Creep Stress Levels Research Articles

Shear creep deformation of rock fracture distrubed by dynamic loading

The long-term stability of jointed rock masses is usually dominated by fault activation, which may be triggered by the dynamic disturbance generated by blasting during mining activities, leading to the occurrence of disasters such as landslides in open-pit and rockbursts in deep mining. The initial stress and dynamic disturbance are key factors that strongly affect the shear creep behavior of rock fractures. In this work, the shear failure instability of rock fractures of sandstone under creep-impact loading was experimentally investigated by using a creep-impact test machine, which allows for applying creep loading and an additional dynamic disturbance on rock fractures. Three stages of shear creep deformation, creep strain rate, and time-to-failure are examined under different creep stress levels and impact energies. Experimental results show that the tangential and normal creep rates increase with the increase of creep stress and impact energy, but the increment of tangential creep rate is higher than that of the normal creep rate. The time-to-failure of the creeping specimen is shortened under high creep stress and large impact energy, while the time-to-failure after the last dynamic disturbance of the specimen is determined by the total impact energy and creep stress level. By using high-speed photography, it is found that the failure types of rock depend on the magnitude of impact energy and creep stress level; that is, rock mainly slides with low stress levels and shears off with high stress levels. In addition, under different impact energy and creep stress levels, the variation of height is between 0.38 and 0.52, while the defined fracture factor, which describes the degree of failure of serrations, is between 0.30 and 0.54. The findings can provide deep insight into the fault sliding mechanism caused by mining activities, which provides theoretical support for the safe mining of ore in fault fracture zones.

Experimental Study on the Accelerating Creep of Rock Under Dry and Wet Conditions Based on a New Test Method

Creep tests under constant stress are among the most important tests for investigating the time-dependent behavior of rock. In previous studies, creep tests were conducted under various loading and environmental conditions; however, it was very difficult to demonstrate accelerating creep at failure at stresses less than 50% of strength in conventional creep tests. In this study, a new test method is proposed that combines an alternating loading-rate test and a creep test in the post-failure region. The accelerating creep as well as the loading-rate dependence of strength were successfully obtained simultaneously from a single specimen. This test method was applied to three rock types under dry and wet conditions. The results of Sanjome andesite under both dry and wet conditions, Kimachi sandstone under dry conditions, and Tage tuff under dry conditions showed a similar relationship between creep strain, creep strain rate, and residual time (i.e., creep lifetime minus elapsed time), from high to low creep stress levels, and a close relationship between accelerating creep and the loading-rate dependence of strength. Further, the mechanism responsible for the time dependence was found not to change significantly from high to low stress levels. In contrast, different results were obtained when high and low creep stress levels were compared in Kimachi sandstone and Tage tuff under wet conditions. The larger increase in creep strain with increasing creep strain rate at lower creep stress levels was related to the shape of the stress–strain curve in the post-failure region.

Long-term mechanical properties of sludge-cured lightweight soil under the superimposed effects of drying-wetting and freezing-thawing

ABSTRACT Sludge-cured lightweight soils have unique advantages in roadbed treatment due to their properties such as low density and high strength. Its long-term mechanical law based on the superposition of drying-wetting and freezing-thawing (D-W-F-T) is studied through creep experiment. The test results show that: with the increase of the number of D-W-F-T, the deformation of the soil gradually increases, and the deformation tends to be stable when it reaches a certain number of times; the stress-strain isochronous curves are obviously nonlinear in general; The long-term strength increases with the increase of density and confining pressure, and decreases with the increase of the number of D-W-F-T; by comparing the isochronous stress-strain curve cluster method and the steady state creep rate vs. The comparative analysis suggests that the steady state creep rate versus stress level curve method be used to determine the long term strength.

