Experimental research on creep behaviors of sandstone under uniaxial compressive and tensile stresses

The consideration of time dependence is essential for the study of deformation and fracturing processes of rock materials, especially for those subjected to strong compressive and tensile stresses. In this paper, the self-developed direct tension device and creep testing machine RLW-2000M are used to conduct the creep tests on red sandstone under uniaxial compressive and tensile stresses. The short-term and long-term creep behaviors of rocks under compressive and tensile stresses are investigated, as well as the long-term strength of rocks. It is shown that, under low-stress levels, the creep curve of sandstone consists of decay and steady creep stages; while under high-stress levels, it presents the accelerated creep stage and creep fracture presents characteristics of brittle materials. The relationship between tensile stress and time under uniaxial tension is also put forward. Finally, a nonlinear viscoelastoplastic creep model is used to describe the creep behaviors of rocks under uniaxial compressive and tensile stresses.

Microstructure,tensile properties and compressive creep resistance of Mg-(5-8.5)%Sn-2%La alloys

The microstructure,tensile properties and compressive creep resistance of permanent-mould cast Mg-(5-8.5)%Sn-2%La (mass fraction) alloys were investigated.The results show that Mg-(5-8.5)%Sn-2%La alloys are all composed ofα-Mg phase, Mg_2Sn and Mg-La-Sn compounds.Compared with those of Mg-5%Sn binary alloy,the grain size and the content of Mg_2Sn compound in Mg-5%Sn-2%La alloy are decreased.With the increase of Sn content in Mg-(5-8.5)%Sn-2%La alloys,the content of Mg_2Sn compound increases,while that of Mg-Sn-La compound changes little.In addition,the investigation suggests that the thermally stable Mg-Sn-La and Mg2Sn compounds can improve the tensile properties and compressive creep resistance of the alloys.

TENSILE STRENGTH AND CREEP RESISTANCE OF Mg-9Al-1Zn BASED ALLOYS WITH CALCIUM ADDITION

Small amount of calcium addition to the Mg-9Al-0.8Zn-0.2Mn (AZ91) alloy resulted in obvious influence on mechanical properties. The yield strength of the alloys increased with the increase of Ca addition and the maximum strength was obtained from the alloy containing 0.15% of Ca. The creep resistance at the temperatures between 150-220°C was also significantly increased with Ca addition. The creep rate (at 200°C, 50 MPa) of the alloy with 0.15% Ca addition was one order of magnitude lower than that of the base alloy (AZ91). Microstructural observations revealed that the addition of calcium refined the microstructure and enhanced the thermal stability of the β precipitates, which accounted for the improvement of creep resistance at high temperatures.

Effect of rare earths on phase transformation in as-cast ZA27 alloy dur-ing compressive creep

The effect of the mixed rare earths of Ce on the phase transformation in ascast ZA 27 alloy during compressive was investigated under 37 MPa and at 160 deg C by X-raydiffraction technique and SEM. The results showed that the as cast microstructure of ZA 27-RE alloyconsisted of a dendritic Al-rich alpha' surrounded by Zn-rich beta' phase, interdendritic epsilonphase and Zn-rich eta phase together with a complex Z phase which was a complex constitute compound,(RE,Cu)Al_5Zn_(16), dispersed in crystal interfaces or branch crystal interfaces and stable duringcompressive creep test at 160 deg C. The phase transformations of ZA 27-RE alloy, decomposition ofbeta' phase arid four transformation, were delayed by the addition of rare earths, also the lamellarstructure and the spheroidized structure in ZA 27-RE alloy were finer than in ZA 27 alloy duringcompressive creep test at 160 deg C at the same creep time, and the compressive creep resistance ofZA 27-RE alloy was higher than that of ZA 27 alloy.

Uniaxial tension and tensile creep behaviors of EPS

The mechanical behavior of EPS(Expanded polystyrene) with three densities at room temperature and under tension loading was studied.The results show that EPS material is characterized by brittle behavior in the tension tests,and tensile properties of EPS increase with the increase of density.Volume fraction has no a significant effect on the modulus of these foams.The tensile creep strain increases with stress for EPS with same density,indicating that the creep behavior is of the stress dependency.And the creep behavior of EPS exhibits density dependency,which the creep strain decreases with densities for a fixed stress value.Moreover the creep behavior under the constant tension load is well in coincidence with the three-parameter solid model.

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.

Effect of the precipitation state on high temperature tensile and creep behaviors of Mg-15Gd alloy

Due to the effective precipitation strengthening effect of the β phase, Mg-Gd alloys exhibit excellent room temperature mechanical behaviors. However, when served at high temperatures, the metastable β phase will transform to other phases, resulting in severe performance degradation. In this study, we investigated the effect of precipitation state achieved by different heat treatments on high temperature tensile and creep behaviors of the Mg-15Gd alloy by comparing the properties of the as-cast, solid-solutioned(T4) and peak-aged(T6) alloys. The results showed that the tensile mechanical properties of the T6 alloy were always highest from room temperature to 300 ℃, in spite of an abnormal strength increase with temperature existed in the T4 alloy. For tensile creep properties, the T6 alloy exhibited the lowest steady creep rate below 235 ℃ while the T4 alloy possessed the best properties above 260 ℃. Microstructure characterization revealed that the transition was caused by the stress-promoted precipitation of β phase in the T4 alloy and rapid phase transformation in the T6 alloy at high temperatures. At 260 ℃, the calculated stress exponent n was 3.1 and 2.8 for the T4 and T6 alloys, respectively, suggesting the creep deformation mechanism was dislocation slip, which was further confirmed by the microstructure after creeping. Our findings can provide new insights into the heat treatment process of Mg-Gd alloys served at high temperatures.

