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High-frequency micrometrical vibrations have been shown to greatly influence the adhesive performance of soft interfaces, however a detailed comparison between theoretical predictions and experimental results is still missing. Here, the problem of a rigid spherical indenter, hung on a soft spring, that is unloaded from an adhesive viscoelastic vibrating substrate is considered. The experimental tests were performed by unloading a borosilicate glass lens from a soft PDMS substrate excited by high-frequency micrometrical vibrations. We show that as soon as the vibration starts, the contact area increases abruptly and during unloading it decreases following approximately the JKR classical model, but with a much increased work of adhesion. We find that the pull-off force increases with respect to the amplitude of vibration up to a certain saturation level, which appeared to be frequency dependent. Under the hypothesis of short range adhesion, a lumped mechanical model was derived, which, starting from an independent characterization of the rate-dependent interfacial adhesion, predicted qualitatively and quantitatively the experimental results, without the need of any adjustable parameters.

The contact between a rigid Hertzian indenter and an adhesive broad-band viscoelastic substrate is considered. The material behaviour is described by a modified power law model, which is characterized by only four parameters, the glassy and rubbery elastic moduli, a characteristic exponent n and a timescale ${\tau}_0$. The maximum adherence force that can be reached while unloading the rigid indenter from a relaxed viscoelastic half-space is studied by means of a numerical implementation based on the boundary element method, as a function of the unloading velocity, preload and by varying the broadness of the viscoelastic material spectrum. Through a comprehensive numerical analysis we have determined the minimum contact radius that is needed to achieve the maximum amplification of the pull-off force at a specified unloading rate and for different material exponents n. The numerical results are then compared with the prediction of Persson and Brener viscoelastic crack propagation theory, providing excellent agreement. However, comparison against experimental tests for a glass lens indenting a PDMS substrate show data can be fitted with the linear theory only up to an unloading rate of about $100 \textrm{ $\mu$}$m/s showing the fracture process zone rate-dependent contribution to the energy enhancement is of the same order of the bulk dissipation contribution. Hence, the limitations of the current numerical and theoretical models for viscoelastic adhesion are discussed in light of the most recent literature results.

The problem of the detachment of a sufficiently large flat indenter from a plane adhesive viscoelastic strip of thickness "b" is studied. For any given retraction speed, three different detachment regimes are found: (i) for very small "b" the detachment stress is constant and equal to the theoretical strength of the interface, (ii) for intermediate values of "b" the detachment stress decays approximately as b^(-1/2), (iii) for thick layers a constant detachment stress is obtained corresponding to case the punch is detaching from a halfplane. By using the boundary element method a comprehensive numerical study is performed which assumes a linear viscoelastic material with a single relaxation time and a Lennard-Jones force-separation law. Pull-off stress is found to consistently and monotonically increase with unloading rate, but to be almost insensitive to the history of the contact. Due to viscoelasticity, unloading at high enough retraction velocity may allow punches of macroscopic size to reach the theoretical strength of the interface. Finally, a corrective term in Greenwood or Persson theories considering finite size effects is proposed. Theoretical and numerical results are found in very good agreement.

We show that the detachment of a flat punch from a viscoelastic substrate has a relatively simple behavior, framed between the Kendall's elastic solution at the relaxed modulus and at the instantaneous modulus, and the cohesive strength limit. We find hardly any dependence of the pull-off force on the details of the loading process, including maximum indentation at preload and loading rate, resulting much simpler than the case of a spherical punch. Pull-off force peaks at the highest speeds of unloading, when energy dissipation is negligible, which seems to be in contrast with what suggested by the theories originated by de Gennes of viscoelastic semi-infinite crack propagation which associated enhanced work of adhesion to dissipation. Further qualitative differences with the dissipation-based model occur to explain the finite size effect.

