Uniaxial Tensile Creep Tests Research Articles

Laboratory Evaluation of Tensile Creep Behavior of Concrete at Early Ages

Short-term uniaxial tensile creep tests were conducted for early-age concrete at different ages in an effort to characterize early-age concrete as an aging viscoelastic material. Based on the test results, the viscoelastic material properties of the test concrete were characterized in terms of the Dirichlet series creep compliance and relaxation modulus functions. Applications of the model to the numerical analysis of the test samples showed a good agreement between the tests and computational results for the samples tested at different ages. This paper presents the test procedures and data analysis with a brief introduction of theoretical background.

An Interpretation between Small Punch Creep and Uniaxial Tensile Creep Test: Computational and Empirical Approach

The relationship between Small Punch (SP) creep testing and uniaxial tensile creep testing is crucial for translating findings from the innovative SP methodology to more conventional testing paradigms. This study involves both computational and empirical approaches to correlate the two testing methods effectively. The computational approach primarily utilizes finite element analysis (FEA) to model and simulate the stress states and deformation behaviors in SP creep tests. The empirical approach involves developing empirical relationships based on experimental data from both SP and uniaxial tensile creep tests. By integrating computational and empirical approaches, researchers can achieve a robust and reliable interpretation of SP creep test data in terms of conventional tensile creep test results.

Creep properties and prediction model of composite solid propellant

In order to describe the creep properties of HTPB propellant, uniaxial tensile creep tests of propellant under different stresses were conducted. Based on the principle of viscoelastic theory, the Burgers model was used to describe the creep properties of the propellant. Combined with the experimental data, the parameters of Burgers model under different stresses were determined, and the model parameters were expressed as a function related to creep stress, and finally the model was verified by using the test data under creep stress 0.2 MPa. The results show that the Burgers model can describe the creep properties of the composite propellant well.

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.

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.

Influence of creep constitutive models for an Al[sbnd]Si[sbnd]Cu alloy on load distribution of bolted joint

Creep of materials under high temperature conditions will greatly affect the reliability of bolted connections. This current paper focuses on the influence of the creep constitutive model for an AlSiCu alloy material on the load-bearing law of threaded teeth under high temperature environment. In order to obtain more accurately the creep law of the AlSiCu alloy in high temperature environment, the uniaxial tensile creep tests at 250 °C, 300 °C and 350 °C were carried out on the AlSiCu alloy specimens. Two creep constitutive model parameters are obtained by fitting the creep test data respectively. Then, the bolt connection combination structure model is established, and the bearing law of the bolt connection combination structure under different temperatures is calculated by ABAQUS, and the influence of the two creep constitutive models on the thread load law is analyzed. The results indicate that the modified Graham creep constitutive model has little effect on the load-bearing distribution of threads at 250 °C, and the time-hardening creep constitutive model has little effect on the load-bearing distribution of threads at 300 °C and 350 °C.

Creep behaviour of WC-12 wt% Co hardmetals with different WC grain sizes tested in uniaxial tensile and compression step-loading tests at 700 °C and 800 °C

At elevated temperatures, the material behaviour of WC-Co hardmetals shows differences under similar loading conditions depending on WC grain size and Co-content. Variations in the chemical composition and microstructure in hardmetals cause different material properties such as strength or creep resistance. In the current work, the influence of WC grain size on creep mechanism and creep resistance was investigated for WC-12 wt% Co hardmetals with 0.4 μm, 0.7 μm and 2.0 μm average WC grain size. Specimens were tested in uniaxial tensile and compression step-loading creep tests at 700 °C and 800 °C under vacuum conditions. Time-dependent creep behaviour with steady-state secondary creep was observed for all hardmetal grades investigated, with specimens creeping faster under tensile than under compressive loading. At 700 °C, the medium-grained hardmetal grade exhibited the highest minimal creep rates ε̇min compared to the submicron and ultrafine-grained grades. In contrast, the ultrafine-grained hardmetal grade showed higher ε̇min at low stresses and 800 °C, because of the high amount of grain boundary area per unit volume, which is advantageous for vacancy diffusion at grain boundaries. Therefore, the ε̇min of the medium-grained hardmetal grade was less affected by temperature than that of the finer-grained grade. Also two stress exponent n-ranges were observed at 700 °C and 800 °C: At low stress levels, n was in the range of about 1. Above a critical stress level, n reached values between about 4 and 6. Beside the influence of the WC grain size on the creep mechanism and creep resistance, damage evolution with increasing stress levels was analysed for the ultrafine-grained grade at 800 °C. The microstructures of three compression step-loading creep tested specimens were examined after maximum stress levels of −350 MPa, −950 MPa and −1350 MPa. Microstructural investigations performed via scanning electron microscopy showed that more and larger cavities had formed at WC/WC interfaces and WC/Co phase boundaries in the specimen tested up to −1350 MPa compared to the ones tested up to −350 MPa and −950 MPa.

