Mechanics of Time-Dependent Materials最新文献

In this paper, we provide detailed variational formulations for the reduced micromorphic model in rectangular and cylindrical coordinates. In these formulations, the material is modeled as consisting of deformable particles that exhibit microstrain and macroscopic strain fields. This microstrain field is independent of the macroscopic strain field of the entire material. In addition, all the kinematical and kinetical variables, equations of motion, and boundary conditions are formulated depending on the displacement and microstrain fields. Here we define the conditions that give the reduced micromorphic model with decoupled equations of motion such that the displacement field is described as independent of the microstrain field. In addition, we show the applicability of the developed formulation to investigate the simple shear behavior of solid-lubricant cylindrical films. An analytical solution for this model is developed, and numerical results are represented to demonstrate the microstructural topology effects on the mechanics of the lubricant film. The formulations and revealed findings of the present study are important for the design of novel coating architectures materials.

Ethylene-tetra-fluoroethylene (ETFE) is a polymer employed in tension membrane structures with mechanical properties that strongly depend on time and temperature effects. A comprehensive understanding of the mutual influence of these variables and a unified viscoelastic constitutive model design can enable wider exploitation of ETFE in sustainable lightweight construction. This study presents a thermomechanical characterisation of ETFE foils through quasi-static tensile experiments spanning two orders of magnitude of strain rates, creep, relaxation, shear and dynamic cyclic tests in a wide range of temperatures suitable for building applications, from (-20^{circ }text{ C}) to (60^{circ }text{ C}). The experimental results in different material orientations are used to identify the limits of the linear viscoelastic domain, define the direction-dependent creep compliance master curves and calibrate the parameters of a plane stress orthotropic linear viscoelastic model, employing the Boltzmann superposition and the time-temperature superposition principles. The model has been numerically implemented using a recursive integration algorithm and its code is provided open source. A validation on independently acquired data shows the accuracy of the constitutive model in predicting ETFE behaviour within the linear viscoelastic regime usually adopted during structural design, with excellent extrapolation capabilities outside the range of the calibration data.

Ultrasonication has widely been used in many industries to develop advanced materials, improve materials behaviors, and enhance mechanical strength to name a few. The present investigation aims to accelerate the densification mechanisms during the hot-pressing process of Ti-6Al-4 V powder through high power ultrasonication. A computational study has been developed and implemented to simulate the consolidation behavior, then compared with the experimental data to ensure the simulation accuracy. The constitutive equations, encompassing thermoplastic and power-law creep models, were implemented in the simulation as UMAT and CREEP subroutines. Finally, the simulation results in densification curves and density distribution have been compared with the results of experimental tests. The comparison of the simulation and experimental results shows a maximum error of 6.8 and 2.8% in predicting the densification behavior of hot pressing without and with ultrasonication, respectively. The results show the good accuracy of the simulation in predicting final relative density and density distribution with ultrasonic vibrations.

This study investigates the role of a breathing crack on a viscoelastic composite rotor-shaft system supported at the ends by journal bearings. A finite element-based mathematical formulation is developed to model the breathing crack. The geometry of the crack configuration is used to derive a time-dependent stiffness matrix. This matrix is then incorporated into the equation of motion for the composite shaft, derived with the Equivalent Modulus Theory (EMT). The equation of motion is of higher order due to the inclusion of the material’s internal damping behavior, modeled using an operator-based viscoelastic model. Upon validating the mathematical model of the breathing crack, we analyzed its effects over one complete shaft rotation. This analysis further compared the strain energy and orbit plots of the cracked shaft with those of an intact shaft.

This work investigates the problem of time- and space-dependent thin film thickness, specifically focusing on the flow of a Carreau–Yasuda (CY) ternary nanofluid over a porous stretching and rotating disk. The study examines how the thin film thickness varies under partial slip conditions. The CY-ternary nanofluid is composed of silver, alumina, and carborundum nanocombination in ethylene glycol. Also, the study takes into account the effect of thermal radiation with the extension of a magnetic field. To solve the unsteady nonlinear problem, it is transformed into a nonlinear problem and solved using the homotopy analysis method (HAM). The acquired data, together with the CY-ternary nanofluid percentage heat transfer augmentation, are shown visually and quantitatively. The results demonstrate that the CY-ternary nanofluid thin film thickness is influenced by the flow parameters. Moreover, a decrease in thin film thickness is facilitated by rotation, magnetic field, and porosity, which significantly boosts heat transfer rates. These findings are practical applications and offer opportunities for improved thermal management in engineering, biomedical, and industrial processes.

Mode I fracture toughness (Kıc) is a critical parameter in rock mechanics that is essential for understanding how rocks behave under tensile loading and crucial for applications ranging from safety assessments to structural design in geotechnical engineering. This study comprehensively investigates the influence of various environmental conditions (dry, saturated, frozen, thermal shock and thermal aging) on the physico-mechanical properties and Kıc of rocks. The primary novelty of this study lies in its comprehensive modeling approach under diverse environmental conditions, providing a nuanced understanding of factors influencing rock fracture toughness. By extending analysis to less-studied conditions such as freezing and thermal shock cycles, the study enhances the predictive capacity of fracture toughness models in practical geotechnical applications. Physico-mechanical properties, including uniaxial compressive strength, point load strength, Brazilian tensile strength (BT), Schmidt hardness, and ultrasonic wave velocity were evaluated across different environmental scenarios. Simple and linear multiple regression models were developed using these properties to predict Kıc. Notably, BT emerged as a significant predictor in the simple regression analyzes. Ten linear multiple regression models were formulated using SPSS 20, combining mechanical tests (UCS, BT, PL) with non-destructive testing methods (Vp, Vs, SH), demonstrating robust predictive capabilities with R² values exceeding 0.95. Performance metrics (mean absolute error, mean absolute percentage error, root mean square error) were used to verify the accuracy of the model.