Triaxial Creep Tests Research Articles

Investigation of a fractional derivative creep model of clay and its numerical implementation

The fractional calculus has been successfully applied to characterize the rheological property of elastic viscoplastic materials. However, soils were seldom involved in fractional derivative constitutive models, and the topic of triaxial creep tests is rarely discussed through fractional calculus. This study presents a fractional derivative model that describes the time-dependent behavior of Shanghai marine clay. The fractional derivative Merchant (FDM) model is established by introducing fractional element into the Merchant model. The comparison of the calculated values of the FDM model and the Merchant model shows that the FDM model can better describe the instant elastic deformation, attenuation creep and steady-state creep stages of the clay. The FDM model was implemented into a finite-element code in ABAQUS, which is used to simulate the creep tests. The simulations demonstrate that the development model has good ability to predict the time-dependent behavior of clay.

Analysis of elastic-viscoplastic creep model based on variable-order differential operator

Based on the variable-order differential operator (VODO), a variable-order elastic-viscoplastic (VO-EVP) creep model of geomaterials is established. Firstly, the variable-order strain rate model is established according to the newly defined Riesz-Caputo (RC) VODO, and the creep equation of the VO-EVP model is solved by Laplace transform. Then, the order function expression suitable for the creep of geomaterials is proposed by analyzing the existing order functions. A series of numerical analyses are carried out on the VO-EVP model and the order function. Finally, combined with the results of triaxial creep tests of Hong Kong marine deposits and Sackville clay, the effectiveness of the model under static load test and the sensitivity of the order function are analyzed. The results show that the VO-EVP model is significantly better than the conventional EVP model. Compared with the constant fractional EVP (CF-EVP) model, the VO-EVP model considering only the internal variables in time does not have obvious advantages in predicting the creep behavior of soil under static load, nor does it show strong time memory and parameter sensitivity. However, the VO-EVP model also has important significance in studying creep behavior.

Moisture content effect on the creep behavior of loess for the catastrophic Baqiao landslide

Understanding the creeping behavior of loess is of great importance as large-scale loess landslides are closely related with creep behavior. At present, it is still challenging to predict and estimate the long-term stability of such landslides. This is in a large degree due to the poor understanding of moisture control on creep behavior of loess. The purpose of this study is to decipher the loess creep behavior under various moisture contents (MCs) using loess specimens obtained from Baqiao landslide, Xi’an of China, using multi-loading triaxial creep tests under different MCs of 9%, 12%, 15%, 18% and 21%. Based on the laboratory test results, a series of relationships between the creep rate at the steady-state creep stage and the initial strain and initial shear modulus are revealed. Meanwhile, a method for obtaining the long-term strength of loess specimens, namely, the Steady-state Creep Rate Slope Method (SCRSM), is proposed. SCRSM resolves the issue in several conventional methods such as the Isochronous Stress-Strain Curve Method, the Tangent Method of Steady Creep Rate when MCs are of concern. Such an improvement is primarily due to a better way of finding the inflection point of the steady-state rate. It is found that SCRSM is robust and accurate to determine the long-term strength of loess specimens. Furthermore, we propose a modified Burgers model with a newly introduced nonlinear parameter n to overcome deficiencies of conventional creep models. This modified Burgers model is flexible to fit the creep test curves of loess, and can describe the curves at the accelerated creep stage more accurately. Lastly, the main factors triggering the Baqiao landslide is analyzed considering stratum lithology, rainfall and excavation. In general, this study provides a basis for understanding the evolutional process of loess landslides as well as guidelines for prevention, controlling and prediction of loess landslides.

