Triaxial creep tests of weak sandstone from fracture zone of high dam foundation

The lithology of fracture zone which was developed at the dam foundation of a hydropower station is weak sandstone with poor integrity and pore cementation contact.Its creep properties have a significant impact on the deformation and stability of the dam.Based on the characteristics of loose organizational structure,high moisture content and poor mechanical properties,the triaxial compression tests and creep tests were carried out,respectively.The results show significant non-linear,low strength and no obvious strength peaks.Both axial and lateral strains are achieved more than 3%when the tests are failed.The weak sandstone has a significant creep property,but only transient and steady state appear under low stress.Increased stress causes creep intensified and lateral strain gradually exceeds axial strain.In the failure stage,it has characteristics of large axial plastic deformation,obvious volumetric ductility dilation and large steady creep rate.The accelerated creep appears shortly after transient loading under confining of pressures 1.0 MPa and 1.5 MPa.Therefore,an improved Burgers creep model considering the non-linear characteristics of weak sandstone is built based on hyperbolic equation and the creep parameters are identified.This model can well describe the creep properties of weak sandstone.

Temperature-controlled triaxial compression/creep test device for thermodynamic properties of soft sedimentary rock and corresponding theoretical prediction

In deep geological disposal of high-level nuclear waste,one of the most important subjects is to estimate long-term stability and strength of host rock under high temperature conditions caused by radioactive decay of the waste.In this paper,some experimental researches on the thermo-mechanical characteristics of soft sedimentary rock have been presented.For this reason,a new temperature-controlled triaxial compression and creep test device,operated automatically by a computer-controlled system,whose control software has been developed by the authors,was developed to conduct the thermo-mechanical tests in different thermal loading paths,including an isothermal path.The new device is proved to be able to conduct typical thermo-mechanical element tests for soft rock.The test device and the related testing method were introduced in detail.Finally,some test results have been simulated with a thermo-elasto-viscoplastic model that was also developed by the authors.

Dynamic stress accumulation model of granite residual soil under cyclic loading based on small-size creep tests

The creep behaviors of granite residual soil with pre-stress of 100 kPa was investigated by a series of small size creep tests. Three different types of strain curves were obtained at different stress levels. Based on creep characteristics of the granite residual soil under different stress levels, a creep model of the granite residual soil was established by rheological theory, and related parameters of the model were determined according to the experimental data at the same time. Further on, based on the established creep model, a theoretical model of dynamic stress accumulation in the granite residual soil under cyclic loading was deduced. It is found that there is a threshold of dynamic stress accumulation in this theoretical model. The dynamic stress accumulation laws of the granite residual soil are different under different cyclic loading stress. Finally, with the dynamic stress accumulation laws in the small-size samples of granite residual soil under different cycle loading studied and the experimental results comparing with the theoretical results, it verifies the validity of the theoretical model.

Computer Simulation of the Indentation Creep Tests on Particle-Reinforced Composites

A systematical simulation has been carried out on the indentation creep test on particle-reinforced composites. The deformation, failure mechanisms and life are analyzed by three reasonable models. The following five factors have been considered simultaneously: creep property of the particle, creep property of the matrix, the shape of the particle, the volume fraction of the particle and the size (relative size to the particle) of the indentation indenter. For all the cases, the power law respecting to the applied stress can be used to model the steady indentation creep depth rate of the indenter, and the detail expressions have been presented. The computer simulation precision is analyzed by the two-phase model and the three-phase model. Two places of the stress concentration are found in the composites. One is ahead of the indentation indenter, where the high stress state is deduced by the edge of the indenter and will decrease rapidly near to a steady value with the creep time. The other one is at the interface, where the high stress state is deduced by the misfit of material properties between the particles and matrix. It has been found that the creep dissipation energy density other than a stress parameter can be used to be the criterion to model the debonding of the interfaces. With the criterion of the critical creep dissipation energy density, a power law to the applied stress with negative exponent can be used to model the failure life deduced by the debonding of interfaces. The influences of the shape of the particles and the matching of creep properties of particle and matrix can be discussed for the failure. With a crack model, the further growth of interface crack is analyzed, and some important experimental phenomena can be predicted. The failure mechanism which the particle will be punched into matrix has been also discussed. The critical differences between the creep properties of the particles and matrix have been calculated, after a parameter has been defined. In the view of competition of failure mechanisms, the best matching of the creep properties of the two phases and the best shape of the particles are discussed for the composite design.