Creep-Fatigue Life Evaluation for Grade 91 Steels with Various Origins and Service Histories

Grade 91 steel is widely used in the boilers and piping of thermal power plants. There has been significant research interest in understanding the variations in creep characteristics among different heats of this steel for effective plant management. In recent years, thermal power plants have been subjected to frequent load changes and startup/shutdown to adjust power supply and demand and stabilize frequencies. These operational shifts have raised concerns regarding the potential for creep-fatigue damage in high-temperature components. Therefore, this research focuses on creep-fatigue properties of Grade 91 steel and their predictability. Tensile, creep, strain-controlled fatigue, and strain-controlled creep-fatigue tests were performed on six Grade 91 steels with different heats and/or histories, and the characteristics in each test were compared. As a result, the variations in creep-fatigue life among the materials were found to be correlated with the difference in creep characteristics and stress level during stress relaxation. Furthermore, the study involved a comparative assessment of the predictive performance of creep-fatigue life using five different approaches: time fraction, classical ductility exhaustion, modified ductility exhaustion, energy-based, and hybrid approaches. Among these approaches, the hybrid approach, based on inelastic strain energy density at fracture formulated as a function of inelastic strain rate, exhibited the most accurate predictive performance.

Power Law Creep Behavior Model of Third Generation Lead-Free Alloys Considering Isothermal Aging

Abstract In realistic applications, the solder joint is continually subjected to thermal-mechanical stress due to the difference in the coefficient of thermal expansion between the printed circuit board substrate and the electronic packaging components. Creep and fatigue processes were the most common causes of failure in electronic assemblies. Under isothermal aging, creep deformation becomes more prominent. The aged microstructure was recognized by intermetallic coarsening and the appearance of intergranular fracture generated by dynamic recrystallization in the bulk solder joint. In this study, the influence of Bi content on the creep behaviors of solder joints was investigated under various aging conditions. Three lead-free solder alloys, including SAC305, SAC-3Bi, and SAC-6Bi, are tested at room temperature. For each alloy, preliminary micro-indentation tests were conducted to define three stress levels for distinct aging conditions. After each test, displacement versus time data was gathered. A novel approach based on an empirical model was developed to systematically examine the development of the steady-state creep rate. A power dependency prediction model was developed to investigate the relationship between creep strain rate and stress levels. The steady-state creep rate of SAC305 is significantly higher than that of SAC-Bi alloys owing to the presence of bismuth (Bi) in the solid solution at room temperature. The creep properties showed less variation after 100 h of aging. SAC-Bi alloys showed less coarsening of the intermetallic compounds precipitates after aging than SAC305. In the SAC-Bi solder alloys, combinations of precipitate and solid solution hardening mechanisms were observed, while Ag3Sn particles were the dominant strengthening mechanism in the SAC305 alloy system.

Triaxial creep damage characteristics of sandstone under high crustal stress and its constitutive model for engineering application

AbstractThe creep characteristics of rock under high crustal stress are of important influence on the long‐term stability of deep rock engineering. To study the creep characteristics and engineering application of sandstone under high crustal stress, this study constructed nonlinear creep damage (NCD) constitutive mode based on the triaxial graded loading‒unloading creep test of sandstone in the Yuezhishan Tunnel. A numerical NCD constitutive model and a breakable lining (BL) model were developed based on FLAC3D and then applied to the stability analysis of the Yuezhishan Tunnel. Based on the creep test results of sandstone, a power function of creep rate and stress level was constructed, by which the long‐term strength was solved. The results show that the long‐term strength of the red sandstone based on the related function of the steady‐state creep rate and stress level is close to the measured stress value in engineering. The NCD model considering damage factors reflects the instantaneous and viscoelastic plasticity deformation characteristics of the red sandstone. The numerical NCD constitutive model and the BL model can reflect surrounding rock deformation characteristics and lining failure characteristics in practical engineering. The research results provide theoretical references for long‐term stability analysis of rock engineering and the deformation control of surrounding rock under high crustal stress.

Dislocation structures after creep in an Al-3.85 %Mg alloy studied using EBSD-KAM technique

The electron backscatter diffraction (EBSD) technique is used to investigate the dislocation structures formed after steady-state creep deformation of an Al-3.85%Mg alloy. This material is crept at two different stress levels, corresponding to the so-called power-law and power-law breakdown regimes. The results show that, regardless of the creep stress level, the strain tends to localize, leading to the formation of intragranular bands. The thickness of such bands is larger when the material is tested at loads corresponding to the power-law breakdown. This suggests enhanced diffusion by dislocation pipes.

