Mechanics of Time-Dependent Materials最新文献

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.

The scope of the present work is to study, experimentally and theoretically, the temperature and strain rate effect on the yielding and postyielding tensile behavior of an epoxy resin. Regarding the theoretical study, a three-dimensional viscoplastic model, namely a Zener B model, associated with the decomposition of the total strain into elastic and viscoplastic part was employed. To have an integrated aspect regarding the various models potentiality, a fractional Zener B viscoelastic model was comparatively utilized. Due to the limited capability of the two well-known models to describe the strain softening, exhibited by the polymeric material, apart from a stress-dependent viscosity related to a nonlinear Eyring-type dashpot, a strain-dependent activation volume was considered to be developed by a distribution function. The strain hardening hereafter was simulated by a back stress, associated with a hyperelastic spring. The strain rate effect could be successfully predicted by the scaling rule valid in viscoelasticity. No essential superior capability simulation was deduced from the comparative study between the two models.

Sandwich panels with honeycomb cores are widely used for structural applications due to their lightweight and impact-resistant properties. However, improving the energy absorption and crashworthiness of these panels remains a significant challenge, particularly when optimizing core materials and skin configurations. This study examines how different core materials, STF-filled honeycomb, water, resin, and semi-rigid foam, affect the impact performance of sandwich panels at low velocities. Additionally, the influence of different skin materials such as aluminum, epoxy-glass composites, and STF-impregnated fabric is analyzed. The panels were fabricated by filling the honeycomb cores with different materials and applying the skins to the cores. Low-velocity impact tests were conducted at drop heights of 100 mm and 500 mm to evaluate energy absorption, mean crushing force, and specific energy absorption. The results demonstrate that STF-filled cores significantly improve energy absorption and impact resistance compared to traditional core materials. Furthermore, STF-impregnated fabric skins enhance overall panel performance, making STF-filled sandwich panels a promising solution for lightweight, high-strength structures in industries such as automotive and aerospace.

Previous thermoelastic models have struggled to accurately capture the complex behavior of materials under thermal and mechanical loads, particularly with regard to nonlocal effects and memory-dependent behaviors. To address this limitation, a new model has been developed to study the behavior of porous materials with voids, which are critical in engineering applications such as construction, aerospace, and biomedicine. The proposed model is based on the dual-phase lag theory (DPL), which accounts for delays in thermal responses within porous materials, where multiple phases influence thermal conductivity. A key innovation of this research is the integration of spatial and temporal nonlocal effects, which are essential for understanding microscopic interactions in porous materials. Furthermore, the introduction of Caputo-tempered fractional derivatives enhances the modeling of memory effects, providing a more precise understanding of how previous deformations and thermal exposures influence the behavior of these materials. The model has been validated by analyzing the transient response of a porous cylindrical medium subjected to a laser-shaped thermal flow. The effects of nonlocal interactions, phase delays, and fractional parameters on the thermomechanical responses have subsequently been compared and examined. The findings underscored the pivotal role of nonlocal time-length scale parameters in nanomaterial models, highlighting their influence on the reduction of heat transfer efficiency and the attenuation of thermal stresses.

The rheological behavior of wood emerges from complex mechanical interactions occurring across multiple length scales. This behavior is characterized by directional dependence, as well as sensitivity to moisture content, loading time, and the degree of loading. This study focuses on the viscoelastic creep response of Norway spruce (Picea abies) tissues under different moisture levels and loading degrees. Using a custom-designed, fully automated test rack with moisture control, we investigate the uniaxial, moisture-dependent creep compliances across all feasible anatomical directions, as well as of isolated earlywood (EW) and latewood (LW) slices to understand their contribution to the cumulative behavior of the growth ring. The creep response is compared to the moisture dependence of the elastic compliance, revealing nontrivial scaling behavior as a function of moisture content. The results show significant directional dependencies and reveal the critical impact of moisture on deformation mechanisms. The transverse directions involve a complex interaction between bending, determining a more compliant and moisture-sensitive creep response, and cell wall stretching in the softest direction compared to loading in grain. These findings offer valuable insights into the moisture-dependent creep mechanisms of wood slices, highlighting the importance of exploring different orientations and tissues at various moisture content to fully understand the creep behavior at the bulk scale.

The Adomian Decomposition Sumudu Transform Method (ADSTM) is applied to solve a fractional-order problem that involves temperature variations in a fully wet convective–radiative longitudinal fin. Darcy’s law is used in formulating the energy balance equation to take into account the porous nature of the fin. The fractional-order energy balance equation for the fin is solved under two situations: a constant convective heat transfer coefficient and a temperature-dependent convective heat transfer coefficient. The ADSTM solution is compared with numerical results, obtained using the Runge–Kutta–Fehlberg approach. A series solution is obtained, and the roles of various parameters of the fractional-order differential equation are analyzed. It is found that the solution to the fractional-order differential equation outperforms the integer-order solution in modeling the temperature profile of the fin. Furthermore, it is observed that improvements in the wet porous characteristics of the fin lead to a reduction in its temperature.