Mechanics of Time-Dependent Materials最新文献

In this work, the effective behavior of viscoelastic composites with ellipsoidal reinforcements and imperfect interface or degraded interphase is investigated through the inclusion replacement concept. The concentration equations have been reformulated as to define the equivalent inclusion’s behavior with imperfect interface or thin coating allowing to evaluate the effective behavior through different homogenization schemes. The correlation between interface and interphase descriptions is formulated in the context of anisotropic behavior of the inclusion and the matrix and for ellipsoidal inclusion shape. In the case of isotropic elasticity, the exact analytical solutions agree with the literature references for spherical and cylindrical inclusion morphologies and linear spring interface model. The replacement procedure was extended to viscoelastic behavior of the components with imperfect interface and/or interphase. Alternative descriptions of the interface behavior are proposed through Maxwell and Kelvin–Voigt models. The combined influence of shape of inclusions and interface parameters is analyzed on the effective relaxation modulus.

The effect of the loading rate on the dynamic fracture behavior of a ZrB₂-SiC ceramic was investigated using a split Hopkinson pressure bar on a single-edge notch beam. The dynamic fracture toughness was measured, and the failure mode of the ZrB₂-SiC ceramic was identified. The rate-dependent constitutive model of JOHNSON_HOLMQUIST II (JH-II) was utilized to analyze the effect of the loading rate on the stress intensity factor and the failure process of the ZrB₂-SiC ceramic. Results show that the dynamic fracture toughness and the energy dissipation rate increase with the increase of the loading rate. The dynamic fracture toughness improved from 9.92 MPa⋅m(^{1/2}) at 6.68×104 MPa⋅s⁻¹ to 31.5 MPa⋅m(^{1/2}) at 28.26×104 MPa⋅s⁻¹. The JH-II model was found suitable to model the dynamic fracture behavior of the ZrB₂-SiC ceramic. Both experimental and numerical results showed that the fracture process of the ZrB₂-SiC ceramic showed dependence on the loading rate. The crack first initiated from the plane of the notch induced by the tensile stress applied near the crick tip. At high loading rates, the ZrB₂-SiC ceramic specimen absorbed more energy and fractured to a larger number of small fragments than at a lower rate.

This study aims to explore for the first time the thermoelastic buckling behavior of functionally graded porous plates and shells using an efficient finite element model based on the first-order shear deformation theory (FSDT) with the improvement of the shear strains via the introduction of a quadratic function that able to take into account the parabolic distribution of transverse shear stresses without any need of shear correction factors as standard (FSDT) theory. In this research, different sets of functionally graded metal/ceramic combinations, as well as porosity distributions, namely uniform (or even) and random (or uneven) porosity patterns, are also considered, and the effective material properties of the graded porous structure are determined via a modified power-law function. Two types of applied thermal loads are considered, namely Uniform and nonuniform thermal load (UT, NUT) with temperature-dependent (TD) and independent (TID) mechanical properties. The Green-Lagrange formulation, variational method, and a numerical iterative algorithm are applied to solve the governing equations with porosity and thermal dependent coefficients. To verify our results, various numerical comparisons are conducted on critical temperature buckling of plates and spherical shells, and they are compared with available results where a close correlation is observed. The influence of thermal loads, porosity volume fraction, types of porosity patterns, temperature dependency, and geometrical aspects on the thermal buckling behavior of FG porous plates and shells are scrutinized through different parametric studies.

Surrogate materials were fabricated to investigate the creep damage characteristics of coal pillars in the long wall mining method. Similarity ratios for coal creep viscosity coefficient and creep rate were determined from analyzes derived from the fractional creep constitutive equation. Surrogate materials consisting of sand, paraffin, vaseline, and silicone oil, were prepared to simulate creep behavior. The creep characteristics of these surrogate materials were identified, and the compositions including the ratios of aggregate to binder, and paraffin to vaseline and silicone oil were determined. A physical similarity model was established to calculate the stress and deformation, and determine the damage characteristics of a coal pillar. The results indicate that the stress in the coal pillar decreases over time, and the maximum principal stress shifts toward the center before eventually taking an arc-shaped distribution. The vertical and horizontal distortions of the coal pillar decrease gradually from the coal wall on each side toward the center, resulting in a convex and inverted S-shaped deformation pattern, respectively. The coal pillar develops progressive damage on both sides, with the damaged area gradually increasing toward the middle section, ultimately leading to the collapse of the entire coal pillar. These findings provide valuable insight into the preparation of creep surrogate materials and the management of coal pillar stability.

The article deals with fractional viscoelastic models, including conformable derivatives. The Maxwell model and Zener model involving conformable derivative are studied for relaxation modulus as well as for creep compliance. We obtain some mechanical properties from both models, which is very useful for studying material viscoelasticity. Interesting results are extracted and compared to experimental data.