Creep Characteristics of Soil in the Sliding Zone of Huangtupo Landslide

The reservoir water level in the Three Gorges Reservoir (TGR) of the Yangtze River is adjusted between 145 m and 175 m throughout the year. The landslide below the reservoir water level bears periodic dynamic seepage pressure on the basis of the original steady-state water pressure. In the process of rising reservoir water levels in particular, the effective stress on the sliding zone soil below the reservoir water level line is reduced, and the sliding zone soil shows an unloading state. In order to study the creep characteristics of landslides in a reservoir area, direct shear creep tests of the sliding zone soil in the Huangtupo landslide in an unloading state were carried out in this study. The test results show that the creep characteristics of the sliding zone soil are obvious. The creep curve of the sliding zone soil presents an attenuation creep stage with low shear stress, which is mainly manifested as an elastic creep. However, with the increase in the shear stress, the creep curve of the sliding zone soil presents a steady creep stage, which is mainly manifested as viscoelastic creep. The nonlinear creep characteristics of the sliding zone soil are related to the creep time, stress level, and soil porosity. The longer the creep time, the greater the stress level and the denser the soil, the more obvious the nonlinear creep characteristics of the sliding zone soil. In this study, the Burgers model was used to fit the creep curve of the sliding zone soil, and the fitting effect was good, which indicates that the Burgers model can sufficiently describe the unloading state creep characteristics of the sliding zone soil in the Huangtupo landslide.

Nonlinear creep properties and time-to-failure prediction of sandstone under saturated and dry conditions

The moisture content of rock significantly influences its time-dependent deformation behavior. This paper studied the creep behavior of sandstone under saturated and dry conditions under different driving stress ratio (not the same creep stress level).Their impacts on the time-to-failure (TTF) of sandstone were also investigated. The results show that under an identical driving stress ratio, the dry specimens' creep strain and steady-state creep rate are relatively higher than those of the saturated ones. Nevertheless, the water-saturated specimen has a shorter creep strain path and TTF due to the softening effect of water. It is evident that in terms of creep deformation and creep rate, the impact of creep stress is much more significant than that of moisture content; however, in terms of creep TTF, the moisture content of sandstone plays a dominant role. The rationality of the generalized Kelvin, Burgers, and power-law creep models were evaluated based on the experimental data. The power-law model was found to be capable of fitting the test data, and better predicting sandstone creep deformation than other two models. Finally, based on the assumption of a critical strain value εcri, a method for predicting creep TTF from limited laboratory data was proposed, providing a foundation for incorporating TTF prediction into continuum numerical simulations.

A Fractional Order Creep Damage Model for Microbially Improved Expansive Soils

Microbial Induced Calcite Precipitation method was used to improve the expansive soils of Nanning, Guangxi. The nonlinear shear creep behavior of microbially improved expansive soil was studied by triaxial consolidation drainage shear test. The results show that when the expansive soil was applied a small partial stress, the creep curve of soil exhibits transient deformation and decay creep. When the partial stress reaches a certain value, there is decay creep, steady-state creep and accelerated creep successively showed on the creep curve. The stress-strain isochronous curves reflect there are obvious nonlinear characteristics in the creep process of improved expansive soils. The degree of this nonlinearity is related to the creep time and stress level. The longer the creep time as well as the higher the stress level, the higher the degree of nonlinearity. Based on the fractional calculus theory and statistical damage theory, the probability density function of Weibull distribution was introduced, and the damage degradation of soft component viscosity coefficient was considered. As a result, a fractional-order damage creep model which can describe the shear creep evolution of microbially improved expansive soils is established. Compared with the Kelvin creep model of integer order and the Burgers creep model of fractional order, the fractional order damage creep model has not only better comparative evaluation results but also more higher computational accuracy. It indicates that the fractional-order damage creep model can better describe the whole process of shear creep in microbially improved expansive soils. The above findings provide a theoretical basis for the study of deformation analysis of microbially improved expansive soils under long-term loading.

