Uniaxial Creep Tests Research Articles

Investigation on internal mechanical response of asphalt mixture during creep test utilizing the Smart Aggregate

• Internal mechanical response of asphalt mixture to the creep behavior was investigated using Smart Aggregate. • Effects of particle location, stress level and temperature on the internal mechanical response were analyzed. • Relationship between internal mechanical response and creep behavior was discussed for asphalt mixture. The micro-mechanical behaviors of asphalt pavement play the essential role in its macro-mechanical properties and damage evolution. However, the efforts to research the micro-mechanical mechanism of asphalt mixture are still not enough, especially from a particle level in the laboratory. This study aims to investigate the internal mechanical response of asphalt mixture to the creep behavior with the assistance of built-in Smart Aggregate. After calibrating the force measurement for the Smart Aggregate, it was embedded in the asphalt mixture to monitor the inter-particle forces during the uniaxial static creep tests. Meanwhile, various test conditions, including particle location, stress level and temperature, were concerned to analysis their influences on the mechanical response of the Smart Aggregate. In addition, the relationships between internal mechanical responses and creep behavior were discussed. Results indicate that the applied force has a nonlinear relation to measured voltage of the Smart Aggregate, and can be obtained according to the regression model after calibration. During creep test, the Smart Aggregate performs well in acquiring the internal mechanical response of asphalt mixture. The inter-particle forces show uneven along the direction of loading stress during the creep test in asphalt mixture. The measured force increases monotonously whilst the strain increasing during creep evolution. Regarding the same temperature, there are significantly linear relations between instantaneous elastic modulus and measured force at the beginning and between eventual stiffness modulus and measured force at the end in dual logarithmic scales. Meanwhile, the slope k f of force curve in the steady shows strongly linear relation to the slope k s in equi-velocity creep stage on the Cartesian coordinates.

Creep Properties of Mudstone Interlayer in Bedded Salt Rock Energy Storage Based on Multistage Creep Test: A Case Study of Huai’an Salt Mine, Jiangsu Province

Salt rock is internationally recognized as an excellent medium for energy storage. However, most of the domestic salt mines are lacustrine laminated formations with mudstone interlayers, which have different creep characteristics to pure salt rocks. Therefore, in this paper, uniaxial compression creep tests were carried out on a mudstone interlayer in Huai’an, Jiangsu, China, and the multistage creep mechanical behavior of the mudstone interlayer was investigated using acoustic emission (AE) technology. The results show that at low stress levels, the axial deformation of the interlayer is greater than the lateral deformation. As the increase of the stress level, the lateral deformation becomes more significant, and the corresponding axial and lateral steady-state creep rates both exhibit nonlinear acceleration with the increase of the loading stress. In the initial loading stage of each stress level, the mudstone interlayer releases a large number of AE signals. After the creep stabilization, the AE signals decreased obviously, and a large number of AE signals were released at the last stress level. Based on the creep test data, an improved fractional viscoelastic-plastic model was fitted to it and compared with the Nishihara model. The applicability of the model was also tested, and the model was found to be a good description of the mudstone interlayer after oil erosion. The research results reflected the creep law of the mudstone interlayer to a certain extent and are expected to serve as a reference to studies on the long-term stability of the surrounding rock of a bedded salt rock energy storage.

Mechanical Behavior of Human Cancellous Bone in Alveolar Bone under Uniaxial Compression and Creep Tests.

