• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2020-01-16
Feng Jin, Guoqing Song, Zhuoping Shao

Bamboo with high specific strength is a renewable biomaterial. Studying the rheological properties of bamboo is helpful to improve the performance and quality of bamboo products. In this paper, four-element Burgers model was applied to describe the creep behavior of moso bamboo (Phyllostachs pubescens) in compression perpendicular to grain under hot-pressing process. The relationship between creep components and experimental factors (temperature, moisture content and stress level) was investigated. More importantly, four rheological parameters in Burgers model were also determined at different temperatures, moisture contents and stress levels. And the effect of experimental factors on rheological parameters was quantitatively explored. The results showed that, when compressive stress was below the yield limit, the amount of three components of creep was proportional to experimental factors, but the increase in temperature and moisture content could reduce the proportion of elastic deformation, and improve the proportion of viscoelastic deformation and viscous deformation. Besides, rheological parameters were insensitive to stress level when temperature and moisture content remained unchanged. But they were greatly affected by temperature and moisture content, presenting a linear inverse proportion to them.

更新日期：2020-01-16
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2020-01-15
Libor Navrátil, Louis Leveuf, Vincent Le Saux, Yann Marco, Jérôme Olhagaray, Sylvain Leclercq, Sylvain Moyne, Matthieu Le Saux

更新日期：2020-01-15
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2020-01-02
Imran Siddique, Iskander Tlili, Syeda Mahwish Bukhari, Yasir Mahsud

Unsteady free convection flows of an incompressible differential type fluid over an infinite vertical plate with fractional thermal transport are studied. Modern definitions of the fractional derivatives in the sense of Atangana–Baleanu (ABC) and Caputo Fabrizio (CF) are used in the constitutive equations for the thermal flux. Exact solutions in both cases of the (ABC) and (CF) derivatives for the dimensionless temperature and velocity fields are established by using the Laplace transform technique. Solutions for the ordinary case and some well-known results from the literature are recovered as a limiting case. Expressions for Nusselt number and Skin friction coefficient are also determined. The influence of the pertinent parameters on temperature and velocity fields are discussed graphically. A comparison of ordinary model, and (ABC) and (CF) models are also depicted. It is found that memory of the physical aspects of the problem is well explained by fractional order (ABC) and (CF) models as compared to ordinary one. Further it is noted that the (ABC) model is the best fit to explain the memory effect of the temperature and velocity fields.

更新日期：2020-01-02
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-12-31
Guangyu Luo, Wendong Yang, Chunjie Bo, Lianzhen Zhang, Kang Duan, Wenjun Jing, Yan Zhao

Creep properties of interlayered rock mass often encountered in rock engineering, significantly affect the safety and long-term stability of practical engineering. In this study, the theoretical equations of creep strain for interlayered rock specimens when the uniaxial compression loading was applied perpendicular to or parallel with bedding plane were derived. The laws of viscoelastic deformation for soft-hard interlayered rock specimens, soft-soft interlayered rock specimens and hard-hard interlayered rock specimens were also discussed, respectively. The creep deformation characteristics obtained from the numerical simulation and the theoretical analyses were consistent, confirming the correctness of the calculation method proposed in this paper. The results showed that the viscoelastic deformation of interlayered rock specimens was not only related to the volume ratio of rock layers, but it also was affected by the spatial position of the layered structure. In addition, creep properties of hard and soft rock layers were characterized by stable creep model and unstable creep model, respectively. The influences of volume content, number of strips, buried depth, spacing and angle of soft rock interlayer on the creep properties of soft-hard interlayered rock specimens were also analyzed.

更新日期：2019-12-31
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-12-31
Q. Z. Zhang, Q. Pan, C. Zhao, Y. Chen, B. A. Jang

Abstract The creep behavior of rock mass, including structural plane creep and rock creep, is different from that of intact rock. It is generally recognized that the characteristics of structural planes play a crucial role in such creep behavior. However, currently there are few comprehensive systematic studies on the rheological properties of structural planes, including experimental and theoretical aspects. In this paper, marble with green schist’s weak structural planes from the Jinping secondary hydropower station were selected as samples, and multi-stage loading shear creep tests were carried out in order to analyze the shear creep characteristics of such structural planes under different normal stress conditions. The test results showed that the shear creep process of structural planes can be divided into three stages, namely the attenuation stage, the constant stage, and the accelerated creep stage. Furthermore, the degree of structural plane development and normal stress level have a great influence on the creep behavior of rock mass. Moreover, based on the testing results, we established a nonlinear viscoelastic-plastic shear creep model by adding a new rheological element to the classic Nishihara model. In addition, the comparison between the test data and the fitting curve showed that this new model could describe the viscoelastic and viscoplastic deformation of weak structural planes more accurately.