Long‐Term and Short‐Term Creep Characteristic Analysis for HTPB Propellant

AbstractA solid rocket motor is in storage for most of its life cycle, and propellant creep occurs subjected to gravity load. To analyse the creep characteristics of HTPB propellant during long‐term storage, MSC Marc is used to calculate the maximum von Mises stress of the motor under horizontal and vertical storage conditions. Based on the numerical results, the short‐term reciprocating creep test and long‐term creep test are designed and carried out. The results illustrate that the process of propellant creep from deformation to final failure can be divided into four stages: instantaneous deformation stage, attenuation creep stage, steady‐state creep stage and accelerated creep failure stage. When the creep stress level is less than 0.1 MPa, the creep characteristics tend to be stable. When the creep strain exceeds a certain critical value, the propellant will enter the steady‐state creep stage. The damage caused by creep rises with the increase of creep time and stress. When the maximum creep strain is less than 10 %, the creep damage can be fully recovered. The splicing curve of short‐term creep test data is in good agreement with that of long‐term creep test data. A mechanical long‐term creep test method is provided, and the parameters of the modified Burgers model are fitted. The text methods and conclusions can provide a reference for the evaluation of structural integrity and storage life of solid motor.

Undrained creep behavior of CO2 hydrate-bearing sand and its constitutive modeling considering the cementing effect of hydrates

A technique for carbon dioxide (CO2) capture and storage using CO2 hydrates where CO2 is stored as solid hydrates in the seabed ground, is attracting attention. Shallow sediments may be the most suitable seabed ground for CO2 hydrate storage because these unconsolidated soil sediments satisfy the limitation for the low-temperature condition. Hence, the deformation properties and long-term stability of gas hydrate-bearing sediments during and after gas storage must be investigated. In this study, a series of undrained triaxial creep tests were conducted on artificially made CO2 hydrate-bearing sand specimens to study the fundamental time dependent property of hydrate-bearing sediment. We extended an elasto-viscoplastic constitutive model by introducing a cohesion component and its degradation on surfaces and applied the proposed model to creep tests on gas hydrate-bearing sand.Three findings were obtained from the experiments and modelling. First, CO2 hydrate-bearing sand specimens showed accelerated creep behavior, which was characterized by the creep stress ratio level, regardless of the hydrate saturation. Second, creep accelerated under undrained conditions before the stress reached the critical state line obtained from the monotonic loading tests, and the stress ratio at the occurrence of acceleration creep was higher for specimens with a higher hydrate saturation. Third, the elasto-viscoplastic constitutive model which considered the cementing effect of hydrates was able to well reproduce the undrained creep behavior of hydrate-bearing sand with different hydrate saturations under relatively high creep stress levels.

Deformation recovery characteristics of asphalt pavement material on steel bridge decks under nondestructive conditions

The composition and properties of high-performance asphalt materials for steel bridge deck pavements are quite different. At present, research on the properties of steel bridge deck pavements mainly focuses on the strength law and damage behavior at the loading stage; however, little research exists on the deformation recovery characteristics of materials during unloading. In order to evaluate the deformation recovery ability of steel bridge deck pavement materials in the unloading stage, the repeated creep and step-load recovery test is herein put forward as an alternative to the deficient creep step-load test. The internal stress and residual strain of three typical steel deck pavement materials—a gussasphalt mixture, an epoxy asphalt mixture, and a stone mastic asphalt mixture—were tested. By comparing the results of CSR test and RCSR test, it is found that RCSR test is more suitable for steel bridge deck paving materials.The recovery modulus under three different creep stress levels was calculated to evaluate the ability of the three materials to drive deformation recovery. At the same time, based on the basic theory of viscoelasticity, the fitting parameters of creep compliance were determined, and the strain recovery rates of three kinds of materials under different interval times were calculated. The results reveal that the recovery modulus of the epoxy asphalt mixture is the largest, about 7 times and 4 times as much as stone mastic asphalt and gussasphalt, respectively, and the self-driving ability of epoxy asphalt mixture is better than that of gussasphalt and stone mastic asphalt. Moreover, within the same time interval, the strain recovery rate of the gussasphalt mixture is the fastest; in 3.6 s, the strain recovery rate can reach 82%.

