scholarly journals A Creep Model of Asphalt Mixture Based on Variable Order Fractional Derivative

2020 ◽  
Vol 10 (11) ◽  
pp. 3862
Author(s):  
Wenbo Luo ◽  
Bo Li ◽  
Yongjun Zhang ◽  
Boyuan Yin ◽  
Jingling Dai
Keyword(s):  
Mechanical Properties ◽  
Stress Level ◽  
Asphalt Mixture ◽  
Primary Creep ◽  
Creep Model ◽  
Creep Process ◽  
Variable Order ◽  
Time Varying ◽  
Steady Creep ◽  
Creep Stage

In order to quantitatively describe the time-varying mechanical properties of asphalt mixture during creep process, a nonlinear viscoelastoplastic creep model was proposed, by using variable-order fractional calculus. The differential order of the variable-order fractional element of the model is no longer constant, but a variable that changes with time, which reflects the changes of the mechanical properties of the material during the creep process. Whereas the tertiary creep phase is modeled by the viscoplastic element with time-varying viscosity, which is attributed to damage evolution. The uniaxial creep compression tests of AC-13C asphalt mixture under different stress levels (0.7 MPa, 0.9 MPa, 1.1 MPa, 1.3 MPa, 1.5 MPa, 1.7 MPa) were carried out with MTS-809 testing machine at 25 °C, and the test results were analyzed by the model using Levenberg–Marquardt optimization algorithm. It is shown that creep damage occurs when the applied stress exceeds a certain critical value, and the damage incubation time depends on the applied stress level. The higher stress decreases the damage incubation time. The model is in good agreement with the experimental results, and is applicable to describe the entire creep process, which consists of primary, steady and tertiary stages. Moreover, the variation of the model parameter can describe the change of viscoelastic properties of the material during the creep process. The differential order of the variable-order fractional element is constant during the primary creep stage, indicating that the creep behavior of the asphalt mixture is linear viscoelastic in small strain range. For the same stress level, the fractional order of the steady creep stage is greater than that of the primary creep stage, and it increases with the increasing stress level, which shows that the viscous behavior in the steady creep is more remarkable than that in the primary creep, and the higher the stress level, the more prominent the viscous performance exhibits.

scholarly journals Study on Multi-Step Creep Aging Behavior of Al-Li-S4 Alloy

Metals ◽  
2019 ◽  
Vol 9 (7) ◽  
pp. 807
Author(s):  
Yunlong Ma ◽  
Feng Xia ◽  
Lihua Zhan ◽  
Yongqian Xu
Keyword(s):  
Mechanical Properties ◽  
Steady State ◽  
Creep Rate ◽  
Yield Strength ◽  
Stress Level ◽  
New Technology ◽  
Steady State Creep ◽  
Steady Creep ◽  
Slow Increase ◽  
Creep Stage

Creep age forming (CAF) is a new technology developed for manufacturing large aluminum components in the aerospace industry. Aluminum–lithium alloys may be used in aerospace components because of their high modulus, specific strength and specific stiffness. Therefore, the creep deformation, mechanical properties and aging precipitation of Al-Li-S4 alloy under CAF conditions were studied. It was found that the creep behavior presents double steady state creep stages during the creep aging process. With the increase of stress level, the first steady creep rate increased, but the second steady creep rate was slightly reduced. Coincidentally, in the first steady state creep stage, the yield strength of the studied alloy also showed a slow increase stage. TEM observation showed that Al-Li-S4 alloy mainly contains two precipitation phases, T1 phase and θ’ phase. A few precipitates form during the first steady creep stage. Then, a lot of nucleation and growth of T1 phase resulted in rapid increase of yield strength. At the same time, the increase of stress level effectively inhibited the growth of T1 phase, which resulted in these strengthening phases being more uniform, and thus improved the mechanical properties of materials. On this basis, the relationship between the multi-step behaviors of creep, mechanical properties and aging precipitates are discussed. It is considered that the main reasons for the multi-step phenomenon of creep and mechanical properties are strongly related to the nucleation, growth and distribution of T1 phase.

