Mechanics of Time-Dependent Materials最新文献

This study presents a novel numerical and artificial neural network (ANN) investigation of three-dimensional magnetohydrodynamic (MHD) bioconvective flow of a Carreau ternary hybrid nanofluid (THNF) over a bidirectional-stretching Riga plate embedded in a porous medium. The mathematical model adopted is derived from the Buongiorno model and includes the effects of thermal radiation, Joule heating, Brownian motion, thermophoresis, heat generation/absorption, activation energy, and binary chemical reaction. Similarity variables are introduced into the governing partial differential equations to convert them into ordinary differential equations, which are then solved numerically using the MATLAB solver bvp4c. A Levenberg-Marquardt-based backpropagation neural network (LMS-BPNN) is trained to predict the key transport properties, using 70% of the data for 15% of training to validate and test the network. The results show that the velocity profiles decrease with the increase of the Weissenberg number because of the dominant elastic forces, and the temperature profile increases with the increase of the Brownian motion and thermophoresis parameters. The Nusselt number increases with the increase of Brownian motion and thermophoresis, while the skin friction coefficient increases with the increase of the magnetic and chemical reaction parameters. The results obtained from ANN agree fairly well with the numerical results, and the mean square error value is in the range of (1 0^{-9}) to (1 0^{-8}). The high agreement between the numerical and ANN predictions with previous studies demonstrates the accuracy and robustness of the proposed computational framework, which has the potential to predict complex transport phenomena in advanced thermal systems.

The steady Casson nanofluid flow over a curved stretched sheet is numerically analyzed in this work while taking viscous dissipation and mixed convection into account. Nanoparticles improve heat transport, the non-Newtonian behavior is described by the Casson model. To represent boundary-layer flow over a curved surface, a local non-similarity transformation converts the nonlinear PDEs into linked ODEs with variable coefficients. MATLAB bvp4c is used to numerically solve the resulting equations. Analysis is done on the effects of the Eckert number, mixed convection, curvature, Casson parameter, and nanoparticle volume percent. The findings indicate that while viscous dissipation and mixed convection have opposing thermal effects, curvature greatly modifies the flow. In addition to improving thermal performance, the introduction of nanoparticles increases the system’s sensitivity to buoyancy and viscosity effects. The results directly affect high-precision heat systems in polymer extrusion, microfluidics, and bioengineering applications and advance our knowledge of intricate transport processes in curved geometries.

Reinforced concrete structures exposed to aggressive environments often deteriorate due to carbonation and the penetration of harmful agents through the pore network. Cementitious protective mortars are an effective solution to improve surface durability. This study investigates the mechanical and durability behavior of cementitious mortars incorporating binary sand mixtures of limestone sand and dune sand, two abundant local materials in arid regions of Algeria. Twelve mortar mixtures were prepared with different proportions of limestone and dune sands while maintaining a constant water-cement ratio of 0.55. Mechanical strength, immersion water absorption, capillary water absorption, and accelerated carbonation resistance were evaluated. The results indicate that binary sand significantly improves mortar performance. The highest compressive strength, averaging 46 MPa at 28 days, was obtained for mixtures containing 60–70% limestone sand and 30–40% dune sand. Immersion water absorption ranged from 6.85% to 7.46%, with most binary mixtures showing slightly lower values than the reference mortar. Capillary absorption was also reduced, with cumulative water uptake decreasing from approximately 15 kg/m² for the control mixture to 8.5–10 kg/m² for the optimized mixtures, indicating improved pore structure and reduced permeability. Accelerated carbonation tests revealed a significant improvement in carbonation resistance. After 56 days, carbonation depth decreased from 10.2 mm for the reference mortar to 5.13–5.97 mm for the optimized mixtures, corresponding to a reduction of about 40–50%. These results demonstrate that binary limestone-dune sand mortars provide improved durability and represent a sustainable solution for protective cementitious coatings.

In this study, the stress relaxation behavior of coarse-grained (CG) and gradient nanostructured (GNS) pure copper was systematically investigated. Stress relaxation tests revealed that the gradient nanostructure enhances resistance to time-dependent softening, as reflected by a lower normalized stress drop. Both the apparent activation volumes ((V_{a})) and effective activation volumes ((V*)) are significantly reduced, with (V_{a}) decreasing from average 1099.1 (b^{3}) to 315.5 (b^{3}) and (V*) decreasing from average 726.2 (b^{3}) to 199.4 (b^{3}) for the CG and GNS samples, respectively. The reduced activation volumes suggest a transition from conventional dislocation-forest-controlled deformation toward predominantly short-range obstacle- or interface-controlled processes in the nanostructured surface layer. Further analysis reveals that the evolution of the mobile-dislocation fraction is consistent with more rapid immobilization of mobile dislocations in the GNS sample. These findings offer guidance for designing gradient architectures with improved long-term mechanical stability.