Effects of the preparation, curing and hygrothermal conditions on the viscoelastic response of a structural epoxy adhesive

This paper addresses the viscoelastic behaviour of a commercially available cold-curing structural epoxy adhesive, under different preparation, curing and hygrothermal conditions. The main parameters studied were the preparation method (influence of the degassing and the temperature of the initial curing), the creep stress level, and the hygrothermal conditions. Tensile creep tests last up to 2400 h. Test results revealed that the preparation method has great influence on the tensile properties of the adhesive, particularly on the viscoelastic response where degassing and curing at 20 °C showed lower creep deformations. Specimens under 98% of relative humidity faced tertiary creep and then rupture. For the adopted levels of creep stress, the adhesive shows a linear creep behaviour, being parameterized using the Burgers and the modified Burgers equations.

Effect of temperature and filler volume fraction on the creep and recovery behaviour of MWCNT–COOH–reinforced polypropylene films

Nonlinear viscoelastic behaviour of –COOH functionalized multi-walled carbon nanotubes reinforced polypropylene nanocomposites is examined up to 2% MWCNT concentration. Isochronal creep and strain recovery experiments are conducted in dynamic mechanical analyser with oscillatory inputs. Creep stress level of 5 MPa is selected and successive creep and recovery experiments are performed at $$-\,20\,^\circ $$ C, $$25\,^\circ $$ C and $$50\,^\circ $$ C to analyse the effect of temperature and MWCNT concentration on strain developed during creep loading and strain recovery stages. A well-known Schapery nonlinear viscoelastic solid model with Zapas–Crissman viscoplastic term is fitted to the experimental observations. Stress-dependent nonlinear model parameters are obtained under temperature and MWCNT loading. Viscoelastic and viscoplastic strains are predicted at the end of recovery period. High temperatures and low concentrations of MWCNT are observed to cause higher viscoplastic strain. Strengthening MWCNT up to 1% improves the thermomechanical behaviour and viscoelastic recovery and decreases the viscoplastic strain in nanocomposites. Temperature-dependent compliance behaviour of nanocomposites is investigated with Williams–Landel–Ferry model and Arrhenius model using time–temperature superposition principle.

Prediction of triaxial drained creep behaviors of interactive marine-terrestrial deposit soils by fractal derivative

The mechanical properties of interactive marine-terrestrial (IMT) deposit soils differ from those of other deposits as a result of the complex sedimentary environments in which they are found. The creep behaviors of IMT deposits in Ningbo, China were studied via triaxial drained creep testing. The theory of fractal derivatives was then applied to develop a fractal merchant model, in which a fractal dashpot replaces the conventional Newtonian dashpot, to characterize the triaxial creep behaviors of IMT deposits. The model results indicate that Ningbo deposits demonstrate typical nonlinear creep behaviors. As the creep time and stress level increase, the creep process shows characteristics of transient creep attenuation and shear contraction. To validate the proposed fractal model, its predictive results were compared with the experimental results and those of conventional and fractional merchant models. It was found that the fractal merchant model results agree closely with the experimental results and that it was more accurate and efficient than the other models even though it applies the same number of parameters and has a simpler computational approach than the fractional merchant model.

Experimental and numerical investigation of cyclic creep and recovery behavior of bovine cortical bone

An experimental and numerical study of the creep behavior of the bovine metatarsal cortical bone is presented. The experimental procedure included the multiple cycle tensile creep-recovery tests at eleven creep stress levels between 20 and 120 MPa. The threshold values of stress above which viscoplastic strain and damage start to accumulate are determined. Furthermore, based on the experimental data, a one-dimensional constitutive model has been developed. We demonstrate that Abdel-Tawab's viscoelasticity/damage model combined with the viscoplastic model proposed by Zapas and Crissman can predict the nonlinear behavior of cortical bone tissue subjected to creep-recovery loading conditions. Within the framework of numerical investigations, an efficient algorithm for the integration of the proposed constitutive model at the material point level is derived and implemented into the finite element software ABAQUS using user subroutines. The computational algorithm shows a good capability to describe the tensile behavior of bovine cortical bone for the specific mechanical conditions analyzed.