scholarly journals Improved Nonlinear Nishihara Shear Creep Model with Variable Parameters for Rock-Like Materials

2020 ◽  
Vol 2020 ◽  
pp. 1-15 ◽  
Author(s):  
Hang Lin ◽  
Xing Zhang ◽  
Yixian Wang ◽  
Rui Yong ◽  
Xiang Fan ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Shear Strength ◽  
Creep Test ◽  
Creep Property ◽  
Variable Parameter ◽  
Nonlinear Function ◽  
Creep Model ◽  
Creep Process ◽  
Shear Creep

Creep property is an important mechanical property of rocks. Given the complexity of rock masses, mechanical parameters change with time in the creep process. In this work, a nonlinear function for describing the time-dependent change of parameters was introduced and an improved variable-parameter nonlinear Nishihara shear creep model of rocks was established. By creating rock-like materials, the mechanical properties of rocks under the shear creep test condition were studied, and the deformation characteristics and long-term shear strength of rocks during creep were analyzed. The material parameters of the model were identified using the creep test results. Comparison of the model’s calculated values and experimental data indicated that the model can describe the creep characteristics of rocks well, thus proving the correctness and rationality of the improved model. During shear creep, the mechanical properties of rocks have an aging effect and show hardening characteristics under low shear stress. Furthermore, according to the fact that Gk of the nonlinear model can characterize the creep deformation resistance, a method to determine the long-term shear strength is proposed.

scholarly journals A Nonlinear Fractional Viscoelastic-Plastic Creep Model of Asphalt Mixture

Polymers ◽  
2021 ◽  
Vol 13 (8) ◽  
pp. 1278
Author(s):  
Yongjun Zhang ◽  
Xiu Liu ◽  
Boyuan Yin ◽  
Wenbo Luo
Keyword(s):  
Permanent Deformation ◽  
High Stress ◽  
Asphalt Mixture ◽  
Creep Model ◽  
Creep Process ◽  
Creep Tests ◽  
Model Predictions ◽  
Stress Levels ◽  
Plastic Creep ◽  
Good Agreement

The mechanical behavior of asphalt mixture under high stresses presents nonlinear viscoelasticity and permanent deformation. In this paper, a nonlinear fractional viscoelastic plastic (NFVEP) creep model for asphalt mixture is proposed based on the Nishihara model, with a Koeller spring-pot replacing the Newton dashpot. The NFVEP model considers the instantaneous elasticity, viscoelasticity with damage and time-hardening viscoplasticity with damage concurrently, and the viscoelastic response is modeled by fractional derivative viscoelasticity. To verify the model, uniaxial compressive creep tests under various stresses ranging from 0.4 MPa to 0.8 MPa were carried out at room temperature. The NFVEP model predictions are in good agreement with the experiments. The comparison with the modified Nishihara model and the Burgers model reveals the advantages of the NFVEP model. The results show that the NFVEP model, with the same set of parameters, can not only describe the primary and steady-state creep stages of asphalt mixture under low stress levels but also the whole creep process, including the tertiary creep stage, of asphalt mixture under high stress levels.

scholarly journals A Study on the Creep Characteristics of Airport Viscous Subsoil Based on Unsaturated Stress Level

Geofluids ◽  
2021 ◽  
Vol 2021 ◽  
pp. 1-13
Author(s):  
Jun Feng ◽  
Yue Ma ◽  
Zaobao Liu
Keyword(s):  
Stress Level ◽  
Unsaturated Soil ◽  
Three Dimensional ◽  
Matric Suction ◽  
Creep Model ◽  
Creep Stage ◽  
One Dimensional ◽  
Creep Characteristics ◽  
Stress Levels ◽  
The One