The accumulation of residual stress during welding and additive manufacturing is an important effect that can significantly anticipate the workpiece failure. In this work we exploit the physical and analytical transparency of a 1.5D model to show that the deposition of thermally expanded material onto an elastic substrate leads to the accumulation of strain incompatibility. This field, which is the source of residual stress in the system, introduces memory of the construction history even in the absence of plastic deformations. The model is then applied to describe the onset and the progression of residual stresses during deposition, their evolution upon cooling, and the fundamental role played by the velocity of the moving heat source.

The old asperity model of Fuller and Tabor had demonstrated almost 50 years ago surprisingly good correlation with respect to quite a few experiments on the pull-off decay due to roughness of rubber spheres against roughened Perspex plates. We revisit here some features of the Fuller and Tabor model in view of the more recent theories and experiments, finding good correlation can be obtained only at intermediate resolutions, as perhaps in stylus profilometers. In general we confirm the predictions of the Persson & Tosatti and Bearing Area Model of Ciavarella, as stickiness depends largely on the long wavelength content of roughness, and not the fine features. Therefore, multi-instruments measurements should hopefully not be needed., Comment: 17 pages, 5 figures

It has recently been claimed by Baccini and coauthors that due to ANVUR's bibliometric evaluations of individuals, departments, and universities, in Italy there has been a surge in self-citations in the last ten years, thus increasing the "inwardness" of Italian research more than has happened abroad. We have studied the database of Ioannidis et al. published on 12 August 2019 of the one hundred thousand most "highly cited" scientists, including about two thousand Italians, and we found that the problem of self-citations in relation to this scientific elite is not significant in Italy, while perhaps observing a small deviation in the low scores in the rankings. The effect indicated by Baccini et al. consequently, does not seem worrying for the scientific elite (we quantified it in 2 percent of the total of scientists of the "best" one hundred thousand), and is probably largely concentrated in the further less cited scientists. Evaluation agencies like ANVUR should probably exclude self-citations in future evaluations, for the noise introduced by the young researchers. The overall state of health of the Italian research system and the positive effect of the ANVUR assessments are demonstrated by the number of Italian researchers in the top one hundred thousand, which has increased by comparing the "career" databased of 22 years, with that of the "young" researchers in the "2017" database. Italy, looking at the elite researchers, not only is not the most indulgent in self-citations, but has shown the best improvements, proving that the introduction of ANVUR had a positive effect. Indeed, all countries apart from Italy have suffered a decline, even substantial (-20 percent on a national Japan scale), of the number of researchers present in the 2017 data sets compared to career data. Italy instead shows a +0.2 percent on a global basis and an impressive +11.53 percent on a national basis., Comment: 10 pages, 9 figures, 3 tables, in English

A criterion for the mean stress effect correction in the shift factor approach for variable amplitude life prediction is presented for both smooth and notched specimens. The criterion is applied to the simple idea proposed by the authors in a previous note that Ga{\ss}ner curves can be interpreted as shifted W\"ohler curves. The mean stress correction used has been proposed by Smith, Watson and Topper and, more in general, by Walker. By applying the correction, a new expression for the shift factor G is obtained and, through the application of the theory of the critical distances in its point variant, surprisingly G is demonstrated to be valid for both smooth and notched geometries since it does not seem to depend on the geometry, but only on the fatigue exponent and the loading history. Finally, a comparison with the SAE Keyhole test program data is added to substantiate the findings., Comment: 23 pages, 6 figures, 2 tables, 7588 words, 68 references

