U. Galvanetto - Academia.edu (original) (raw)

Papers by U. Galvanetto

Pattern Recognition Letters, 2002

We introduce an approach to detect cyclical patterns embedded within chaotic data and make use of... more We introduce an approach to detect cyclical patterns embedded within chaotic data and make use of the detected patterns to estimate periodic-like motions in a chaotic process. A chaotic attractor contains many unstable periodic orbits (UPOs). The UPOs are hidden cyclical patterns that dominate the dynamical evolution of the system. Knowledge of UPOs can be used for estimating the trends of chaotic evolutions. A numerical experiment is conducted to illustrate an application on the business cycle detection.

Stable manifolds of saddle points are important in defining the dynamics of smooth nonlinear dyna... more Stable manifolds of saddle points are important in defining the dynamics of smooth nonlinear dynamical systems [1]. The stable manifold theorem for a fixed point states that there are local stable and unstable manifolds tangent to the eigenspaces of the linearised system at the fixed point. The global stable (and unstable) manifold is given by the union of backward (and forward) mappings in time of the local manifold. If we restrict our attention to two-dimensional Poincare maps of three-dimensional flows, at a saddle point of the map, which corresponds to a saddle-limit cycle of the flow, the linearised system will have one-dimensional stable and unstable eigenspaces. At the saddle the local manifolds are tangent to the eigenvectors and in a neighbourhood of the saddle the local manifolds are therefore approximated by these eigenvectors [2]. A numerical procedure to compute the global stable manifolds derives directly from what we just said: a fixed point of a Poincare map is located with standard numerical algorithms [2] and its eigenvectors are computed. If it is a saddle point then an eigenvector is larger than one in modulus and the other is smaller. The stable eigenspace corresponds to the eigenvalue smaller than one. A number of points approximately lying on the stable manifold are then generated by choosing them close to the saddle in the direction of the stable eigenvector. The global stable manifold can be traced by integrating backward in time such a set of points. Numerical issues can affect the computation of the stable manifold; in particular the ‘stretching’ of the manifolds may reduce the degree of accuracy with which the manifold is reconstructed, but most of these problems have been successfully overcome for smooth systems [2].

Damage identification is a key factor in many engineering fields, especially in those where the s... more Damage identification is a key factor in many engineering fields, especially in those where the safety of people is involved, as aeronautics. Many damage detection techniques require a baseline of the intact structure to evaluate the presence of the defect. The Gapped Smoothing Method (GSM) has already been investigated as a possible way to overcome this limitation. However it has been applied to simple structures as cantilever beams and plates, which are not commonly used in the aeronautical field. In the present paper a damage detection technique based on the Proper Orthogonal Decomposition (POD) is presented. The application of the GSM to the curvature of the modes shows the capability of detecting the damage in simple structures, while in the case of a more complex structure as a stiffened panel more manipulation of the damage indices were required in order to overcome the sharp variation on the curvature caused by the stiffener. Different positions and types of damage were investigated

Nonlinear Dynamics, 2013

This paper is a preliminary work to address the problem of dynamical systems with parameters vary... more This paper is a preliminary work to address the problem of dynamical systems with parameters varying in time. An idea to predict their behaviour is proposed. These systems are called transient systems, and are distinguished from steady systems, in which parameters are constant. In particular, in steady systems the excitation is either constant (e.g. nought) or periodic with amplitude, frequency and phase angle which do not vary in time. We apply our method to systems which are subjected to a transient excitation, which is neither constant nor periodic. The effect of switching-off and full-transient forces is investigated. The former can be representative of switching-off procedures in machines; the latter can represent earthquake vibrations, wind gusts, etc. acting on a mechanical system. This class of transient systems can be seen as the evolution of an ordinary steady system into another ordinary steady system, for both of which the classical theory of dynamical systems holds. The evolution from a steady system to the other is driven by a transient force, which is regarded as a map between the two steady systems.

Meccanica, 1993

This paper considers the behaviour of a mechanical oscillator with cubic non-linearity subjected ... more This paper considers the behaviour of a mechanical oscillator with cubic non-linearity subjected to a forcing excitation whose frequency remains constant while the amplitude is ramped, growing until it reaches a predetermined value. We concentrate on the nature of the basins of attraction whose size indicates the stability of the system, in a structural sense. The reduced level of forcing at the initial stages of ramping produces a delay in bifurcational events when compared to the constant sinusoidally forced counterpart. Preliminary results show that for some parameter values the area of basin does not increase monotonically as the length of ramping is varied. SOMMARIO. I1 presente lavoro considera il comportamento di un oscillatore meccanico con una non linearit/t cubica soggetto all'azione di una forzante la cui frequenza ~ costante mentre l'ampiezza varia linearmente nel tempo crescendo da zero sino a raggiungere un valore predeterminato. Si osserva in modo particolare la natura dei bacini di attrazione e la loro estensione che pu6 essere assunta come indicatore della stabilit~i del sistema in senso strutturale. La graduale crescita della forzante determina un ritardo negli eventi biforcativi rispetto allo stesso sistema forzato da una forzante ad ampiezza costante. Per alcuni valori dei parametri si osserva che l'area dei bacini non 6 una funzione monotona del tempo di crescita della forzante.