Flexural Creep Behavior of High-Density Polyethylene Lumber and Wood Plastic Composite Lumber Made from Thermally Modified Wood.

The use of wood plastic composite lumber as a structural member material in marine applications is challenging due to the tendency of wood plastic composites (WPCs) to creep and absorb water. A novel patent-pending WPC formulation that combines a thermally modified wood flour (as a cellulosic material) and a high strength styrenic copolymer (high impact polystyrene and styrene maleic anhydride) have been developed with advantageous viscoelastic properties (low initial creep compliance and creep rate) compared with the conventional WPCs. In this study, the creep behavior of the WPC and high-density polyethylene (HDPE) lumber in flexure was characterized and compared. Three sample groupings of WPC and HDPE lumber were subjected to three levels of creep stress; 7.5, 15, and 30% of the ultimate flexural strength (Fb) for a duration of 180 days. Because of the relatively low initial creep compliance of the WPC specimens (five times less) compared with the initial creep compliance of HDPE specimens, the creep deformation of HDPE specimens was six times higher than the creep deformation of WPC specimens at the 30% creep stress level. A Power Law model predicted that the strain (3%) to failure in the HDPE lumber would occur in 1.5 years at 30% Fb flexural stress while the predicted strain (1%) failure for the WPC lumber would occur in 150 years. The findings of this study suggest using the WPC lumber in structural application to replace the HDPE lumber in flexure attributable to the low time-dependent deformation when the applied stress value is withing the linear region of the stress-strain relationship.

Evaluating Long‐Term Strength and Time to Failure of Sandstone with Different Initial Damage

Long‐term strength (LTS) of rock materials is important for the long‐term stability analysis and the failure prediction of structures in rock engineering. Numerous studies have been carried out on the LTS for various kinds of rock; however, the effects of initial damage on the LTS and creep failure time of rock have not been conducted. In the present study, the creep experiment with controllable initial damage state of rock was designed. Then, the LTS of rock specimens with different initial damage was determined by four methods (i.e., the isochronous stress‐strain curve method, the steady creep discriminated method, the volumetric strain inflexion point determined method, and the intersection of the steady creep rate method). The results show that, with the increase in the initial damage, the LTS of rock decreases and the relationship between the initial damage and the LTS of rock can be described as a linear function. Finally, an evaluation method for predicting the creep failure time of rock under a single stress level was proposed. In addition, the creep failure time of rock with different initial damage under different creep stress levels was obtained by the method. The results indicate that both the initial damage and the creep stress levels have a great influence on creep failure time, i.e., greater initial damage or creep stress leads to a shorter period for rock failure. Thus, for analyzing the long‐term stability of rock mass structure, not only the influence of in situ stress but also the initial damage state of the surrounding rock should be considered.

A two-constituent nonlinear viscoelastic model for asphalt mixtures

The goal of this study is to model the creep and recovery response of fine asphalt mixtures using a thermodynamically consistent nonlinear viscoelastic model. The model considers asphalt mixture to consist of two constituents: aggregate structure incorporating the asphalt–aggregate interface and asphalt binder. The efficacy of the model is evaluated using the response of warm fine aggregate mixtures (WFAM). These materials were produced using a polymer-modified binder of PG 76-22 and three warm mix additives (Sasobit, Advera and Rediset). Unaged and aged samples were subjected to creep stress levels of 75 and 400 kPa followed by rest periods. The model was quite successful in capturing the material behaviour as a single set of parameters were derived from the prediction of shear and normal stress responses for both 75 and 400 kPa stress levels. The presented model offers a unique feature in modelling the energy storage and dissipation of each of the two constituents. As such, one can examine the effect of changes in individual material properties on the material response and performance.