Nonlinear vibration absorber optimal design via asymptotic approach

This paper tackles the classical problem of Vibration Absorbers (VAs) operating in the nonlinear dynamic regime. Since traditional linear VAs suffer from the drawback of a narrow bandwith and numerous structures exhibit nonlinear behavior, nonlinear absorbers are of practical interest. The resonant dynamic behavior of a nonlinear hysteretic VA attached to a damped nonlinear structure is investigated analytically via asymptotics and numerically via path following. The response of the reduced-order model, obtained by projecting the dynamics of the primary structure onto the mode to control, is evaluated using the method of multiple scales up to the first nonlinear order beyond the resonance. Here, the asymptotic response of the two-degree-of-freedom system with a 1:1 internal resonance is shown to be in very close agreement with the results of path following analyses. The asymptotic solution lends itself to a versatile optimization based on differential evolutionary

Payload oscillations control in harbor cranes via semi-active vibration absorbers: modeling, simulations and experimental results

Abstract Semi-active vibration absorbers (SAVAs) are proposed to suppress large amplitude oscillations in container cranes during maneu-vers and wind forcing. The SAVA design and optimization are achieved via suitable nonlinear models, numerical simulations, and laboratory as well as full-scale tests. A comprehensive nonlinear modelling, featuring a full three-dimensional crane model and the adaptive vibration control architecture, is devised. The container is modeled as a rigid body elastically suspended from the trolley traveling along the crane boom. Two identical SAVAs are studied coupling their equations of motion - which include the impact against rubberized end stops - with the container crane dynamics. Suitable parametric analyses are carried out to investigate and optimize the control devices. Full-scale experiments are performed to validate the semi-active control architecture which proves to be a feasible approach

effect of electrospun pvdf fibers orientation for vibration sensing

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Shape-Changing Carbon Fiber Composite with Tunable Frequency and Damping

A shape-adaptable Carbon Fiber Reinforced Composite (CFRC) is proposed to derive a material with tunable mechanical properties in order to optimize its response to external excitations. The composite is bi-stable thanks to internal stresses arising in the manufacturing process and is characterized by a built-in heating system that can control the temperature of the material. This approach allows to gradually change the actual curvature of the material as well as tuning its natural frequencies and damping properties

SELF-ACTUATED MORPHING COMPOSITE WITH TUNABLE FREQUENCY AND DAMPING

In this paper, an innovative approach is proposed to realize a morphing material with loadbearing capability that can self-activate with temperature. In particular the material can provide large-scale shape changes with fast response that can be induced with tuneable energy levels. The above properties are achieved with a simple and reliable multi-scale design of the material which leads to an effective morphing system

Multi-scale design of an architected composite structure with optimized graded properties

A design framework is here presented for the development of an architected solid with targeted mechanical properties thanks to an optimized porosity distribution. A 2D lattice of regular hexagons is considered as core element of a sandwich panel and a Bloch-Floquet-based approach is adopted to derive homogenized equivalent properties. The density distribution of the equivalent continuum is taken as objective function to be minimized in the optimization process. To this end, suitable constraints are designed to avoid empty regions and ensure a minimized density where required by the mechanical actions. A de-homogenization process is carried out on the optimized equivalent continuum to derive the configuration of regular hexagons with optimally varying wall thickness. Static and buckling responses of the optimized architected solid are compared with that of a 2D continuum whose material density distribution is determined through a classical topology optimization. It is shown that the architected 2D solid can absorb higher strain energy, with respect to classically optimized structures, which suffer a buckling-driven collapse below the elasticity threshold. The architected solid is also shown to have an improved energy absorption capability, that may increase considerably its performance, depending on the ductility of the adopted material

Metamaterial beam with embedded nonlinear vibration absorbers

In this work the multi-mode vibration absorption capability of a nonlinear metamaterial beam is investigated. A Euler–Bernoulli beam is coupled to a distributed array of nonlinear spring–mass subsystems acting as local resonators/vibration absorbers. The dynamic behavior of the metamaterial beam is first investigated via the classical approach employed for periodic structures by which the frequency stop bands of the single cell are determined. Subsequently, the frequency response is obtained for the metamaterial beam to study a multi-frequency stop band system by adding an array of embedded nonlinear local resonators. A path following technique coupled with a differential evolutionary optimization algorithm is adopted to obtain the optimal frequency-response curves of the metamaterial beam in the nonlinear regime. The use of the local absorbers, via a proper tuning of their constitutive parameters, allows a significant reduction of the metamaterial beam oscillations associated with the lowest three vibration modes

Three-Dimensional Modeling of Container Cranes

This work deals with three-dimensional (3D) modeling of container cranes including the hoisting cable length commands. The proposed models allow to effectively study the 3D motion caused by the eccentricity of initial conditions or loading conditions such as those induced by wind. The container is modeled as a 3D rigid body while the hoisting cables are treated either as inextensible trusses or as linearly elastic straight, taut cables. The 3D model with inextensible cables is shown to coalesce into existing two-dimensional models under the relevant planarity constraints. Details about the treatment of the internal inextensibility constraints are discussed. Time-marching simulations are carried out to show representative 2D and 3D responses to initial conditions and commanded motion of the trolley. The main differences between the constrained model and that with the elasticity of the cables are highlighted in the framework of a few significant design scenarios. Copyright © 2013 by ASME

CARBON NANOTUBES BASED SENSORS FOR DAMAGE DETECTION

In this paper Multi-walled Carbon Nanotubes (MWNTs) - based nanocomposite films were investigated as dynamic strain sensors for damage detection. After a thorough study to optimize the electromechanical properties of the material as a function of the MWNTs content, the proof-of-concept was carried out by installing the film onto the surface of a fiberglass cantilever. The cantilever was made to oscillate through an impulsive force applied at its free end. The free oscillations of the structure are monitored and the frequencies are identified before and after reducing locally the cross section of the cantilever (to simulate damage propagation). The experimental data show the capability of the manufactured nanocomposite film to clearly identify structural frequencies in dynamic regimes. The obtained experimental data also show that this material has the potential to monitor incipient cracks (nano/micro-scaled cracks) due its sensitivity to nanoscale morphology changes