1,721,193 research outputs found
Numerical and experimental study of the fatigue life of wheels for industrial vehicles
No full textLa presente ricerca riguarda lo sviluppo di metodi sperimentali per la previsione di vita a fatica di ruote per applicazioni industriali. Ad oggi, gli studi su tale argomento riguardano soprattutto il settore automotive, dove i carichi sulle ruote sono più bassi e le condizioni di utilizzo più uniformi, rispetto al settore industriale. Da qui l’esigenza di sviluppare maggiori conoscenze anche in quest’ultimo settore. Infatti, la continua richiesta di miglioramento delle prestazioni dei veicoli dal punto di vista dei carichi di lavoro, della variabilità di utilizzo e della durata complessiva, potrebbe condurre a situazioni dove gli attuali strumenti di validazione risultino insufficienti.
La prima parte della tesi presenta lo stato dell’arte dei metodi di validazione delle ruote industriali, con particolare riferimento alle prove di fatica eseguite in laboratorio secondo standard riconosciuti. Di queste sono evidenziate le principali criticità, relative soprattutto ad applicazioni non comprese in tali standard di prova, o a casi dove gli elevati carichi richiesti non consentono di completare le prove con tempi e costi accettabili per la pratica industriale. Per ovviare a tali criticità, è proposto un approccio basato sul miglioramento della conoscenza degli spettri di carico in esercizio tramite prove sul campo, e su metodi alternativi di caratterizzazione a fatica del materiale dopo il processo produttivo. L’importanza di questi temi è confermata dall’analisi di un caso di rottura in servizio di una macchina per movimentazione container.
Nella seconda parte della tesi, lo studio si concentra sulla caratterizzazione del comportamento a fatica di un componente, un cerchione per applicazioni agricole, per il quale non esistono attualmente casi di rottura né sul campo né tramite prove a fatica tradizionali. Il metodo usato prevede l’esecuzione di prove a fatica su provini ricavati dal componente, studiati in modo da replicarne le condizioni di massima sollecitazione nei punti critici in condizioni di esercizio. Dai risultati si è ricavata una curva a fatica per il componente che considera anche alcuni aspetti determinati dal processo produttivo, come la rugosità superficiale e l’incrudimento dovuto a deformazione plastica localizzata. Non sembrano essere invece incluse le tensioni residue di compressione che, seppure presenti nel cerchione completo, sono state rilasciate durante la realizzazione dei provini. Data la loro riconosciuta influenza sulla vita a fatica dei componenti strutturali, esse dovrebbero comunque essere valutate nell’analisi complessiva, ad esempio misurandole sul componente finito ed aggiungendole, come tensioni medie, agli spettri di carico misurati nelle prove sul campo.
Il metodo esposto è stato replicato su un cerchione differente, per il quale già esistevano dati di rottura a fatica, verificando l’esistenza di un fattore di scala tra i risultati di durata del componente rispetto a quelli dei singoli provini. L’applicazione dello stesso procedimento a casi simili consentirà di valutare la variabilità di tale fattore di scala e la sua estendibilità ad altri componenti.
In conclusione, le procedure utilizzate in questa ricerca consentono un miglioramento nella progettazione a fatica di ruote per veicoli industriali. Questo può portare allo sviluppo di prodotti più affidabili ed ottimizzati, con riduzione di peso a pari resistenza, e conseguente risparmio energetico, aspetto fondamentale nello sviluppo industriale moderno. Dai risultati ottenuti, sono poi emersi ulteriori argomenti di ricerca, riguardanti l’influenza dei processi produttivi sulla resistenza del materiale e la necessità di definire nuovi standard di prova in laboratorio, per ruote di applicazioni critiche, a partire dall’analisi dei dati acquisiti in condizioni di esercizio.This research deals with the development of experimental methods for the fatigue life evaluation of wheels for industrial applications.
To date, the studies about this topic are mainly related to the automotive industry, where the loads on wheels are lower and the conditions of usage more standardised than in the industrial sector. Hence, the need of improving the knowledge also in this sector. In fact, the continuous request of improving the vehicles performance from the point of view of working loads, variability of usage and life-time duration, could lead to situations where the current design validation methods are insufficient.
The first part of this thesis presents the state of the art of the validation methods for industrial wheels, with particular reference to fatigue lab testing according to approved standards. The most critical aspects of these methods are highlighted, mostly regarding applications not included in any test standard, or cases where the high loads required do not allow fatigue tests to be completed with acceptable time and cost for the industrial practice. To overcome these critical issues, an alternative approach is proposed. It is based on improving the knowledge of the service load spectra through appropriate field tests, and on alternative methods for evaluating the fatigue properties of the material after the production process. The importance of these topics is confirmed by a case of failure analysis of a machine for container handling.