更新日期：2019-12-31
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-12-18
Nasrin Jafari, Mojtaba Azhari

This paper presents a new method for bending analysis of moderately thick viscoelastic plates using elasticity responses at asymptotic times. The displacement vector is approximated utilizing two functions of geometrical parameters and one function of time parameter with an unknown coefficient. The unknown coefficient of time function is determined by solving an implicit equation having little computational cost. Numerical results are compared with other existing results to show the accuracy and efficiency of the present method. Also, the effects of geometrical parameters and material properties on the time-dependent central deflection of viscoelastic Mindlin plates are considered for the first time.

更新日期：2019-12-19
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-12-17

This paper investigates the postbuckling analysis of a viscoelastic microbeam embedded in a double layer viscoelastic foundation. This viscoelastic microbeam is modeled using the Kelvin–Voigt model and the modified couple stress theory. A material length scale parameter is utilized to describe the size-dependent behavior of the viscoelastic microbeam. The visco-Pasternak foundation used in this study contains a viscoelastic medium and a shear layer. This microbeam is subjected to an axial compressive load at the beam ends which can change as a function of time. According to the Euler–Bernoulli beam theory and von-Karman nonlinearity, the time-dependent equations of motion are derived by Hamilton’s principle. The nonlinear equations of motion are directly solved under the simply supported boundary condition. Both time-dependent deflection and viscoelastic buckling load are investigated. Finally, the influences of the material length scale parameter, parameters of the visco-Pasternak foundation and the material viscosity coefficient on the dynamic postbuckling response are studied.

更新日期：2019-12-18
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-12-16
V. Falahi, H. Mahbadi, M. R. Eslami

The purpose of this paper is to investigate the effect of stress classification on cyclic viscoplastic response of a structure (here, a thick spherical vessel which has an application in pressure vessel technology). The response is either reverse plasticity and shakedown or ratcheting behavior of a structure. The viscoplastic behavior of thick spherical vessels subjected to thermo-mechanical loads, using the rate-dependent Chaboche unified viscoplastic model with combined kinematic and isotropic hardening theory of plasticity, is evaluated. The material properties of the structure is assumed to be temperature dependent. A combination of generalized differential quadrature method (GDQ) and a precise numerical scheme, using the successive approximation iterative method, is employed to solve the resulting nonlinear differential equations. It is concluded that the Chaboche viscoplastic hardening model, as well as the plastic hardening model, predicts reverse plasticity under thermal cyclic loading with and without the creep consideration (a strain controlled cyclic loading). It is also shown that the rates of loading and unloading do not have significant influence on the cyclic viscoplastic behavior of the vessel, although they have influence on the strain and stress distributions.

更新日期：2019-12-17
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-12-16
Gesheng Xiao, Zhigang Li, Erqiang Liu, Li Qiao, Xuefeng Shu, Ruijing Sun

In this research, the microscale strain rate sensitivity and high-temperature mechanical properties of cured isotropic conductive adhesive (ICA) were investigated using microindentation. The indentation modulus and hardness of cured ICA with high silver content are relatively large. The slopes of contact stiffness–depth curve, modulus and hardness increase with increasing loading strain rate. The elastic modulus, hardness and creep behaviour at high temperature were characterised on the basis of the “rapid loading–holding–rapid unloading” loading mode and the semiempirical method from the generalised Kelvin model. With increasing temperature, the elastic modulus and hardness of cured ICA decrease from 3000–7000 and 100–300 MPa in the glassy state to 6–200 and 1–10 MPa, respectively, in the rubbery state. Creep compliance, which is relatively high in the rubbery state, increases with increasing holding time. On the retardation spectrum, the widened retardation peaks reflect different retardation processes with increasing retardation time.

更新日期：2019-12-17
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-12-12
Dong Wang, Lanying Lin, Feng Fu

Wood is generally considered a linear orthotropic viscoelastic. The creep of cell wall under long-term load is important for the deformation and destruction of wood. The aim of this study was to investigate the difference of creep compliance between compound middle lamella (CML) and secondary S2 layers by nanoindentation creep testing. The results indicated that the creep compliance of cell wall under compression along grain increased with the maximum load and loading rate increasing. Furthermore, the creep compliances and creep compliance percentages of the CML layer were more than that of the secondary S2 layer, and the viscoelastic behavior of the CML layer also was more sensitive to MC compared with the S2 layer. Finally, the Burgers’ model was appropriate for predicting the viscoelastic behavior of wood cell walls. The parameters of Burgers’ model dropped markedly with increased MC. These parameters in the CML layer also were lower than those of S2 layer. The differences of creep properties between the CML and S2 layers can prove that the slippage failure of cell wall under compression along grain occurs in the S2 layer.

