Mechanics of Time-Dependent Materials最新文献

Lung tissue plays a crucial role in biological functions and exhibits significant sensitivity to mechanical loading. Its mechanical properties have garnered increased attention for their potential to guide human protection strategies against collisions and explosions. However, the behavior and underlying mechanisms remain largely undefined, particularly under dynamic loading conditions. In the present study, rabbit lung tissues were subjected to directional compression loadings, both parallel and perpendicular to the trachea. For accurate dynamic measurements, a modified Hopkinson pressure bar was employed. To minimize spike-like stress characteristics, annular specimens were utilized, and a polymethyl methacrylate bar served as the transmission tube, in conjunction with semiconductor strain gauges, to enhance the amplification of transmission signals. Experiments were meticulously conducted using the modified split Hopkinson pressure bar and an Instron machine, covering a strain rate range of 0.0005–3000 s⁻¹. The results revealed a pronounced rate-dependence in the stress–strain curves of lung tissue, characterized by an initial linear elastic regime, a deformation plateau, and ultimate densification. A significant dependency on strain rate was observed, with the strength of tissue increasing a thousandfold from quasi-static to dynamic loading. Anisotropic behavior was evident under both loading directions. Furthermore, both strain rate dependency and anisotropic behavior became more pronounced beyond 0.3 strain under dynamic loading and 0.45 under quasi-static loading. Finally, potential mechanisms involving tissue fluid discharge and the mechanical characteristics of orientated collagen were proposed. These mechanisms were corroborated by staining techniques that demonstrated the predominant orientation of collagen in a specific direction within rabbit lung tissue.

We investigate herein the role of acidic dry-wet cycles and dynamic loading on the mechanical stability of sandstone, which is crucial for managing closed and abandoned mines’ safety. Using a split Hopkinson pressure bar, we conducted dynamic compression tests on sandstone samples exposed to four acidic conditions (pH = 3, 5, 6.5, 7) and five dry-wet cycle frequencies (1, 5, 10, 20, 30) at an impact pressure of 0.70 MPa. Our findings reveal that the dynamic stress-strain response of sandstone entails compacting, elastic, plastic, and failure phases, with peak stress and elasticity decreasing as the acidity and cycle frequency increase. Analytical techniques, including EDS, XRD, and NMR, showed changes in composition and porosity, indicating reduced deterioration compared to untreated stone. Based on Weibull distribution and damage mechanics, a dynamic damage constitutive model was developed to accurately predict the sandstone’s behavior under these conditions. This model, validated by experimental data, effectively captures the dynamic stress-strain characteristics of sandstone, indicating the importance of understanding environmental degradation effects on rock stability in mining contexts.

In this paper, we investigate the role of transverse isotropy on the creep behavior of bedded salt. We conducted a series of triaxial creep tests on prismatic specimens subjected to confining pressures ((sigma _{3})) of up to 24 MPa and a constant octahedral shear stress ((tau _{mathrm{o}})) of 9 MPa. The specimens were oriented with their bedding planes at various angles ((beta )) to the major principal axis to simulate transverse isotropic conditions. Our findings reveal that both instantaneous and creep deformations are most significant when (beta = 0^{circ }), decreasing progressively to a minimum at (beta = 90^{circ }) across all confining pressures. The discrepancy in deformations between these intrinsic angles narrows with increasing (sigma _{3}). Creep deformations for intermediate angles ((0^{circ} < beta < 90^{circ })) follow the elliptical equations. Utilizing the Burgers creep model, we observed that the instantaneous, viscoelastic moduli, and viscoplastic coefficients escalate with (beta ). The degree of anisotropy declines sharply as confining pressures increase, reaching an isotropic state under (tau _{mathrm{o}} = 9text{ MPa}) and (sigma _{3}) around 40 MPa, beyond which transient creep ceases, indicating a transition to Maxwell-material behavior. Employing linear viscoelastic theory, we derived an equation for time-dependent deformation under varying octahedral shear stresses. This enables the formulation of governing equations for Burgers-model parameters, considering bedding plane orientations, loading durations, and the interactions between shear and confining stresses.