Experimental investigation of the path-dependent strength and deformation behaviours of frozen loess

A series of triaxial shear, creep and relaxation tests with different stress paths were conducted on frozen loess at a temperature of −6 °C in order to experimentally study the path-dependent strength and deformation behaviours of the loess. Testing results indicate that the strength, the stiffness and the stress-strain relationships of frozen samples all rely on consolidation pressure and the shear stress path. The effects of K0 (0.625–1) consolidation path in the mechanical behaviours of frozen loess are not significant. Triaxial creep and relaxation tests were performed by seven pre-set paths. The rheological behaviours of frozen loess are also sensitive to the stress path. The initial loading process before rheological tests leads to the microstructure damage and anisotropic behaviours of frozen samples, and further dominates the strengthening of the viscosity properties during the creep (stress relaxation) process. Three types of stress history (consolidation, pure shear and couple history) were selected for preloading on frozen samples before triaxial tests to study the influences of the stress history on the mechanical behaviours in detail. The weakening effects of the pure shear history in the soil properties were experimentally verified. The stiffness behaviours of frozen loess are more susceptible to the damage process (prestress history) than the strength behaviours. Due to the competitive influences of the consolidation and shear history on the soils properties, the rheological behaviours of samples are dependent on the specific path history and maximum stress history of the preloading procedure.

A micromechanical‐based elasto‐viscoplastic model for the Callovo‐Oxfordian argillite: Algorithms, validations, and applications

SummaryFor a potential geological barrier of high‐level radioactive waste repositories in France, the long‐term mechanical behavior of the Callovo‐Oxfordian (COx) argillite is the most concern for engineers. In this paper, a micromechanical‐based elasto‐viscoplastic model is proposed, and its numerical realization is our main object. The COx argillite is considered as a three‐phase composite consists of porous clay, quartz, and calcite. By assigning appropriate constitutive laws to those constituents, the macroscopic elasto‐viscoplastic behavior of the COx argillite is determined with an extended Hill's incremental approach. The numerical aspects includes (a) a new formulation is proposed for the plastic multiplier when adopting the overstress (Perzyna) model to define the viscoplastic strain. Meanwhile, a new formulation is also proposed to solve it within the framework of an implicit returning mapping scheme. (b) The corresponding consistent tangent moduli are strictly derived by extending the method proposed for solving plastic problems; (c) the efficiency of the proposed integration algorithms for the local constitutive equations and the homogenization procedure are validated, receptively, by a built‐in porous plasticity model of a commercial finite element (FE) program ABAQUS and by FE computations of a two‐phase unit cell; and (d) the proposed micromechanical model is finally applied to simulate experiment data in short‐term triaxial compression tests and long‐term triaxial creep tests. And the numerical results show that it is able to reflect the variation of the mechanical behavior with respect to the varied mineralogical compositions.

Fractional derivative-based creep constitutive model of deep artificial frozen soil

A fractional derivative constitutive model of deep artificial frozen soil was established based on the Nishihara model by analyzing the triaxial creep and shear test results of deep artificial frozen soil under different confining pressures and temperatures. Considering the physical and mechanical properties of frozen soil, linear Newton equations were substituted with nonlinear Newton equations, and a flexibility matrix was derived for numerical calculations. A relevant constitutive finite element program was developed and used to simulate a deep artificial frozen curtain of a coal mine through a user subroutine of the ADINA commercial finite element software. The theoretical calculations agreed well with the experimental results of the triaxial creep tests, and the numerical simulation results were consistent with the field measurement data. This fractional derivative constitutive model was verified to describe the non-decaying creep deformation characteristics of deep artificial frozen soil under high confining pressures. It can also be applied to the design and calculation of artificial freezing in deep strata.

Experimental study on the mechanical properties of short-term creep in post-peak rupture damaged sandstone

The creep of rock is a slow stress adjustment deformation and an evolutional process of rock mass destruction. Therefore, rock creep plays a vital role in a variety of rock engineering. To reveal the creep properties of rock, the following work has been done. A series of triaxial creep tests has been conducted considering different low confining pressures. The creep curve is analyzed under different confining pressures, and the creep law of each stage is obtained. Many results were obtained for the influence of confining pressure on the instantaneous modulus of deformation, creep strain, creep rate and instantaneous strain. The proportion of the various stages of creep changes under different confining pressure. Due to the existence of confining pressure, the ultimate failure method of rock has also been affected. Creep data can then be fitted by the improved Nishihara model, revealing a good relationship. The conclusion of this paper can provide some significant guidance for rock engineering.