更新日期：2019-12-13
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-11-26
Ali Dogan

This paper investigates the dynamic behavior of laminated composite shells (LCS) under the effect of dynamic loads. The governing equations for composite shells have been derived by using Hamilton’s principle. Then, these governing equations have been obtained by using Navier solution method. Many researchers do not use the term of ($$1+z/R$$) in the stress equations. However, these equations with the term of ($$1+z/R$$) have been obtained to get a more realistic model in the proposed study. It is aimed to emphasize the importance of the term ($$1+z/R$$) to take into account the effect of the radius of curvature in the calculation of the equations of the shell elements. In addition, the truncated series are used for a better numerical stability in the solution of the motion equations. The differential equations governing the system are derived by using the dynamic virtual displacements. Time-dependent ordinary differential equations are transformed into the Laplace space. Equations dependent on the parameters are then solved in this space. Calculations are transformed from the Laplace space into the time space by the help of modified Durbin’s algorithm. In order to verify the methodology, the results obtained in this study are compared with those obtained by Newmark and ANSYS finite element methods. The results of numerical studies for the dynamic response of laminated composite shells (LCS) are demonstrated and compared with previous studies in the literature.

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-11-26

In this paper, we investigate the history of radial displacement, stresses, electric potential, and magnetic potential of a functionally graded magneto-electro-elastic (FGMEE) hollow cylinder subjected to an axisymmetric hygro-thermo-magneto-electro-mechanical loading for the plane strain condition. The material properties are taken as a power-law function of radius. Using stress-displacement relations, equations of equilibrium, electrostatic and magnetostatic equations, we find a differential equation including creep strains. Initially, eliminating creep strains, we obtain an analytical solution for the primitive stresses and electric and magnetic potential. In the next step, considering creep strains, we find the creep stress rates by applying the Norton law and Prandtl–Reuss equations for steady-state hygrothermal boundary condition. Finally, using an iterative method, we find the time-dependent creep stresses, radial displacement, and magnetic and potential field redistributions at any time. In numerical section, are comprehensively investigate the effects of grading index, hygrothermal environmental conditions, rotating speed, and temperature- and moisture-dependency of elastic constant of FGMEE.

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-11-21
Chongfeng Chen, Tao Xu, Guanglei Zhou, Tao Qin

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-11-06
Shuang-jian Niu, Wen-lin Feng, Jin Yu, Chun-sheng Qiao, Sheng-xiang Liu, Yong-biao Sun

The creep of rock is a slow stress adjustment deformation and an evolutional process of rock mass destruction. Therefore, rock creep plays a vital role in a variety of rock engineering. To reveal the creep properties of rock, the following work has been done. A series of triaxial creep tests has been conducted considering different low confining pressures. The creep curve is analyzed under different confining pressures, and the creep law of each stage is obtained. Many results were obtained for the influence of confining pressure on the instantaneous modulus of deformation, creep strain, creep rate and instantaneous strain. The proportion of the various stages of creep changes under different confining pressure. Due to the existence of confining pressure, the ultimate failure method of rock has also been affected. Creep data can then be fitted by the improved Nishihara model, revealing a good relationship. The conclusion of this paper can provide some significant guidance for rock engineering.

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2018-08-20
Yifei Sun, Cheng Chen

The creep of coral sand is a critical factor that needs to be considered in geotechnical design of engineering facilities along coastal areas. In this study, a new fractional order creep model for coral sand is developed by extending the previous work on fractional (visco)plasticity. Unlike traditional fractional creep models that use viscoelasticity, this model uses a fractional order flow rule to capture the creep behaviour of coral sand. To validate the proposed model, test results of coral sand from the available literature are simulated, where good agreement between the model predictions and test results is observed.

