108 research outputs found
Equilibrium states and stability of pre-tensioned adhesive tapes
No full textIn the present paper we propose a generalization of the model developed in Afferrante, L.; Carbone, G.; Demelio, G.; Pugno, N. Tribol. Lett. 2013, 52, 439–447 to take into account the effect of the pre-tension in the tape. A detailed analysis of the peeling process shows the existence of two possible detachment regimes: one being stable and the other being unstable, depending on the initial configuration of the tape. In the stability region, as the peeling process advances, the peeling angle reaches a limiting value, which only depends on the geometry, on the elastic modulus of the tape and on the surface energy of adhesion. Vice versa, in the unstable region, depending on the initial conditions of the system, the tape can evolve towards a state of complete detachment or fail before reaching a state of equilibrium with complete adhesion. We find that the presence of pre-tension in the tape does not modify the stability behavior of the system, but significantly affects the pull-off force which can be sustained by the tape before complete detachment. Moreover, above a critical value of the pre-tension, which depends on the surface energy of adhesion, the tape will tend to spontaneously detach from the substrate. In this case, an external force is necessary to avoid spontaneous detachment and make the tape adhering to the substrate
Theoretical estimation of drop shape and apparent contact angles of regular micro-structured superhydrophobic surfaces
No full text
Corrugation Models and the Roaring Rails Enigma: a Simple Analytical Contact Mechanics Model Based on a Perturbation of Carter's Solution
No full textCorrugation in railways, and especially short pitch corrugation (30-80 mm), is still considered something of an enigma, despite extensive research. Models based on repeated impacts or differential wear, such as Grassie and Johnson's (1985) and Bhaskar et al.'s (1997), seem not to be conclusive, or not to suggest the correct wavelength. Further models have been suggested, either linear (Frederick, Valdivia, Hempelmann, Vassilly and Vincent) or nonlinear (Mueller), but most suggest a constant frequency mechanism invariably connected to vertical resonances of the system either in the low frequency range (50-100 Hz, the resonance of the vehicle's unsprung mass on the track stiffness referred to here as the "P2 resonance", close to the Hertz contact resonance), or at about 1000 Hz (pinned-pinned resonance, in which the rail vibrates almost as if it were a beam pinned at sleepers), or even higher frequencies still (1700-1800 Hz). The experimental data available, by contrast, do not fit these frequency ranges. The discrepancy is tentatively explained with "contact filtering" and varied traffic ideas, but do not convince completely. In this paper, we stress the importance of wheel inertia in coupling the oscillations of normal load, with the variations of tangential load and longitudinal creepage. A simple zeroth order perturbation of the classical rolling contact solutions is suggested, which obtains good qualitative agreement with experimental evidence. The model also leads to the recognition that vertical resonances are not crucial in explaining corrugation, as believed in previous models, since we use an extremely simple model of an Euler beam with no elastic support, having no resonances. Important factors for the growth of corrugation are the friction coefficient and the tractive ratio. High longitudinal creepage is needed to promote rapid development, and this can arise from curving, hunting motion or misaligned axles, and is probably exacerbated by high contact conformity, since this increases the fluctuating component of longitudinal creepage due to the movement of the contact point. With discrete supports, we expect a modulation of corrugation wavelength and amplitude, but this requires a separate investigation, not just the inclusion of pinned-pinned resonance
A novel probabilistic approach to assess the blade throw hazard of wind turbines
No full textThe increasing number of wind turbine power plant installations and the recent trend to locate them in proximity of build-up areas raise safety concerns as the rotor failure may result in blade throws that can endanger people living/working close to the wind farm. Therefore, it becomes strictly necessary to define setback distances and/or buffer zones to minimize the risk of damage or injury from components failure. However, according to the existing standards, buffer zones and/or setbacks distances are defined by 'rule of thumbs', usually based on the height of the wind tower, and are often overestimated, resulting in too large distances, which may result incompatible with the needs of increasing the number wind power installations. This explains why the scientific community is, now more than in the past, spending a lot of effort in the attempt of developing reliable methodologies able to assess the impact risk in the areas surrounding the wind farm.
In the present paper a very novel and computationally efficient method is presented to estimate the blade throw hazard of wind turbines. The method combines a 3D dynamic model of the detached blade fragment with a rigorous probabilistic approach. Results are shown in terms of safe (white) and unsafe (dark) zones, which are estimated on the basis of an acceptable risk threshold
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