DETERMINATION OF CREEP PROPERTIES OF THERMAL BARRIER COATING(TBC)SYSTEMS FROM THE INDENTATION CREEP TESTING WITH ROUND FLAT INDENTERS

Indentation creep behavior with cylindrical flat indenters on the thermal barrier coating (TBC) was studied by finite element method (FEM). On ike constant applied indentation creep stress, there is a steady creep rate for each case studied for different creep properties of the TBC system. The steady creep depth rate depends on the applied indentation creep stress and size of the indenters as well as the creep properties of the bond coat of the TBC and the substrate. The possibilities to determine the creep properties of a thermal barrier system from indention creep testing were discussed. As an example, with two different size indenters, the creep properties of bond coat of the TBC system can be derived by an inverse FEM method. This study not only provides a numerical method to obtain the creep properties of the TBC system, but also extends the application of indentation creep method with cylindrical flat indenters.

Aging Coefficient, Creep Coefficient and Extrapolating Aging Coefficient from Short Term Test for Sealed Concrete

Aging coefficient and creep coefficient are important parameters for creep analysis of any structure. With the aim to obtain those parameters, an experimental investigation is carried out on sealed concrete. Altogether ten specimens were tested, out of which four were for creep, two for shrinkage and remaining four for relaxation test. A year of relaxation test and two years of creep test results were analyzed to compute aging coefficient and creep coefficient. From regression analysis of observed and calculated aging coefficient using formula of Bazant and Kim, modification for aging coefficient is performed and extrapolation equations are generated. Instead of ACI model, B3 model has been used to obtain compliance function as the parameters of B3 model seems more relevant to creep mechanism of concrete compared to that of ACI model.

Microstructural Evolution on the T91 Dissimilar Metal Joints during Creep Rupture Tests

T91 steel is one of the new materials presently employed in power plant pipe components. The creep rupture strength and microstructure of the T91+10CrMo910 and T91+13CrMo44 welded joints were analyzed during creep rupture tests. Creep transgranular ductile rupture occurred at the 10CrMo910 matrix in the T91+10CrMo910 welded joints and creep intergranular brittle rupture occurred at the 13CrMo44 HAZ in the T91+13CrMo44 joints. Microhardness measurements showed high hardness at the heat affected zone (HAZ) of T91 and a sharply drop at the 13CrMo44 HAZ during creep rupture. The metallographic tests showed that no obvious microstructure degradation was observed in the 10CrMo910 HAZ and matrix, while creep cracks appeared at the 13CrMo44 HAZ. T91 steel had relatively high creep resistant strength in the welded joints tested. Recovery occurred in the T91 HAZ with the growth of subgrain size and the decrease of dislocation density during creep. It was concluded that the dissimilar joints of T91 and low alloy heat-resistant steel should have close creep strength matching to increase the service life of the overall joints at elevated temperature.

Rheological Characteristics of Gluten after Modified by DATEM, Ascorbic Acid, Urea and DTT Using Creep-Recovery Test

The effects of diacetyl tartaric acid ester of monoglycerides (DATEM), ascorbic acid (AA), urea, and dithiothreitol (DTT) on viscoelastic properties of commercial hard red winter wheat gluten were investigated. A constant shear stress of 40 Pa was applied to gluten during creep-recovery test. Experimental creep-recovery compliance responses were fitted into a Burgers model with four elements accounting for characteristics of pure elastic (spring), viscoelastic (spring-dashpots elements), and viscous flow (dashpot). DATEM decreased the elasticity and viscoelasticity, but increased viscosity of gluten. The addition of AA, urea, and DTT, resulted in opposite rheological properties when compared with DATEM. Relationship among physical properties was also studied with principal component analysis (PCA) including gluten viscoelasticity, dough mixing and baking properties. Regressed coefficients from Burgers model accounted for higher percent of explained variance and were independent from flour content, baking and dough mixing properties.

STUDY ON TESTING TECHNIQUE FOR MEASUREMENT OF FATIGUE－CREEP INTERACTION RESISTANCE

By using dropped stress creep method a new testing technique for measurement of the fa-tigue-creep interaction resistance is developed. At varied adjusted mean stresses the creep testingwas performed repeatedly When an unlimited extensive incubation period with zero creep rate oc-curred. the stress cavsing zero creep rate is defined as fatigue-creep resistance. The developped test-ing technique was used to measure the fatigue-creep resistance in F anc C regions. The dynamic ef-fective stress could yield a better descrption of fatigue and creep interaction. The fatigue-creep rateequations with varied exponents inF of C region are established. The different deformation mecha-nisms in F or C regions are indicated

Introduction of creep-rupture testing capability and proficiency at Baosteel

To characterize the long-term service performance and predict the lifetime of heat-resistant steel applications,more than 300 test pieces can be loaded simultaneously at Baosteel to determine the relationship between stress and rupture duration at specific temperatures. In addition, versatile testing methods have been established that evaluate creep-rupture, stress-relaxation, and creep-fatigue, as well as state-of-the-art tensile testing at hyper-high temperatures exceeding 2 100 ~2. Based on the Larson-Miller method, according to API 530, the accumulation of large volumes of data with respect to stress, temperature, and rupture time of actually tested steels ensures the reliability of predicting the allowable working stress over 100 000 h at any given temperature. For examples ,the stress tendencies against rupture time of T/P91 and T/P92 steel tubes, as obtained by Baosteel, are consistent with those from NIMS and ECCC. As an accredited provider of the Proficiency Testing schemes in ISO/ IEC 17025 and ISO/IEC 17043, Baosteel has periodically organized national proficiency testing with respect to creep and rupture, thereby providing an efficient way for all parties to assess their technical competence when performing uniaxial creep testing by the methods in ISO 204 and ASTM E139.

