查看更多>>摘要：The present work proposes a variational-asymptotic homogenization technique for non-conventional thermoviscoelastic periodic microstructured materials. According to second sound theories, the heat flux vector linearly depends upon the history of temperature gradient and the heat conduction tensor represents the kernel of the relative hereditary integral, thus overcoming the paradox inferred from the usual Fourier's law of thermal waves propagating at infinite speed. Down-scaling relations have been provided in the frequency domain, relating the transformed micro displacement and relative temperature fields to the corresponding macro variables and their gradients through frequency-dependent perturbation functions which convey the influence of the underlying microstructural heterogeneity. Average field equations of infinite order have been derived. Transformed field equations of the equivalent first-order medium have been obtained as Euler- Lagrange equations of a suitable functional whose first variation is properly truncated. They are characterized by frequency-dependent overall constitutive tensors, whose closed form has been provided. A benchmark test assesses the capabilities of the proposed homogenization method, where the solutions relative to a periodic, bi-phase, thermo-viscoelastic material and to the equivalent homogenized medium match under the hypothesis of periodic source terms.
查看更多>>摘要：In the current study, the 3D shape optimization of a hyperloop pod's head and tail is carried out to improve the pod's aerodynamic performance. For shape exploration, a new shape design method called multi-resolution morphing is applied with three baseline hyperloop models, where each has its own aerodynamic characteristics. As a result, an optimal hyperloop pod with a drag value that is 7.5% smaller with respect to the baseline model and optimal pods with well-balanced drag and lift values are obtained. From further investigations, several meaningful results are also discovered. First, the shape of the tail has a greater impact on the drag and lift of the hyperloop pod than the shape of the head, and this difference is more remarkable in the drag than the lift. Second, changing the shape of the head does not elicit a large effect on the aerodynamic performance of the tail and vice versa. Accordingly, the optimization of a hyperloop pod's head and tail can be implemented independently. A high correlation between the strength of the turbulent kinetic energy around the tail and drag is also found from a flow comparison between the optimal models. Finally, the local flow characteristics are similar for the models that are constructed with the same coefficients of the high-degree spherical harmonics. This indicates that it is possible to transfer the local flow characteristics of one model to the other via the multi-resolution morphing method.
查看更多>>摘要：This paper introduces a novel physics-informed multi-agents constitutive model to propose prediction in quasi-static constitutive behavior of cross-linked elastomer and the loss of mechanical performance during environmental aging. The presented model is used to simulate the effect of single-mechanism chemical aging (i.e. thermal-inducedor hydrolytic aging) on the behavior of the material in this hybrid framework. Those environmental single-mechanism damages change the polymer matrix over time due to massive chain scission, chain formations, and changing the arrangement of molecules in the polymer matrix. We propose a data driven super-constrained machine-learned engine to represent damage in the polymer matrix and capture the changes in material behavior, including its inelastic features such as Mullins effect and permanent set in the course of aging. We have simplified the 3D stress-strain tensor mapping problem into a small number of super-constrained 1D mapping problems by means of a sequential order reduction. An assembly of multiple replicated conditional neural-network learning-agents (L-agents) is trained to systematically simplify the high-dimensional mapping problem into multiple 1D problems, each represented by a different type of agent. Our hybrid framework is designed to capture the effect of deformation history, aging time, and aging temperature. The model is validated with respect to a comprehensive set of experiments specifically designed to benchmark model capabilities and also against available data in the literature. Thermodynamic consistency and frame independency have been verified. Besides acceptable predictive abilities, a significant reduction of computational cost to predict behavior at multiple states of deformation is the most significant feature of this model.
查看更多>>摘要：Ballistic impact experiments are conducted to investigate high-speed penetration of polycarbonate by steel spheres at impact velocities ranging from & SIM;600 to 2400 m s(-1). High-speed photography is applied to capture the penetration processes, including projectile trajectories and cavity/damage evolution. Postmortem projectiles are characterized with three-dimensional laser scanning, and postmortem targets, with optical imaging and micro computed tomography. The maximum penetration depth varies nonmonotonically with impact velocity i, and decreases with increasing i above about 1777 m s(-1). With increasing nu(i), the damage of polycarbonate targets increases significantly in amplitude and volume. The damage networks, consisting of cracks and voids, exhibit fractal features with similar fractal dimensions. At low nu(i), plate-shaped cracks are formed in the cavity, while needle-shaped cracks along with a large amount of melting-induced small voids, at high i. Numerical simulations of impact penetration of polycarbonate reproduces well the experimental observations. Considering projectile distortion, a modified Poncelet model is derived to describe the nonmonotonic relation between the maximum penetration depth and nu(i).
查看更多>>摘要：The study brings forth extended criteria for electrostatically actuated micro-plates, which take into account the presence of prestress. Following previous studies that analysed the veracity of a reduced order (RO) model, based on Berger's plate approximation, we present here an analysis of a single degree-of-freedom (DOF) model. The resulting simplified model has enabled a semi-analytical derivation of criteria, for which the plate must adhere to in order to gain bistability in the presence of electrostatic load. The study entails a precursor analysis of a "mechanically"loaded plate, so as to inspect the reliability of Berger's RO (BRO) model, whilst laying the analytical-numerical foundation for the subsequent electrostatic analysis. The end result is a set of criteria in the space of the plate elevation, thickness-to-gap ratio, and membrane load, splitting it into mono-and bistable domains. Numerical fits, obtained for various cases are also presented. In the process of the study, an upper bound for the condition was also disclosed, representing a necessary condition for bistability. The resulting criteria were compared to numerically obtained ones, extracted from the validated Foppl-von-Karman's RO (vKRO) model, as well as from direct finite differences (FD) based solutions (vKFD). The result has shown a remarkable qualitative resemblance, especially in low enough thickness-to-gap ratios, providing a much needed tool in the design of such structures.
