Mechanics of Time-Dependent Materials最新文献

Uniaxial tensile creep tests were conducted at various stress levels to investigate the creep properties of hydroxy-terminated polybutadiene (HTPB) propellant. Due to the limitations of the classical time-hardening model and the Burgers model in predicting the nonlinear creep behavior of HTPB propellant, a new creep damage model was developed. This model combines linear viscoelasticity theory with continuum damage theory. Utilizing the user material subroutine UMAT provided by ABAQUS for the secondary development of the models, simulation calculations were performed on dumbbell specimens. A comparative analysis was conducted with the results from the time-hardening model and the Burgers model, and the simulation results were validated through experimental testing. The findings indicate that HTPB propellant exhibits a distinct three-stage process characterized by decay creep, stable creep, and accelerated creep. The creep damage model effectively describes the accelerated creep stage, with the simulation results demonstrating an error margin of less than 5%. This confirms the feasibility of the creep damage model for creep analysis of HTPB propellant.

This research aims to explore the creep behavior of the AZ31B alloy under various temperature and stress conditions using the impression technique. The experimental studies were carried out at temperatures ranging from 150 °C to 250 °C and stress levels ranging from 121 to 401 MPa. The results showed that the stress exponent and activation energy of this alloy varied depending on the test conditions, resulting in different creep mechanisms. The stress exponents obtained at high-stress values were 7.2, 4.44, 9.59, and 11.35 at 150, 175, 200, and 250 °C, respectively. In low-stress regimes, the values were 1.12, 3.25, 4.77, and 4.91, respectively. Results highlighted that at lower stress levels, grain boundary diffusion, dislocation viscous glide, and dislocation climb were the dominant creep mechanisms, while at high stress levels, dislocation climb and a combination of dislocation climb, glide, and cross-slip mechanisms governed the material’s deformation behavior. The activation energy was determined to be 87.61 kJ/mol at low-stress conditions, indicating grain boundary diffusion and pipe diffusion as the rate-controlling mechanisms. Under high-stress conditions, it reached 103.7 kJ/mol, suggesting pipe diffusion or Mg lattice self-diffusion. The upper-bound analysis results were also used to establish the correlation between creep properties obtained from impression ((P) and (dot{U}), which are punch pressure and velocity, respectively) and conventional ((sigma ) and (dot{varepsilon } ), representing stress and strain rate, respectively) tests. Conversion factors of (P)/(sigma ) = 3.72 and (dot{varepsilon } )/(dot{U}) = 2.23 were calculated to relate these parameters. These findings provide valuable insights for guiding design decisions in the industrial applications of magnesium alloys.

This paper aims to investigate the mathematical modeling of Casson hybrid nanofluid flow, which uses pure blood as the base fluid and incorporates the impacts of titanium dioxide (TiO₂) and silver (Ag) nanoparticles on a Riga plate that helps to stabilize and disperse drug molecules efficiently through a drug-delivery system. The bottom plate is assumed to be implemented with thermal-source effects where the fluid flow has time-dependent attributes. The squeezing characteristics are considered to be induced on the surface of the upper Riga plate that is moving with some speed. A set of suitable variables are incorporated to convert modeled equations to dimensionless form. The problem was initially solved through homotopy analysis method (HAM) and then the artificial neural network (ANN) is used on the basis of HAM. Medical diagnostics could benefit from this model, particularly in the process of drug delivery and the flow dynamics of the microcirculatory mechanism. It has been observed in this study that, with growth in the modified Hartman number, as well as the volumetric fraction of titanium dioxide nanoparticles, the velocity distribution was retarded for both Ag/blood nanofluid and Ag+TiO_2/ blood hybrid nanofluid. For an increase in the volumetric fraction of silver nanoparticles and thermal-source factor there is a corresponding progression in thermal distribution both for Ag/blood nanofluid and Ag+TiO_2/ blood hybrid nanofluid. The heat-transfer rate determines the sustainability of drug delivery by ensuring its safe administration. It is observed that using the 5% nanoparticle volume fraction the obtained results show that a 10.06% increase has been achieved using the hybrid nanofluid in comparison with Ag nanofluid that has increased the heat-transfer rate up to 7.79%. With an increase in the squeezing factor (S) such that (S = 0.0, - 0.2, - 0.4, - 0.6, - 0.8, - 1.0, - 1.2) there is a reduction in the thermal distribution. The optimal model performance is observed at epochs 211, 179, 115, 181, and 168, as indicated in the data displayed at these stated epochs throughout the training. For all five scenarios gradient values are linked at (9.94 times 10^{ - 8}), (9.88 times 10^{ - 9}), (9.90 times 10^{ - 8}), (9.90 times 10^{ - 8}), and (9.93 times 10^{ - 8}). Medical diagnostics could benefit from this model, particularly in the process of drug delivery and the flow dynamics of the microcirculatory mechanism.