The present study takes the ratio of the matric suction to the net vertical stress and the ratio of the matric suction to the net mean stress as new unsaturated stress levels f and F , respectively. Based on the laboratory tests and theoretical derivation, the modified one-dimensional Mesri creep model and three-dimensional creep model were established, which takes the unsaturated stress level into account. Then, the one-dimensional and three-dimensional creep characteristics of the unsaturated viscous subsoil of an airport under different unsaturated stress levels were analyzed. The following conclusions could be drawn: (1) under different stress levels, the one-dimensional creep deformation of unsaturated soil has a power function relationship with time, and the change rate exponentially decreases with the stress level, which can be well-expressed by the proposed modified one-dimensional Mesri creep model; (2) under different stress levels, the three-dimensional creep strain of the unsaturated soil shows a hyperbolic curve with time and a near-linear relationship at the semilogarithmic coordinate, which can be well-expressed by the proposed modified three-dimensional creep model; (3) under different stress levels, both the one-dimensional creep and three-dimensional creep of the unsaturated soil can be divided into two stages, which are the accelerated creep stage and stable creep stage.

scholarly journals Study on the Porosity of Saturated Fragmentized Coals during Creep Process and Constitutive Relation

2020 ◽  
Vol 2020 ◽  
pp. 1-10
Author(s):  
Peng Gong ◽  
Zhanguo Ma ◽  
Yongheng Chen ◽  
Shixing Cheng ◽  
Kelong Li
Keyword(s):  
Grain Size ◽  
Stress Level ◽  
Creep Model ◽  
Creep Process ◽  
Deformation Characteristics ◽  
Test Device ◽  
Porosity Variation ◽  
Grain Sizes ◽  
Voigt Model ◽  
Creep Constitutive Model

Pore abundance and deformation characteristics of saturated fragmentized coals during creep process are of significant meaning to the study on ground sediment in the mined-out area. The law of porosity variation of saturated fragmentized coals during creep process and its creep constitutive model were studied by using the self-developed multiphase coupling creep test device. And, results have indicated that the porosity logarithm of fragmentized coal during creep process shows a linear negative correlation with the time ln(n−a) = −ct + lnb, and the porosity decrease is evidently divided into three phases. In addition, when the stress level is relatively low, the porosity decreases slowly; when the stress level rises up, the porosity decreases quickly; when the stress level remains stable finally, the porosity is smaller. Under the equal stress, as the grain size of fragmentized coals decreases, the porosity tends to decrease, and as the grain size of fragmentized coal tends to be stable, the porosity tends to increase; the creep constitutive equation of fragmentized coals with different grain sizes was established by using the Kelvin–Voigt model, and the correlation analysis shows that the Kelvin–Voigt creep model of fragmentized coals is reasonable.

scholarly journals Effects of T5 Treatment on Microstructure and Mechanical Properties at Elevated Temperature of AZ80-Ag Alloy

Materials ◽  
2019 ◽  
Vol 12 (19) ◽  
pp. 3214 ◽  
Author(s):  
Zeng ◽  
Liu ◽  
Gao ◽  
Jiang ◽  
Yu ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Tensile Strength ◽  
Elevated Temperature ◽  
Elevated Temperatures ◽  
Volume Fraction ◽  
Primary Creep ◽  
Dislocation Creep ◽  
Precipitation Process ◽  
Creep Stage ◽  
Microstructure And Mechanical Properties

Effects of T5 treatment on microstructure and mechanical properties at elevated temperature of hot-ring-rolled (HRRed) AZ80-Ag magnesium alloy were systematically investigated. Results show that, after aging at 175 °C for 36 h, discontinuous and continuous precipitates form inside grains, with the former one taking up a volume fraction of ~64.9%. T5 treatment improves the tensile strength at ambient temperature of the alloy but weakens its tensile strength and creep resistance at elevated temperatures (120–175 °C), indicating opposite effects of T5 on mechanical properties at ambient and elevated temperatures. During creep at 120–175 °C and under 70–90 MPa, the dynamic precipitation process in HRRed specimen is accelerated with increasing temperature. At 150–175 °C massive nucleation and growth of dynamic discontinuous precipitates could result in an atypical primary creep stage, consisting of deceleration and acceleration creep stages, which is reported in wrought Mg-Al-based alloy for the first time. Such primary creep stage can be eliminated by T5 treatment. Besides, diffusion-controlled dislocation creep is the dominant creep mechanism for both HRRed and T5 specimens.