TENSILE PROPERTIES AND CREEP RESISTANCE OF Fe_3Al－BASED ALLOYS CONTAINING NIOBIUM

Small amount(0.5-0.8at.%) of niobium addition to the multicomponent alloys based on Fe3Al(Fe-28Al-5Cr-0.5Mo-0.05Zr-0.05B-0.1Ce) results in the microstructural refinement and the increase in the yield strength at temperatures up to 600℃,but is not beneficial to room temperature ductility.Results of microanalysis indicate that the addition of niobium causes the formation of precipitates with complex compositions which strengthen both matrix and grain boundaries and enhance the creep resistance of the alloys.

Microstructures,mechanical properties and compressive creep behaviors of as-cast Mg-5%Sn-(0-1.0)%Pb alloys

The microstructures,tensile properties and compressive creep behaviors of Mg-5%Sn-(0-1.0)%Pb (mass fraction) alloys were studied. The microstructures of the Mg-Sn-Pb alloys consist of dendritic α-Mg and Mg2Sn phase. The addition of Pb can refine the size of Mg-2Sn phase and grain size,reduce the amount of Mg-2Sn phase at grain or inter-dendrite boundaries and change the distribution of Mg-2Sn phase. Pb exists in the Mg-2Sn phase or dissolves in α-Mg matrix. The mechanical properties of the tested alloys at room temperature are improved with the addition of Pb. When the Pb content is over 0.5%,the mechanical properties are decreased gradually. The Mg-5%Sn-0.5%Pb shows the best ultimate tensile strength and elongation,174 MPa and 14.3%,respectively. However,the compressive creep resistance of the Mg-Sn-Pb alloys is much lower than that of the Mg-Sn binary alloy at 175 °C with applied load of 55 MPa,which means that Pb has negative effects on the compressive creep resistance of the as-cast Mg-Sn alloys.

Effect of Zr on behavior of compressive creep in as cast ZA27 alloy

The effect of Zr on the behavior of compressive creep in as cast ZA27 alloy was investigated in the temperature range of 20160 ℃ and under different compressive stresses in the range of 50137.5 MPa with special apparatus. The results show that the primary compressive creep strains and steady creep rates of ZA27 Zr alloy and ZA27 alloy increase with increasing temperature and stress. However, the primary compressive creep strain and steady creep rate of the ZA27 Zr alloy are lower than that of the ZA27 alloy below 100 ℃, but higher at 160 ℃. The compressive creep behaviors in both ZA27 Zr alloy and ZA27 alloy obey an empirical equation ln t=C-n ln σ+ Q/RT , and the exponent stress n is 3.63 for ZA27 Zr alloy and 3.46 for ZA27 alloy, respectively, the activation energy Q is 87.32 kJ/mol for ZA27 Zr alloy and 81.09 kJ/mol for ZA27 alloy. Different material structural constants are associated with different compressive creep behaviors in the alloy. The compressive creep rate in the alloy is controlled by the lattice diffusion of zinc and dislocation limb.

Tensile and creep properties of Ti-600 alloy

Ti-600 is one of the high performance titanium alloys used at 600 ℃,which was developed in Northwest Institute for Nonferrous Metal Research(NIN) in China. The tensile and creep properties of Ti-600 alloy with different thermal treatment conditions were investigated. The results indicate that Ti-600 alloy possesses favorite comprehensive properties solution-treated at 1 020 ℃ for 1 h,then air-cool,and aged at 650 ℃ for 8 h,finally air-cooling,especially possesses quite good creep resistance. The residual deformation is less than 0.1% for the alloy exposed at 600 ℃ for 100 h with the stress of 150 MPa,and the bimodal microstructures of the alloy are almost the same as that of the alloy treated by duplex thermal treatment,only needle primary α phases became relatively thicker and coarsened. The ultimate strength and the elongation of the alloy tested at ambient temperature are 1 080 MPa and 12%,respectively;while at 600 ℃,they are 690 MPa and 16%,respectively. The ductility of the alloy tested at room temperature is no less than 5% after thermal exposing at 600 ℃ for 100 h.

EFFECTS OF TENSILE STRESS AND IMPURITIES ON CREEP ACTIVATION ENERGY OF PLATINUM

The effect of tensile stress and trace amounts of impurities on the creep activation energyof platinum was studied and the creep mechanism was disscussed.