Experimental investigation of creep behavior of clastic rock in Xiangjiaba Hydropower Project

There are many fracture zones crossing the dam foundation of the Xiangjiaba Hydropower Project in southwestern China. Clastic rock is the main media of the fracture zone and has poor physical and mechanical properties. In order to investigate the creep behavior of clastic rock, triaxial creep tests were conducted using a rock servo-controlling rheological testing machine. The results show that the creep behavior of clastic rock is significant at a high level of deviatoric stress, and less time-dependent deformation occurs at high confining pressure. Based on the creep test results, the relationship between axial strain and time under different confining pressures was investigated, and the relationship between axial strain rate and deviatoric stress was also discussed. The strain rate increases rapidly, and the rock sample fails eventually under high deviatoric stress. Moreover, the creep failure mechanism under different confining pressures was analyzed. The main failure mechanism of clastic rock is plastic shear, accompanied by a significant compression and ductile dilatancy. On the other band, with the determined parameters, the Burgers creep model was used to fit the creep curves. The results indicate that the Burgers model can exactly describe the creep behavior of clastic rock in the Xiangjiaba Hydropower Project.

Experimental determination of mechanical properties and short-time creep of AISI 304 stainless steel at elevated temperatures

The high temperature properties of AISI 304 stainless steel were studied. Basic data about the employed experimental equipment, testing procedures, and specimen geometry were given. The experimental setup was used to obtain stress-strain diagrams from tensile tests at room temperature as well as several elevated temperatures. Furthermore, the specimens were subjected to short-time creep tests at various temperatures. Stress levels for creep testing were established as a percentage of yield stress. The results indicate that at lowered temperatures and lower stress levels, AISI 304 stainless steel can be used as a sufficiently creep resistant material.

TENSILE PROPERTIES AND CREEP RESISTANCE OF AZ91 ALLOY CONTAINING ANTIMONY

Small amount of antimony addition to the Mg-9Al-0.8Zn-0.2Mn(AZ91) alloy results in the obvious increase of tensile strength at both ambient and elevated temperatures. The creep resistance at the temperatures up to 200°C is also improved significantly by antimony addition. Microstructural observations revealed that the addition of antimony modifies morphology of the β(Mg17Al12) phase and causes the formation of some rod-shaped precipitates Mg3Sb2 at grain boundaries. These precipitates have high thermal stability and play an important role for strengthening grain boundaries at elevated temperatures.

Study of electromagnetic and acoustic emission in creep experiments of water-containing rock samples

Based on biaxial shear creep tests conducted on rock samples with different water contents, we present the results of our study on the regularities of electromagnetic and acoustic emission during the process of creep experiments in which we have analyzed the contribution of water to the occurrence of electromagnetic radiation. The result shows that in the creep-fracturing course of rock samples, when the water content increases, the initial frequency and amplitude of electromagnetic and acoustic emission also increases, but at a decreasing growth rate caused by loading stress. This can be used as a criterion for the long-term stability of rock masses under conditions of repeated inundation and discharge of water.

Microstructural evolution and creep properties of Mg-4Sn alloys by addition of calcium up to 4 wt.%

The effects of addition of calcium up to 4 wt.% on the microstructure and creep properties of Mg-4 Sn alloys were investigated by the impression creep test. Impression creep tests were performed in temperature range between 445 and 475 K under normalized stresses σ/G(where σ is the stress;G is the shear modulus) between 0.0225 and 0.035. Optical microscopy and scanning electron microscopy were used to study the microstructure of samples. It is observed that the addition of Ca more than 2 wt.% suppresses less stable Mg Sn2 phase, and instead forms more thermally stable phases of Ca-Mg-Sn and Mg2 Ca at the grain boundaries which improve the creep resistance of Mg-4 Sn alloys. According to the stress exponents(6.04<n<6.89) and activation energies(101.37 k J/mol<Q<113.8 k J/mol) which were obtained from the impression creep tests, it is concluded that the pipe diffusion climb controlled dislocation creep is the dominant creep mechanism.