In the process of orthodontic treatment, the remodeling of cancellous bone in alveolar bone (in this paper, cancellous bone in alveolar bone is abbreviated as CBAB) is key to promoting tooth movement. Studying the mechanical behavior of CBAB is helpful to predict the displacement of teeth and achieve the best effect of orthodontic treatment. Three CBAB samples were cut from alveolar bone around the root apex of human teeth. A uniaxial compression test was used to study the transient elastic properties of CBAB. A creep test was used to study the time-dependent viscoelastic properties of CBAB. Both tests were carried out at the loading rates of 0.02 mm/min, 0.1 mm/min and 0.5 mm/min. The results revealed that CBAB is a nonlinear viscoelastic and hyperelastic material. The stress–strain curve obtained from the uniaxial compression test could be divided into three stages: the collapse stage of the front section, the exponential stage of the middle section and the almost linear stage of the rear end. According to the strain–time curve obtained from the compression creep test, a trend of increasing strain over time was relatively obvious within the first 30 s. After 200 s, the curve gradually tended to plateau. Four hyperelastic models and three viscoelastic models were used to fit the test data. Finally, the fifth-order polynomial hyperelastic model (coefficient of determination “R2 > 0.999”) was used to describe the hyperelastic properties of CBAB, and the seven-parameter model of the generalized Kelvin modified model (“R2 > 0.98”) was used to describe the viscoelastic properties of CBAB.

Study on Creep Mechanical Properties of Non-Water Reacting Polyurethane Grouting Material

Non-water reacting polyurethane is a new type of grouting material for support reinforcement and emergency rescue, whose creep mechanical behavior is directly related to the stability and safety of the reinforcement project. To investigate the creep properties of non-water reacting polyurethane grouting material, a series of multistage loading creep tests were performed under uniaxial compression conditions. Then, the creep curves, creep strain rate, and creep failure modes of non-water reacting polyurethane grouting material were obtained. In addition, the effects of temperature and density on the creep properties of non-water reacting polyurethane grouting material were also analyzed. Finally, based on the finite difference program, the numerical simulation of the uniaxial compression creep test of non-water reacting polyurethane grouting material was conducted by employing the Burgers–Mohr creep model. The simulated creep curve was in good agreement with the test curve, which verified the feasibility of the model to describe the creep characteristics of non-water reacting polyurethane grouting material. The research results can provide a reference for studying the creep behavior of polyurethane grouting material and also contribute to the development and application of polyurethane material grouting technology.

An initial study on the impression creep behaviour of stir-cast GNP reinforced AA6061 composite (AMMC)

Impression creep testing technique is used to screen out creep behaviour of materials as it is relatively faster method, requires small volume of the specimen, and is relatively material non-invasive technique when compared to conventional uniaxial tensile creep test method. This technique also possesses the benefit of maintaining constant stress throughout the experiment because a flat cylindrical indenter is used to apply load on the specimen. Owing to commercial unavailability of the materials under development such as metal matrix composites (MMC), the impression creep testing approach for such materials becomes a best method. In present work, creep behaviour of stir-cast graphene nanoplatelets (GNP) reinforced AA6061 composite (AMMC) is studied by using impression creep tests. The AMMC was prepared by adding GNPs in AA6061 through stir casting process by melting the alloy up to 1073 K. The impression creep experimentation included stress levels of 140 MPa, 170 MPa, and 200 MPa at a temperature of 573 K, 603 K and 633 K. The tests were conducted at constant stress by varying the temperature and vice-versa in vacuum. Two distinct regions i.e., primary and secondary/steady-state were observed and minimum creep strain rate was obtained in steady-state region. Tertiary region was absent, since the load/stress applied was compressive. During the experiment, depth of indentation of 0.03 mm to 0.14 mm was produced in the specimen. The microstructure of the composite has been analyzed using optical microscope and scanning electron microscope (SEM) to correlate with impression creep behaviour of the studied materials.

Research on the mesoscopic viscoelastic property of semi-flexible pavement mixture based on discrete element simulation