In the second part of this thesis, the study focuses on the evaluation of the fatigue properties of a specific component, a rim for agricultural applications, for which, to date, no fatigue failure has ever happened, neither in field service nor in traditional lab testing. The method used consists in the fatigue testing of section specimens obtained from the component, designed to replicate the maximum stress in the critical points of the full-scale component in service conditions. The results allowed to obtain a fatigue curve for the component, which also considers some aspects related to the production process, such as the surface roughness and the work hardening due to local plastic deformation. However, the compressive residual stresses, which were present in the full-scale rim, were released during the production of the specimens and they seem not considered by this approach. Given their known influence on the fatigue life of structural components, they should be evaluated anyway, in the general analysis. For example, they could be measured on the finished component and added, as mean stresses, to the stress spectra acquired during field tests.
The approach discussed was also applied to a different rim size, for which fatigue failures were already reached through standard testing. This further investigation showed the existence of a scale factor between the fatigue life results of the full-scale component and the ones of the rim section specimens. The further application of the same procedure to similar cases will allow to evaluate the variability of this scale factor and its possible use for other components.
In conclusion, the methods investigated in this research can be used to improve the design against fatigue of wheels for industrial vehicles. That may lead to the development of more reliable and optimised products, with possible weight reduction at the same fatigue strength, and consequent energy saving, which is a fundamental aspect in modern industry. Moreover, further topics for new possible research emerged, regarding the influence of the production process on the fatigue strength of the components and the need of developing new lab test standards, for critical applications, starting from the analysis of field tests data in real service conditions.
The themes discussed and the methods used, here applied to wheels for industrial vehicles, are of general interest in mechanical engineering and can be extended to other structural components
From single neuron activity to network information processing: Simulating cortical local field potentials and thalamus dynamic regimes with integrate-and-fire neurons
No full textCortical and subcortical neural activity has been modeled for decades by means of recurrent networks of integrate and fire neurons (IFN). Such networks have proved to be able to capture a variety of neural functions ranging from sensory processing to short term memory and decision making. Some network scale phenomena, however, were considered too complex to be simulated with simple basic components as IFN. Namely, Local Field Potentials (LFP) were simulated with multi-compartmental models, as IFN lacked the ability to cope with the spatial features of the signal, and thalamic oscillations were modeled with Hodgkin and Huxley and similar neuron models, as IFN were thought not be able to mimic the rebound properties of the neurons of the area. Here we will show instead how it is possible to capture both phenomena with IFN networks. First, in a series of works spanning almost a decade, we were able to build flexible LFP proxies simulating the extracellular signal from IFN network dynamics. We identified several key properties of the neuronal structure, based on the dipolar approximation of LFP generation, and we implemented them in the IFN model. This led to a dimensionality reduction of the system parameters of two order of magnitudes, while capturing almost entirely the LFP dynamic. Actually, the resulting model of LFP is so efficient to be able to reconstruct spiking dynamics from LFP recorded in the primary visual cortex. Second, we were able to reproduce with IFN two dynamic regimes of the thalamus displaying different characteristic oscillations bands and different information processing properties. The key advancement was to highlight the connection between single neuron dynamics and network regimes. Spindle oscillations preventing information transmission to the cortex occur when the neurons fire exclusively because of rebound from inhibition. When the external excitatory inputs are sufficiently strong, instead, thalamocortical relay neurons start firing because of depolarization, weakening spindle oscillations and leading to information transmission. Starting from these results we will discuss the possibility of capturing the whole process of encoding of sensory stimuli by means of a chain of spiking neuron model covering the whole path from peripheral sensors to primary sensory cortex. Building the whole structure with simple and elegant IFN will make the analysis and the comparison with experimental results sharper
From neuronal networks to behavior: dynamics of spontaneous activity and onset of movement in the leech
Full text linkAnimal behavior was once seen as a chain of reactions to stimuli from the environment. From
chemotaxis in bacteria to mammals withdrawing from painful stimuli, most of the actions taken by
animals are clearly driven by external inputs. Reflexes were among the first phenomena to be
studied to have an insight on the dynamics of the nervous system. Later, a step forward was the
discovery of central pattern generators: once a behavior is started by a stimulus, some neuronal
networks are able to maintain it without further inputs from the environment. The nervous system of
all animals, however, is so complex that is displaying a rich dynamics even in the absence of
external inputs or, in a more realistic situation, when no single input is able to drive a clear-cut
reaction. In the same way, at the motor output level, animals keep moving in the absence of evident
stimuli. These spontaneous behaviors are still far from being understood.
Difficult problems are often easier to solve in simple systems. The leech has a relatively simple
nervous system, composed of ~103 neurons disposed in a regular structure, but at the same time
displays a variety of different behaviors. It seems then a good preparation to approach the
spontaneous dynamics problem.
The aim of my PhD research is to describe the spontaneous behavior of the leech and the
spontaneous activity of its nervous system.
A first, necessary step for this study was to develop a method of automatic classification and
analysis of the leech movements. Thanks to this method we described accurately the properties of
the different behaviors: we focused particularly on the largely unknown irregular exploratory
behavior, which is found to display a broad range of oscillation frequencies and displacement
speeds, but with some recurrent movement patterns. Finding the complete list of the leech
spontaneous behaviors, and the probability of the transitions between them, it was possible to
demonstrate that decision making in the leech is a Markovian process.
