of hydraulic concrete, uniaxial compression creep tests with four loading ages (3d, 14d, 28d, and 60d) and conventional triaxial compression creep tests with three loading ages (14d, 28d, and 60d) are conducted under unified test conditions. And high-precision creep data of transverse and longitudinal for multi-age loading and unloading are obtained. We then denoise the creep data by using a Savitzky-Golay filter. Furthermore, based on the denoised creep data, we calculate the Poisson ratio at each stage for uniaxial compression and conventional triaxial compression. Finally, we compare and analyze the Poisson ratio of each stage in different stress states. The results show that the creep tests have the same noise when conducted under the same test conditions, and the optimal denoising by the Savitzky-Golay filter is obtained with parameters W (window width) = 9 and N (polynomial order) = 6. Under different stress states, the elastic Poisson ratio, creep Poisson ratio and effective creep Poisson ratio are not equal to each other, and the elastic Poisson ratio is greater than the creep Poisson ratio. In addition, under the conventional triaxial stress state, the effective creep (creep recovery) Poisson ratio in each direction is different, which indicates that the creep (creep recovery) in each direction is dependent under multi-axial stress state.

Creep performance of carburized 316H stainless steel at 550 °C

The creep behaviour of two ex-service type 316H austenitic stainless steel casts with different Mn contents and austenite grain size, that were pre-carburized for either 8000 h at 550 °C or 3000 h at 600 °C in simulated CO2-rich gas environment of UK Advanced Gas-cooled Reactors, was evaluated by uniaxial creep testing at 550 °C in air. The low Mn (0.98 wt.%) steel grade with an average austenite grain size of 142 ± 110 µm presents a higher creep deformation rate and creep ductility compared to the high Mn (1.52 wt.%) steel cast with an austenite grain size of 81 ± 67 µm. Below an applied stress of 318 MPa, the presence of the sub-surface carburized layer reduces the minimum creep rate, likely due to the significantly modified material properties of the carburized layer and the presence of local compressive stresses. The stress exponent determined for the high Mn steel carburized at 600 °C (n = 9.6), and its reference non-carburized microstructure (n= 9.8), identifies the dominant creep mechanism to be dislocation climb & glide. However, crack evolution through M23C6-decorated grain boundaries within the carburized layer was observed in all the tested material's conditions at creep strains of 0.50%. The average crack length increases with creep strain and attains values close to the carburized layer thickness at creep strains of ε ≥ 1.00%. The prevalence and size of cracking increases with the local hardness carburized layer depth. At applied stresses σ ≥ 318 MPa, cracking on-loading prior to forward creep already occurs in the carburized layer, leading to a sharp increase in minimum creep rates of carburized specimens. These results demonstrate the impact a relatively thin layer of modified material can have on the materials creep performance; emphasising the requirement to consider the effects of the service environment in structural assessment procedures, particularly for high temperature applications including Generation IV and advanced modular reactor designs.

Viscoelastic Damage Characteristics of Asphalt Mixtures Using Fractional Rheology.

The mechanical behavior of asphalt mixtures at high stress levels are characterized by non-linear viscoelasticity and damage evolution. A nonlinear damage constitutive model considering the existence of creep hardening and creep damage mechanisms in the entire creep process is proposed in this study by adopting the fractional rheology theory to characterize the three-stage creep process of mixtures. A series of uniaxial compressive creep tests under various stresses were conducted at different temperatures to verify the model. The results indicated that the model predictions were in good agreement with the creep tests. The relationship between the model parameters and applied stresses was established, and the stress range in which the mixture exhibited only creep consolidation was obtained. The damage to the asphalt mixture was initiated in the steady stage; however, it developed in the tertiary stage. A two-parameter Weibull distribution function was used to describe the evolution between the damage values and damage strains at different stress levels and temperatures. The correlation coefficients were greater than 0.99 at different temperatures, indicating that a unified damage evolution model could be established. Thus, the parameters of the unified model were related to material properties and temperature, independent of the stress levels applied to the mixtures.