1,720,999 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
On the Role of Roughness in the Indentation of Viscoelastic Solids
Full text linkA 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
VALORI, DISVALORI E CRIMINE NELL'ITALIA ALLE SOGLIE DEL DUEMILA. LA PERCEZIONE SOCIALE DEL CONCETTO DI REATO
No full text
Elastic contact of rough surfaces: A simple criterion to make 2D isotropic roughness equivalent to 1D one
No full textWe analyse the periodic contact between an elastic half-space and two types of rough substrates: (i) a perfect isotropically rough rigid substrate (2D isotropic roughness), and (ii) a perfect anisotropically rough rigid substrate, i.e. a substrate with roughness in only one direction (1D roughness). The analysis is carried out with the aid of proprietary codes, that we have developed (both in real and Fourier space) to deal with this type of contacts. Of course, 1D contacts differ from 2D isotropic contacts. However our results and theoretical arguments suggest a possible criterion to make 2D contacts equivalent to 1D ones from the point of view of contact area and separation calculations. The rule consists in replacing the 2D Power Spectral Density (PSD) of the isotropic surface into an equivalent 1D PSD. Interestingly the trasformation rule does not depend on the statistical properties of the surface roughness, hence seems to have a universal character for isotropic surfaces
LA CRIMINALITA' COLPOSA DEL TRAFFICO STRADALE.RILEVANZA DEL PEDONE NEGLI INCIDENTI STRADALI
No full textSelf-excited vibrations due to viscoelastic interactions
No full textSelf-excited vibrations represent a big concern in engineering, particularly in automotive, railway and aeronautic industry. Many lumped models have been proposed over the years to analyze the stability of such systems. Among the instability mechanisms a falling characteristic of the friction law and mode coupling have been shown to give friction-excited oscillations. The mass-on-moving-belt system has been studied extensively in Literature, very often adopting a prescribed form of the friction law and linearizing the contact stiffness. Instead, in this work, the case of a spherical oscillator excited by a moving viscoelastic halfspace is considered. The friction law and the nonlinear normal contact stiffness have been computed via boundary element numerical simulations for varying substrate velocity and indentation depth, then they have been adopted in time-marching dynamical numerical simulations. It is shown that the horizontal and vertical dynamics of the oscillator are tightly connected each other through the viscoelastic substrate. Furthermore, for certain normal forces/substrate velocity, the system has multiple stable solutions, which may include lift-off of the oscillator, which are selected based solely on the system initial conditions
Thermoelastic effects in the contact mechanics of 1D+1D rough profiles
No full textRough contact mechanics is a challenging topic that has attracted the interest of many scientists in the past and recent years. Notwithstanding a large amount of literature on the topic, there is a lack of studies investigating the contact behaviour of rough elastic bodies exchanging heat at the interface. For this reason, we propose a deterministic model to investigate the thermoelastic contact of a linear elastic half-plane indented by a rigid rough punch. Surprisingly, an increase in the temperature difference between the contacting solids does not change the relationship between contact area and load as well as that between interfacial mean separation and load. However, the thermal expansion causes an increase in the force required to sustain the contact at a given penetration. In addition, thermal contact resistance (TCR) is predicted to be a decreasing function of the contact pressure in agreement with the trend suggested by experimental data available in the literature. On the contrary, the dependence on the temperature difference ΔT seems to be strictly related to the characteristics of the materials and, for the elastic case investigated in this work, TCR is found to be almost independent of ΔT
- …