A nonlinear elasto‐viscoplastic model for clayed rock and its application to stability analysis of nuclear waste repository

AbstractThe creep behavior is one of the most important topics on the stability analysis of rock mass in underground engineering. In order to characterize the creep deformation of clayey rock more accurately, this study proposes a nonlinear elasto‐viscoplastic (EVP) model based on a new yield surface and a modified Mohr‐Coulomb (MC) flow potential. First, the finite element (FE) formulation for the constitutive creep behavior is derived and a user subroutine to define the material's mechanical behavior (UMAT) is implemented in the commercial finite element package ABAQUS. The numerical simulation is validated against the triaxial creep tests of a cylinder, and the results show that the proposed model can effectively predict the creep responses of clayey rock and thus compensate for the deficiency of the creep model in ABAQUS. Then, the proposed model is used to study the long‐term stability of clayey rock formation surrounding an underground experimental site. Based on the results from triaxial creep tests and in situ monitoring of the lining deformation, the time‐dependent material creep parameters are obtained for rock by using the back analysis method. The predicted lining deformation is in good agreement with the field data with most of the relative errors less than 3.5%. The creep zone in the surrounding rock demonstrates that the horizontal scope is larger than the vertical one. The proposed model is proven to provide accurate prediction on the creep characteristics of clayey rock, showing a potential to assess the long‐term stability and reliability of large‐scale underground engineering structures.

Study on sandstone creep properties of Jushan Mine affected by degree of damage and confining pressure

Creep characteristics of rock mass are related not only to the confining pressure but also to the degree of internal fracture damage. Revealing the creep characteristics of rock mass under different confining pressures and degrees of damage is of great significance for selecting a reasonable supporting time and optimum supporting strategy in engineering. Sandstone is the main medium of the coal seam roof in the Jushan Mine and has poor physical and mechanical properties. To study the creep behavior of sandstone, triaxial creep tests and simulations are conducted under different confining pressures and degrees of damage. The results show that the appearance of the creep stage is related not only to the stress but also to the confining pressure and the degree of damage. With an increase in the confining pressure or the degree of damage, the creep parameters will have clear change, especially under high stress level. The creep parameters and confining pressures show different changing laws, and there are also different relationships between the creep parameters and the degrees of damage. Three surface equations are proposed to visually describe the variation in the creep parameters with the confining pressure and the degree of damage. The sudden increase in cracks is affected by both the confining pressure and the degree of damage, and it can be used as an early warning of rock failure. Thus, the influence of the confining pressure and damage should be simultaneously considered in the long-term stability of rock mass.

The Effect of Initial Creep Damage on Unloading Failure Properties of Sandstone from Macro-mesoscopic Perspective

The aim of this research was to show the effect of initial creep damage on unloading failure of rock from macro-mesoscopic perspective. A series of triaxial creep tests were carried out on sandstone to simulate initial creep damage and then unloading confining pressure tests were performed, respectively. A creep damage variable was proposed to analyze the degree of initial creep damage and the relationship of it with the macroscopic strength parameters was established. The results showed that the unloading amount of confining pressure and residual strength all tend to decrease when the degree of initial creep damage increases. The critical challenge was how to describe the effect of initial creep damage from mesoscopic perspective. This aim was achieved through two steps. In the first step, the mesoscopic properties were analyzed using experimentally obtained SEM images of the rock samples with different levels of initial creep damage. By comparative analysis of porosities in different magnifications, it can be concluded that porosity can’t reflect the effect of initial creep damage very well, thus, other pore parameters are further proposed. In the second step, three pore parameters were calculated by using the Matlab and IPP software, then, the average value of mean pore diameter is determined as the proper evaluation parameter and, finally, the agreement was verified between the mesoscopic pore parameter and creep damage variable.

