Mechanics of Time-Dependent Materials最新文献

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.

Prestress loss poses a significant risk to structural safety and must be carefully considered in the design of prestressed anchors. This study begins by analyzing the relationship between creep in the anchorage zone and prestress loss. Subsequently, creep tests were conducted to determine the creep parameters of the anchorage zone and the differences in creep behavior under varying anchorage lengths were compared. Based on the test results, numerical simulations were performed to investigate the evolution of mechanical behavior during prestress loss and the influence of anchorage length on long-term performance. The results indicated that the coupled process of prestress loss and creep could be characterized as creep behavior under variable load, where the load was correlated with the total displacement of the anchor. During prestress loss, the shear load distribution in the anchorage zone transitioned from a concentrated to a uniform pattern. Specifically, the shear load near the free section decreased while the shear load near the bottom increased. The boundary between these two regions remained relatively stable, ranging between 0.5 m and 0.75 m, regardless of anchorage length. Increasing the anchorage length significantly reduced the prestress loss during the first year. This reduction was attributed to the smaller shear force and creep deformation in the rear section, which constrained deformation in the front section and minimized retraction in the free section of the anchor. However, the effectiveness of increasing anchorage length diminished as the length continued to expand. These findings offer valuable insights into the influence of anchorage length on the long-term service capacity of prestressed anchors, guiding structural design and optimization.

One of the most important ways to reduce energy consumption in belt conveyor transportation along horizontal routes of significant length is to ensure limited belt indentation rolling resistance. This type of resistance results from the cyclical compression of the bottom cover of the belt by idlers as the belt moves over successive idler sets. This paper presents a method for testing the dynamic properties of rubber subjected to cyclical compression. The presented analysis is multiaspect. The method parameters have been optimized to match one of the theoretical models of indentation rolling resistance calculations. The tests of dynamic properties were performed for five different rubber types and included calculations of the belt damping factor and modulus of elasticity. The theoretical model allowed for the calculations of belt indentation rolling resistance. The results were verified in indentation rolling resistance tests performed on a test rig designed for steel-cord belts. The measurements were performed for belt specimens with covers made of the tested rubber types. The calculation results highly correlated with the measurement results. The proposed method for testing the dynamic properties of rubber can not only provide data required in the calculations of the power demand from belt conveyors but also serve conveyor belt manufacturers in their search for rubber compounds used in belts that generate lower indentation rolling resistances.

Relaxation behavior of fiber-reinforced polymer (FRP) bars is one of the main issues considered in prestressing applications. Recently, basalt FRP (BFRP) bars have been developed as an alternative to other types of FRP bars. However, the available studies on their relaxation behavior are still limited, especially under severe environmental conditions. In this study, the long-term relaxation behavior of BFRP bars was studied, the initial stress level under different environmental conditions are the variables considered in this investigation. A total of twenty-four specimens of BFRP bars of nominal diameter 6 mm were used in this study. The relaxation behavior and the residual tensile strength of BFRP after relaxation were experimentally assessed. The expected relaxation loss after a million hours was estimated based on the extrapolated statistical analysis of the experimental results. The test results showed that the relaxation behavior of BFRP bars under different initial stress levels have linear relationship with logarithm of time, similar to other types of FRP bars, and are significantly affected by the initial stress level. Furthermore, the test results demonstrated that the relaxation behavior of BFRP bars is slightly affected by seawater (pH 8.1) and can be recommended for use in marine structures. On the other hand, the test results showed that the acidic (5% HCl) and alkaline (10% NaOH) environmental conditions greatly impacted the relaxation behavior of BFRP bars, such that although BFRP bars stressed to 50% of their ultimate strength survived 1000 h under normal conditions, exposure to different environmental conditions resulted in their rupture under the same initial stress level. Finally, the expected million-hour relaxation losses of BFRP bars range between 7.76% and 13.75% under normal conditions based on the initial tensile stress level, while introducing seawater can increase this ratio to an average of 16.13%.

Stress relaxation is a time-dependent mechanical behavior of textile materials, which can affect the performance of the fabrics in various applications, especially technical applications such as tensile structures, geotextiles, and medical textiles. The present study aims to consider the effect of fabric structure (Tricot, Locknit, Satin, Queen’s cord, Reverse Locknit, and Sharkskin) on the tensile stress relaxation of biaxial warp-knitted fabrics. According to the results, the fabric structure remarkably influences its stress relaxation. The stress relaxation of the fabric in the course direction increases by increasing the length of underlaps in the front and back guide bars. Using elastic yarns as weft yarns in the fabric structure reduces the stress relaxation of the fabric in the course direction without affecting the fabric’s stress relaxation in the wale direction. Finally, to improve the functionality of pressure garments under stress relaxation, using elastic weft yarns in the fabric structure and bounding the warp and weft yarns in the biaxial warp-knitted fabrics by the Tricot structure is proposed.

This study examines the MHD channelized flow of Maxwell fluid with suction/injection at the boundaries to control fluid velocity and boundary layer development. The governing equations, accounting for momentum, energy, heat generation, and chemical reactions, are analytically solved using the Laplace transform. Explicit solutions for velocity, temperature, and concentration fields are derived in exponential form and further expressed using summation notation for efficient inversion. Graphical analysis reveals that suction reduces velocity, while injection enhances it, significantly improving the heat transfer process. The findings highlight the critical role of suction/injection in optimizing heat and mass transfer, offering valuable insights for engineering applications.