1,721,021 research outputs found
On DMT methods to calculate adhesion in rough contacts
Full text linkIn this paper, we compare different rough contact-mechanics theories with the assumption of weak interfacial adhesion. Two different approaches for the local modeling of adhesion are also considered: the DMT force approach (DMT-F) and the Maugis’ approximation (DMT-M). The first approach is based on the idea of summing up attractive interactions that act outside the contact zone; the latter considers a constant adhesive load for each asperity in contact. A comparison with numerical data proves the DMT-F approach is very accurate when hard solids and low adhesive interactions are considered. The DMT-M approach shows, instead, less accuracy especially at low fractal dimensions
Separated steady state solutions for two thermoelastic half-planes in contact with out-of-plane sliding
No full textWhen two materials are placed in contact along an interface, thermoelastic effects can separate the surfaces and create "hot spots" when there is sufficient frictional heating fVp generated at the interface, even if the two surfaces are nominally flat. Additionally, heat can flow because the bodies are generally at different temperatures, and this is an independent cause of separation, generally when heat flows into the less distortive material. These two effects have been considered separately, and here we consider the case with interaction of the two effects, showing possible non-existence, multiplicity and instability of solutions. Approximate Hertzian solutions for the separated contact regime are very limited, particularly for the frictional heating case. Hence, a new efficient full numerical solution is developed, and compared with direct FEM results, the latter permitting also the assessment of stability in the transient regime. Connection to previous results for simple rod models is made. The case of heat flow into the more distortive material is discussed. (c) 2005 Elsevier Ltd. All rights reserved
Short pitch corrugation of railway tracks with wooden or concrete sleepers: An enigma solved?
No full textBritish Railways (BR) in the 1970s observed an increased level of short pitch corrugation after the electrification of the West Coast main line, which involved (among other things) the change from wooden to concrete sleepers. Here, using a simple model for the two systems, we find a different sensitivity to lateral creepage, a doubled growth at the "pinned-pinned" resonance regime shifted from about 600 to about 1000 Hz, but a reduced growth in the 300-600 Hz range, where most data for both systems seem to lye. Hence, despite it would be tempting to associate the increased corrugation simply to the increased (doubled) peak of normal load, there is discrepancy in the corresponding wavelength predicted by the model. Hence, there is still an "enigma" about the reasons for the enhanced corrugation
On the Long and Short-Range Adhesive Interactions in Viscoelastic Contacts
No full textRecently, tribologists have shown increasing interest in rate-dependent phenomena occurring in viscoelastic fractures. However, in some cases, conflicting results are obtained despite the use of similar theoretical models. For this reason, we try to shed light on the effects that long and short-range adhesion has on the pull-off force in the contact of viscoelastic media by exploiting a recently developed numerical model. We find that, in the limit of long-range adhesion, the unloading velocity has little effect on the pull-off force, which is close to the value predicted by Bradley for rigid bodies. In such case, the detachment process is characterized by a uniform bond-breaking of the contact area, and viscous dissipation involves the bulk material. For medium(short)-range adhesion, the pull-off force is instead a monotonic increasing function of the pulling velocity and, at high speeds, reaches a plateau that is a function of the adiabatic surface energy. In this case, the detachment process is similar to the opening of a circular crack, and viscous dissipation is localized at the contact edge
Size effects in adhesive contacts of viscoelastic media
No full textIs the maximum force required to detach a rigid sphere from a viscoelastic substrate dependent on the initial value of the contact radius? Experimental and theoretical investigations reported in the literature have given opposite responses. Here, we try to answer the above question by exploiting a fully deterministic model in which adhesive interactions are described by Lennard-Jones potential and the viscoelastic behaviour with the standard linear solid model. When the approach and retraction phases are performed under quasi-static conditions, the substrate behaves as an elastic medium and, as expected, the pull-off force FPO (i.e., the maximum tensile force) is found to be independent of the maximum contact radius amax reached at the end of loading. Size-dependent effects are instead observed (i.e., the pull-off force FPO changes with amax) when transient effects occur as the larger the contact area, the greater the size of the bulk volume involved in the dissipation. Results are also discussed in the light of viscoelastic crack Persson's theory, which is modified to capture size effects related to amax
Modeling the Adhesive Contact of Rough Soft Media with an Advanced Asperity Model