International Journal of Mechanical Sciences, 1994

AbsWact-This paper considers the behaviour of a two degree-of-freedom autonomous system with stat... more AbsWact-This paper considers the behaviour of a two degree-of-freedom autonomous system with static and dynamic friction consisting of two blocks linked by springs on a moving belt. This system is the simplest model which has been used to simulate the dynamics of seismic faults. The friction force is assumed to be a decreasing function of the relative sliding velocity. The motion of the blocks is composed of a uniform stick motion, during which the divergence of the system is zero, and an accelerated slip motion, during which the divergence is positive. The mathematical model by definition concentrates the dissipation on the point where the slip motion ceases. It is assumed that slip occurs only in one direction. A three-dimensional Poincar6 map and a scalar single variable map are discussed which characterize the dynamics of the system in a simple way. The one-dimensional map can be used to diagnose the chaotic behaviour of the full system, and quantities, similar to Lyapunov exponents, can be easily calculated which provide information regarding the system-sensitive dependence on initial conditions. The system dynamics illustrate the idea of studying the earthquake generation mechanism as a chaotic phenomenon.

AIAA Journal, 2008

ABSTRACT The present paper describes a new structural damage detection method based on the monito... more ABSTRACT The present paper describes a new structural damage detection method based on the monitoring of vibrational properties of the structure. Sensors record the accelerations of several points of the structure. The recorded data are then used to compute the proper orthogonal modes. The comparison between the proper orthogonal modes of the undamaged structure and those of the damaged structure provides information on damage location. Experimental results are presented.

International Journal of Simulation Modelling, 2010

Motorcycle crash-related fatalities and injuries have a relatively increasing tendency compared t... more Motorcycle crash-related fatalities and injuries have a relatively increasing tendency compared to other vehicles. The new development of safety devices and technologies for prediction of their behaviour are therefore also increasingly important. Motorcycles have the least amount of protective devices amongst vehicles. A small disturbance in the motion of motorcycles can expose the riders to severe impacts leading to injuries especially in the appendicular part of the body, but the severest injury is usually to the head. Head injuries are the most common cause of death amongst motorcyclists (approximately 45 %). Thus, naturally, the main protective equipment preventing motorcyclists from fatal injuries is the helmet. In this study, detailed finite element models of helmet and human head are used to simulate and analyse the impacts on a protected and unprotected head in a scenario typical for motorcycle-related collisions.

A number of Finite Element Analyses (FEA) were performed on a helmeted Hybrid III dummy and the h... more A number of Finite Element Analyses (FEA) were performed on a helmeted Hybrid III dummy and the helmeted detached head of the dummy both making impacts onto a flat anvil, in order to investigate the effect of the body on head injury predictors founded on the kinematics of the head as well as the predictors defined at tissue level. For the latter, an FE model of the human head was employed. It was shown that for the impact velocity at which the helmet had been drop tested, and certified as per ECE 22.05, the helmet was unable to protect the head when the entire body was used in the virtual impact test.

Chaos, Solitons & Fractals, 1995

... 23. A. Wolf, J-B. Swift, H, L. Swinney and JA Vastano, Determining Lyapunov exponents from a ... more ... 23. A. Wolf, J-B. Swift, H, L. Swinney and JA Vastano, Determining Lyapunov exponents from a time series. Physica D 16, 285-317 (1985). 24. ... 25. J. Huang and DL Turcotte, Evidence for chaotic fault interactions in the seismicity of the San Andreas fault and Nakai trough. ...

International Journal of Crashworthiness, 2018

Neck protective devices for motorcyclists have been introduced fairly recently but there is no st... more Neck protective devices for motorcyclists have been introduced fairly recently but there is no standard method to evaluate their performance. The goal of this study is to compare the response of riders' necks to direct impacts on the helmet with and without such a device. We investigate three common types of cervical injury mechanisms i.e. hyperflexion, hyperextension and lateral bending using finite-element method. The rotational movement of the head with respect to the torso, the neck shearing and axial loads and the stress distribution throughout the cervical vertebrae show that using the investigated type of neck protective device, which is designed to restrain the head-neck motion, can in some cases increase the risk of neck injury. Hence, the design of such devices needs further study and their assessment requires the introduction of relevant standards of evaluation.

