Meccanica最新文献

Some plain considerations are provided on the influence of axial deformation on the stability of the upper equilibrium position of the Kapitza pendulum with respect to the linearisation or non-linearisation of the associated Lagrange’s equations. Following a very uncomplicated approach and fully accounting for the non-linearity of the problem, it is shown that in the case of the extensible Kapitza pendulum the dynamical behaviour of the system cannot be always correctly captured by a simple linearisation about the upper equilibrium point and a phenomenon related to the degree of approximation can take place for this dynamic system that replicates what happens in the case of the stability of equilibrium of simple axially extensible systems. Also, it is remarked that the introduction of axial deformation may play the same role as the addition of damping.

The motion of the hand’s fingers allows humans to perform many activities. A mechanical model of these limbs can be used in industry and healthcare applications. Due to the sophisticated structure of such limbs, the generation of mechanisms to emulate them is complex but can be addressed with computational intelligence techniques such as metaheuristics. Current models consist of closed, open, or hybrid kinematic chains. Each alternative has advantages and disadvantages in terms of cost, energy, precision, variety of movements, and anthropometric and anthropomorphic characteristics. These mechanisms are derived from information obtained from hand biomechanical studies or clinical experience, so they are not considered customizable and are hardly anthropometric and anthropomorphic. This work presents an approach for the intelligent synthesis of customizable mechanical fingers with anthropomorphic and anthropometric features. This approach aims to exploit the relatively low cost, high precision, and complex trajectories that can develop the one-degree-of-freedom Stephenson III six-bar mechanism to perform cyclic flexion and extension movements as a human finger would. For this, the dimensional synthesis problem of the six-bar mechanism is proposed as an optimization one. So, anthropometric characteristics of the finger are accounted for by using a reference trajectory derived from precise measurements of the subject’s cyclic flexion and extension movements relative to the metacarpophalangeal joint. On the other hand, anthropomorphic features are incorporated by imposing constraints that induce dimensions of the mechanism that resemble the human finger, regulate the size of the links corresponding to hand bones, and place fixed points in locations that mirror the metacarpal structure. The characteristics obtained through this approach have not been found in any design similar to this one to date. With the proper synthesis of the mechanism, it is intended to track an anthropometric reference trajectory collected from the finger of a healthy individual through a commercial low-cost optical hand sensor and conditioned using the spectral clustering unsupervised learning technique. This approach successfully synthesized a customized mechanical finger for a test subject using a genetic algorithm. The design was implemented through low-cost additive manufacturing. After several analyses, the proposal proved to be accurate in tracking the finger movements of different individuals, flexible to anthropometric data, and possessing advantages over other alternative metaheuristics approaches.

We model ductile fracture for geometrically linear deformations by coupling plasticity and phase-field fracture models in a variationally consistent framework. The main aim of the proposed model is to account for the effect of stress triaxiality, in order to accurately reproduce ductile fracture, in particular, the instant and location of fracture initiation. For this purpose, we couple the modified Cam-Clay plasticity model with a phase-field fracture model. We study the behaviour of the model analytically in terms of homogeneous material responses, and numerically on plane-strain and axisymmetric specimens under tension with different notches.

In this paper, we analyze the local linear stability of plane Poiseuille flow of an upper convected Maxwell (UCM) fluid through a periodic channel under two flow regimes, i.e., inertial (Re (ne) 0) and purely elastic (Re (equiv) 0). The analysis is conducted with respect to the dimensionless control parameters: Reynolds number (Re), elasticity number (E), and Weissenberg number (We). We focus on the stability of two-dimensional perturbations, using spectral methods and Chebyshev collocation to discretize the dispersion equations. For creeping flow, we perform a numerical study to explore the combined effects of periodic modulation ((epsilon)), section (x), and control parameters (E, We) on the stability of UCM fluid flow, and to examine the elasto-inertial interplay in flow stability. Our results reveal two key findings: first, the existence of a critical position ((x_{c})=(frac{pi }{2n})) and ((x_{c})=(frac{3pi }{2n})) for small wavenumbers (n); and second, insights into the structure of the full elasto-inertial eigenspectrum, consisting of multiple discrete modes influenced by the section (x) and channel amplitude ((epsilon)).