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2018-07-03
M. M. Muhammad, M. Abdulhameed, I. Khan

This problem deals with steady driven pressure flow and heat transfer of electromagneto-hydrodynamic micro-pump of third-grade fluids between two micro-parallel plates embedded in a porous medium. The effects of thermal radiation and electro-kinetics have been taken into account. The flow forced by the Lorentz force, produced by the interaction of a vertical magnetic field and an externally horizontal imposed electrical field, is assumed to be unidirectional and one dimensional. Based upon the velocity field, the thermally fully developed heat transfer with radiation effect is analyzed by taking the viscous dissipation, the volumetric heat generation due to the Joule heating effect and the electromagnetic couple effect into account. Analytical solutions using the perturbation method are obtained in series forms, under the assumption that the third-grade non-Newtonian parameter has small values. Using the homotopy perturbation method, approximate analytical solutions for any values of the third-grade non-Newtonian parameter were obtained. The effects of the permeability of the porous medium $$K$$, the dimensionless electrical strength parameter $$H$$, the Hartmann number $$\mathit{Ha}$$, the non-Newtonian parameter Lambda, the constant pressure $$P$$, the thermal radiation $$\mathit{Nr}$$, and the non-dimensional parameter Brinkman number gamma1 on the velocity and temperature are investigated graphically and discussed in detail.

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2018-05-28
Meysam Rahmat

A significant number of materials show different mechanical behavior under dynamic loads compared to quasi-static (Salvado et al. in Prog. Mater. Sci. 88:186–231, 2017). Therefore, a comprehensive study of material dynamic behavior is essential for applications in which dynamic loads are dominant (Li et al. in J. Mater. Process. Technol. 255:373–386, 2018). In this work, aluminum 6061-T6, as an example of ductile alloys with numerous applications including in the aerospace industry, has been studied under quasi-static and dynamic tensile tests with strain rates of up to $$156~\mbox{s}^{-1}$$. Dogbone specimens were designed, instrumented and tested with a high speed servo-hydraulic load frame, and the results were validated with the literature. It was observed that at a strain rate of $$156~\mbox{s}^{-1}$$ the yield and ultimate strength increased by $$31\%$$ and $$33\%$$ from their quasi-static values, respectively. Moreover, the failure elongation and fracture energy per unit volume also increased by $$18\%$$ and $$52\%$$, respectively. A Johnson–Cook model was used to capture the behavior of the material at different strain rates, and a modified version of this model was presented to enhance the capabilities of the original model, especially in predicting material properties close to the failure point. Finally, the fracture surfaces of specimens tested under quasi-static and dynamic loads were compared and conclusions about the differences were drawn.

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2018-08-03
Kapil Kumar Kalkal, Suresh Kumar Sheokand, Sunita Deswal

The present manuscript is aimed at studying the propagation of plane waves in a fiber-reinforced, anisotropic, thermoelastic half-space with diffusion. The formulation is applied to generalized thermoelasticity based on the Green–Lindsay (G–L) theory. A thermal shock is applied on the surface of the half-space, which is taken to be traction free. The analytical expressions for the displacement components, stresses, concentration and temperature field are obtained in the physical domain by using normal mode analysis. Moreover, the derived expressions are computed numerically, and corresponding graphs are plotted to illustrate and compare theoretical results. Comparisons are made within the theory in the presence and absence of fiber-reinforcement and diffusion. The effect of time on the physical fields is also observed. Some particular cases of interest have been deduced from the present investigation.

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2018-08-23
Yue Mu, Anbiao Chen, Guoqun Zhao, Yujia Cui, Jiejie Feng, Foufei Ren

Viscoelasticity is an important characteristic of many complex fluids such as polymer solutions and melts. Understanding the viscoelastic behavior of such complex fluids presents mathematical, modeling and computational challenges, particularly in the case of fluids affected by elastic turbulence at high Weissenberg number. A numerical methodology based on the penalty finite element method with a decoupled algorithm is presented in the study to simulate three-dimensional flow of viscoelastic fluids. The discrete elastic viscous split stress (DEVSS) formulation in cooperating with log-conformation formulation transformation is employed to improve computational stability at high Weissenberg number. The momentum equation is calculated after introducing an ellipticity factor and the constitutive equation is calculated based on the logarithm of the conformation tensor. The finite element-finite difference formulations of governing equations are derived. The planar contraction as a representative benchmark problem is used to test the robustness of the numerical method to predict real flow patterns of viscoelastic fluids at different Weissenberg numbers. The simulation results predicted with differential constitutive models based on the logarithm of the conformation tensor agree well with Quinzani’s experimental results. Both the stability and the accuracy are improved compared with traditional calculation method. The numerical methodology proposed in the study can well predict complex flow patterns of viscoelastic fluids at high Weissenberg number.