Study on the creep behavior of Tianjin marine soft clays and its improved Mesri model

Marine soft clays are widely distributed in coastal areas. Owing to their strong creep properties, soft clay foundations undergo significant post-construction settlement, thereby affecting the life cycle of buildings. In this study, the creep behavior of Tianjin marine soft clays was investigated by consolidated-undrained triaxial creep tests. Based on the creep test results and the Mesri creep model and by treating the final clay deformation under each increment of load as the initial strain, an improved Mesri creep model that describes both the stress-strain and strain-time relationships using hyperbolic functions was established. The creep curves under various levels of cell pressure and deviatoric stress showed notable decay and stable creep characteristics but contained no accelerated creep stage. The stress-strain isochronous curves exhibited notably nonlinear characteristics. Relative to the results calculated by the Singh-Mitchell and Mesri models, the results calculated by the improved Mesri model were in better agreement with the test results, which suggests that the established model is suitable for describing the creep behavior of Tianjin marine soft clays and can provide a reference for predicting the long-term settlement of foundations.

Nonlinear creep damage model considering effect of pore pressure and analysis of long-term stability of rock structure

Water changes natural seepage in the surrounding geological environment and weaken rock mass of valley during reservoir impounding, which result in large time-dependent deformation of foundation of dam. A nonlinear creep damage model considering water effect is proposed by taking into account the influence of pore pressure on the spherical stress tensor and yield surface. The elastic-viscoelastic model is consisted of a spring and several cascaded Kelvin models. The model is implemented in the ABAQUS user material subroutine. The accuracy of the model is verified by the triaxial creep test of saturated sandstone and dry sandstone. Then it is applied in the valley contraction analysis during impounding of Xiluodu high arch dam.

A creep model for frozen sand of Qinghai-Tibet based on Nishihara model

The tri-axial creep tests of frozen sand with different dry densities and grain size distributions were performed at 0.1 MPa confining pressure at different temperatures. The results show that the rheological properties of frozen sand are significantly affected by the temperature, the creep deformation of the frozen sand is obvious even under a small load when the temperature is higher than - 1.5 °C. Firstly, based on the Nishihara model, the elastic modulus of viscoelastic elements is modified to be a function of stress, the coefficient of viscosity in viscoelastic part is modified considering the effect of time and stress, and the viscoplastic element is improved by introducing the damage variable. Then a new non-linear creep constitutive model for frozen sand is proposed, and the new model is verified based on test results. It can be found that the predicted results agree well with the corresponding experimental results from low stress to high stress, and the proposed model can describe the whole creep process of frozen sand with different dry densities and grain size distributions accurately under different shear stress levels and temperatures. Besides, the function relationship between the model parameters and the shear stress under different temperatures is obtained through the regression analysis. Finally, the influences of the temperature and shear stress on the parameters are analyzed as well. This study is important to further study the stability of permafrost subgrade, and can accumulate experimental data for the study of creep theory.

Effect of Loading and Unloading on Steady-State Creep Rate of Single-fracture Rocks with Different Angles

Soft rock-like specimens with different single-fracture angles were prepared by using similar materials. Uniaxial and triaxial creep tests and confining pressure unloading tests were carried out for single-fracture rock-like specimens with different angles. The results show that the steady-state creep rate shows a parabolic trend with the change of angle under the same load. The steady-state creep rate increases first with the increase of angle, then decreases with the increase of angle. The steady-state creep rate peaks at about 30 degrees, and the steady-state creep rate of the complete specimen is the smallest.

Experimental investigation of creep behavior of loess under different moisture contents

The creep behavior of loess under different moisture content and pressure conditions has been investigated. For this purpose, triaxial creep tests were conducted by using a soil rheological triaxial testing machine. The results show that the creep behavior of loess is significant at high moisture levels, and that less time-dependent deformation occurs at high confining pressures. The creep failure mechanism of loess was analyzed on the basis of the micro-structure of the original loess and its typical creep failure characteristic. Finally, a new two-element creep model is proposed for describing the deformation of loess during creep based on fractional calculus and continuum damage mechanics. Through the validation of fitting, it is shown that the theoretical model properly depicts the creep properties of loess and thus provides an excellent description of the overall creep deformation phases of loess.

The creep behaviour and permeability evolution of breccia lava in triaxial creep tests

Breccia lava is a comparatively weaken rock material that has been widely distributed in the Southwest China. Its creep behaviour and permeability evolution are significantly important to the long-term safety and stability of rock engineering. This paper presents the experimental results of breccia lava under triaxial creep tests. The microstructure of the breccia lava before and after the creep tests was studied by scanning electron microscopy and optical microscopy. The results suggested that the erosion of calcite induced by pore-water pressure contributes greatly in increasing the permeability in creep stage.