The spontaneous activity in the isolated leech ganglion was found to be characterized by long-term
correlations and a large variability in bursts size and duration. The same dynamics was observed in
dissociated culture of rat hippocampal neurons, despite the difference in the structure between the
two networks. We studied the effects of pharmacological modulations of inhibitory and excitatory
processes on the spontaneous activity, and the role of single identified motor neurons in spontaneous bursts. Finally we proposed a simple statistical model accounting for experimental
results.
We studied then the spontaneous activity of the leech ganglion when it was connected to the other
ganglia and in the semi-intact moving animal. Inputs received from the head and tail brain caused a
drastic change in the activity of the ganglion, increasing synchronization among neurons and
leading to a regime dominated by very large bursts. By recording at the same the movements of the
leech and its nervous activity it was possible to have a better understanding of the relationship
between the motor neuron bursts and the onset of movements
Can extremism guarantee pluralism?
No full textMany models have been proposed to explain opinion formation in groups of
individuals; most of these models study opinion propagation as the interaction
between nodes/agents in a social network. Opinion formation is a complex
process and a realistic model should also take into account the important
feedbacks that the opinions of the agents have on the structure of the social
networks and on the characteristics of the opinion dynamics. In this paper we
will show that associating to different agents different kinds of
interconnections and different interacting behaviours can lead to interesting
scenarios, like the coexistence of several opinion clusters, namely pluralism.
In our model agents have opinions uniformly and continuously distributed
between two extremes. The social network is formed through a social aggregation
mechanism including the segregation process of the extremists that results in
many real communities. We show how this process affects the opinion dynamics in
the whole society. In the opinion evolution we consider the different
predisposition of single individuals to interact and to exchange opinion with
each other; we associate to each individual a different tolerance threshold,
depending on its own opinion: extremists are less willing to interact with
individuals with strongly different opinions and to change significantly their
ideas. A general result is obtained: when there is no interaction restriction,
the opinion always converges to uniformity, but the same is happening whenever
a strong segregation process of the extremists occurs. Only when extremists are
forming clusters but these clusters keep interacting with the rest of the
society, the survival of a wide opinion range is guaranteed
EMG-based prediction of step direction for a better control of lower limb wearable devices
Full text linkBackground and objectives: Lower-limb wearable devices can significantly improve the quality of life of subjects suffering from debilitating conditions, such as amputations, neurodegenerative disorders, and stroke-related impairments. Current control approaches, limited to forward walking, fall short of replicating the complexity of human locomotion in complex environments, such as uneven terrains or crowded places. Here we propose a high-level controller based on two Support Vector Machines exploiting four surface electromyography (EMG) signals of the thigh muscles to detect the onset (Toe-off intention decoder) and the direction (Directional EMG decoder) of the upcoming step. Methods and materials: We validated a preliminary version of the approach by acquiring EMG signals from ten healthy subjects, performing steps in four directions (forward, backward, right, and left), in three different settings (ground-level walking, stairs, and ramps), and in both steady-state and static conditions. Both the Toe-off intention and Directional EMG decoders have been tested with a 5-fold cross-validation repeated five times, using linear and radial-basis-function kernels, and by changing the classification output timing, from 200 ms before to 50 ms after the toe-off. Results: The Toe-off intention decoder reached a median accuracy of 83.34 % (interquartile range (IQR): 6.48) and specificity of 92.72 % (IQR: 3.62) in its radial-basis-function version, while the Directional EMG decoder's median accuracy ranged between 73.92 % (IQR: 5.8), 200 ms before the toe-off, to 92.91 % (IQR: 4.11), 50 ms after the toe-off, with the radial-basis-function kernel implementation. For both the Toe-off intention and Directional EMG decoders the radial-basis-function version achieved better performances than the linear one (Wilcoxon signed rank test, p < 0.05). Conclusions and significance: The combination of the two decoders proved to be a promising solution to detect the step initiation and classify its direction, paving the way for wearable devices with a broader range of movements and more degrees of freedom, ultimately promoting usability in uncontrolled settings and better reactions to external perturbations. Additionally, the encumbrance of the setup is limited to the thigh of the leg of interest, which simplifies the implementation in compact devices, concurrently limiting the sensors worn by the subject
Development of an innovative test machine for tyre, wheel and suspension systems for automotive and industrial vehicles
No full text
Le società miste quotate in mercati regolamentati (dalla "Golden Share" ai fondi sovrani)
No full textThe essay examines the special treatment which under Italian law is applied to listed companies having among their shareholders the Italian State or any of the Italian local public entities (Regions, municipalities); it also takes into consideration any specific rules concerning the holding of participations in listed Italian companies by foreign sovereign funds. It concludes that the special treatment awarded to the Italian public entities as shareholders of listed companies causes an unnecessary privilege for such shareholders
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