Understanding the deterioration of shotcrete exposed to flowing water is essential for improving the durability of tunnel support structures. This work investigated the effects of alkali-free accelerator dosage, flow velocity, and curing duration on shotcrete deterioration using accelerated leaching tests in 6 mol/L ammonium chloride solution. Accelerator dosages of 0–8%, flow velocities of 0–2 m/s, and curing durations of 7–28 days were examined. Compressive strength, mass loss, and apparent leaching depth were measured, while low-field nuclear magnetic resonance, X-ray diffraction, and backscattered electron imaging with energy-dispersive X-ray spectroscopy characterized changes in pore structure, phase composition, and calcium distribution. Exposure increased porosity, particularly within the 0.01–0.1 μm pore-size range, and depleted calcium-bearing phases in the affected regions. Flow velocity induced the largest deterioration changes within the investigated ranges. For specimens containing 8% accelerator and cured for 28 days, increasing flow velocity from 0 to 2 m/s increased compressive strength loss from 69.3% to 91.7%, mass loss from 1.5% to 8.2%, and apparent leaching depth from 6.8 mm to at least 15 mm after 28 days of exposure. Higher accelerator dosages and shorter curing durations also intensified deterioration, consistent with their greater initial porosity and less compact microstructures. The findings support a mechanism in which flowing solution promotes calcium removal, while dissolution of calcium-bearing phases increases porosity and weakens the cementitious skeleton. Under the accelerated conditions investigated, controlling accelerator dosage, reducing flow velocity, and extending curing before exposure mitigated deterioration.

The increasing demand for sustainable pavement materials has promoted the use of waste-derived modifiers as partial alternatives to conventional petroleum bitumen. This study investigates the effects of waste cooking oil (WCO) and rice husk ash (RHA) on the physical, rheological, temperature-susceptibility, temperature-sensitivity, chemical, and workability characteristics of VG-30 bitumen. Waste cooking oil-modified bio-bitumen (WCOMBB) was prepared using 5–15% WCO, while rice husk ash-modified bio-bitumen (RHAMBB) was prepared using 3–9% RHA by weight of base binder. Conventional binder tests, viscosity analysis, mixing and compaction temperature evaluation, activation energy analysis, Dynamic Shear Rheometer (DSR)-based frequency sweep, master curve analysis, Fourier transform infrared spectroscopy (FTIR), and Grey Relational Analysis (GRA) were conducted under unaged and short-term-aged conditions. The results showed that WCO significantly softened the binder by increasing penetration up to 307.9% and reducing absolute viscosity up to 83.1%, thereby improving workability and reducing processing temperatures. In contrast, RHA improved binder stiffness by reducing penetration up to 57.1% and increasing viscosity up to 19.8%, indicating better thermal stability and high-temperature consistency. Rheological results showed that WCOMBB-10% exhibited improved flexibility, whereas RHAMBB-9% showed enhanced stiffness and rutting-related response. FTIR indices indicated physical blending for WCOMBBs and filler reinforcement for RHAMBBs without major new chemical bond formation. GRA identified 15% WCO and 9% RHA as the best-ranked dosages within the selected binder-level criteria. Overall, WCO is promising for warm-mix applications, while RHA is suitable for high-temperature binder improvement.

Layered rocks exhibit pronounced mechanical anisotropy, and their mechanical response depends on specimen geometry. This work investigates the effects of bedding angle and height-to-diameter ratio on the mechanical response, fracture morphology, and crack evolution of layered rocks. Experimental characterization combined uniaxial compression tests with three-dimensional fractography, while numerical simulations utilized two-dimensional discrete-element method. Specimens with bedding angles of 0°, 30°, 60°, and 90° were tested, and the resulting fracture surfaces were evaluated using fractal dimension, joint roughness coefficient, and statistical morphological descriptors. Numerical models incorporating random bedding distributions were developed for height-to-diameter ratios of 0.5, 1.0, 2.0, and 3.0. The results show that bedding orientation governs compressive strength, elastic modulus, failure mode, and fracture-surface morphology. Compressive strength exhibits a U-shaped dependence on bedding angle, while both compressive strength and elastic modulus generally decrease as the height-to-diameter ratio increases. Fracture surfaces formed at bedding angles of 0° and 30° are relatively rough and geometrically complex, whereas those formed at 60° and 90° are smoother because failure is increasingly controlled by bedding-parallel slip or separation. Increasing the height-to-diameter ratio reduces end-constraint effects, promotes shear-crack development and damage localization, and enhances the continuity of the principal failure zone. These findings clarify the coupled effects of bedding anisotropy and specimen geometry on the failure behavior of layered rocks and provide a basis for evaluating the stability of layered rock masses.