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2018-08-02
Jieying Zhi, Shenping Wang, Mengjie Zhang, Haiqing Wang, Hongli Lu, Wenjun Lin, Congde Qiao, Changxu Hu, Yuxi Jia

The hysteresis loss and the heat generation of rubber compounds under different ambient temperatures and frequencies were predicted in terms of the viscoelastic theory. The cross-linking densities and the dynamic compressive properties were measured in order to partially reveal the microstructures and the viscoelastic response of rubber specimens. Based on the experimental data of storage and loss moduli, a piecewise fitting method was established to determine the viscoelastic constitutive model parameters. Then the dynamic compressive process of natural rubber specimen was analyzed by the finite element method. The obtained hysteresis loss and the energy dissipation were used to predict the transient and steady-state temperature field. The results reveal that the dependence of rubber hysteresis loss on the frequency at a lower ambient temperature plays a more important role in predicting the heat generation of rubber compounds; the dependence becomes weaker as the ambient temperature increases.

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-01-02
Nawras H. Mostafa

In this study, the stress relaxation effect on the fatigue cycles-to-failure of the biaxial elastic fibre prestressed woven composite (E-glass/polyester) was investigated. The fibre pretension load was applied prior and during matrix cure, and then it has been released to induce compressive residual stresses within the matrix. The longevity of these stresses is questionable, and it needs investigation. The time of residual stress redistribution or relaxation was estimated experimentally for the E-glass fibre prestressing level be equal to 50 MPa. Residual stresses within the polyester matrix have declined by (27%) throughout 110 days leading to reduce the improved fatigue life by about 14% due to the stress relaxation process within the polyester matrix material. The study showed that even though the stress relaxation in the matrix reduced the improved fatigue cycles of the biaxial elastic prestressed E-glass fabric/polyester resin system, some improvement still is possible for long-term performance.

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-09-26
W. M. Hasona

This paper aims to investigate the influences of variable viscosity and thermal conductivity on peristaltic flow of Carreau–Yasuda nanofluid in a 2D tapered asymmetric channel. Viscosity is considered as a function on the temperature of the fluid. Consequently, all dimension parameters that are functions of viscosity such as, thermophoresis and Brownian motion, and Prandtl, local temperature, and local nanoparticle Grashof numbers has also been performed as variable within the flow. For the pertinent problem, the flow equations are first established, and then reformulated under the assumption of low Reynolds number and long wavelength. Numerical results have been obtained for the pressure gradient as well as the velocity, temperature, and nanoparticle concentration distributions. Moreover, numerical integration has also been performed to assess the expressions for the pressure rise. It is worth mentioning that increases in the variable viscosity parameter cause diminishes in temperature. Hence, the use of nanofluids with high viscosity is favorable in application of solar energy to get higher performance (by acting as a cooling system for solar cells) and lower operating costs. Increases in Brownian motion and thermophoresis parameters cause better transport of heat, and highly absorption in the solar range.

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-09-26
S. M. Mirkhalaf, M. Fagerström

Polylactic acid (PLA) is one of the highly applicable bio-polymers in a wide variety of applications including medical fields and packaging. In order to quantitatively model the mechanical behavior of PLA and PLA based bio-composite materials, and also tailor new bio-composites, it is required to characterize the mechanical behavior of PLA. In this study, thin films of PLA are fabricated via hot-pressing, and tensile experiments are performed under different strain rates. To model the mechanical behavior, an elasto-viscoplastic constitutive model, developed in a finite strain setting, is adopted and calibrated. Using the physically-based constitutive model, all regimes of deformation under uniaxial stress state, including post-yield softening, were adequately captured in the simulations. Also, the rate dependency of the stress–strain behavior was properly modelled.

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-09-23
Giulia Fredi, Andrea Dorigato, Alessandro Pegoretti

