Mechanics of Time-Dependent Materials最新文献

Glycidyl azide polymer (GAP)-based propellants, known for their high energy efficiency, exhibit unique nonlinear variations in viscoelastic behavior during thermal aging, which is distinct from the monotonic trends observed in traditional propellants. This paper investigates the relaxation behavior of GAP-based propellants subjected to thermal aging at 60 °C. Nuclear Magnetic Resonance and high-performance liquid chromatography analyses are conducted to reveal the underlying mechanisms driving the nonlinear relaxation response. The aging process is classified into three distinct stages: an initial phase dominated by post-curing reactions, followed by competing effects from crosslink network scission, and plasticizer degradation. These competing mechanisms affect the relaxation through microscopic changes in free volume, resulting in complex viscoelastic responses. A predictive model is developed for the relaxation modulus to take into account of these aging mechanisms, with capability to capture the nonlinear fluctuations in the aging shift factor. The proposed model provides accurate predictions of relaxation behavior during thermal aging, including the long-term performance of GAP-based propellants.

We address a non-destructive testing of bitumen insulations. A new approach to its in situ monitoring is proposed. It is based on single impact microindentation. To describe the straining process of a bitumen coating, we analyzed Maxwell and Voigt rheological models. It is shown experimentally that Maxwell model suits well for this task. Temporal changes of the rigidity coefficient in the coating depending on the ambient temperature were measured. It has been established that microindentation-based method is effective for the assessment of the insulation aging. Thermal aging experiments and measurements were carried out to confirm the applicability of the proposed approach.

An experimental study and numerical simulation of short- and long-term shear stress relaxation behaviors of nonaligned and aligned magnetorheological elastomers (MREs) were investigated. The aligned MRE was created by aligning micro-size carbonyl iron particles in chains in silicon rubber using an external magnetic field during the curing process, while the nonaligned MRE was fabricated without applying a magnetic field. The effects of permanent magnetic fields on the shear stress relaxation of the nonaligned and aligned MREs were examined using the double-lap shear stress relaxation test with a short-term period of 1200 s and a long-term period of (1.08 times 10^{6}text{ s}). The shear stress and relaxation modulus of the nonaligned and aligned MREs increased considerably with the rise of magnetic flux density to about 500 mT and then enhanced slightly above 500 mT. The shear stress and relaxation modulus of the aligned MRE were considerably higher than those of the nonaligned one. The shear stress relaxation of the nonaligned and aligned MREs was numerically simulated using the fractional derivative viscoelastic Kelvin–Voigt model. The model parameters were identified by fitting the relaxation modulus to the short-term measured data of the MREs. The shear stress estimated from the investigated model with fitted parameters was in excellent agreement with the short-term experimental data of the MREs measured under different magnetic fields. Besides, the short-term model-fitted parameters were used to predict the long-term shear stress relaxation of the nonaligned and aligned MREs. The largest difference between model-predicted and long-term measured results for the nonaligned and aligned MREs was less than 1%. Therefore, the studied model can be used to predict the long-term shear stress relaxation of the nonaligned and aligned MREs.

Elastomeric isolation systems are often used as seismic isolation devices for buildings and bridges. These systems are typically designed based on the nominal properties of the elastomer. However, key properties such as stiffness and damping can vary with environmental temperature, affecting the performance of the elastomeric isolation. The coupled thermo-mechanical dynamic behavior of the elastomer must be considered for accurate response evaluation. Experimental assessment of the coupled thermo-mechanical response in a laboratory setting presents a significant challenge. This paper presents a laboratory testing methodology for evaluating the thermo-mechanical dynamic response of elastomeric isolation systems using real-time hybrid simulation (RTHS). The test system consists of a superstructure resting on an elastomeric isolation system. In RTHS, the elastomeric isolation system itself is tested, while an electromagnetic shaker is used to resemble the behavior of different superstructures. The temperature around each elastomeric isolator is controlled using two L-shaped radiation heaters. The control strategy for the RTHS is validated through virtual simulations for different superstructures. After the numerical validation, experiments are conducted at different temperatures to demonstrate the impact of temperature on the dynamic response of the system. The proposed methodology proves to be effective and can be utilized for studying the coupled thermo-mechanical behavior of elastomeric isolation systems.

The primary objective of this study was to evaluate the use of available off-the-shelf finite element software like ABAQUS Standard™, ANSYS Workbench™, and Sandia National Laboratory Sierra Mechanics™ to model linear viscoelastic materials and compare their results to an analytically exact model. The study makes use of a standard beam under constant extension loading originally proposed by R.H. MacNeal and R.L. Harder in 1984 for testing the accuracy of finite element analysis tools. The results indicate that these finite element codes approximate the viscoelastic effects of the analytical formulation. When mesh and time step convergence studies were performed, the displacement results obtained diverged by (pm 6%) from the analytical solution for a 3000-hour analysis as stipulated by ASTM D2990 and by (pm 16%) for a 12-year analysis. The computed results show a continuous divergence between the computational and analytical solutions in time. A parametric study on the effect of Poisson’s ratio on the tip displacement was also considered. The parametric studies suggest that the finite element algorithms apply a constant Poisson’s ratio for viscoelastic case studies.