No full textAdhesive interactions strongly characterize the contact mechanics of soft bodies as they lead to large elastic deformations and contact instabilities. In this paper, we extend the Interacting and Coalescing Hertzian Asperities (ICHA) model to the case of adhesive contact. Adhesion is modeled according to an improved version of the Johnson, Kendall & Roberts (JKR) theory, in which jump-in contact instabilities are conveniently considered as well as the lateral interaction of the asperities and the coalescence of merging contact spots. Results obtained on complex fractal geometries with several length scales are accurate as demonstrated by the comparison with fully numerical simulations and experimental investigations taken from the literature. Also, the model quite well captures the distributions of the contact stresses, gaps, and contact spots
Adhesion between self-affine rough surfaces: Possible large effects in small deviations from the nominally Gaussian case
No full textIt is shown that even small deviations from the ideal Gaussian random roughness case seem to lead to dramatic increase in adhesion of rough surfaces: this could be due to a finite number of asperities, or to a finite tail in the height distribution, particularly realistic at low fractal dimensions D, which is the case of most practical interest. It is emphasized that the assumption of a perfect Gaussian heigth distribution, including infinite tails, may be a strong one when studying adhesion in rough surfaces
TEDI (ThermoElasto-Dynamic Instability): A new mechanism for squeal & TEI
No full textA new mechanism of dynamic instability is found, generated by the thermo-elastic deformations. In particular, it is found that even if coupling between dynamics and heat transfer seems apparently very weak (due to the very different time scales involved), the dynamic modes become unstable for arbitrarily small speeds, in a simple model involving an elastic layer compressed between two rigid plates and sliding out-of-plane. The present analysis neglects the effect of out-ofplane deformations and possible stick-slip in that direction
Frictionally excited thermoelastic instability in the presence of contact resistance
No full textIn sliding systems, frictional heating generates a well-known instability above a certain critical speed Vcr, which is a function of geometrical and material properties only. Similar instabilities are known to occur in the static problem, driven by temperature differences, in the presence of thermal contact resistance. Thermal contact resistance at the interface has seldom been considered and gives rise to full coupling of the problem. Generally, the resistance decreases non-linearly when pressure is increased. Here, the critical condition (in terms of heat flux and sliding speed) for the stability of the uniform pressure solution for a half-plane in frictional contact with a rigid wall at fixed temperature is studied for a general resistance function R(p). It is found that the heat flux direction increases the instability as in the case of zero speed, i.e. when directed into the half-plane (which is the only distortive material), whereas frictional heating can have also a stabilizing effect, for a given heat flux, specifically when R(p) + pRprime(p) < 0. Also, an isothermal critical speed has been defined, and the actual critical speed is found to be smaller or larger depending on the temperature difference sign. Longer wavelengths are always more unstable so that the critical wavelength is still dictated by the real size of the system
Microstructured superhydrorepellent surfaces: effect of drop pressure on fakir-state stability and apparent contact angles
No full textIn this paper we present a generalized Cassie-Baxter equation to take into account the effect of drop pressure on the apparent contact angle theta(app). Also we determine the limiting pressure p(W) which causes the impalement transition to the Wenzel state and the pull-off pressure p(out) at which the drop detaches from the substrate. The calculations have been carried out for axial-symmetric pillars of three different shapes: conical, hemispherical-topped and flat-topped cylindrical pillars. Calculations show that, assuming the same pillar spacing, conical pillars may be more inclined to undergo an impalement transition to the Wenzel state, but, on the other hand, they are characterized by a vanishing pull-off pressure which causes the drop not to adhere to the substrate and therefore to detach very easily. We infer that this property should strongly reduce the contact angle hysteresis as experimentally observed in Martines et al (2005 Nano Lett. 5 2097-103). It is possible to combine large resistance to impalement transition (i.e. large value of p(W)) and small (or even vanishing) detaching pressure p(out) by employing cylindrical pillars with conical tips. We also show that, depending on the particular pillar geometry, the effect of drop pressure on the apparent contact angle theta(app) may be more or less significant. In particular we show that in the case of conical pillars increasing the drop pressure causes a significant decrease of theta(app) in agreement with some experimental investigations (Lafuma and Quere 2003 Nat. Mater. 2 457), whereas theta(app) slightly increases for hemispherical or flat-topped cylindrical pillars
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