Study on the Time-Dependent Mechanical Behavior and Springback of Magnesium Alloy Sheet (AZ31B) in Warm Conditions.

In this study, the time-dependent mechanical behavior of the magnesium alloy sheet (AZ31B) was investigated through the creep and stress relaxation tests with respect to the temperature and pre-strain. The microstructure changes during creep and stress relaxation were investigated. As the tensile deformation increased in the material, twinning and dynamic recrystallization occurred, especially after the plastic instability. As a result, AZ31B showed lower resistance to creep and stress relaxation due to dynamic recrystallization. Additionally, time-dependent springback characteristics in the V- and L-bending processes concerning the holding time and different forming conditions were investigated. We analyzed changes of microstructure at each forming temperature and process. The uniaxial tensile creep test was conducted to compare the microstructures in various pre-strain conditions with those at the secondary creep stage. For the bending process, the change of the microstructure after the forming was compared to that with punch holding maintained for 1000 s after forming. Due to recrystallization, with the holding time in the die set of 60 s, the springback angle decreased by nearly 70%. Increased holding time in the die set resulted in a reduced springback angle.

Tensile creep mechanical behavior of periodontal ligament: A hyper-viscoelastic constitutive model

ObjectiveIn orthodontic treatment, the biomechanical response of periodontal ligament (PDL) induces tooth movement. Coupling modeling of PDL can effectively reflect its biomechanical response. The nonlinear creep mechanical behavior of PDL was studied by uniaxial tensile creep test and a new hyper-viscoelastic constitutive model. Two coupling modeling methods with limitations were excluded. MethodsPDL specimens were prepared from the central incisors of pig mandible. The theoretical step function was replaced by static loading with a total loading time of 1 s. The creep loading with the constant stresses of 0.05, 0.1, and 0.15 MPa was selected and kept unchanged for 1000 s. The instantaneous hyperelastic mechanical behavior and time-dependent nonlinear viscoelastic mechanical behavior of PDL were characterized by coupled instantaneous third-order Ogden hyperelastic and time-dependent nonlinear creep models. ResultsThe results showed that the instantaneous elastic curve of PDL increases in the form of hyperelastic index. The creep strain and creep compliance curves increase rapidly before 200s, and then increase slowly in steady state. The creep strain increased with an increase in the constant stress; conversely, the creep compliance decreased with an increase in the constant stress. The results showed that the experimental data were highly consistent with the hyper-viscoelastic constitutive model (R2>0.97). SignificanceWe normalize the framework of hyper-viscoelastic coupling modeling (Instantaneous hyperelastic model + time-dependent nonlinear viscoelastic model). Which can be extended to other nonlinear viscoelastic biomaterials.

Thermo-Mechanical Behavior of Poly(ether ether ketone): Experiments and Modeling.

Observations are reported on poly(ether ether ketone) (PEEK) in uniaxial tensile tests, relaxation tests and creep tests with various stresses in a wide interval of temperatures ranging from room temperature to 180 °C. Constitutive equations are developed for the thermo–mechanical behavior of PEEK under uniaxial deformation. Adjustable parameters in the governing equations are found by matching the experimental data. Good agreement is demonstrated between the observations and results of numerical simulation. It is shown that the activation energies for the elastoplastic, viscoelastic and viscoelastoplastic responses adopt similar values at temperatures above the glass transition point.