DETERMINATION OF CREEP PARAMETERS FROM INDENTATION CREEP EXPERIMENTS

The possibilities of determining creep parameters for a simple Norton law material are explored from indentation creep testing. Using creep finite element analysis the creep indentation test technique is analyzed in terms of indentation rates at constant loads. Emphasis is placed on the relationships between the steady creep behavior of indentation systems and the creep property of the indented materials. The role of indenter geometry, size effects and macroscopic constraints is explicitly considered on indentation creep experiments. The influence of macroscopic constraints from the material systems becomes important when the size of the indenter is of the same order of magnitude as the size of the testing material. Two methods have been presented to assess the creep property of the indented material from the indentation experimental results on the single-phase-material and two-phase-material systems. The results contribute to a better mechanical understanding and extending the application of indentation creep testing.

Quantifying Creep of Concrete Filled Steel Tubes

Using age adjusted effective modulus(AAEM)method,creep of concrete filled steel tube(CFST)member was formulated considering of creep coefficient and aging coefficient.Ten CFST specimens were tested including eight for creep and two for shrinkage.The experimental result was compared with the computed result using AAEM in which the creep coefficient was taken from calibration of ACI model based on experimental result on sealed concrete,and aging coefficient was supplied from relaxation test on sealed concrete specimen.Furthermore,the creep of CFST member was analyzed using author's own subroutine to input concrete properties through user programmable feature(UPF)in ANSYS software.Comparison was made on authors' own experimental database,some existing experimental results,and results from AAEM and numerical analysis.Finally,the conditions of applicability of AAEM method are put forward,and numerical approach to compute creep of CFST specimen is delineated.

Shear creep parameters of simulative soil for deep-sea sediment

Based on mineral component and in-situ vane shear strength of deep-sea sediment, four kinds of simulative soils were prepared by mixing different bentonites with water in order to find the best simulative soil for the deep-sea sediment collected from the Pacific C-C area. Shear creep characteristics of the simulative soil were studied by shear creep test and shear creep parameters were determined by Burgers creep model. Research results show that the shear creep curves of the simulative soil can be divided into transient creep, unstable creep and stable creep, where the unstable creep stage is very short due to its high water content. The shear creep parameters increase with compressive stress and change slightly or fluctuate to approach a constant value with shear stress, and thus average creep parameters under the same compressive stress are used as the creep parameters of the simulative soil. Traction of the deep-sea mining machine walking at a constant velocity can be calculated by the shear creep constitutive equation of the deep-sea simulative soil, which provides a theoretical basis for safe operation and optimal design of the deep-sea mining machine.

Creep properties and a creep equation of delay outburst coal and its adjacent mudstone

The study of the creep properties of coal and its adjacent mudstone is very important for understanding the mechanism of delay outburst coal. The samples of delay outburst coal and its adjacent mudstone collected from Yongshanqiao mine were used to carry out triaxial creep tests. The influence of confining pressure and axial compression on the creep test was analyzed. An accelerated creep model was constructed in parallel with a nonlinear viscous component and plastic component. It is connected with the traditional Burges creep model in series. A creep model which can describe the nonlinear viscoelastic-plastic creep model of rock was established and the corresponding creep equation was derived.According to the results of the creep test, the related parameters of the equation were fitted. The results show that, under the same confining pressure, instantaneous creep strain, creep strain of deceleration phase and constant rate creep of the coal and its adjacent mudstone are increased with an increase in the deviatoric stress. But at the same axial pressure, all of the above decrease with an increase of confining pressure. The duration time of the deceleration creep phase increases with the increase in the deviatoric stress. The theoretical values of the creep equation are in good agreement with the experimental results. It indicates that the creep properties of the delayed outburst coal and its adjacent mudstone can be well described by the creep model established in this paper.

Effects of stress conditions on rheological properties of granular soil in large triaxial rheology laboratory tests

In order to study the rheological properties of red stone granular soil,a series of rheological experiments were executed on large tri-axial rheological apparatus.Under 100,200 and 300 kPa confining stress conditions,the rheological tests were carried out.These experiment results showed that the stress conditions,especially the stress level were the critical influencing factors of the rheological deformation properties.Under the low stress level(S=0.1),the granular soil showed the elastic properties,and there was no obvious rheological deformation.Under the middle stress level(0.2<S≤0.6),creep curves showed the linear viscoelastic rheological properties.However,under the high stress level(S>0.8) creep curves showed the non-linear viscous plastic rheological properties.Especially,under the stress level of S=1.0,the accelerated rheological phase of creep curves occurred at early time with a trend of failure.The stress level had obvious effects on the final rheological deformation of the soil sample,and the final rheological deformation increments nonlinearly increased with stress level.The final rheological deformation increment and step was little under low stress level,while it became large under high stress level,which showed the nonlinearly rheological properties of the granular soil.The confining pressure also had direct effects on final rheological deformation,and the final rheological deformation linearly increased with confining pressure increments.