Mechanics of Time-Dependent Materials最新文献

Wood, favored for its renewable nature and ease of shaping, is widely used as a structural construction material. However, once in service, wood undergoes creep. This paper delves into the nonlinear creep behavior of Entandrophragma cylindricum wood, known in Cameroon as Sapele, using rheological models based on fractional derivatives. The classical eight-parameter Kelvin–Voigt rheological model and the fractional rheological models of Zener, Thomson, and Burger are employed for modeling. The rheological parameters for these models are determined through an optimization algorithm. The results reveal that the classical model encounters difficulty in describing the experimental data, whereas the fractional models offer a better fitting. Notably, among the fractional models, the Thomson model predicts Sapele creep with an accuracy of 98%.

Abstract

The objective of this study was to synthesize a Gum Ghatti-cl-poly(NIPAm)/CoFe₂O₄ (GGNICAF) hydrogel through free radical copolymerization. The key components used in the synthesis included gum ghatti as a biopolymer (GG), methylene bis-acrylamide (MBA), Potassium Persulfate (KPS), and Ammonium Persulfate (APS) as a cross-linker. Additionally, varying quantities (0–50 mg) of cobalt ferrite (CoFe₂O₄) magnetic nanoparticles (CFMNPs) were incorporated as fillers, synthesized through a coprecipitation route.

The hydrogels were characterized using TGA and FTIR studies. Notably, the swelling study in water demonstrated remarkable water absorption properties. Rheological properties were observed at room temperature using a rheometer with a parallel plate at a 1 mm gap. The rheological and microstructural behavior of the composites were investigated through steady-state flow curves, creep-recovery tests, and small amplitude oscillatory shear tests.

Higher biopolymer content in the mixtures resulted in a more elastic and compact structure, characterized by higher values of both (mathrm{G}') and (mathrm{G}'') . Flow curves indicated shear-thinning behavior. Oscillatory tests revealed an increase in the strength of the hydrogel network with higher crosslinker concentrations, decreasing at low polymer concentrations. Within the linear viscoelastic region (LVR), (mathrm{G}') values consistently exceeded (mathrm{G}'') , indicating a predominantly elastic character. Tan (delta ) values consistently remained below one, signifying an elastic structure throughout a wide range of concentrations (0–5) for all GGNIPACF samples.

Viscosity vs. shear rate profiles were assessed using the Power Law model, while shear stress vs. shear rate curves were analyzed using the Bingham model and Herschel-Bulkley model.

Abstract

In order to study the effect of high-temperature and water-cooling on the argillaceous siltstone creep mechanical behavior, the samples were treated at 200 °C, 600 °C, and 1000 °C respectively, and then cooled with water. Then, the uniaxial compression creep mechanics test was carried out, and the acoustic emission (AE) characteristics were monitored in the entire creep process in synchronization. The results show that: (1) With the increase in temperature, the creep failure strength of argillaceous siltstone decreases, and its macroscopic failure mode transition from shear failure mode to split failure mode. (2) High temperature inhibits both the instantaneous strain and the creep strain and steady creep rate are significantly reduced after high-temperature treatment. (3) The creep curves were fitted and identified by the L-M optimization algorithm under different high-temperature and water-cooling conditions. The Burgers creep model can better describe the argillaceous siltstone creep characteristics. Elastic coefficients (E_{1}) , and (E_{2}) , and viscosity coefficients (eta _{1}) , and (eta _{2}) decreased after high-temperature and water-cooling treatment. The viscosity is enhanced, and the damage-hardening characteristics are obvious. (4) The AE ringing count rate decreases at the initial loading moment and the steady creep stage after high-temperature treatment. The evolution trend of the AE event ringing count rate at the steady creep stage is consistent with that of the creep rate.

The present research investigates the energy absorption of honeycomb core sandwich panels (HCSP) loaded with shear thickening fluid (STF) with different structural parameters at various impact velocities. The HCSP was aluminum, and the skin was constructed of different materials: (i) aluminum, (ii) glass-epoxy composite (GEC), and (iii) STF-impregnated fabric (STF/fabric) with a weight fraction of 15% silica particles. The experiment tests were carried out at 100 mm and 500 mm falling heights. Specific energy absorption HCSP/SF/S compared to HCSP/Al/S and HCSP/G/S increased by 6 47.49% and 23.04%, respectively. According to the results, the energy absorption of skin made of impregnated fabric is better than skin made of aluminum and composite. When the STF is under a high shear rate, the flow changes because of hydrocluster formation and changing the molecular structure from “order” to “disorder.” These changes increase the viscosity notably, causing the STF to become a solid material, resulting in energy absorption.

To design and analyze structures and materials that are subjected to changing thermal environments, it is essential to take into account thermal shock events, which are characterized by rapid and dramatic changes in temperature. In this study, a new thermal conductivity model was used to consider the thermal response of an isotropic thermoelastic medium heated by a moving heat source. This model uses memory-dependent higher derivatives and the concept of the Moore–Gibson–Thompson equation. Using the vector-matrix differential equation form, the basic equations are formulated. The model was applied to consider the thermomechanical behavior of a semi-infinite thermoelastic solid. In the field of the Laplace transform, the technique known as the eigenvalue approach deals with the mathematical formulation and solution of the problem. The inversions of Laplace transforms are found numerically using the Honig and Hirdes approximation approach. A graphical representation is provided showing the fluctuation in temperature, displacement, and stress distributions with changing values of kernel functions and higher orders, as well as the velocity of the heat source. Tables are also included to show comparisons and a full analysis of thermomechanical responses and how they affect the way system variables behave.