Jet dispensing is a foundational technology in microelectronics packaging. Piezoelectric-driven jet dispensing involves short-distance jet processes, typically ranging between 0.7 and 1.2 mm. During this process, jet breakup occurs after the droplet contacts the substrate, leading to substrate contamination, irregular adhesive dot shapes, and various other challenges. The underlying mechanism of jet breakup is complex, and the formation mechanism for satellite droplets remains unclear. To address this issue, in this paper, a certain epoxy resin adhesive is investigated to analyze the mechanism of satellite droplet formation during short-distance jet dispensing through experiments, theoretical model, and numerical simulations. The results indicate that the second jet breakup above satellite droplets is most likely to form scattered spots, and the probability of satellite droplet generation is negatively correlated with jet breakup time. Reducing jet velocity, lowering jetting height, and adjusting nozzle temperature can effectively increase jet breakup time and reduce the generation of satellite droplets. Additionally, decreasing jet velocity by modifying parameters such as the length-to-diameter ratio of the micro-jet orifice and the nozzle cone angle can help suppress satellite droplet formation.

Free vibrations of third-order nanobeams are investigated by exploiting an effective nonlocal methodology. Notably, the stress-driven nonlocal theory, which is a consistent tool for modeling nanostructures, is here generalized and combined with a refined shear deformation beam theory. The need for shear correction factors, which is a crucial point in nonlocal continuum mechanics, is thus bypassed by adopting a third-order nanobeam theory. The governing elastodynamic problem is mathematically described by an integro-differential formulation. An equivalent purely differential problem is derived to reduce computational burdens. Parametric analyses are carried out to investigate size dependent free vibration responses. The relevant eigenproblem is thus solved for exemplar structural schemes assessing the relative nonlocal natural frequencies and eigenfunctions. The obtained numerical outcomes can be conveniently exploited in modeling and design of ultrasmall components of smart devices.

The introduction of micro-texture on gear tooth surface can potentially enhance the contact fatigue life and operational reliability of the gear pair, and is attracting increasing attention in an effort to provide improved tribological properties and contact performances. In this work, a new tribological model of micro-textured gear tooth in elastohydrodynamic lubrication (EHL) contact considering the coupled effect of rough surface topography is developed. The combined effect of micro-texture, surface roughness topography, elastic deformation of the tooth surface and lubrication on the contact characteristics of the meshing interface are included to obtain a revised oil film thickness equation. The rough surface contact of gear pair is characterized by the real rough morphology of gear tooth. The coefficient of friction at the transient meshing point of the micro-textured tooth surface is derived with the effect of flash temperature included. The sub-surface stress–strain distributions of the micro-textured gear are determined and the contact fatigue life is evaluated based on the Brown-Miller-Morrow multiaxial fatigue life criterion. Effects of surface roughness and micro-texture parameters on the lubrication behavior, friction coefficient and fatigue life are investigated and discussed. Experimental validation is performed and good agreement is observed between the model predictions and experimental results.

Spherical contact under combined normal and cyclic tangential loading is a simple but representative issue in contact mechanics. This work developed a generalized three-phase constitutive model considering elastic, yield plateau, and strain hardening characteristics and contributed an energy conservation model of quasi-static contact under various sliding conditions. The von Mises stress, equivalent plastic strain, contact behaviors, and energy components were examined under different material parameters and loading conditions. The results indicate that the stress concentration zone appears in the heading area of the sliding direction and vanishes in the trailing area; junction growth derives from a new increased zone and an accumulated plastic deformation inside the original contact zone; large normal force decrement, substantial junction growth, and considerable energy dissipation occur in the material more prone to higher plastic deformations; there are more complex responses related to single frictional dissipation and plastic dissipation.

This paper focuses on a novel and computationally efficient large-displacement methodology utilizing a corotational approach for the stability analysis of masonry structural elements. By virtue of the flexibility offered by the Virtual Element Method, each brick is modeled using a single Virtual Element. In contrast, the mortar layer is modeled through multiple cohesive damage-frictional elements. Furthermore, the adopted Virtual Element formulation does not require stabilization. The advantages of the proposed approach are showcased through several examples demonstrating the striking accuracy of the obtained results compared to analytical solutions. The proposed approach is used to assess the sensitivity of the load-bearing capacity and ductility of masonry walls under vertical loading to mortar tensile strength, boundary conditions, load eccentricity, and block irregularity.