Meccanica最新文献

The automation of transportation systems inevitably faces the challenge of enhancing both the safety and intelligence of passenger vehicles. In this transitional stage toward full automation, advanced driver assistance systems (ADAS) play a critical role in bridging the gap. A key component of ADAS is vehicle stability control (VSC), which ensures motion stability during highly nonlinear handling maneuvers. This paper addresses the system nonlinearity under critical driving conditions and the loop delays within feedback processes by proposing a delay-tolerant feedback structure for VSC. The approach utilizes only the linearized dynamics along the trajectory of the maneuver, where the target-tracking performance is optimized. A nonlinear vehicle model is first constructed, followed by an investigation of its open-loop characteristics through equilibrium analysis and local linearization. Time delays arising from control sampling and actuation are incorporated into the feedback torque, yielding a delayed nonlinear system. A semi-discretized method is developed to construct stability charts of the tunable control gains, whose aggregation yields a conservative delay-tolerant domain. Two gain scheduling strategies are proposed to achieve maximum target-tracking performance, tailored for either real-time (RT) or offline implementation. The proposed method is designed for stable tracking of dynamic motion references under nonlinear conditions and is validated using experimental data-based simulations. The results demonstrate that a linearized control law, when properly designed, can deliver high-performance VSC with strong adaptability across different control loops subject to varying delays.

In this work, a computationally efficient multiscale strategy is proposed for accurately predicting failure in composite materials under general loading conditions. The main ingredient of this strategy is a data-driven surrogate model for damaging anisotropic microstructures, to be obtained through several nonlinear hierarchical homogenization processes performed on the same repeating unit cell subjected to different macrostrain paths. The adopted macroscale constitutive model considers the overall secant elastic moduli as internal variables, and introduces a general fourth-order damage surface tensor, representing the macroscale anisotropic damage evolution, which depends on both the overall secant moduli and applied macrostrains. A deep neural network (DNN) approach is used to derive an approximate functional form for this damage surface tensor, based on the best fitting of nonlinear micromechanical results. Then, the numerical accuracy of the proposed data-driven multiscale model is assessed by comparing the relevant results with those coming from a nonlinear periodic homogenization approach, with reference to a regularly perforated microstructure subjected to arbitrary macrostrain histories, involving both proportional and nonproportional paths.

This paper introduces a systematic method for analyzing the free and forced vibrations of hybrid double-beam systems under axial force, utilizing the linear multibody system transfer matrix method. The hybrid double-beam system consists of two types of elements, the double-beam segments and the spring-supported rigid bodies. This configuration is commonly found in research and engineering applications. The frequency equation of the system can be directly obtained through successive multiplication of the element transfer matrices, accommodating arbitrary boundary conditions. The transfer equation for the axially loaded Timoshenko beam are derived analytically, thereby avoiding the accuracy loss due to spatial discretization. And there is no need to discuss the derivation for different cases. The orthogonality of the augmented eigenvectors of the hybrid double-beam system is mathematically proven. The forced vibration of the system is solved using the modal superposition method. Three numerical examples verify the systematicity, simplicity and high accuracy of the proposed method. Furthermore, the effects of axial force, spring support stiffness, and rigid body mass on the vibration characteristics of the hybrid double-beam system are analyzed, providing valuable insights for optimizing designs and avoiding undesirable vibrations.

Ultra-High Performance Fiber-Reinforced Concrete (UHP-FRC), a construction material that has been introduced and refined over the past two decades, offers exceptional advantages that set it apart from traditional concrete. The mechanical response of UHP-FRC is often evaluated in the laboratory using tests that include compression, tensile, and three-point bending, in which non-homogeneous deformation fields develop, resulting in the localization of failure processes. Here, we utilize a second gradient continuum theory, applicable to UHP-FRC, developed within the granular micromechanic framework to model the deformation and failure behavior. The granular micromechanic framework accounts for the variability in grain-pair orientations within a continuum material point, integrating interactions across the orientational space to capture the evolving macroscale behavior of UHP-FRC. A key outcome of the model is the prediction of directional evolution in damage and plasticity, leading to emergent anisotropy in the material’s response. As a result, a comprehensive micromechanic framework is developed that can characterize the deformation behavior of UHP-FRC, providing a robust connection between microscale processes and macroscale performance. This method incorporates Piola’s ansatz to link granular micromechanics with the continuum scale and introduces objective kinematic descriptors to represent grain-to-grain relative displacements under finite deformations. Evolution equations for damage and plastic variables, derived using Karush–Kuhn–Tucker (KKT)-type conditions, govern the interactions at the grain level. The model’s applicability is demonstrated through numerical simulations and comparisons with experimental tests in terms of the force–displacement curves. A parametric analysis is also conducted to assess the influence of input parameters on the simulation results. The model replicates the superior tensile and residual strength, excellent crack control, and remarkable resistance to crack propagation that enhance durability and structural integrity of UHP-FRC. The theoretical insights and analysis capability offered by the described model can form a basis for exploiting the immense potential of UHP-FRC for innovative and resilient applications in structural engineering.

Multiple bifurcations due to symmetry often occur when analyzing nonlinear structural motifs with axial symmetry. The identification of multiple bifurcation points and the tracing of bifurcation paths become significant challenges in numerical analysis. In this paper, we address a numerical problem of nonlinear bifurcation in a symmetric structure exhibiting double bifurcation points. By focusing on the initial imperfection vector corresponding to the partial irreducible representation of its symmetry, we propose a modified stiffness method. This method utilizes the orthogonalization transformation differences to separate the multiple bifurcation points of the second-order irreducible representation of the stiffness matrix into a single bifurcation point. As a numerical example, bifurcation analysis of an axially symmetric fullerene truss structure is conducted to demonstrate the effectiveness of the proposed method. This study successfully addresses the issue of multiple bifurcations in axially symmetric structures by incorporating group-theoretic bifurcation theory and modifying the stiffness method, as validated by the numerical analysis of a fullerene truss structure.

Monolayered pantographic waveguides are mechanical apparatus consisting of flexible elements hinge-connected in a diamond shape, that store elastic deformation energy on stretch gradients and exhibit an extensible-to-inextensible transition in tension. This yields exotic dispersion properties and leaves room to the hypothesis that elastic solitary waves may propagate through these waveguides. The present communication delves into this issue by exploiting a homogenized continuum description based on Hookean interaction potentials at the micro-scale—where flexible elements are considered to be inextensible—to derive admissibility conditions for solitary wave propagation. It is found that monolayered pantographic waveguides admit elastic rarefaction solitary waves. Solitary waveforms and their spectral stability are analyzed numerically.

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.