Dynamic-mechanical analysis (DMA) was performed to investigate the viscoelastic response of multifunctional laminates for thermal energy storage (TES). The laminates were constituted by a microencapsulated paraffinic phase change material (PCM), a carbon fiber fabric, and an innovative reactive acrylic resin (Elium®). In the Elium®/PCM systems, the PCM fraction affected neither the glass transition temperature ($$T _{\mathrm{g}}$$) of the resin, found at 100–120 ∘C, nor the activation energy of the glass transition, determined with multifrequency scans from the position of the $$\tan\delta$$ peaks. On the other hand, the low-temperature (0–40 ∘C) transition detected on the neat resin was hidden by the PCM melting, evidenced by a step in $$E'$$ and peaks in $$E''$$ and $$\tan \delta$$. In the laminates, the amplitude of the $$E'$$ step and the intensity of the $$\tan \delta$$ peak associated to the PCM melting presented a linear correlation with the PCM content and the melting enthalpy. Cyclic heating/cooling DMA tests showed that the decrease in $$E'$$ due to PCM melting was almost completely recovered (90–95%) upon crystallization. The difference between the $$\tan \delta$$ peak positions on heating and on cooling decreased from 30 to 12 ∘C when the heating/cooling rate changes from 3 to 1 ∘C/min. Multifrequency tests highlighted that the activation energy of the glass transition of the laminates was lower than that of the matrices, and it did not follow a trend with the PCM fraction. Interestingly, also the $$E''$$ and $$\tan \delta$$ peaks related to PCM melting depended on the testing frequency, and their asymmetric shape could be interpreted by considering a progressive melting of the PCM in the microcapsules during heating.

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-09-23
Hongwei Zhou, Teng Su, Huilin Deng, Rui Wang, Jiawei Zhao, Xiaotong Sun, Lu An

Accurate description of creep behavior is of great significance to the safety evaluation of high-level radioactive waste disposal in granite. In this study, a fractional derivative constitutive model is proposed to depict the creep process of Beishan granite, based on the conformable derivative. A variable-coefficient Abel dashpot considering the effect of damage evolution on creep properties is introduced to describe the nonlinear dilatancy strain in the accelerating stage. A new model is proposed and further generalized to three-dimensional creep equations by adopting Drucker–Prager criterion, and the analytic solutions are given employing the Laplace transform. The parameters of new creep model are determined based on experimental data and fitting results are compared. The fitting curves present a good agreement with experimental data, indicating that the conformable derivative creep model achieves a good performance in describing all three creep stages. The proposed model is validated by the comparison of different experiments and degradation of fractional derivative model. In addition, a sensitivity analysis is carried out to show the effects of stress level, fractional derivative order, damage exponent, and other parameters.

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-09-17
Marco Contino, Luca Andena, Vincenzo La Valle, Marta Rink, Giuliano Marra, Stefano Resta

According to linear elastic fracture mechanics the stress intensity factor and the energy release rate are two fracture parameters linked by the elastic modulus and Poisson’s ratio of the considered material. This concept has been extended to the analysis of linear viscoelastic materials, by introducing time-dependent quantities; it is also used for nonlinear viscoelastic polymers, even if its accuracy in this case is still an open question. In this work the slow crack growth and the environmental stress cracking resistance of two high-density polyethylene grades were investigated, differing for their molecular weight distribution and fracture resistance. The description of the fracture behavior of the two materials provided by the stress intensity factor or the energy release rate turned out to be equivalent, despite the nonlinear mechanical behavior exhibited by the two polymers. Moreover, a time-dependent effective modulus, related to the two fracture parameters, was evaluated: its value was in good agreement with the modulus experimentally determined from tensile tests. A constant relevant effective strain was found despite the different testing conditions (i.e. applied mechanical loading, temperature and presence of an active environment), its value being equal for the two considered polyethylenes.

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-09-03
Wangnan Li, Shaojuan Lu, Guijie Liang, Xiaohong Cheng, Jingyang Wang, Zhicheng Zhong

Based on the micro-mechanics of failure (MMF) theory, the accelerated testing methodology (ATM) and the VUMAT subroutine in ABAQUS, this study put forward a novel microscopic analysis approach for the large deformation failure analysis of composite. The predicted strength accuracy of the MMF approach for the bolted structure was then compared with those calculated based on the Tsai–Wu and Hashin theories, and with the experimental results. Results showed that the MMF method enjoyed the highest strength accuracy with an error of 7.2%, and the component damage initial occurred in matrix of the 90-degree layer and the initial failure of fiber occurred in the 0-degree layer. The failure mode predicted by the MMF approach behaved as shearing mode, which was in agreement with the experimental results. The long-term static strength of the bolted structure was predicted based on the MMF/ATM approach, and a comparison with the test results was conducted.

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-08-30
K. Sakaue, M. Ochiai, S. Endo, H. Takaoka

The mechanical characteristics of short-fiber-reinforced thermoplastics should be evaluated based on viscoelastic theory because of the time and temperature dependent characteristics of the matrix resin. In this study, the viscoelastic characteristics of short-fiber-reinforced polybutylene terephthalate, PBT composite, are evaluated by material testing and micromechanics model. Firstly, tensile tests are carried out PBT composite with three fiber volume ratios under various temperature and strain rate conditions. The results show that the viscoelastic characteristics of the PBT composites can be obtained by the time-temperature superposition principle using shift factor of the PBT resin. Next, the Eshelby model combined with the Mori–Tanaka theory is used to predict to the viscoelastic characteristics of the PBT composites based on the measurement of fiber length and fiber orientation angle. The predicted viscoelastic characteristics of the PBT composites are in good agreement with the tensile test.