In this work, we investigate the viscoelastic properties of hydrogels through stress relaxation experiments to better understand the force-dependent dynamics of these materials with the aspiration of expanding their application envelope within the biomedical field and beyond. We experimentally studied the viscoelastic behavior of 4 different types of hydrogels: covalently crosslinked polyacrylamide (PAAm), covalently crosslinked PAAm network immersed in a viscous alginate solution, ionically crosslinked alginate along with crosslinked PAAm-alginate double network. Through our investigations, we demonstrate that we can tailor the viscoelasticity of a covalently bonded PAAm network by tuning the viscosity of the solution in the gel. Moreover, based on the stress relaxation test of ionically crosslinked alginate gel and the double network gel, we have revealed the quantitative correlation between the ionic bond dissociation and force-dependent viscoelastic behavior of gels containing ionic crosslinks.

Thermoplastics have a crystal structure. It has been pointed out that the crystalline structure affects viscoelastic behavior in crystalline polymers, which must be taken into account in MD simulations. In this study the crystalline lamellar structure of Polyethylene (PE) was reproduced via molecular dynamics. To investigate the mechanical behavior and deformation behavior of the lamellar structure of PE, deformation was applied to the model under a constant tensile rate and constant tensile load as tensile and creep analyses, respectively. A tensile analysis indicated localized cracking, and a creep analysis revealed molecular-chain undulation along the tensile direction. To clarify the reason for the difference in deformation distribution between tensile and creep analyses, the potential energy during tensile loading was examined. In the tensile analysis, all the potential energies increased at the start of tension development and decreased rapidly at the break. As revealed in the creep analysis, the bond stretching and bond angle potential energies did not change when deformation started at a strain of approximately 0.20. These results indicated that the deformation behavior depended on the loading configuration, such as tensile and creep loading, and that deformation behaviors vary because of differences in displacement distribution and potential energy.

The fracture process zone (FPZ) has been assumed to activate microcrack evolution and influence the mechanical parameters of the rock specimen. This can be linked to the grain size of the rock specimens located in the path of the crack propagation. However, few studies have considered the effect of the grain distribution on the size of the FPZ, especially under dynamic loadings. In this paper, we analyze the mechanism by which the strain rate and grain distribution affect the FPZ and the dynamic mechanical parameters. We selected three kinds of sandstone specimens to represent the mesostructure heterogeneities characterized by the fractal dimensions. Also, the size of the FPZ can be calculated by the grain size and the dynamic fictitious crack length under the quantified mesostructure heterogeneities and the concept of the box dimension method. Based on the results, the dynamic strength and fracture toughness can be obtained with unknown coefficients. The unknown coefficients were then determined via the dynamic fracture test, in which the processed semicircle bending (SCB) specimens were used for the pendulum hammer-driven split Hopkinson pressure bar (SHPB) apparatus. Finally, the results were validated using the existing experimental methods recommended by the International Society for Rock Mechanics (ISRM). This study provides a valid and simpler method for the simultaneous determination of the dynamic fracture toughness and tensile strength of rock specimens.

Determination of optimum rejuvenator dosage is critical to the performance of 100% hot recycled asphalt mixtures. Further, at the optimum dosage, the rejuvenated binder is expected to have chemical and mechanical properties similar to the targeted virgin/control binder. The present study used waste engine oil (WEO) and tall oil (TO) to rejuvenate recycled asphalt pavement (RAP) binders obtained from two different sources. The optimum dosages of the rejuvenators were evaluated using different test procedures. The chemical, morphological, and performance characteristics of the RAP binders rejuvenated at the optimum dosages were studied. True fail temperature was identified as the most suitable parameter for estimating the optimum rejuvenator dosage. The optimum rejuvenator dosages of WEO were found to be 19% and 18%, respectively, for the two RAP sources considered in this study. The corresponding dosages for TO were estimated as 17% and 14%, respectively. Saturates-aromatics-resins-asphaltenes (SARA) analysis indicated that the rejuvenators were able to restore the chemical properties of the RAP binders, the degree of restoration being a function of the rejuvenator type and stiffness of the RAP binder. Results from atomic force microscopy (AFM) analysis confirmed that the rejuvenated binders showed the formation of new structures that were unique for different combinations of RAP binder and rejuvenator. Rutting and fatigue characteristics, evaluated using multiple stress creep and recovery (MSCR) and linear amplitude sweep (LAS) tests, respectively, improved after rejuvenating the RAP binders. In terms of rejuvenation and performance characteristics, TO showed better results in comparison to WEO.

Cavity bifurcation is an important mechanism of damage and fracture failure of various materials. The thermal cavitation problem of a composite sphere composed of two kinds of viscoelastic materials subjected to a uniform temperature field was studied in this paper. Based on the finite deformation dynamics theory, a nonlinear mathematical model describing cavity movement in a composite sphere was established by employing the Kelvin–Voigt constitutive equation of thermo-viscoelasticity. Adopting the dimensionless transformation, a parametric cavitated bifurcation solution describing the cavity radius with the temperature was obtained. The dynamic variation curves of the cavity radius, which increase with external temperature, radius ratios, and material parameters, were also discussed. It was proved that the dynamic growth of an infinitely large sphere, including a small sphere, can be achieved by a finitely composite sphere.