Comprehensive study on mechanical properties of coated biaxial warp-knitted fabrics

In recent years, coated fabrics have become the major material used in membrane structures. Due to the special structure of base layer and mechanical properties, coated biaxial warp-knitted fabrics are increasingly applied in pneumatic structures. In this article, the mechanical properties of coated biaxial warp-knitted fabrics are investigated comprehensively. First, off-axial tensile tests are carried out in seven in-plane directions: 0°, 15°, 30°, 45°, 60°, 75°, and 90°. Based on the stress–strain relationship, tensile strengths are obtained and failure modes are studied. The adaptability of Tsai–Hill criterion is analyzed. Then, the uniaxial tensile creep test is performed under 24-h sustained load and the creep elongation is calculated. Besides, tearing strengths in warp and weft directions are obtained by tearing tests. Finally, the biaxial tensile tests under five different load ratios of 1:1, 2:1, 1:2, 1:0, and 0:1 are carried out, and the elastic constants and Poisson’s ratio are calculated using the least squares method based on linear orthotropic assumption. Moreover, biaxial specimens under four load ratios of 3:1, 1:3, 5:1, and 1:5 are further tensile tested to verify the adaptability of linear orthotropic model. These experimental data offer a deeper and comprehensive understanding of mechanical properties of coated biaxial warp-knitted fabrics and could be conveniently adopted in structural design.

Elevated Temperature Tensile Creep Behavior of Aluminum Borate Whisker-Reinforced Aluminum Alloy Composites (ABOw/Al-12Si).

In order to evaluate the elevated temperature creep performance of the ABOw/Al–12Si composite as a prospective piston crown material, the tensile creep behaviors and creep fracture mechanisms have been investigated in the temperatures range from 250 to 400 °C and the stress range from 50 to 230 MPa using a uniaxial tensile creep test. The creep experimental data can be explained by the creep constitutive equation with stress exponents of 4.03–6.02 and an apparent activation energy of 148.75 kJ/mol. The creep resistance of the ABOw/Al–12Si composite is immensely improved by three orders of magnitude, compared with the unreinforced alloy. The analysis of the ABOw/Al–12Si composite creep data revealed that dislocation climb is the main creep deformation mechanism. The values of the threshold stresses are 37.41, 25.85, and 17.36 at elevated temperatures of 300, 350 and 400 °C, respectively. A load transfer model was introduced to interpret the effect of whiskers on the creep rate of this composite. The creep test data are very close to the predicted values of the model. Finally, the fractographs of the specimens were analyzed by Scanning Electron Microscope (SEM), the fracture mechanisms of the composites at different temperatures were investigated. The results showed that the fracture characteristic of the ABOw/Al–12Si composite exhibited a macroscale brittle feature range from 300 to 400 °C, but a microscopically ductile fracture was observed at 400 °C. Additionally, at a low tensile creep temperature (300 °C), the plastic flow capacity of the matrix was poor, and the whisker was easy to crack and fracture. However, during tensile creep at a higher temperature (400 °C), the matrix was so softened that the whiskers were easily pulled out and interfacial debonding appeared.

Uniaxial tensile creep properties of ETFE foils at a wide range of loading stresses subjected to long-term loading

As a typical fluorine-based copolymer with excellent chemical, thermal, electrical and physical properties, ETFE (ethylene tetrafluoroethylene) is usually produced into thin films as an essential building and construction material especially for constructing the permanent membrane structures in civil engineering. This paper is thus aimed to investigate the uniaxial tensile time-dependent behaviors of ETFE foils with applying long loading period and various creep stresses experimentally. To this end, 30-day and 1-day uniaxial tensile creep tests were accomplished for the dumbbell specimens of the original ETFE foils by employing a new method and the traditional method, respectively. High efficiency, feasibility and precision of the selected new method could be firstly confirmed while comparing the 1-day creep curves of the original ETFE foils between two results from different methods. The experimental results of creep tests demonstrate that the creep curves of the original ETFE foils are strongly nonlinear with the faster strain during the initial stage and the slower strain during the final stage, and also prove that the creep stress is an essential factor in influencing the time-dependent behaviors. Meanwhile, it could be found that the ETFE foils subjected to the larger creep stresses were still deformed slowly even after the 30-day creep test. Additionally, uniaxial tensile stress-strain test was performed on the deformed ETFE foils that have undergone the 30-day creep test to evaluate the post-deforming mechanical properties. The experimental result of the additional stress-strain test shows that the elastic modulus of the deformed ETFE foils would increase with enlarging the creep stress. These findings would be significant in understanding the time-dependent behaviors of ETFE foils and especially for estimating the structural safety of ETFE membrane structure during the long-term service condition.