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-08-30

In this paper, the thermo-mechanical dynamic response of annular/circular viscoelastic graphene plates embedded in visco-Pasternak foundation has been investigated using nonlocal first and modified higher-order shear deformation theories. The modified higher-order shear deformation theory is assumed to obtain the results of thick (high ratio of thickness to size) plates too. Also, the sheet is considered in thermal environment in order to study the thermal effects on the viscoelastic analysis. The viscoelastic behavior of the plate is simulated based on the Kelvin–Voigt model and the nonlinear von Kármán strains have been considered. The dynamic governing equations have been derived using the Hamilton stationary of minimum potential energy based on the nonlocal first and modified higher-order shear theories and have been solved applying semi-analytical polynomial method solving method. The solving methodology is unique and has not been used in any other dynamic analysis before and its ability for solving the dynamic governing equations has also been confirmed. The time-dependent deflection of the sheet under uniform and non-uniform loads has been obtained. Different effects on the problem, including boundary conditions, damping coefficient of visco-Pasternak foundation, viscoelastic coefficient of the plate, small-scale effects, loading, and nonlinear effects have been discussed more precisely.

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2019-08-12
Nofar Stivi, Arieh Sidess, Daniel Rittel

更新日期：2019-12-11
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2018-12-14
Heinz E Pettermann,Antonio DeSimone

A constitutive material law for linear thermo-viscoelasticity in the time domain is presented. The time-dependent relaxation formulation is given for full anisotropy, i.e., both the elastic and the viscous properties are anisotropic. Thereby, each element of the relaxation tensor is described by its own and independent Prony series expansion. Exceeding common viscoelasticity, time-dependent thermal expansion relaxation/creep is treated as inherent material behavior. The pertinent equations are derived and an incremental, implicit time integration scheme is presented. The developments are implemented into an implicit FEM software for orthotropic material symmetry under plane stress assumption. Even if this is a reduced problem, all essential features are present and allow for the entire verification and validation of the approach. Various simulations on isotropic and orthotropic problems are carried out to demonstrate the material behavior under investigation.

更新日期：2019-11-01
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2016-01-01
M H Motta Dias,K M B Jansen,J W Luinge,H E N Bersee,R Benedictus

The influence of fiber-matrix adhesion on the linear viscoelastic creep behavior of 'as received' and 'surface modified' carbon fibers (AR-CF and SM-CF, respectively) reinforced polyphenylene sulfide (PPS) composite materials was investigated. Short-term tensile creep tests were performed on ±45° specimens under six different isothermal conditions, 40, 50, 60, 65, 70 and 75 °C. Physical aging effects were evaluated on both systems using the short-term test method established by Struik. The results showed that the shapes of the curves were affected neither by physical aging nor by the test temperature, allowing then superposition to be made. A unified model was proposed with a single physical aging and temperature-dependent shift factor, aT,te . It was suggested that the surface treatment carried out in SM-CF/PPS had two major effects on the creep response of CF/PPS composites at a reference temperature of 40 °C: a lowering of the initial compliance of about 25 % and a slowing down of the creep response of about 1.1 decade.

更新日期：2019-11-01
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2017-01-01
J Campbell,J Dean,T W Clyne

This study concerns a commonly-used procedure for evaluating the steady state creep stress exponent, n , from indentation data. The procedure involves monitoring the indenter displacement history under constant load and making the assumption that, once its velocity has stabilised, the system is in a quasi-steady state, with stage II creep dominating the behaviour. The stress and strain fields under the indenter are represented by "equivalent stress" and "equivalent strain rate" values. The estimate of n is then obtained as the gradient of a plot of the logarithm of the equivalent strain rate against the logarithm of the equivalent stress. Concerns have, however, been expressed about the reliability of this procedure, and indeed it has already been shown to be fundamentally flawed. In the present paper, it is demonstrated, using a very simple analysis, that, for a genuinely stable velocity, the procedure always leads to the same, constant value for n (either 1.0 or 0.5, depending on whether the tip shape is spherical or self-similar). This occurs irrespective of the value of the measured velocity, or indeed of any creep characteristic of the material. It is now clear that previously-measured values of n , obtained using this procedure, have varied in a more or less random fashion, depending on the functional form chosen to represent the displacement-time history and the experimental variables (tip shape and size, penetration depth, etc.), with little or no sensitivity to the true value of n .

