The mechanism of detachment of thin films from a flat smooth rigid substrate is investigated. In particular, analytical solutions in closed form are proposed for the double peeling of an elastic tape as well as for the axisymmetric peeling of a membrane. We show that in the case of double peeling of an endless elastic tape, a critical value of the pull-off force is found, above which the tape is completely detached from the substrate. In particular, as the detachment process advances, the peeling angle is stabilized on a limiting value, which only depends on the geometry of the tape, its elastic modulus and on the interfacial energy. This predicted behavior agrees with the ‘‘theory of multiple peeling’’ and clarifies some aspects of this theory. Moreover, it is also corroborated by experimental results (work in progress) we are carrying out on a standard adhesive tape adhered to a smooth flat poly(methyl methacrylate) surface. In the case of the axisymmetric adhering membrane, a different behavior is observed. In such case, the system is always stable, and the detached area monotonically increases with the peeling force, i.e., the elastic membrane can sustain in principle any applied force. Results are validated by a fully numerical analysis performed with the aid of a finite element commercial software.

In this paper, we present numerical investigation of the contact between an elastic solid and a randomly rough surface. In agreement with recent results, we find that the contact area vs load relation depends on the statistical parameters only through the root mean square slope of the heights distribution. Such result extends to contact pressure regimes where the area/load relation is non-linear. Moreover, we show that fractal self-affine surfaces give a good representation of real surfaces from both topographical and contact mechanics points of view. Finally, we investigate how the network of non-contact areas evolves as the real contact area is increased, finding that the percolation threshold is smaller than the one predicted by Bruggeman's theory

La definizione della soluzione del problema di contatto è oggi richiesta da una sempre più ampia varietà di applicazioni. Le sollecitazioni, le deformazioni e la dissipazione all’interfaccia di contatto sono, nello specifico, parametri da settare nel design ottimizzato di tali componenti. In mancanza di soluzioni analitiche universali, si è fatto finora ampio ricorso ai Metodi FEM (Finite Elements Method): gli stessi tuttavia sono poco efficienti nella definizione di un’accurata soluzione di contatto. In particolare, ciò risulta vero quando si voglia tenere in conto la rugosità superficiale dei componenti a contatto. Al fine di superare tali limitazioni, sono stati sviluppate dagli autori metodologie agli Elementi di Contorno in grado di definire accuratamente i parametri di contatto più significativi: area di contatto, rigidezza, dissipazione ed attrito sono analizzate in funzione della configurazione di carico. Particolare attenzione è dedicata allo sviluppo di tecniche di mesh efficienti per la riduzione dei costi computazionali.

In 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.

In this paper, we discuss the mechanism of detachment of thin pre-stressed films from a flat smooth rigid substrate. Indeed, we develop an analytical solution in closed form which shows how the critical value of the pull-off force strongly depends on the press-stress P0 . In detail, the critical pull-off force needed to detachment is shown to be higher for pre- stressed tapes. Furthermore, we notice that, when a high pre- stress is present, tapes may behave in different manner and spontaneously detach from the rigid substrate.

Superhydrophobic surfaces are effective in practical applications provided they are “robust superhy-drophobic”, i.e. able to retain the Cassie state, i.e. with water suspended onto the surface protrusions,even under severe conditions (high pressure, vibrations, high speed impact, etc.). We show that for ran-domly rough surfaces, given the Young angle, Cassie states are robust when a threshold value of theWenzel roughness factor, rW, is exceeded. In particular, superhydrophobic nano-textured surfaces havebeen generated by self-masked plasma etching. In view of their random roughness, topography features,acquired by Atomic Force Microscopy, have been statistically analyzed in order to gain information onstatistical parameters such as power spectral density, fractal dimension and Wenzel roughness factor(rW), which has been used to assess Cassie state robustness. Results indicate that randomly rough sur-faces produced by plasma at high power or long treatment duration, which are also fractal self-affine,have a rWhigher than the theoretical threshold, thus for them a robust superhydrophobicity is predicted.In agreement with this, under dynamic wetting conditionson these surfaces the most pronounced super-hydrophobic character has been appreciated: they show the lowest contact angle hysteresis and resultin the sharpest bouncing when hit by drops at high impact velocity.

We analyse in terms of efficiency and traction capabilities a recently patented traction drive, referred to as the double roller fulltoroidal. variator (DFTV). We compare its performance with the single roller full-toroidal variator (SFTV) and the single roller. half-toroidal variator (SHTV). Modeling of these variators involves challenging tribological issues; the traction and efficiency. performances depend on tribological phenomena occurring at the interface between rollers and disks, where the lubricant. undergoes very severe elastohydrodynamic lubrication regimes. Interestingly, the DFTV shows an improvement of the mechanical. efficiency over a wide range of transmission ratios and in particular at the unit speed ratio as in such conditions in which the DFTV. allows for zero-spin, thus strongly enhancing its traction capabilities.The very highmechanical efficiency and traction performances. of the DFTV are exploited to investigate the performance of a flywheel-based Kinetic Energy Recovery System (KERS), where. the efficiency of the variator plays an important role in determining the overall energy recovery performance. The energy boost. capabilities and the round-trip efficiency are calculated for the three different variators considered in this study.The results suggest. that the energy recovery potential of the mechanical KERS can be improved with a proper choice of the variator.