Semi-flexible pavement (SFP) mixture is a relatively new pavement material that combines the characteristics of the flexibility of asphalt mixtures and the rigidity of cement concrete. In this research, the discrete element method was used to reveal the viscoelastic mechanical behavior mechanism of the SFP mixture at a mesoscopic scale. First, the influence of the matrix asphalt mixture air voids on the viscoelastic performance of the SFP mixture was studied through laboratory tests to clarify its basic macroscopic mechanical properties. The results showed that the SFP mixture combines good high and low temperature performances when the air void is 25%. Then, the discrete element simulation software PFC3D was applied to establish an SFP mixture model, and the simulation of the dynamic modulus test and uniaxial compression creep test was conducted to study the mesoscopic mechanical composition and force evolution. The results demonstrated that when subjected to dynamic loading, each constituent inside the SFP mixture is subjected to compressive forces, most of which are borne by the internal contact of aggregate-aggregate, while the tensile forces are mainly borne by the contacts of asphalt mortar-asphalt mortar, cement stone-cement stone, and aggregate-asphalt mortar. When subjected to a constant static loading, the value order of the contact force in the SFP mixture is: aggregate-aggregate > cement stone-cement stone > aggregate-asphalt mortar > asphalt mortar-cement stone > cement stone-aggregate > asphalt mortar-asphalt mortar. As the creep progresses, the contact force of aggregate-aggregate increases slightly; the contact forces of cement stone-cement stone, aggregate-asphalt mortar, and cement stone-aggregate decrease slightly; and the contact forces of asphalt mortar-asphalt mortar and asphalt mortar-cement stone decrease rapidly.

Multi-scale creep analysis of river sand and manufactured sand concrete considering the influence of ITZ

This paper investigated the multi-scale prediction of long-term creep in river sand (RS) concrete and manufactured sand (MS) concrete and analyzed the effect of the interfacial transition zone (ITZ) around the aggregate on the creep properties of concrete. Uniaxial compressive creep tests were conducted on the concrete for a duration of 360 d. The creep kinetic properties of the paste matrix and ITZ were obtained by microindentation test. The properties of ITZ were determined by the backscattered electron (BSE) image threshold segmentation technique and the Lu & Torquato model. Finally, the predicted concrete creep modulus was obtained through the calculation of creep homogenization, and the accuracy of multi-scale creep prediction when considering different ITZ cases was discussed. The results show that under the same water-binder ratio, the thickness of ITZ around MS was larger, the creep modulus was smaller and the volume fraction was larger compared with that of ITZ around RS. In the multi-scale homogenization calculation of the concrete long-term creep, the prediction accuracy of the computational model increased with the raised of the considered ITZ types. The prediction accuracy obtained when all ITZs were considered was improved by more than 46.3% compared to that when no ITZs were considered.

Modified Burgers model of creep behavior of grouting-reinforced body and its long-term effect on tunnel operation

Grouting has been widely employed to mitigate geohazards in underground projects (e.g., the tunnel). However, few studies have focused on the effects of the creep behavior of a grouting-reinforced body on the stability of tunnel operation. In this study, the curtain grouting-reinforced Nanshibi tunnel was adopted as an example to explore this issue. More specifically, uniaxial compressive creep tests were firstly performed to study the time-dependent behavior (i.e., the creep behavior) of the grouted specimen. Then, the improved nonlinear Burgers model was proposed to characterize the observed creep behavior of the grouted specimen. Finally, a numerical simulation of the tunnel operation post grouting was conducted using the newly-proposed nonlinear Burgers model. The following results were obtained: (1) The creep behavior of the grouted specimen under different loadings undergoes three stages: decaying creep, steady creep, and accelerated creep. The steady creep rate is attributed to the homogeneity of the grouting-reinforced body. (2) The nonlinear Burgers model can reasonably quantify the creep behavior of the grouting-reinforced body. (3) The creep behavior of the grouting circle of the tunnel has a significant influence on the tunnel deformation and thus the lining safety. More specifically, the tunnel deformation exhibits nonlinear growth and mainly occurs at the vault and inverted arch. Furthermore, owing to the shape of the lining structure, the arch spring is most vulnerable to cracking under the eccentric tension state. The improved creep model is proved to more accurately predict the structural deformation of a curtain grouting-reinforced tunnel. As such, it can help devise grouted-tunnel maintenance strategies for long-term safety.