更新日期：2019-11-01
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2015-01-01
Gail M Thornton,Soraya J Bailey,Timothy D Schwab

Ligaments are dense fibrous tissues that connect bones across a joint and are exposed daily to creep and fatigue loading. Ligaments are tensile load-bearing tissues; therefore, fatigue loading will have a component of time-dependent damage from the non-zero mean stress and cycle-dependent damage from the oscillating stress. If time-dependent damage is not sufficient to completely predict the fatigue response, then cycle-dependent damage could be an important contributor. Using data from normal ligaments (current study and Thornton et al., Clin. Biomech. 22:932-940, 2007a) and healing ligaments (Thornton and Bailey, J. Biomech. Eng. 135:091004-1-091004-6, 2013), creep data was used to predict the fatigue response considering time-dependent damage. Relationships between creep lifetime and test stress or initial strain were modelled using exponential or power-law regression. In order to predict fatigue lifetimes, constant rates of damage were assumed and time-varying stresses were introduced into the expressions for time-dependent damage from creep. Then, the predictions of fatigue lifetime were compared with curvefits to the fatigue data where exponential or power-law regressions were used to determine the relationship between fatigue lifetime and test stress or initial strain. The fatigue prediction based on time-dependent damage alone greatly overestimated fatigue lifetime suggesting that time-dependent damage alone cannot account for all of the damage accumulated during fatigue and that cycle-dependent damage has an important role. At lower stress and strain, time-dependent damage was a greater relative contributor for normal ligaments than healing ligaments; however, cycle-dependent damage was a greater relative contributor with incremental increases in stress or strain for normal ligaments than healing ligaments.

更新日期：2019-11-01
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2018-07-03
T Y Qiu,M Song,L G Zhao

This paper studied the mechanical performance of four bioresorbable PLLA stents, i.e., Absorb, Elixir, Igaki-Tamai and RevaMedical, during crimping and expansion using the finite element method. Abaqus CAE was used to create the geometrical models for the four stents. A tri-folded balloon was created using NX software. For the stents, elastic-plastic behaviour was used, with hardening implemented by considering the increase of yield stress with the plastic strain. The tri-folded balloon was treated as linear elastic. To simulate the crimping of stents, a set of 12 rigid plates were generated around the stents with a radially enforced displacement. During crimping, the stents were compressed from a diameter of 3 mm to 1.2 mm, with the maximum stress developed at both inner and outer sides of the U-bends. During expansion, the stent inner diameter increased to 3 mm at the peak pressure and then recoiled to different final diameters after balloon deflation due to different stent designs. The maximum stress was found again at the U-bends of stents. Diameter change, recoiling effect and radial strength/stiffness were also compared for the four stents to assess the effect of design variation on stent performance. The effect of loading rate on stent deformation was also simulated by considering the time-dependent plastic behaviour of polymeric material.

更新日期：2019-11-01
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2018-07-03
J Sweeney,P E Spencer,D Vgenopoulos,M Babenko,F Boutenel,P Caton-Rose,P D Coates

An established statistical mechanical theory of amorphous polymer deformation has been incorporated as a plastic mechanism into a constitutive model and applied to a range of polymer mechanical deformations. The temperature and rate dependence of the tensile yield of PVC, as reported in early studies, has been modeled to high levels of accuracy. Tensile experiments on PET reported here are analyzed similarly and good accuracy is also achieved. The frequently observed increase in the gradient of the plot of yield stress against logarithm of strain rate is an inherent feature of the constitutive model. The form of temperature dependence of the yield that is predicted by the model is found to give an accurate representation. The constitutive model is developed in two-dimensional form and implemented as a user-defined subroutine in the finite element package ABAQUS. This analysis is applied to the tensile experiments on PET, in some of which strain is localized in the form of shear bands and necks. These deformations are modeled with partial success, though adiabatic heating of the instability causes inaccuracies for this isothermal implementation of the model. The plastic mechanism has advantages over the Eyring process, is equally tractable, and presents no particular difficulties in implementation with finite elements.

更新日期：2019-11-01
• Mech. Time Depend. Mat. (IF 1.831) Pub Date : 2018-07-03
J A W van Dommelen,R Estevez

更新日期：2019-11-01
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