The 3D Nonstationary Creep Constitutive Model of Sandstone Based on Fractional Order

Based on nonlinear creep characteristics of rock, triaxial creep tests were conducted on sandstone by using an MTS812.02 test machine. The creep properties of rocks under different conditions were analyzed. Establishing a creep model that describes parameter degradation is necessary to address the deterioration of rock creep parameters over time. A nonstationary Abel pot was constructed on the basis of laws of damage accumulation and deterioration. Then, the nonstationary creep model was established according to fractional order theory. The parameters of the triaxial creep test curve under different confining pressures were identified on the basis of Levenberg–Marquardt algorithm in Hengda coal mine in Fuxin, China. By comparing the model curve with the experimental curve, we found that the nonlinear model not only accurately reflected the creep characteristics of the decay and steady creep stages but also overcame the limitation of traditional creep model, that is, difficulty in describing the accelerated creep. Finally, the influence of fractional order and nonstationary parameters on rock creep was obtained by parameter sensitivity analysis. Overall, the model had high fitting degree as well as good prediction and analysis for 3D creep test data.

Binary medium creep constitutive model for frozen soils based on homogenization theory

Based on test results of creep deformation characteristics of frozen soils, the creep breakage mechanisms of frozen soils are analyzed in detail by the concept of binary medium model under the framework of breakage mechanics for geomaterials and a new binary medium creep constitutive model is proposed for frozen soils. The binary medium creep model for frozen soils is formulated based on the homogenization theory by defining the creep constitutive model of the bonded elements, the creep constitutive model of the frictional elements and the breakage ratio, which can reflect the creep deformation characteristics of frozen soils under different stress states. Finally, the determination of these parameters of the binary medium creep constitutive model are presented through the triaxial creep tests of frozen soils and the creep test data of related literatures. The simulation results demonstrate that the binary medium creep constitutive model proposed here can describe the whole creep process and grasp the salient features of frozen soils relatively well.

Creep Void Formation and Rupture Lifetime in Multiaxial Stress States

This paper discusses creep void formation and rupture lifetimes in multiaxial stress states for a SUS 304 stainless steel at elevated temperatures. Biaxial and triaxial tension creep tests were performed using a cruciform and a cubic specimen, respectively. These two types of the specimens were designed to achieve uniform equi-biaxial and equi-triaxial stress distributions by a finite element analysis in the gage parts. Void formation at grain boundaries was observed by intermitting biaxial creep tests and by interrupting triaxial creep tests. Creep rupture lifetimes were also obtained in biaxial and triaxial creep tests. Biaxial stresses increase the void formation but give a little influence on a creep rupture lifetime in the correlation with von Mises equivalent stress. Triaxial stresses also increase the void formation and drastically reduce a creep rupture lifetime in the correlation with von Mises equivalent stress. Evident void formation in an equi-triaxial stress condition demonstrates that von Mises equivalent stress is not a suitable measure to evaluate creep damage development in multiaxial stress states. A new equivalent stress is proposed to evaluate creep rupture lifetimes in biaxial and triaxial stress states.

Analysis of Mechanical Parameters and Creep Characteristics of Surrounding Rock in Fully Weathered Sandstone Tunnel

Based on the geological conditions of the fully weathered sandstone stratum in the expressway of the mountainous area, the Huizhou-Qingyuan section of the Yuzhan Expressway is used to analyze the mechanical parameters and creep characteristics of the surrounding rock of the tunnel through the indoor model test. It is concluded that: 1) The uniaxial compression deformation test measured the structural parameters of the fully weathered sandstone stratum structure, and obtained that the elastic modulus and internal friction angle φ of the fully weathered sandstone was 14.20°, and the cohesion c was 0.343Mpa. 2) The failure mode of fully weathered sandstone specimens was obtained by triaxial creep test. The creep curves of different weathered loads were analyzed and the Xiyuan rheological model was established. 3) The creep data was analyzed by MATLAB software, and the surrounding rock parameters under different surrounding rock were obtained.