查看更多>>摘要：Surface integrity after shot peening is affected by various parameters. Improper selection of shot peening parameters brings challenges in improving the component's fatigue life or even damages the original surface. An insight into the effects of shot peening parameters on surface integrity is a prerequisite for the appropriate setting of process parameters. This study used the computational fluid dynamics (CFD) method to establish a two-phase flow model describing relationships between shot velocity and pneumatic shot peening machine parameters, such as air pressure and peening distance. A random multi-shots finite element model (FEM) was established to study the effects of shot velocity and shot diameter on surface integrity. Results indicate that the peening distance and air pressure significantly affect shot velocity. Compared to increasing shot velocity, peening with larger diameter shots is more effective in generating compressive residual stress (CRS) and dislocation cell refinement layers. However, it results in a decline in the magnitude of surface compressive residual stress (SCRS). Double shot peening combines the characteristics of different diameter shots, which introduces high SCRS and considerable CRS layer thickness.
查看更多>>摘要：Inspired by the fish tail, we propose a piezoelectric energy harvester based on flow induced vibration with topological strong vortex by forced separation for low-velocity water flow. Observed using computational fluid dynamics, the high surface curvature at the up and down edges of the elliptic cylinder forces the boundary layer to separate and forms strong vortex squeezed on the bluff body. The extended Hamilton's principle is used to derive the governing equation of the electromechanical coupling system and the Galerkin procedure is introduced to calculate the analytical output of the energy harvester. An experimental study with macro fiber composite in a circulating open U-shaped channel is conducted. The output voltage for the elliptic cylinder with an aspect ratio of 2.5 under the water velocity of 0.45 m/s is 38.4 V, 21.5 times higher than that for the circular cylinder. The hydrodynamic negative and positive damping determined by the shape of the bluff body balance the energy pumped in and dissipated in a period and fix its phase portraits at a stable limit cycle, resulting in the self-excited vibration of the bluff body. Optimal resistance analysis shows that changing parameters of the bluff body such as reducing its mass or increasing its length also improves the energy harvesting efficiency. The results are helpful for the optimal design of energy harvesters at low water velocity.
Kim, Min-SeongKim, Jeong GyunKwon, Sang-ChulKim, Sun-Tae...
16页
查看更多>>摘要：In this study, the formation mechanism of deformation bands (DBs) observed in AA1100 after plane forging (PF) and return-plane forging (R-PF) during multi-axis diagonal forging (MADF) was unraveled via microtexture analysis using a rate-sensitive model and crystal plasticity finite element method (CPFEM). Blurry bands with an orientation relationship of mainly LAGBs were developed in the deformed grains in the PF specimen. The heterogeneity of the orientation stability (Q) that occurred within the blurry bands and their surrounding regions contributed to the development of the DBs in the deformed grains in the R-PF specimen. Microstructure-based CPFEM successfully simulated the development of DBs within deformed grains during R-PF. CPFEM revealed that heterogeneity of the crystallographic orientation that occurred within the blurry bands and their surrounding regions inside the PF specimen was the main cause of the formation of DBs in the deformed grains during R-PF.
查看更多>>摘要：To effectively implement constrained layer damping (CLD) for vibration reduction of connected cylindrical shell structure, it is necessary to establish a dynamic model of the composite cylindrical shell to guide vibration reduction design. The nonlinear vibration behavior of bolted double cylindrical shells with partially attached CLD under base excitation is studied by semi-analytical method. Based on Lagrange energy equation and Donnell's thin shell theory, the dynamic equation of double cylindrical shell is established. The orthogonal polynomial is introduced as the displacement admissible function, and the continuous variable stiffness elastic constraint is employed to simulate the bottom boundary conditions. The influence of CLD materials on structural vibration is ingeniously introduced into the nonlinear model of bolts, and the equivalent stiffness and equivalent damping of bolts are obtained by Fourier series expansion to accurately describe the nonlinear behavior of bolted connections. In addition, the incremental harmonic balance method is adopted to solve the nonlinear equations. Then the rationality of the semi-analytical model of bolted double cylindrical shells with CLD is verified by constructing an experimental test system, comparing it with ANSYS simulation and references. Finally, the nonlinear vibration behavior of bolted double cylindrical shells with partially attached CLD under different excitation levels, connection stiffness and damping thickness values (including viscoelastic layer and constrained layer) are investigated.
Wang, ChunjinLoh, Yee ManCheung, Chi FaiChen, Kaiwen...
16页
查看更多>>摘要：Thin-walled components have been widely used in different kinds of fields such as aviation, automobiles, medical, etc. However, it is difficult to strike a balance between polishing efficiency and accuracy in the polishing of such components. Hence, this paper presents a novel magnetic field-assisted batch superfinishing (MABS) process which makes use of a magnetic field applied by two pairs of magnetic poles rotating outside an annular chamber mounted with a number of workpieces concurrently. The rotating magnetic brushes comprise magnetic particles and abrasives formed inside the chamber which impinge and remove materials from the workpiece. A theoretical and experimental investigation of the material removal in MABS is conducted on typical thin-walled components, including kinematic analysis of the brush motion, simulation of the magnetic field distribution and material removal distribution model. The experimental results indicate that the MABS process can be successfully used for batch polishing of thin-walled components while obtaining nanometric surface roughness. The developed material removal distribution model can be used to predict the material removal, so as to provide theoretical guidance of process optimization.
NSTL
Elsevier