Rheology of partially molten plagioclase containing wetting silica-rich anhydrous melt

Abstract This work explores the effects of melt chemistry on diffusion controlled creep of partially molten labradorite plagioclase (An50) at anhydrous conditions. Using sol-gel and hot pressing techniques we produced: (1) nominally melt-free samples, with <1 vol. per cent residual glass confined solely to multiple-grain junctions; (2) two types of partially molten samples, containing respectively ∼1 and ∼5 vol. per cent silica-rich partial melts, wetting numerous grain boundaries by thin (<10 nm) amorphous films. Energy dispersive X-ray analysis showed that the amorphous phases of the latter materials contained ∼85 and 95 wt. per cent SiO2, thus representing different polymerization degrees. Infrared spectroscopy showed that the initial traces of water (∼0.05 wt. per cent) were dried out by annealing in air above 1100 °C. Uniaxial creep tests performed at temperatures and flow stresses ranging, respectively, between 1100–1250 °C and 3–60 MPa showed dominantly linear viscous flow, with a strong grain size dependence indicating grain boundary diffusion control. Counter-intuitively strength and activation energy increased with the content of melts. However, for the sample suite silica content covaries with melt proportion, and thus our results suggest that the kinetics of grain boundary diffusion controlled creep strongly depends on melt chemistry. Instead of acting as shortcut for diffusion, thin films of highly viscous amorphous phases may in turn considerably hinder grain boundary transport properties.

Creep properties and constitutive model for salt rock subjected to uniaxial trapezoidal cyclic loading

Uniaxial trapezoidal cyclic loading creep tests with different loading and unloading rates are performed on salt rock to study its creep properties. The results indicate that the axial strain produced by salt rock under trapezoidal cyclic loading occurs mainly in the 1st cycle, and other cycles generate less axial strain. The existence of constant high pressure stage leads to an obvious horizontal segment in the stress-strain curve, while constant low pressure stage has little effect on its stress-strain curve. The stress-strain curve in the 1st cycle includes elastic stage, plastic stage, creep stage and deformation recovery stage. The axial strain generated at plastic deformation stage gradually tends to 0 with increasing cycle numbers, then the curve in a single cycle includes only elastic stage, creep stage and deformation recovery stage. When the cycle numbers is small, the irreversible deformation produced by salt rock in a single cycle mainly consists of plastic deformation produced at the loading stage and creep deformation generated at the constant high pressure stage. After several cycles, the irreversible deformation in a single cycle mainly consists of creep deformation generated at the constant high pressure stage. With increasing cycle numbers, the minimum value of creep rate in a single cycle basically does not change, and the maximum value of creep rate decreases firstly and then keeps constant. Moreover, with increasing loading rate, the mean value of minimum creep rate basically remains unchanged and the mean value of maximum creep rate gradually increases. With increasing unloading rate, both the mean values of minimum and maximum creep rate decrease gradually. Finally, the creep equation for the Burgers model subjected to trapezoidal cyclic loading is deduced, and the reasonableness of which is checked.

A Core Damage Constitutive Model for the Time-Dependent Creep and Relaxation Behavior of Coal

The creep and stress relaxation behaviors of coal are common in coal mining. The unified constitutive model is suitable to describe and predict both the creep and relaxation evolution characteristics of rocks. The generalized Kelvin model is the core element for traditional and improved component models to reflect both the nonlinear creep and relaxation. In this paper, an improved core damage model, which could both reflect the creep and stress relaxation in relation to the damage evolution, was established based on a comparison of the traditional and improved component models, and the responding constitutive equations (creep and stress relaxation equation) at constant stress/strain were deduced. Then, the core damage model was validated to the uniaxial compressive multistage creep and stress relaxation test results of coal, showing that the model curves had great accordance with the experimental data. Moreover, the model comparisons on accuracy, parameter meaning, and popularization among the core damage model, hardening-damage model, and the fractional derivative model were further discussed. The results showed that the parameters in the core damage model had clear and brief physical significances. The core damage model was also popularized to depict the time-dependent behaviors of other rocks, showing great accuracy.