Integrating mechanical nanosensors with biological structures allows evaluating the mass, displacements, and forces in subcellular and cellular activities. On the other hand, studying bio-nanosensors is crucial for identifying biological, chemical, and physical structures. Therefore, the vibration analysis and critical damping behavior of Lipid/Graphene sandwich viscoelastic nanoplates must be studied. The current work investigates a bio-nanostructure referred to as sandwich viscoelastic nanoplates. The differential equations of bio-nanostructure embedded on the viscoelastic substrate have been derived based on the principle of Hamilton and solved numerically using a general differential quadrature method (GDQM) to predict the vibration behaviors of the bio-nanostructure. The differential quadrature method is utilized to extract the natural frequency and critical damping of the Lipid/ Graphene sandwich nanoplates with structural damping for the first time, and also examines the impact of the viscoelastic medium and the size effect (nonlocal parameter) on the vibration behavior of the bio-nanostructure. The findings of this study indicate that the frequencies of nanostructures decrease noticeably as the structural damping and the damping coefficients of the viscoelastic foundation increase. Moreover, by increasing the damping coefficient values of the viscoelastic foundation, the critical damping of Lipid/Graphene sandwich nanoplates (bifurcation curve) occurs at lower values of the nonlocal parameter. On the contrary, with the increase of structural damping, the critical damping of this bio-nanostructure occurs at higher nonlocal parameter values. These findings can be advantageous for the design and production of nanoscale equipment, including bio-nanosensors, resonators, and nano-devices, which require high precision and sensitivity.

The liquid film is mainly used in coating, cooling, lubrication, thermal, and mechanical engineering. The viscosity of a cross fluid is governed by its shear rate, which lies in the class of non-Newtonian fluids. Furthermore, this model correctly distinguishes the flow region into both high and low shear rates regions. The current study concentrates on the electromagnetohydrodynamic (EMHD) liquid-film flow of the cross nanofluid over an inclined disk for heat- and mass-transfer applications. The cross-nanofluid flow of the liquid film is considered time dependent and variable in thickness. The solution of the problem is obtained through the homotopy analysis method (HAM). The HAM results are then handled through the Least Mean-Square (LMS)-based Artificial Neural Network (ANN). The proposed (LMS-ANN) models are tested for dependability, capability, validity, and reliability through regression, error analysis, and histograms. The ANN outputs are drawn in figures and tables and are discussed. Epochs 218, 96, 297, 180, 213, 184, 173, and 155 marked the best performance for the fluid model. The various parameters reveal that cross nanofluids enhance heat-transfer efficiency by promoting convective heat transfer.

The paper is concerned with the impact of memory-dependent (MD) derivatives in the hygrothermal (HTE) three-phase-lag (3PL) hollow cylinder under thermal and moisture loading. We derive equations for temperature, moisture, displacement, and stress components. We solve these HTE field quantity equations using the variable separation method and Laplace transform. We then perform numerical calculations via Laplace transform inversion. We use Mathematica software to understand the hygrothermal behavior of fiber-reinforced 3PHL hollow cylinders. The model validity is assessed by comparing it to existing results. The analysis focuses on the effect of MD derivatives in the HTE 3PL model by examining their impact on heat and moisture field quantities in the presence of time delay parameters and singular kernel functions. This study further highlights the significant influence of employing various kernel functions on the behavior of the HTE hollow cylinder. The author believes that this research will help develop more robust and efficient methods for incorporating memory effects in mathematical models.

As a basic constituent of micro- and nanoelectromechanical systems, the analysis of the thermodynamic properties of rotating nanobeams is crucial for the safe operation of the systems. However, the classical continuum mechanics theory and Fourier’s law of heat conduction can no longer accurately predict the size-dependent effect in the elastic deformation of micro- and nanostructures and the thermal hysteresis effect in the heat transfer process of micro- and nanostructures, respectively. Therefore, in this paper, a new mathematical model based on the concept of memory derivatives is proposed to analyze the properties of viscoelastic rotating nanobeams surrounded by a magnetic field as well as excited by a heat source. The size-dependent effects of this rotating nanobeam are characterized using the nonlocal modified coupled stress theory, and the controlling equations are constructed in the context of generalized thermoelasticity taking into account the memory-dependent effects using the concepts of the Euler–Bernoulli beam theory, Maxwell electromagnetic equations, and fractional-order Kelvin–Voigt viscoelasticity model. The rotating nanobeam deflection, thermodynamic temperature, displacement, and bending moment are numerically solved using the Laplace transform and its inverse transform technique. The effects of time-delay factors, kernel functions, nonlocal parameters, and internal characteristic parameters of the material on the dimensionless field quantities of the rotating nanobeam are also investigated and characterized graphically.