This paper considers the creep instability analysis of time-dependent sandwich cylindrical and spherical shell panels of quadrilateral planforms having elastic faces and viscoelastic cores according to the first-order shear deformation theory. The viscoelastic properties of the core are extracted based on the Boltzmann integral law. The equilibrium equation is expressed utilizing the virtual work principle. The space and time parts of the displacement vector are approximated using the simple HP-cloud mesh-free method (which has H refinement and P enrichment properties), and the exponential time function, respectively. The stiffness and geometry matrices are constructed in the Laplace–Carson domain. Finally, the time behavior of viscoelastic sandwich shell panels under in-plane compressions is predicted by solving the eigenvalue problem in the Laplace–Carson domain. Also, the maximum compressive load is determined which can be applied to the time-dependent sandwich shell panels without any creep instability. This critical compression is less than the buckling load of the viscoelastic sandwich shell panel at time zero.

The main objective of this work is to create a new thermoelastic model for hyperbolic thermoelasticity in the context of double porosity structure based on nonlocal elasticity theory and the dual-phase-lag model. Nonlocal elasticity theory is used to construct new constitutive relations and equations. In a homogeneous, isotropic thermoelastic material, thermomechanical interactions are studied using normal mode analysis. A time-dependent thermal shock is applied on the boundary surface. This study also produces a few unique situations, which are compared with previous results of other researchers. The normal and tangential stresses, temperature, displacement components, change in void volume fractions, and equilibrated stress vectors concerning distances and time intervals are all calculated numerically. The physical quantities mentioned above are also visually displayed for various thermoelastic models to compare and illustrate the theoretical results. A comparative analysis and graphical presentation of the effects of nonlocal parameters and porosity on various physical characteristics have been performed. The figures show that most of the physical variables decrease with the increase in distance and show oscillatory behavior with the increase in time. The behavior of the void volume fraction field of the first kind is opposite to the behavior of the void volume fraction field of the second kind with the increase in distance. It is also noticed that the behavior of equilibrated stress of the first kind is opposite to the behavior of the second kind.

Stress relaxation happens to a wire cable that is often wrapped around a pulley during a long-term service process, which causes mechanical performance degradation of the wire cable. In order to investigate the stress relaxation performance of a three-layered wire cable subjected to a combined tension and bending load, a finite element simulation model of a wire cable–pulley device is established using methods of parametric modeling and modified time-hardening modeling, during which the coupling effect between the stress relaxation and contact property is considered. The distributions and evolutions of creep strain, von Mises stress, and contact pressures are obtained. The influence of the lay angles of the helical layers, the axial load, and wrap angle on the stress relaxation behavior of the wire cable is analyzed. The results show that the maximum contact pressure, maximum equivalent stress, and maximum creep strain all occur in the middle region of the three-layered wire cable in the wrap section and locate near the pulley side. The stress relaxation causes smaller magnitudes and more uniform distributions of the contact pressure and equivalent stress, which reduces the risk of severe local contact. The interwire contact pressure, relaxation rate, creep rate, and bending moment reduction of the three-layered wire cable increase with the increments in the lay angles of helical layers, the axial force, and the wrap angle. Under the combined tension and bending load, the stress distribution of the wire cable is mainly concentrated in the middle of the bending section, where stress yielding and other dangerous conditions are most likely to occur. The stress relaxation behavior of the wire cable is sensitive to the lay angle of the intermediate layer and the wrap angle.

Basic creep plays an important role in assessing the risk of early-age cracking in massive structures. In recent decades, several models have been developed to characterize how the hydration process impacts the development of basic creep. This study investigates the basic creep of various concrete mixes across different ages at loading. The analysis focuses on the very early stages (less than 24 hours) and early stages (less than 28 days) of concrete development. It is shown that a logarithmic expression that contains two parameters describing the material can accurately model basic creep from a very early age. One parameter relates to the creep amplitude and depends solely on the composition of the concrete. The other relates to the kinetics of creep and depends on the age of the material at loading and the nature of the concrete mixture. The logarithmic expression corresponds to a rheological model consisting of a single dashpot wherein viscosity exhibits a linear evolution over time. The model offers the advantage of eliminating the need to store the entire stress history for computing the stress resulting from the restriction of the free deformation. This approach significantly reduces computation time. A power-law correlation is also observed between the material aging parameter and the degree of hydration. This relationship depends on the composition. At least two compressive creep tests performed at two different degrees of hydration are needed to calibrate the material parameters and consider the effect of aging on basic creep compliance.

Three-dimensional concrete printing is a transformative technology ushering in revolutionary architectural design and construction automation changes. With recent advancements of this technology, a notable absence of theoretical models predicting structural buildability is required. This investigation aims to bridge this knowledge gap by introducing an innovative theoretical model for estimating the total number of layers printed by a concrete 3D printer. This proposed model considers material behavior, building rate, and failure criteria. The material properties are depicted by modeling structural buildability in two cases, (i) bilinear and (ii) exponential. The buildability is characterized by three subcases, namely (i) constant, (ii) increasing, and (iii) decreasing building rates. These subcases hinge on printing velocity, treated as a function of time. Furthermore, the failure modes of 3D printable concrete structures are delineated based on (i) the Mohr–Coulomb theory and (ii) elastic and plastic failure criteria. Additionally, a strength-correction factor is employed to consider the confinement effect of the printed layer. The ultimate expression of the proposed model embodies an exponential approach to gauging the structural buildability of the printed structures. The study encompasses model validation and extensive parametric analysis to scrutinize the impact of printing velocity, structuration rate, printing path, density, and yield stress.