Three-dimensional finite element analysis for structural parameters of asphalt pavement: A combined laboratory and field accelerated testing approach

To solve the issues that material parameters measured under specific laboratory conditions are extremely different from the actual pavement, a method of structural parameters correction in semi-rigid base asphalt pavement combining numerical simulations, laboratory tests, and accelerated pavement testing (APT) was proposed. The dynamic modulus and the uniaxial static compression creep tests were conducted to explore the effect of temperature, size, and fatigue. Then, these factors were coupled to the modified Burgers model that can reflect the creep law of the whole process. A constitutive model reflecting the real parameters of the pavement was subsequently constructed with the ABAQUS user-defined material (UMAT) subroutine. Experimental results illustrated it was verified that the modified subroutine has good feasibility both in single-layer and multi-layer materials from the actual pavement. Furthermore, simulated results also showed excellent agreement with field-tested findings. Finally, the rutting deformation curves and the demarcation points of each stage were obtained, which could guide the rutting prediction and maintenance decision of semi-rigid asphalt pavement.

Creep rupture behavior of 2.25Cr1Mo0.25V steel and weld for hydrogenation reactors under different stress levels

Abstract In the present research work, the 2.25Cr1Mo0.25V steel plates with a thickness of 112 mm were welded using the multi-pass submerged automatic arc welding process. The creep specimens were prepared from the base metal (BM) and weld metal (WM) in the welded joint after heat treatment process. The uniaxial creep tests were performed to investigate the creep deformation and rupture behaviors at 550°C under different applied stress levels. The microstructure and fracture surface morphology of crept BM and WM samples were also characterized using the scanning electron microscope with energy-dispersive X-ray spectroscopy. The results showed that typical three-stage creep deformation curves are observed in both BM and WM specimens, and the BM exhibits a faster deformation rate than the WM. Both the creep rupture time and uniaxial creep ductility are found to be increased with a decrease in applied stress. Furthermore, the relationship between the minimum creep rate and time to rupture of both BM and WM samples was obtained, and it can be described using a unified Monkman–Grant equation. In addition, it is found that the creep fractures of the BM and WM are a transgranular ductile failure. The creep damages of both materials are mainly associated with the microstructural degradations, that is, the initiation and coalescence of creep cavities at second phase particles such as carbide and inclusion particles along the loading direction.

Laboratory Study and Simulation Analysis on Anticrack Properties of Open-Graded Friction Course Based on Creep Tests

Cracking is a common early disease of asphalt pavements. To evaluate the temperature shrinkage cracking resistance of an open-graded friction course (OGFC), the viscoelastic parameters of an OGFC mixture at different temperatures were obtained by uniaxial compression creep tests. According to the viscoelastic parameters, numerical models of the creep, relaxation, and temperature shrinkage cracking of an OGFC were established. Subsequently, the reliability of the numerical model was verified based on the creep test results and the extensive Maxwell model. The crack initiation temperature of the OGFC mixture at different cooling rates was analyzed by combining indirect tensile strength and temperature shrinkage stress. The results showed that the simulation results were close to the model results, and the average errors of the simulation results for creep compliance and relaxation modulus did not exceed 7.18% and 1.83%, respectively. The faster the cooling was, the greater the temperature shrinkage stress and the higher the crack initiation temperature were. The crack initiation temperature of the OGFC at a cooling rate of 20°C/h was about −3.5°C. This research provides a reference for quickly obtaining the crack initiation temperature of asphalt mixtures at different cooling rates.

Discrete element modelling of mechanical response of crumb rubber-modified asphalt pavements under traffic loads

ABSTRACT This study aims to predict the mechanical response of crumb rubber-modified (CRM) asphalt pavements under traffic loads using three-dimensional (3D) discrete element method (DEM). First, irregular-shaped aggregates were generated in Python language, and discrete element models of six different asphalt layers, which considered the temperature gradient and fatigue damage, were established using 3D DEM. Then, model parameters were obtained through the uniaxial creep test for asphalt mastics at different temperatures. The fatigue damage was implemented by introducing a damage factor into the Burgers model. Finally, three mechanical response parameters (namely, permanent deformation, shear stress and transverse strain) of the six asphalt layers under traffic loads were analysed and compared. Results show that the mechanical response of CRM asphalt layers under traffic loads is significantly influenced by number of repeated loads, temperature, and asphalt layer materials. The shear stress and transverse strain at the wheel centre vary along the depth of asphalt layers, and the middle layer is the principal area of shear stresses in asphalt pavements.