Discrepancies between laboratory-based predictions and field performance of asphalt mixes in terms of fatigue life can be reduced by taking into account the self-healing characteristics of asphalt in experimental protocols. In this study, an unmodified binder and a polymer-modified binder are used to compare their relative performance in terms of healing both in the presence and absence of a warm mix additive (WMA). During the test, rest periods of varied durations (10, 15, and 30 minutes) are introduced at 25%, 50%, and 75% of damage levels prior to reaching failure to examine their influence on the further evolution of damage. The addition of the WMA resulted in an improved healing index of both unmodified and modified binders at all the damage levels pre-failure. The results suggest the potential of WMA additives to enhance the healing of bituminous mixes, in addition to their established benefits in lowering temperatures.

Fluctuations in gas pressure within salt cavern storage and the creep behavior of salt rock are key factors influencing the deformation of surrounding rock and the stability of salt caverns. Considering the operational characteristics of salt cavern storage, this study conducted triaxial graded loading creep tests on an impurity-containing salt rock to systematically analyze its creep deformation, strength characteristics, and failure modes under different confining pressures. The findings reveal that as axial stress increases, creep strain gradually becomes the dominant deformation component in an impurity-containing salt rock, while the proportion of instantaneous compressive strain decreases. When axial stress levels are similar, increasing confining pressure reduces both instantaneous compressive and steady-state creep strain rates. Under similar deviatoric stress conditions, a higher confining pressure leads to varying degrees of increase in instantaneous elastic strain, creep strain, and total strain of an impurity-containing salt rock. Under different confining pressures, the evolution of the steady-state creep strain rate and the viscosity coefficient follows an inverse function relationship. Based on the creep characteristics of salt rock and the geometric features of creep models in the nonaccelerated creep stage, a nonlinear integer-order viscous dashpot is proposed to describe the strain surge in the accelerated creep stage. A nonlinear viscoelastic-plastic creep model capable of capturing the entire creep process of salt rock is developed and further extended to a three-dimensional stress state. Comparative analysis demonstrates that the proposed creep model effectively describes the full creep process of different types of salt rock, particularly the accelerated creep stage.

In this paper, an investigation was conducted on the temperature–time dependence of the electrical durability of high-pressure polyethylene (HPPE) films. The role of organic additives, phthalic anhydride and phthalic acid, was identified. The additive content in HPPE compositions was adjusted with a range from 0.01 to 0.1 mass percent. Results show that incorporating appropriate amounts of these additives increases the electrical strength of HPPE films by approximately 50% compared to unmodified HPPE. Measurements were also conducted on the electrical strength of HPPE films and their modified compositions under varying mechanical stress levels. The activation energy of electrical breakdown ((U)) and its intrinsic value ((U_{0})) remained consistent across both unmodified HPPE and its optimally modified forms. However, modifications with phthalic anhydride and phthalic acid altered the structure-sensitive coefficient ((beta )), reflecting changes in HPPE properties. The value of (U_{0}) aligns with the activation energy of chemical bonds, indicating that electrical breakdown in these polymers primarily occurs through bond disruption.

An inverse analysis technique is proposed for identifying characteristics from experimental data using the virtual fields method with stress-sensitivity-based virtual fields. Two-dimensional digital image correlation is used to obtain the inplane displacement and strain distributions on the specimen surface. To determine the stress distributions under the plane-stress condition, the numerical Laplace transform is used to obtain the through-thickness strain from the inplane strains based on the correspondence principle. Using the virtual displacement fields based on the stress sensitivity, the bulk and shear relaxation moduli, which represent the viscoelastic characteristics, are simultaneously identified. The various stress states generated by the specimen shape and their time variation in a single test and the use of the stress-sensitivity-based virtual fields make it possible to simultaneously determine two independent viscoelastic material properties. Therefore, this method is expected to make a significant contribution to the mechanics of viscoelastic materials.