The superlative adhesive properties of some biological attachment systems, such as those of geckos, spiders, and insects, have inspired researchers from different fields (e.g. biology, physics and engineering) to conceive and design man-made microstructured surfaces that might mimic their performance. Among the several proposed designs, very recently mushroom-shaped adhesive microstructures have drawn the interest of scientists and engineers, because experiments have proved their superiority compared to other micro- and nano-structures. In this article, we explain theoretically the physical mechanism behind the enhanced adhesion of such microstructures, and provide for the first time a useful tool to predict adhesive performance depending on the geometry, mechanical properties of the material, and energy of adhesion. Our theoretical predictions are strongly supported by the available experimental data. The present study can streamline the optimisation of adhesive microstructures for industrial applications.

A novel set up is proposed to measure the energy release rate versus crack propagation speed in viscoelastic materials. The new test is referred to as the square sample tear test. In comparison to other techniques as the pure shear specimen and the pure tensile specimen tests, the square sample geometry guarantees at the same time: (i) a constant flux of elastic energy at the crack tip, (ii) the full development of the annular dissipative region around the crack tip, where the material is in the transion region of the viscoelastic response spectrum. Both conditions allow to precisely control the crack propagation speed and to test the predictions of existing theories. By employing the new set-up we measure the energy release rate as a function of the crack propagation speed. We also measure with a high precision infrared camera the increase of temperature at the crack tip, showing that, notwithstanding the low speed regime considered during the experiments, the viscoelastic dissipation at the crack tip leads to a measurable temperature increase of about 1◦C

In this paper we analyse the adhesion between a rubber block and a rigid randomly rough profile. The focus of the investigation is on the influence of the work of adhesion and of the fractal dimension Df of the rough profile on the contact behaviour. In particular, we analyse how the contact area and the power spectral density of the deformed profile are affected by the two aforementioned quantities. We find that at sufficiently small loads the influence of Df is negligible. However, the scenario strongly changes at higher loads as Df strongly affects the number of contact spots. Calculations show that the contact area depends linearly on the work of adhesion, whereas only a negligible influence of the work of adhesion is found on the power spectral density (PSD) of the deformed profile.

In this paper, we analyze in terms of efficiency and traction capabilities a recently patented toroidal traction drive variator: the so-called double roller full-toroidal variator (DFTV). By employing a relatively simple model of the elastohydrodynamic contact behavior between the disks and rollers, we compare the performance of the DFTV with classical solutions as the single-roller full-toroidal variator (SFTV) and the single-roller half-toroidal variator (SHTV). Interestingly, the DFTV shows an improvement of the mechanical efficiency over a wide range of transmission ratios, and in particular at the unit speed ratio, as in such conditions the DFTV allows for zero-spin thus strongly enhancing its traction capabilities. The relation between the torque transmission and the operational volume is also investigated for the three toroid geometries. In this case, the better performance is achieved by the SHTV, whereas the other two geometries show a similar behavior.

We present measurements of friction coefficient of lubricated laser surface textured (LST) microstructures with two different geometries. The former is made of a square lattice of microholes; the latter is constituted by a series of microgrooves. We analyze sliding velocities spanning more than two orders of magnitude to cover the entire range from the boundary to the hydrodynamic regime. In all cases, the interfacial pressure is limited to values (relevant to particular manufacturing processes) which allow to neglect macroscopic elastic deformations, piezo-viscosity and oil compressibility effects. The measured Stribeck curves data are compared with those obtained for the flat control surface and show that the regular array of microholes allows to reduce friction over the entire range of lubrication regimes with a decrease of about 50 % in the hydrodynamic regime. On the contrary, the parallel microgrooves lead to an increase of friction compared to the flat control surface with a maximum increase of about 80–100 % in the mixed lubrication regime. These remarkably opposite friction results are then explained with the aid of numerical simulations. Our findings confirm that LST may have cutting edge applications in engineering, not only in classical applications (e.g., to reduce piston-ring friction losses in internal combustion engines) but also, in particular, in technological processes, such as hydroforming, superplastic forming, where the mapping of the frictional properties of the mold has a crucial role in determining the final properties of the mechanical component.

Surface micro-texturing has been widely theoretically and experimentally demonstrated to be beneficial to friction reduction in sliding contacts under lubricated regimes. Several microscopic mechanisms have been assessed to concur to this macroscopic effect. In particular, the micro-textures act as lubricant reservoirs, as well as traps for debris. Furthermore, they may produce a local reduction of the shear stress coupled with a stable hydrodynamic pressure between the lubricated sliding surfaces. All these mechanisms are strongly dependent both on the micro-texturing geometry and on the operating conditions. Among the various micro-machining techniques, laser ablation with ultrashort pulses is an emerging technology to fabricate surface textures, thanks to the intrinsic property of laser light to be tightly focused and the high flexibility and precision achievable. In addition, when using sub-ps pulses, the thermal damage on the workpiece is negligible and the laser surface textures (LST) are not affected by burrs, cracks or resolidified melted droplets, detrimental to the frictional properties. In this work several LST geometries have been fabricated by fs-laser ablation of steel surfaces, varying the diameter, depth and spacing of micro-dimples squared patterns. We compared their frictional performance with a reference nontextured sample, on a range of sliding velocities from the mixed lubrication to the hydrodynamic regime. The measured Stribeck curves data show that the depth and diameter of the microholes have a huge influence in determining the amount of friction reduction at the interface. Different theoretical interpretations to explain the experimental findings are also provided.