Bulk and shear relaxation functions need not have proportional time dependence. This study develops a quasi-static framework for pressurized, radially graded viscoelastic cylinders in which the two spectra are specified separately. A generalized-plane-strain formulation uses radial displacement, radial stress, and an axial-force integral as state variables, avoiding radial derivatives of the material functions. The transformed problem is solved by the complementary functions method and verified against an independent time-domain finite-element model. The examples show that spatial variation in relaxation times can temporarily change which wall governs the maximum hoop stress. This behaviour reflects competition between stress maxima rather than continuous motion of a single peak. Cross-solver identification tests use outer-surface displacement and axial strain. Repeated-noise tests show that the reduced model can retain useful response predictions as measurement noise increases, although individual material parameters become substantially less precise. Adding radial-profile coefficients worsens conditioning and can prevent convergence under the same measurement and optimization protocol. A separate Gaver–Stehfest audit identifies sensitivity to transform-sample errors and reduced accuracy near a pressure-ramp corner. A material-informed epoxy example illustrates the errors caused by proportional bulk–shear approximations. These results support numerical analysis and experiment design, but not unrestricted parameter recovery or laboratory validation. Axial measurements must be interpreted as gauge averages, and the opposing radial gradients used in the examples remain hypothetical material profiles.

Ordinary concrete is characterized by high brittleness and poor crack resistance. The structural reliability of concrete artificial dam in coal mine underground reservoirs (CMUR) under complex conditions cannot be ensured by traditional concrete. Fiber concrete is utilized in the construction of artificial dam in the CMUR is characterized by excellent toughness and crack resistance. In order to investigate the mechanical properties and the damage accrual process of fiber concrete artificial dam under imposed stressing, a damage model for fiber concrete artificial dam in underground coal mine reservoir is proposed. The damage model can present the characteristics of multi-phase component synergistic load-bearing in the fiber concrete matrix-fiber-interfacial transition zone (ITZ) and the random distribution direction of fibers. Meawhile, the damage model is proposed on the basis of statistical damage mechanics, characterized by a three-parameter Weibull distribution. Moreover, the damage model is validated by the experimental results of fiber concrete under uniaxial and triaxial compression. A sensitivity analysis of the Weibull distribution parameters is conducted to examine the influence on the mechanical response and damage evolution in the fiber concrete artificial dam. Additionally, parametric analyses are performed to explore the impacts of fiber orientation intensity, ITZ thickness, and ITZ stiffness on the elastic response of fiber concrete. The research results serve as a theoretical foundation for the design of the fiber concrete artificial dam in the CMUR.

The overlying strata of coal seams are subjected to stepped cyclic-constant loads induced by mining and stopping operations. Nevertheless, the damage accumulation characteristics of rock under this type of stress path and how to consider the effect of prior cyclic loading in constitutive models remain unresolved issues requiring further deliberation. Accordingly, conventional triaxial loading tests and stepped loading tests were carried out in this study to clarify damage evolution characteristics of sandstone under stepped cyclic-constant loading. The results show that upon stress sudden change, the sudden variations of elastic modulus and residual strain gradually decrease, while that of energy dissipation ratio increases continuously. The damage effect is prominent in the cyclic loading stage at (0.7sim 0.9sigma _{mathrm{max}}), manifested as an overall increase in residual strain. The differences in damage accumulated from the cyclic disturbance stress history exert an influence on the mechanical response in the creep stage, and all sandstone specimens eventually exhibit accelerated creep characteristics as they approach failure. The Non-Viscous Plastic-Bending (NVPB) body and stress history influence factor (H_{mathrm{t}}) are introduced on the basis of the classical Nishihara model to characterize the nonlinear deformation feature in the accelerated creep stage and the strain accumulation effect induced by prior loading history, respectively. The determination coefficients (mathrm{R}^{2}) for the final creep stage and the whole creep stage exceed 91% and 96%, respectively, which verifies the applicability of the improved model. The research results can provide new insights for the stability analysis of overlying strata of coal seams.