Creep damage analysis of thin-walled pressure vessel based on continuum damage model under static loading

UNS N10003 alloy is wildly used as a heat-resistant material for components to be used in high temperature thorium molten salt reactor (TMSR). Because of high temperature operating environment, prediction of its creep behavior is required for safety assessment of reactors. In this study, a series of uniaxial tensile creep tests has been conducted at 650°C in an applied stress range from 220 MPa to 380 MPa. A creep damage model for UNS N10003 alloy based on Lemaitre's work has been derived as well as a creep constitutive equation for modelling creep strain. The experimental data have been used to determine parameters in the constitutive equation and the damage model. The model has been applied to structural integrity assessment of a thin-walled pressure vessel design for TMSR. The numerical results indicate that the pressure vessel design for TMSR is sound and safe in its designated 10-year design life.

A modified reference area method to estimate creep behaviour of service-exposed Cr5Mo based on spherical indentation creep test

In this study, indentation creep tests under different conditions were conducted using a self-designed vacuum high-temperature indentation creep device to investigate the creep properties of service-exposed Cr5Mo. A microstructure test was also conducted and a prominent pile-up was found on the surface of the specimen. We performed a comprehensive finite element analysis to evaluate the non-uniform deformation characteristics of the spherical indentation creep test. The stress distribution and stress evolution for indenters with different diameters were also estimated during the test. Subsequently, considering the difficulty of calculating the equivalent stress owing to the area variation during spherical indentation, a modified reference area method was proposed. Based on a load relaxation simulation and geometric factor analysis, a general formula for the reference areas of indenters with different diameters was given. Thus, the equivalent stress during spherical indentation could be calculated, and the creep behaviour of service-exposed Cr5Mo was estimated. The results when using the modified method in this study demonstrated good consistency with the results obtained by the standard method. To sum up, the spherical indentation creep test has prominent advantages over the uniaxial tensile creep test in the analysis of in-service components.

Morphological Evolution of S-Phase in 2024 Aluminum under Tensile Creep at 448-463 K

The morphological evolution behavior of S-phase in 2024 aluminum alloy under uniaxial tensile creep test at 448 and 463 K under 175, 200 and 225 MPa was studied in detail by HRTEM. Results showed that the S-phase was arranged in the zigzag manner, similar to a zebra pattern, under tensile stress at 448-463 K. And it maintained cubic structure, remarkably grew along the tension direction under uniaxial tensile stress, but grew minimally perpendicular to the stress direction. Compared with applied stress, the temperature had a greater effect on the growth of S-phase. The length of S-phase reached 100 ± 20 nm after testing for 24 h at 448 K under 200 MPa and 200 ± 50 nm after testing for 18 h at 463 K under 175 MPa. The S-phase not only grew significantly in length but also coarsened at 463 K, the lamellar structure becoming fuzzy, which caused the alloy to enter the tertiary creep stage, unsuitable for creep-aging forming.

Assessment of Thermal Stresses in Asphalt Mixtures at Low Temperatures Using the Tensile Creep Test and the Bending Beam Creep Test

Thermal stresses are leading factors that influence low-temperature cracking behavior of asphalt pavements. During winter, when the temperature drops to significantly low values, tensile thermal stresses develop as a result of pavement contraction. Creep test methods can be suitable for the assessment of low-temperature properties of asphalt mixtures. To evaluate the influence of creep test methods on the obtained low-temperature properties of asphalt mixtures, three point bending and uniaxial tensile creep tests were applied and the master curves of stiffness modulus were analyzed. On the basis of creep test results, rheological parameters describing elastic and viscous properties of the asphalt mixtures were determined. Thermal stresses were calculated and compared to the tensile strength of the material to obtain the failure temperature of the analyzed asphalt mixtures. It was noted that lower strain values of creep curves were obtained for the Tensile Creep Test (TCT) than for the Bending Beam Creep Test (BBCT), especially at lower temperatures. Results of thermal stress calculations indicated that higher reliability was obtained for the viscoelastic Monismith method based on the TCT results than for the simple quasi-elastic solution of Hills and Brien. The highest agreement with the TSRST results was also obtained for the Monismith method based on the TCT results. No clear relationships were noted between the predicted failure temperature and different methods of thermal stress calculations.