In this study, a time-dependent constitutive model of a coal pillar was developed using the Hoek–Brown strain-softening model, which is useful for studying the strength deterioration of a coal pillar over time. A database of 32 failed cases of coal pillars of different ages from the Witbank Coalfield has been utilized to deduce the strength parameters of the coal seam through back analysis. A three-dimensional finite-difference method (FDM) has been chosen to simulate the failed cases. The simulation results have been obtained in terms of pillar strength and FOS of the pillar concerning time. Based on the simulation results the life of the pillar is considered when FOS is nearly equal to 1. The appropriate strength parameters have been derived as peak strength parameters: (m_{i} = 1.47) and (s_{i} = 0.01); residual parameters: (m_{r} = 0.125) and (s_{r} = 0.00001); strength-reduction parameters: (alpha = 0.04), (beta = 200) for a coal mass. 39 stable cases from the same coalfields (Witbank) have been considered to validate the strength parameters. The simulation results of all the stable cases were showing FOS > 1. The proposed constitutive model is suitable for assessing a pillar’s time-dependent strength deterioration and creep behavior. The deterioration/yielding of the pillar is observed to be initiated from the skin/side, extending deeper into the pillar’s core with time and ultimately forming an hourglass shape. It is also observed that the FOS of the pillar decreases with time.

This article investigates the influence of an electromagnetic field, angular velocity, and internal heat sources on two-dimensional thermoelasticity in a micropolar thermoelastic medium using a generalized model of higher-order (multi-phase-lag) heat conduction. The governing coupled partial differential equations are transformed through the normal mode analysis method. The eigenvalue approach is then applied to determine analytically the displacement components, stress components, couple stresses, and temperature distributions from the vector-matrix differential equation. The study’s findings are validated through boundary conditions, and graphical representations highlight the influence of angular velocity, magnetic field, and heat sources in this multi-phase-lag model. The graphical comparison of different thermoelastic models is presented, and the inclusion of tabular data enhances clarity, facilitating a comparative analysis of field variables.

The main objective of this work is to introduce a new thermal conductivity model that can be utilized to solve the infinite thermal diffusion problem in the Green and Naghdi type III model. This proposed model incorporates two key concepts: the fourth-order Moore–Gibson–Thompson (MGT) concept and thermal relaxation. By incorporating higher-order terms, the fourth-order MGT model provides a more accurate representation of the thermal behavior of the material. The thermal behavior of a functionally graded (FG) infinite medium containing a spherical gap is then studied using this model. A rectified sine wave heating system is applied to the traction-free gap surface. Power functions are utilized to model the uniform radial variation of the physical properties of the FG medium. The physical variables under investigation were meticulously examined, considering the impacts of heterogeneity, relaxation duration, and thermal frequency. These variables were estimated numerically using a suitable technique for Laplace transformations. Through this work, the expected outcomes may be able to make a significant contribution to the field of thermoelastic analysis in advanced and FG materials, as well as to engineering applications.

This study examines the dynamics of the three different fluid types, mono-, di-, and trihybrid nanofluids, emphasizing the distinction between the three types of fluids. Also, the report highlights the significance of the microgravity environment: (g*(tau ) = g_{0}(1+acos (pi omega {t}))), and a gravitational field plus a temperature gradient typically produce buoyant convective flows in a variety of different situations, most likely in environments of low gravity or microgravity. One of the reasons for the growing interest in trihybrid nanofluids is their unique ability to improve thermal performance, which is really useful in various heat exchangers. The leading governing equations of linear momentum and energy of the developed problem are transmuted into nondimensional nonlinear coupled PDEs by using appropriate similarity modifications. The obtained systems of partial differential equations are solved via the finite-element method (FEM) in a MATLAB environment. The FEM is the most reliable, powerful, efficient, and fast convergence rate technique. The fluid velocity decreases as a function of the increasing strength of the magnetic ((M)) and Casson (beta ) parameters. However, the temperature distribution increases as a function of these parameters. It is observed that both temperature and velocity functions for trihybrid nanofluid flow obtain peak values as compared to mono- and bihybrid cases. The Nusselt number exhibits an increasing behavior by (15%) as compared to mono- and trihybrid nanofluids and (5%) when comparing bihybrid cases with trihybrid cases. Furthermore, the shear stress and Nusselt number are enhanced against increasing amplitude modulation.

The objective of this study is to analyze the thermo-mechanical interactions occurring in a nonlocal transversely isotropic functionally graded (nonhomogeneous) micropolar thermoelastic half-space when subjected to an inclined load, based on the Lord and Shulman (LS) theory. The material properties are assumed to be graded exponentially along the (z)-direction. Utilizing the normal mode technique, the exact expressions for physical fields such as normal displacement, normal stress, shear stress, temperature field, and couple stress are derived. Numerical computation of the derived results is performed for a material resembling a magnesium crystal, and graphical representations are presented to illustrate the impacts of nonhomogeneity parameter, material’s anisotropy, time, nonlocal parameter, microinertia, and the inclination angle of the applied load on the variations of different physical fields. Some specific cases of interest have been deduced from the present investigation.