MICROSTRUCTURAL EVOLUTION OF AS-CAST ZA27 ALLOY DURING COMPRESSIVE CREEP

Phase transformation and micro structural change of an as-cast ZA27 alloy were investigated during compressive creep by X-ray diffraction technique, SEM and TEM. Compressive creep induced decomposition of α metastable η' phase and a four-phase transformation, α + ε → T' + η and T' → θ, occurred during the compressive creep testing. The occurrence of negative creep in the alloy resulted from the volume expansion caused by the four-phase transformation. A micro structural change was also observed from a lamellar structure into a spheroidized structure in higher creep strain of tested specimens. It provided evidence of compressive creep induced phase transformation which occurred in ageing process.

Investigation of Compressive Refractory Creep

Creep of refractories at high temperatures may have significant influences on the thermomechanical behavior of refractory linings in industrial vessels,and thus the consideration of creep in finite element modeling of industrial vessels is necessary. The present paper introduces an advanced high temperature compressive creep device and a proper inverse procedure using Levenberg-Marquardt algorithm to identify Norton-Bailey creep parameters. In addition,finite element thermomechanical modeling results of a RH-snorkel are presented to illustrate the impact of creep,creep formulations and creep models on the circumferential stress and joint opening of the working lining.

Negative creep during compressive creep of as-cast ZA27 alloy

The negative creep during compressive creep deformation of as cast ZA27 alloy was investigated at the temperature range of 20160 ℃ and at compressive stress levels from 50137.5 MPa with special apparatus. Results show that the negative creep in the alloy occurred respectively at 20 ℃ (50 MPa, 87.5 MPa and 100 MPa), 60 ℃(50 MPa and 87.5 MPa) and 100 ℃(50 MPa). According to the phase transformation and theoretical analysis, the negative creep resulted from volume expansion caused by four phase transformation α+ε→T′+η in the alloy. The theoretical analysis is consistent with the experiment results. And the values of negative creep depended on the difference between the compressive creep deformation and the volume expansion.

Linear extrapolation for prediction of tensile creep compliance of polyvinyl chloride

The universal creep equation is successful in relating the creep （ε） to the aging time （te）, coefficient of retardation time （β）, and intrinsic time （t0）. This relation was used to treat the creep experimental data for polyvinyl chloride （PVC） specimens at a given stress and different aging times. The βgs found by the “polynomial fitting” method in this work instead of the “middle-point” method reported in the literature. The unified master line was constructed with the treated data and curves according to the universal equation. The master line can be used to predict the long-term creep behavior and lifetime by extrapolating.

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.

Investigation on relations between grain crushing amount and void ratio change of granular materials in one-dimensional compression and creep tests

Grain crushing plays an important role in one-dimensional (1D) compression and creep behaviors of granular materials under high stress. It is clear that the macro-properties of granular materials are closely related to the micro-fracture properties of grains in 1D compression and creep tests. In this paper, a series of 1D compression and creep tests were performed on Ottawa sand to investigate the deformation and grain crushing properties of granular materials, and it shows that the void ratio is correlated to the grain crushing amount (the quantity of crushed grains) for granular materials subjected to grain crushing. The test results, combining with the existing test data related to grain crushing of granular materials, were used to verify the relation. Moreover, the implications of these relations on the yield of granular material, and the equivalent effect of stress and time in changing soil fabric are presented.

Nonlinear creep damage model for soft rock under uniaxial compression

A nonlinear creep-damage model for soft rock under uniaxial compression waspresented,which takes into account both nonlinear creep and damage growth with time.The model is based on the hardening theory,The model is validated through comparisonwith experimental results.

MECHANICAL PROPERTIES OF FINE PITCH DEVICES SOLDERED JOINTS BASED ON CREEP MODEL

Nonlinear analyses of quad flat package （QFP） on printed circuit board （PCB） assemblies subjected to thermal cycling conditions are presented. Two different solders are considered, namely, Sn37Pb and Sn3.5Ag. The stress and strain response of fine pitch devices soldered joints was investigated by using finite element method based on Garofalo-Arrheninus model. The simulated results indicate creep distribution of soldered joints is not uniform, the heel and toe of soldered joints, the area between soldered joints and leads are the creep concentrated sites. The similar phenomena of stress curves simulated based on Garofalo-Arrheninus model and Anand equations is confirmed, and the creep strain value of Sn3.5Ag soldered joints is lower than that of Sn37Pb soldered joints. Thermal cycling results show that Sn3.5Ag strongly outperforms Sn37Pb for QFP devices under the studied test condition. This is well matched with the experimental outcome analyzed. In addition, the soldered devices were tested by micro-joints tester, the tensile strength of Sn3.5Ag soldered joints is found to be higher than that of Sn37Pb soldered joints. By analyzing the fracture microstructure of soldered joints, it is found that fracture mechanism of Sn3.5Ag soldered joints is toughness fracture, while fracture mechanism of Sn37Pb soldered joints includes brittle fracture and toughness fracture. The results of this study provide an important basis of understanding the mechanical properties of fine pitch devices with traditional Sn37Pb and Sn3.5Ag lead-free soldered joints.