Failure analysis of a gripping assembly loaded at temperatures above 1000 °C

Abstract The aim of the present work was to study the high-temperature behavior of a Ni-15Cr-18Co-4Al-3.5Mo powder-processed polycrystalline superalloy. This superalloy has been used to make the gripping assembly of a new commercial creep testing machine up to 1200 °C. The gripping assembly was exposed to a temperature of 1100 °C for 10 days at a nominal stress of 0.3 MPa and a local peak stress of 3.5 MPa. During this creep test, unacceptable creep deformation of the gripping assembly occurred. Three states of the superalloy microstructure were studied: (i) the state at receipt; (ii) the heat-treated state (annealing, 1200 °C/10 min/air); and (iii) the state following its use as a gripping assembly adapter during the creep test. Microstructural degradation resulted from exposed γ′ phase face-centered cubic Ni3Al particles that were preferentially created at the grain boundaries. Uniaxial tensile creep tests were conducted at 870 °C over a range of applied stresses from 100 to 220 MPa, and two tests were performed at 1000 °C and 1100 °C at 100 MPa and 10 MPa, respectively. A rapid drop in both the creep strength and the oxidation resistance was observed above 1000 °C. After deformation, the microstructure was investigated using advanced SEM and TEM techniques. Based on this investigation, the studied material is not recommended for use in creep testing machine gripping assemblies at temperatures above 1000 °C.

Mechanical behavior of sandstone under uniaxial constant cyclical compressive and tensile loading

In order to investigate mechanical behavior of red sandstone under cyclical compressive and tensile loads, a series of short-term uniaxial constant cyclical compressive and tensile loading tests and uniaxial constant cyclical compressive and tensile loading creep tests were conducted. First, based on the results of short-term uniaxial constant cyclical compressive and tensile loading experimental, the permanent residual strain, strains at peak stress were analyzed. Results show that the specimen shows permanent residual strain after suffering short-term cyclic loads; the residual strains and strains at peak stress show the “decay increase” and “steady-state increase” stages with the cyclic number N; the relationship between the strain at peak stress (tensile or compressive stress) and cyclic number can be well described by a modified Burger model. And then, in accordance with the creep experimental results, the creep behavior of the red sandstone was analyzed. Results show that there is obvious instantaneous strain, decay creep, and steady creep under each stress level of tensile or compressive stress stages; the specimen appears accelerated creep stage under the 5th tensile stress of 1.19 MPa. It was also found that power function can better express the relationship between steady strains and cycle number under tensile or compressive stress levels. In the end, a viscoelastic-plastic creep model was proposed; comparison of the test results with the proposed viscoelastic-plastic creep model predictions indicates that the proposed model is capable of describing the creep behavior of red sandstone subjected to cyclic tensile and compressive stress loading.

On the applicability of boundary condition based tensile creep model in predicting long-term creep strengths and lifetimes of engineering alloys

Creep behaviour of metal alloys for critical high temperature engineering applications is usually studied by uniaxial tensile creep tests. The stress and temperature dependence of steady-state or minimum creep rate observed in such tests is rationalised at present on the basis of either Norton or MBD equation. But, the stress exponent determined on such basis depend strongly and often irregularly on test temperature and the creep activation energy varies with stress or stress range. Hence, these parameters cannot be used for long-term predictions. Here, it is shown that these difficulties can be removed if a new creep model, which incorporates tensile strength, is used to rationalise the creep data. The stress exponent determined then does not depend on temperature although it depends on stress range, and the creep activation energy does not depend on stress. Thus, there is no relationship between stress exponent value and creep mechanism. Consequently, the new tensile creep model can be used in combination with Monkman-Grant relationship to predict the long-term tensile creep strengths and lifetimes at different temperatures using creep parameters determined from short-term tensile creep tests. Both the deficiencies of the Norton and MBD equations and the predictive quality of the new tensile creep model are demonstrated here using uniaxial tensile creep data and tensile strength data measured for two totally different types of creep resistant engineering alloys: 9Cr-1Mo steel and Ni base superalloy Ni-16Cr-8.5Co-3.5Al-3.5Ti-2.6W-1.8Mo-0.9Nb.