1,721,026 research outputs found
Bio-inspired solution for optimal adhesive performance
No full textIn recent years there has been a growing interest into high performance bioinspired adhesives. This communication focuses on the adhesive behavior of a rigid cylinder that indents an elastic layer coated on a rigid substrate. With the assumption of short range adhesive interactions (JKR type) the adhesive solution is obtained very easily starting from the adhesiveless one. We show that ultrastrong adhesion (up to theoretical material strength) can be reached in line contact by reducing the thickness of the layer, typically down to the nanoscale size, which suggests a new possible design for "optimal adhesion". Adhesion enhancement occurs as an increase of the actual pull-off force, which is further enhanced by Poisson's ratio effects in the case of nearly incompressible layer. The system studied could be an interesting geometry for an adhesive system, but also a limit case of the more general class of layered systems, or FGMs (Functionally Graded Materials). The model is well suited for analyzing the behavior of polymer layers coated on metallic substrates
Axisymmetric JKR-type adhesive contact under equibiaxial stretching
No full textAn axisymmetric frictionless adhesive contact problem for a spherical indenter pressed against an isotropic elastic incompressible half-space under equibiaxial stretching is studied in the framework of the generalized Johnson-Kendall-Roberts (JKR) theory, which accounts for the effect of weak coupling between fracture modes I and II by means of a phenomenological mode-mixity function. The model predicts that contact area can withstand a larger level of the substrate stretch under moderate pre-pulling force. We have provided simple formulas to evaluate the pull-off force and the critical contact radius at the detachment point
Viscoelastic normal indentation of nominally flat randomly rough contacts
No full textViscoelastic materials are receiving increasing attention in soft robots and pressure sensitive adhesives design, but also in passive damping techniques in automotive and aerospace industry. Here, by using the correspondence principle originally developed by Lee and Radok and further extended by Ting and Greenwood, we transform the elastic solutions of Persson for contact of nominally flat but randomly rough surfaces to viscoelastic indentation. As an example, the cases of step loading and of the response to a single cycle of harmonic loading are studied. For the latter, the effect of the loading frequency, of the ratio between the rubbery and the glassy moduli of the material, and of the mean normal load on the dissipated energy per cycle is studied in detail for a standard viscoelastic material. The results shown are significant for the engineering applications involving cyclic indentation of soft materials, such as in tire-road contact, seals, pick-and-place manipulators and gripper
Viscoelastic dissipation in repeated normal indentation of an Hertzian profile
No full textSimple exact solutions are known for the indentation problem of a viscoelastic halfspace by a rigid sphere only as long as the contact area is growing. We consider instead a more general cyclic repeated indentation with a pulsating load with a period of zero load. We show that a combination of exact with empirical relaxation solutions coming from simple uniaxial cases is sufficiently accurate to estimate the energy dissipated per cycle, which we report for the standard "3-elements" solid and periodic half-sine loading for various parameters. The theoretical predictions favourably compare with boundary element numerical simulations. We find more energy is dissipated during the first indentation cycle with respect to the subsequent ones, due to the residual indentation left in the viscoelastic half-space. In load controlled systems, the maximum dissipation is reached at an angular frequency that is close to the reciprocal of the relaxation time of the material both for the first and subsequent cycles, but this is in general not true when displacement controlled systems are considered, when dissipation is much lower for subsequent cycles
Adhesion enhancement in a dimpled surface with axisymmetric waviness and rate-dependent work of adhesion
No full textSurfaces showing macroscopic adhesion are rare in industry but are abundant in Nature. Adhesion enhancement has been discussed mostly with geometrical systems (e.g. patterned surfaces), more rarely with viscoelasticity, and has the goal of increasing hysteresis and the detachment force at separation. Soft materials are common, and these have viscoelastic properties that result in a rate-dependent increase in toughness. Here the detachment of two half-spaces is studied, one being flat and the other having a dimple in the limit of short-range adhesion and rate-dependent work of adhesion, in the presence of axisymmetric single-scale waviness, which itself results in adhesion enhancement, similarly to the Guduru model of rough spheres. The dimpled surface shows pressure-sensitive adhesion, and when waviness is added, the roughness-induced toughening is enhanced. It is shown that when a rate-dependent work of adhesion is also accounted for, as in the present paper, assuming a power-law dependence of the effective work of adhesion on the crack tip velocity, the two enhancements are not completely multiplicative, as the "viscoelastic" one tends to prevail and indicates a new avenue of research for pressure-sensitive adhesion
