1,720,971 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
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
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
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
Rate-dependent adhesion of viscoelastic contacts. Part II: Numerical model and hysteresis dissipation
Full text linkIn this paper, we propose a numerical model to describe the adhesive normal contact between a glass spherical indenter and a viscoelastic model rough substrate of PDMS material. The model accounts for dissipative process under the assumption that viscoelastic losses are localized at the (micro)-contact lines. Numerical predictions are then compared with experimental measurements, which show a strong adhesion hysteresis mostly due to viscous dissipation occurring during pull-off. This hysteresis is satisfactorily described by the contact model which allows to distinguish the energy loss due to material dissipation from the adhesion hysteresis due to elastic instability. Our analysis shows that the pull-off force required to detach the surfaces is strongly influenced by the detachment rate and the root mean square (rms) roughness amplitude, but it is almost unaffected by the maximum load from which unloading starts. Moreover, the increase in the length of the boundary line separating contact and non-contact regions, which is observed when moving from smooth to rough contacts, negligibly affects the viscous dissipation. Such increase is much less significant than the reduction in contact area, which therefore is the main parameter governing the strong decrease in the effective surface energy for the specific rough geometry considered in the present work
Stickiness of randomly rough surfaces with high fractal dimension: is there a fractal limit?
No full textTwo surfaces are ?sticky? if breaking their mutual contact requires a finite tensile force. At low fractal dimensions D, there is consensus stickiness does not depend on the upper truncation frequency of roughness spectrum (or ?magnification?). As debate is still open for the case at high D, we exploit BAM theory of Ciavarella and PerssonTosatti theory, to derive criteria for all fractal dimensions. For high D, we show that stickiness is more influenced by short wavelength roughness with respect to the low D case. BAM converges at high magnifications to a simple criterion which depends only on D, in agreement with theories that includes Lennard-Jones traction-gap law, while Persson-Tosatti disagrees because of its simplifying approximations
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
Finite deformations induce friction hysteresis in normal wavy contacts
No full textSince Hertz's pioneering work in 1882, contact mechanics has traditionally been grounded in linear elasticity, assuming small strains and displacements. However, recent experiments clearly highlighted linear elasticity limitations in accurately predicting the contact behavior of rubbers and elastomers, particularly during frictional slip, which is governed by geometric and material nonlinearity. In this study, we investigate the basic scenario involving normal approach-retraction contact cycles between a wavy rigid indenter and a flat, deformable substrate. Both frictionless and frictional interfacial conditions are examined, considering finite strains, displacements, and nonlinear rheology. We developed a finite element model for this purpose and compared our numerical results with Westergaard's linear theory. Our findings show that, even in frictionless conditions, the contact response is significantly influenced by geometric and material nonlinearity, particularly for wavy indenters with high aspect ratios, where normal-tangential stresses and displacements coupling emerges. More importantly, interfacial friction in nonlinear elasticity leads to contact hysteresis (i.e., frictional energy dissipation) during normal loading–unloading cycles. This behavior cannot be explained in a linear framework; therefore, most of the experiments reporting hysteresis are typically explained invoking other interfacial phenomena (e.g., adhesion, plasticity, or viscoelasticity). Here we present an additional suitable explanation relying on finite strains/displacements with detailed peculiarities, such as vanishing pull-off force. Moreover, we also report an increase of hysteretic losses as for confined systems, stemming from the enhanced normal-tangential nonlinear coupling
On stickiness of multiscale randomly rough surfaces
No full textA new stickiness criterion for solids having random fractal roughness is derived using Persson’s theory with DMT-type adhesion. As expected, we find that stickiness, i.e. the possibility to sustain macroscopic tensile pressures or else non-zero contact area without load, is not affected by the truncation of the PSD spectrum of roughness at short wavelengths and can persist up to roughness amplitude orders of magnitude larger than the range of attractive forces. With typical nanometre values of the latter, the criterion gives justification to the well-known empirical Dalhquist criterion for stickiness that demands adhesives to have elastic modulus lower than about 1 MPa
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