scholarly journals Prediction Models of Shear Parameters and Dynamic Creep Instability for Asphalt Mixture under Different High Temperatures

Polymers ◽  
2021 ◽  
Vol 13 (15) ◽  
pp. 2542
Author(s):  
Junxiu Lv ◽  
Xiaoyuan Zhang
Keyword(s):  
Shear Stress ◽  
Shear Strength ◽  
Stress Level ◽  
Prediction Models ◽  
Asphalt Mixture ◽  
Confining Pressure ◽  
Shear Strength Parameters ◽  
Dynamic Creep ◽  
Shear Stress Level ◽  
Creep Instability

This study mainly investigates the prediction models of shear parameters and dynamic creep instability for asphalt mixture under different high temperatures to reveal the instability mechanism of the rutting for asphalt pavement. Cohesive force c and internal friction angle φ in the shear strength parameters for asphalt mixture were obtained by the triaxial compressive strength test. Then, through analyzing the influence of different temperatures on parameters c and φ, the prediction models of shear strength parameters related to temperature were developed. Meanwhile, the corresponding forecast model related to confining pressure and shear strength parameters was obtained by simplifying the calculation method of shear stress level on the failure surface under cyclic loading. Thus, the relationship of shear stress level with temperature was established. Furthermore, the cyclic time FN of dynamic creep instability at 60 °C was obtained by the triaxial dynamic creep test, and the effects of confining pressure and shear stress level were considered. Results showed that FN decreases exponentially with the increase in stress levels under the same confining pressure and increases with the increase in confining pressure. The ratio between shear stress level and corresponding shear strength under the same confining pressure was introduced; thus, the relationship curve of FN with shear stress level can eliminate the effect of different confining pressures. The instability prediction model of FN for asphalt mixture was established using exponential model fitting analysis, and the rationality of the model was verified. Finally, the change rule of the parameters in the instability prediction model was investigated by further changing the temperature, and the instability forecast model in the range of high temperature for the same gradation mixture was established by the interpolation calculation.

scholarly journals A Phenomenological Primary–Secondary–Tertiary Creep Model for Polymer-Bonded Composite Materials

Polymers ◽  
2021 ◽  
Vol 13 (14) ◽  
pp. 2353
Author(s):  
Xiaochang Duan ◽  
Hongwei Yuan ◽  
Wei Tang ◽  
Jingjing He ◽  
Xuefei Guan
Keyword(s):  
Composite Materials ◽  
Creep Behavior ◽  
Reference Model ◽  
Creep Model ◽  
Creep Process ◽  
Validation Dataset ◽  
Testing Conditions ◽  
Proposed Model ◽  
Testing Data ◽  
Bonded Composite

This study develops a unified phenomenological creep model for polymer-bonded composite materials, allowing for predicting the creep behavior in the three creep stages, namely the primary, the secondary, and the tertiary stages under sustained compressive stresses. Creep testing is performed using material specimens under several conditions with a temperature range of 20 °C–50 °C and a compressive stress range of 15 MPa–25 MPa. The testing data reveal that the strain rate–time response exhibits the transient, steady, and unstable stages under each of the testing conditions. A rational function-based creep rate equation is proposed to describe the full creep behavior under each of the testing conditions. By further correlating the resulting model parameters with temperature and stress and developing a Larson–Miller parameter-based rupture time prediction model, a unified phenomenological model is established. An independent validation dataset and third-party testing data are used to verify the effectiveness and accuracy of the proposed model. The performance of the proposed model is compared with that of an existing reference model. The verification and comparison results show that the model can describe all the three stages of the creep process, and the proposed model outperforms the reference model by yielding 28.5% smaller root mean squared errors on average.

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