A very interesting recent paper by Dalvi et al. has demonstrated convincingly with adhesion experiments of a soft material with a hard rough material that the simple energy idea of Persson and Tosatti works reasonably well, namely the reduction in apparent work of adhesion is equal to the energy required to achieve conformal contact. We demonstrate here that, in terms of a stickiness criterion, this is extremely close to a criterion we derive from BAM (Bearing Area Model) of Ciavarella, and not very far from that of Violano et al. It is rather surprising that all these criteria give very close results and this also confirms stickiness to be mainly dependent on macroscopic quantities., Comment: 10 pages

letter to the Editor of Trib.Int., Comment: 4 pages

We study experimentally and theoretically the equilibrium adhesive contact between a smooth glass lens and a rough rubber surface textured with spherical microasperities with controlled height and spatial distributions. Measurements of the real contact area $A$ versus load $P$ are performed under compression by imaging the light transmitted at the microcontacts. $A(P)$ is found to be non-linear and to strongly depend on the standard deviation of the asperity height distribution. Experimental results are discussed in the light of a discrete version of Fuller and Tabor's (FT) original model (\textit{Proceedings of the Royal Society A} \textbf{345} (1975) 327), which allows to take into account the elastic coupling arising from both microasperities interactions and curvature of the glass lens. Our experimental data on microcontact size distributions are well captured by our discrete extended model. We show that the elastic coupling arising from the lens curvature has a significant contribution to the $A(P)$ relationship. Our discrete model also clearly shows that the adhesion-induced effect on $A$ remains significant even for vanishingly small pull-off forces. Last, at the local asperity length scale, our measurements show that the pressure dependence of the microcontacts density can be simply described by the original FT model.

The contact between rough surfaces with adhesion is an extremely difficult problem, and the approximation of the DMT theory (to neglect deformations due to attractive forces), originally developed for spherical contact of very small radius, are receiving some new interest. The DMT approximation leads to extremely large overestimate of the adhesive forces in the case of spherical contact, except at pull-off. For cylindrical contact, the opposite trend is found for larger contact areas. These findings suggest some caution in solving rough contacts with DMT models, unless the Tabor parameter is really low. Further approximate models like that of Pastewka & Robbins' may be explained to work only for a coincidence of error cancellation in their range of parameters., Comment: 11 pages, 5 figures

Recently, a simple and elegant "dimple" model was introduced by McMeeking et al. (Adv Eng Mat 12(5), 389-397, 2010) to show a mechanism for a bistable adhesive system involving a surface with a shallow depression. The system shows, at least for intermediate levels of stickiness, that external pressure can switch the system into a "strong adhesive" regime of full contact, or into weak adhesion and complete pull-off, similarly to the contact of surfaces with a single scale of periodical waviness. We add to this model the effect of roughness, in the simple form of axisymmetric single scale of waviness, permitting a very detailed study, and we show that this induces a resistance to jumping into full contact on one hand (limiting the "strong adhesion" regime), and an enhancement of pull-off and of hysteresis starting from the partial contact state on the other (enhancing the "weak adhesion" regime). We show the system depends only on two dimensionless parameters, depending on the ratio of work of adhesion to the energy to flatten the dimple or waviness, respectively. The system becomes pressure-sensitive also in the intermediate states, as it is observed in real adhesive rough systems. The model obviously is specular to the Guduru model of rough spheres (Guduru, JMPS, 55, 473--488, 2007), with which it shares the limitations of the analysis assuming a connected contact (crack) area, and serves also the purpose of showing the effect of a depression into an otherwise periodic rough contact, towards the understanding of adhesion with multiple scales of roughness., Comment: 17 pages

We show the full multiscale Persson's theory for rubber friction due to viscoelastic losses can be approximated extremely closely to simpler models, like that suggested by Persson in 1998 and similarly by Popov in his 2010 book (but we don't make any use of the so-called "Method of Dimensionality Reduction"), so it is essentially a single scale model at the so called large wavevector cutoff. The dependence on the entire spectrum of roughness is therefore only confusing, and we confirm this with actual exact calculations and reference to recent Lorenz et al (2015) data. The multiscale aspect is irrelevant with respect to the real critical issue: the choice of the "free parameter" best fit truncation cutoff, which shows the models are mainly fitting equations. In this sense, we provide at least a very simple one., Comment: This paper contained some crucial errors in then derivation