Development of Ultra-Miniature Creep Testing Method

A uniaxial creep test method using an ultra-miniature creep (Ultra-MC) specimen with a size equivalent to that of a small punch creep (SPC) specimen was developed in this study. It was confirmed that the devised Ultra-MC specimen had the self-alignment function and geometry optimized by FEM analysis. In addition, by comparing the creep test results of the Ultra-MC specimen with the standard specimen (ϕ6), the applicability of Ultra-MC testing method was confirmed. The creep rupture time of the Ultra-MC specimen was found to be good agreement with that of the standard specimen, and the deviation of the creep rupture time was small. On the other hand, the creep deformation of the Ultra-MC specimen was larger than that of the standard specimen. By FEM analysis, this was found to be due to the creep deformation in the shoulder portion of the Ultra-MC specimen. So, a modification procedure for adjusting the creep property of the Ultra-MC specimen to that of the standard specimen was proposed.

Compressive creep behavior of Ti-6Al-4V-xSi-ySc alloys under high-pressure at room temperature

One of the characteristics of Titanium alloys is that they are prone to creep due to the service conditions under high-pressure, such as deep-sea equipment. The compressive properties and deformation mechanism of Ti-6Al-4V-xSi-ySc alloys at room temperature are investigated in this study. The alloy samples are subjected to uniaxial compressive and creep tests. The obtained compression creep properties at room temperature follow the power-law function, and creep is mainly found in the first and second stage. Si and Sc are observed to improve the creep properties where the latter shows significant effect. This can be attributed to the strengthening of the solid solution of Si and Sc. Sc and Si strengthen the α and β phases respectively. The stability of these phases contributes to increasing the creep resistance of Titanium alloys. The creep resistance mechanisms of the alloys involve dislocation movement and twins.

Multiphase modeling of the effect of external loading on the shrinkage of concrete and its consequences for modeling tensile creep

Multiphase modeling is a powerful framework for the simulation of the early-age and long-term behavior of concrete. Nevertheless, state-of-the-art multiphase models do not capture the experimentally observed instantaneous change in internal relative humidity upon external loading, which also manifests in the lack of predicting load-induced shrinkage. The approach presented in this publication is a step to overcome this deficiency. It is based on a poromechanical multiphase model for coupled creep and shrinkage of concrete, which is enhanced by a strong hygro-mechanical coupling, introduced via a load-dependent desorption isotherm. The enhanced model is evaluated for sealed and drying uniaxial compressive and tensile creep tests. Instantaneous changes in internal relative humidity upon loading are recovered by the enhanced model. In compression, the influence on the mechanical response is small. In contrast, the representation of tensile creep is significantly improved by considering load-induced shrinkage.

Microstructural characterisation of creep tested advanced austenitic stainless steel super 304H

ABSTRACT Microstructural characterisation was performed on the gauge section of two creep rupture samples manufactured from Super 304H; an advanced austenitic stainless steel that is used as superheater and reheater tubes in power plants. The samples had been exposed to uniaxial creep tests (Sample A: 700°C and 75Mpa, Sample B: 650°C and 120Mpa) where both exhibited low creep ductility. Six phases were identified: MX (Nb-rich), sigma, Cu-rich, M23C6, modified Z phase and Fe (BCC). Quantification of Nb-rich and Cu-rich particles revealed ~10% and ~16% more Nb-rich and Cu-rich particles respectively in Sample A versus Sample B. The amount of sigma and the average particle size for the Nb and Cu-rich particles was similar for both samples. There appears to be an association between the sigma phase and creep cavities as shown in 2D microstructural characterisation which could have contributed to the low creep ductility exhibited by these two creep samples.

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

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