Communications in Numerical Methods in Engineering, 2000

ABSTRACT

$Research paper thumbnail of An adaptive multi-grid peridynamic method for dynamic fracture analysis$

International Journal of Mechanical Sciences

Abstract The standard way of implementing Peridynamics is a meshfree approach which uses a unifor... more Abstract The standard way of implementing Peridynamics is a meshfree approach which uses a uniform discretization. This is inefficient when a very dense grid spacing for a localized area is required. In this paper, a radically new strategy to couple grids with different spacing is put forward. It is free of ghost forces in static cases and spurious waves in dynamic problems can be controlled and made negligible thanks to proper discretization. There is no loss of volume due to non-uniform discretization at the interface between different grids. An efficient algorithm is developed to apply the refinement adaptively. It permits to increase the resolution of the analysis only in the critical zones. The performance is investigated by solving dynamic problems, including cases of crack propagation in brittle materials. We compare the solutions of the proposed method with those of a standard peridynamic model, which employs uniform discretization, and show that the same accuracy is obtained at a much smaller computational cost.

Computers & Mathematics with Applications

Abstract Parallel processing is one of the major trends in the computational mechanics community.... more Abstract Parallel processing is one of the major trends in the computational mechanics community. Due to inherent limitations in processor design, manufacturers have shifted towards the multi- and many-core architectures. The graphics processing units (GPUs) are gaining more and more popularity due to high availability and processing power as well as maturity of development tools and community experience. In this research we describe a rather general approach to using OpenCL implementation of 3D Peridynamics model on GPU platform. Peridynamics is a non-local continuum theory for describing the behavior of material used especially when damage and crack nucleation or propagation is of interest. The steps taken for developing an OpenMP code from the serial one as well as the comparison between OpenCL and OpenMP codes are provided. Optimization techniques and their effects on the performance of the code are described. The implementations are tested on some 3D benchmarks with hundred of thousands to millions of nodes. The behavior of codes in terms of being memory or compute bound are analyzed. In all test cases reported, the OpenCL implementation consistently outperforms serial and OpenMP ones and paves the road for the development of high performance Peridynamics codes.

Applied Composite Materials, 2016

The chin bar of motorcycle full-face helmets is the most likely region of the helmet to sustain i... more The chin bar of motorcycle full-face helmets is the most likely region of the helmet to sustain impacts during accidents, with a large percentage of these impacts leading to basilar skull fracture. Currently, helmet chin bars are designed to mitigate the peak acceleration at the centre of gravity of isolated headforms, as required by standards, but they are not designed to mitigate the neck force, which is probably the cause of basilar skull fracture, a type of head injury that can lead to fatalities. Here we test whether it is possible to increase the protection of helmet chin bars while meeting standard requirements. Fibre-reinforced composite shells are commonly used in helmets due to their lightweight and energy absorption characteristics. We optimize the ply orientation of a chin bar made of fibre-reinforced composite layers for reduction of the neck force in a dummy model using a computational approach. We use the finite element model of a human head/neck surrogate and measure the neck axial force, which has been shown to be correlated with the risk of basilar skull fracture. The results show that by varying the orientation of the chin bar plies, thus keeping the helmet mass constant, the neck axial force can be reduced by approximately 30% while ensuring that the helmet complies with the impact attenuation requirements prescribed in helmet standards.

Mechanics Research Communications, 1993

... Bande transporteuse; Conveyor belt; banda transportadora; Frottement; Friction; Vibration méc... more ... Bande transporteuse; Conveyor belt; banda transportadora; Frottement; Friction; Vibration mécanique; Mechanical vibrations; Contact mécanique; Mechanical ...

Advances in Space Research, 2013

ABSTRACT This paper discusses the impact behavior of a self-healing ionomeric polymer and compare... more ABSTRACT This paper discusses the impact behavior of a self-healing ionomeric polymer and compares its protection capability against space debris impacts to that of simple aluminium-alloy bumpers. To this end, 14 impact experiments on both ionomer and Al-7075-T6 thin plates with similar surface density were made with 1.5 mm aluminium spheres at velocity between 1 and 4 km/s.First, the perforation extent in both materials was evaluated vis-à-vis the prediction of well known hole-size equations; then, attention was given to the damage potential of the cloud of fragments ejected from the rear side of the target by analysing the craters pattern and the momentum transferred to witness plates mounted on a ballistic pendulum behind the bumpers.Self-healing was completely successful in all but one ionomer samples and the primary damage on ionomeric polymers was found to be significantly lower than that on aluminium. On the other hand, aluminium plates exhibited slightly better debris fragmentation abilities, even though the protecting performance of ionomers seemed to improve at increasing impact speed.