Effects of finite thickness on crack propagation in viscoelastic materials
No full textCrack propagation in viscoelastic materials is a problem of considerable importance, now relatively well understood after early paradoxical results have been addressed with the use of cohesive models. However, finite size effects have received limited theoretical attention so far. Here, following suggestions of Persson (2017), we derive simple results for a crack propagating in a finite size specimen for a model of a single relaxation time material (but extension to many relaxation times is trivial). We show results for the maximum velocity above which the crack may become unstable and the toughness enhancement reduction with respect to that of the infinite system, which corresponds to the ratio of instantaneous to relaxed elastic moduli. Agreement with the literature is dubious, since de Gennes (1996) predicts instability but same amplification as the infinite system, whereas a more recent theory of Persson (2021) suggests same amplification of that of the infinite system, but without instability. A clarification of these qualitative differences is hoped for the future
Extensions and comparisons of BAM (Bearing Area Model) for stickiness of hard multiscale randomly rough surfaces
No full textIn the present paper, we consider a recent very simple model for the estimate of adhesion between elastic (hard) rough solids with Gaussian multiple scales of roughness (BAM, Bearing Area Model), and compare it in particular with very recent extensive results from the numerical method of Joe, Thouless and Barber (JTB theory), in the range of non-hysteretic behaviour. BAM shows no sensitiveness to rms slopes and curvatures for the pull-off load or the apparent surface energy, in agreement with the JTB theory, but in contrast with the criterion proposed by Pastewka and Robbins for stickiness, especially in the fractal limit. Results show also reasonable accuracy with the JTB theory, and BAM theory is simpler than that of Persson and Scaraggi which involves convolution of adhesion tractions in the regions of separation
A generalized Johnson parameter for pull-off decay in the adhesion of rough surfaces
No full textThere is no simple theory at present to predict accurately the decay of pull-off in the adhesion of randomly rough surfaces. The asperity model of Fuller and Tabor has shown significant error in recent numerical investigations by Pastewka and Robbins of self-affine random roughness from micrometer to atomic scale which corresponds to low values of Tabor parameter. For sinusoidal contact, the Johnson parameter, originally introduced for the JKR regime (from Johnson–Kendall–Roberts) is the dominant parameter ruling the pull-off at intermediate Tabor values. Hence, we define a generalized Johnson parameter as the ratio between the adhesive energy to the elastic strain energy to flatten the surface in the case of multiscale roughness and find that it correlates very well with the data of Pastewka and Robbins spanning almost five orders of magnitude of reduction from theoretical strength, improving significantly with respect to other possible single parameter criteria. For the most important case in practice, that of low fractal dimensions, this suggests the product of amplitude and slope of the largest wavelength components of roughness dominate pull-off decay, and not small scales features like slopes and curvatures, as suggested by Pastewka and Robbins
Discussion: "The Effect of Anisotropy on the Percolation Threshold of Sealing Surfaces" (Yang, Z., Liu, J., Ding, X., and Zhang, F., 2019, ASME J. Tribol., 141(2), p. 022203)
No full textThe effect of wear on ThermoElastic Instabilities (TEI) in bimaterial interfaces
No full textThere is ample evidence of ThermoElastic Instabilities (TEI) occurring in sliding contacts. The very first experiments of JR Barber in 1969 suggested wear interacts in the process of localization of contact into “hot spots”. However, studies on the interaction of TEI with wear are scarce. We consider the case of two sliding halfspaces and make a perturbation analysis permitting the formation of waves migrating over the two bodies, in presence of wear. We find that for exactly identical bodies wear does not affect the stability boundary. In the other limit case of bad conductor against a good conductor, wear tends to suppress TEI completely. Intermediate cases show a complex range of possible effects: for certain thermomechanical properties wear may even reduce the critical speed
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