Spacer fabric is a type of 3D textile characterized by the presence of two distinct fabric layers that are interconnected by spacer yarns. This investigation focused on analyzing the impact of the weft-knitted spacer fabrics’ thickness resulting from different spacer layers’ knit patterns on the compressional behavior of the spacer fabrics designed specifically for shoe soles. In this regard, the static and dynamic compressional behavior of spacer fabrics was examined according to the simulation carried out for walking. The results obtained from the application of static compressive force on spacer fabrics revealed that by increasing the spacer yarn’s length, the compression and recovery energy, dissipated compression energy, relative compressibility, and thickness recovery of spacer fabrics decreased; in contrast, the surface thickness changes increased. In addition, by examining the results of dynamic compressibility, it was observed that after the first loading cycle, the mechanical properties of spacer fabrics have changed significantly; however, with the increase in the number of loading cycles, the variation rate of these properties has decreased and reached a constant value. Also, with the increase of loading cycles, compressibility, and recovery work, dissipated compression energy and the elastic strain decreased, while the residual strain and ratcheting strain increased. Finally, an analysis of the compressional behavior of weft-knitted spacer fabrics based on viscoelastic models showed that the three-component model with a nonlinear spring can successfully correlate with the experimental results.

The objective of this study is to investigate the long-term stability of high-speed rail tunnel basements in diatomite stratum. A series of experiments that included scanning electron microscopy, X-ray diffraction, and creep tests of diatomite were carried out to investigate its microscopic mechanism, chemical composition, and mineral composition. The Burgers model was employed to reveal and describe the creep and deformation characteristics of diatomite. Additionally, the deformation characteristics of the tunnel basement in diatomite stratum under different conditions were investigated using FLAC3D finite-difference software. The results show that different diatomites contain a large number of disc-shaped diatomites and cylindrical diatomites, which are composed of complete diatomites, diatomite fragments, and clay minerals. Under the same load, the creep deformation of white diatomite and blue diatomite with increasing saturation shows the trend of decreasing and then increasing. The deformation of tunnel basement uplift in diatomite stratum with the increase of time under different conditions exhibits a law of change characterized by a rapid rise, attenuation, and subsequent tendency towards stability. The long-term deformation of the stratum 1 m under the tunnel basement and below the center of the tunnel basement (up to the model boundary) is in the order of high saturation > low saturation > saturation, white diatomite > blue diatomite, and full coverage > semi-full coverage > under coverage. The research results can be an important reference for the geotechnical stability research of diatomite affected by the creep effect.

This study investigates the flow behavior of a Casson fluid under specific conditions relevant to engineering, astrophysics, and biofluid mechanics. Blood, which exhibits Casson-fluid properties, interacts with magnetohydrodynamics (MHD), and understanding this behavior can aid in designing medical devices such as blood pumps and diagnostic tools for conditions like hypertension. The research examines the unsteady MHD free-convective rotational flow of an incompressible, electrically conducting Casson fluid over an impulsively moving, infinite, vertical porous plate. The study incorporates a ramped wall temperature and mass concentration while considering the effects of Hall current and ion slip. A uniform magnetic field is applied perpendicular to the flow direction, assuming a low magnetic Reynolds number, which renders the induced magnetic field negligible. The Rosseland approximation is used to model radiative-heat transfer in the energy equation. Analytical solutions to the governing equations are obtained using the Laplace-transform method. The influence of key parameters on velocity, temperature, and mass-concentration distributions is investigated through graphical representations. Additionally, shear stress, heat-transfer rates, and mass-transport rates are examined using tabulated data. Results indicate that Hall current and ion slip enhance the resultant fluid velocity. Significant differences in velocity profiles are observed between ramped and isothermal boundary conditions. Furthermore, this study has implications for thermal management in spacecraft components and industrial applications involving Casson fluids, such as molten plastics and polymers. The findings also provide insights into extrusion and molding processes under varying conditions.