170,153 research outputs found
Indenting viscoelastic thin layers: A numerical assessment
No full textNormal indentation of viscoelastic bodies is a technique widely employed to characterize the viscoelastic material properties. Here, a numerical Boundary Element methodology is developed to model the indentation process also when the solids in contact are thin layers. Specifically, two boundary conditions for the thin slabs are considered: the case of a confined layer, perfectly bonded to a rigid substrate, and that one of a free layer, supported by a constant pressure. Numerical analyses focus, firstly, on creep and relaxation indentation tests and show that finite values of the contacting layers thickness produce dramatic quantitative changes in comparison with what obtained under the half-plane (HP) assumption. Similar effects are found also in the case of loading and deformation indentation cycles: this results crucial for vibrational phenomena and confirms the opportunity of introducing the numerical technique here presented
The influence of temperature on viscoelastic friction properties
No full textViscoelastic friction strongly depends on temperature, which determines the material stiffness and, therefore, given a constant load, the volume that is deformed and dissipates energy. We compare the results obtained by a numerical approach introduced by Carbone and Putignano (2013) [1] with measurements that separate viscoelastic losses from Coulomb contribution. This is done for a range of temperatures. We show that viscoelastic friction curves for different temperatures can be arranged into a single master curve using a frequency shift coefficient, which can be found from the characterization of the viscoelastic material response. This shows that it is possible to accurately (a) use dynamic material analysis data to extrapolate viscoelastic friction measurements to values outside the tested range, and (b) use a tribometer to obtain fundamental viscoelastic material properties
On the Role of Roughness in the Indentation of Viscoelastic Solids
No full textA numerical boundary element methodology is employed to understand how fractality intervenes when a 1D rigid rough profile indents a linear viscoelastic half-plane. The focus is, in particular, on the viscoelastic dissipation and how this is influenced by the profile statistical parameters, namely, the mean square roughness h(rms) of the profile, the mean square slope m(2) and the Hurst exponent H. Our numerical investigation, properly supported by a dimensional analysis, reveals that, in the one-dimensional case under investigation, the leading role is played by h(rms) and, thus, mainly by the large scales of the rough spectrum. Clearly, on an experimental level, this implies that a simple measure of the roughness parameter h(rms) is sufficient to determine the viscoelastic dissipation
Mechanics of rough contacts in elastic and viscoelastic thin layers
No full textContact mechanics between rough solids usually relies on the half-space approximation, which assumes that the contact area dimension is much smaller than the thickness of the layers of materials that characterize the surfaces of the contacting bodies. However, such simplifying assumption is often inadequate when industrially relevant applications are considered, in particular those of biomechanical interest. Indeed, a large variety of systems, including not only classical engineering applications such as gear boxes, shafts, tyres, etc., but also biological tissues such as human skin, is characterized by superficial coatings; very often the mechanical properties of these coatings are very different from those of the bulk region of the bodies in contact. The aim of this paper is to shed light on the role played by the thickness of the layer of material used as a coating, with specific focus on the contact between a rigid rough surface and a thin deformable layer bonded to a rigid substrate. Starting from a recently developed boundary element formulation (Carbone and Putignano, 2013), we derive a methodology which accounts for finite thickness by a corrective coefficient modulating the classical Greens function, and extends our analyses to periodic domains. This enables to avoid border effects and provides an innovative tool to tackle viscoelastic contacts with realistic roughness. This is exploited to perform a thorough investigation of the mechanisms responsible for frictional losses in layered systems characterized by different materials, thickness and loading conditions. Results show that decreasing the layer thickness corresponds to an increase in the contact stiffness. Furthermore, in the case of viscoelastic layer, particular attention has to be paid to the changes in the viscoelastic dissipation due to the finite thickness of the surface layer
PolSOM and TexSOM in polarimetric SAR classification
No full textThe polarimetric observables in a SAR image possess an intrinsic physical information, what makes polarimetric data fit to unsupervised classification, without need of a-priori information on the scene. Indeed, in natural targets, like vegetation, surface, volume and sometimes double-bounce scattering mechanisms are mixed, while backscattering from man-made targets can be usually attributed to dihedrons, trihedrons and bare surfaces. In many cases a radar resolution cell hosts more than one mechanism, although an average or dominant scattering mechanism can be identified for the purposes of classification. Following Chandrasekhar's pioneering target decomposition and the generalized and systematic theory by Huynen, a number of approaches to the interpretation of the scattering processes and to the identification of scatterers have appeared in the open literature. Target decomposition theory laid down the basis for the classification of radar images. In particular, the formalism worked out by Cloude, led to the introduction of an unsupervised classification scheme, further augmented and improved by subsequent contributions, also connecting the fuzzy logic theory with Wishart's statistical approach and electromagnetic modeling.
Neural Network Algorithms (NNA) have been used in multispectral images classification and for change maps, but their application to polarimetric SAR image classification is more limited. In supervised schemes, the NNA were trained by Huynens parameters, or by the polarimetric coherence matrix [T], H and alpha. Unsupervised Neural Net classifiers, based on Self-Organizing Maps (SOM), have exploited MÏ ller matrix directly, polarization signatures, or parameters derived from decomposition, like Huynen's, or Freeman's.
In this Thesis two novel unsupervised classification algorithms, named PolSOM and TexSOM, for polarimetric data are proposed. Both algorithms are SOM-based and have been tested on complex Italian landscapes, where classification can become quite challenging and a limited use of polarimetric
data has been reported for undulating, heterogeneous and fragmented scenarios. AIRSAR fully polarimetric data from MAC-Europe Campaign and RADARSAT-2 data acquired for a SOAR project (SOAR-1488) have been classified and confusion matrices have been computed from ground truth maps. PolSOM and TexSOM performances have been compared with each other and with consolidated and commonly used classification method, to assess their potential. The Neural Network algorithms have been carefully designed based on an in-depth analysis of their operation and, for the first time at the author's knowledge, both object-based and pixel-based information are jointly used in Radar polarimetric image analysis. The proposed classification algorithms are proving to be fairly versatile and not strictly confined to polarimetric images, like the other considered algorithms
VALORI, DISVALORI E CRIMINE NELL'ITALIA ALLE SOGLIE DEL DUEMILA. LA PERCEZIONE SOCIALE DEL CONCETTO DI REATO
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