1,721,191 research outputs found
A crack-like notch analogue for a safe-life fretting fatigue design methodology
No full textVarious analogies have recently been proposed for comparing the stress fields induced in fretting fatigue contact situations, with those of a crack and a sharp or a rounded notch, resulting in a degree of uncertainty over which model is most appropriate in a given situation. However, a simple recent approach of Atzori–Lazzarin for infinite-life fatigue design in the presence of a geometrical notch suggests a corresponding unified model also for fretting fatigue (called Crack-Like Notch Analogue model) considering only two possible behaviours: either 'crack-like' or 'large blunt notch.' In a general fretting fatigue situation, the former condition is treated with a single contact problem corresponding to a Crack Analogue model; the latter, with a simple peak stress condition (as in previous Notch Analogue models), simply stating that below the fatigue limit, infinite life is predicted for any size of contact. In the typical situation of constant normal load and in phase oscillating tangential and bulk loads, both limiting conditions can be readily stated. Not only is the model asymptotically correct if friction is infinitely high or the contact area is very small, but also remarkably accurate in realistic conditions, as shown by excellent agreement with Hertzian experimental results on Al and Ti alloys. The model is useful for preliminary design or planning of experiments reducing spurious dependences on an otherwise too large number of parameters. In fact, for not too large contact areas ('crack-like' contact) no dependence at all on geometry is predicted, but only on three load factors (bulk stress, tangential load and average pressure) and size of the contact. Only in the 'large blunt notch' region occurring typically only at very large sizes of contact, does the size-effect disappear, but the dependence is on all other factors including geometry
Conditions of yield and cyclic plasticity around inclusions
Full text linkn this paper the stress field in the proximity of a circular (cylindrical) inclusion is considered. The conditions for in-plane plastic flow in the matrix are examined from a classical elasticity solution obtained by Goodier. Elementary cases are considered such as remote loading ranging from pure tensile and pure shear to equibiaxial tension. For proportional loading, it is argued that the upper bound to the shakedown limit is always twice the elastic limit; therefore, within the limits of our assumptions, if the elastic stress concentration for the equivalent stress is greater than two, there is a possibility of cyclic plasticity before bulk yielding, which means that possibly an arbitrarily large plastic strain can cumulate with increasingly high risk of exhaustion of ductility and void nucleation or detachment of the interface Consequently, conditions under which it is possible to reach twice the elastic limit before full-scale yielding are shown in the Dundurs plane representing all possible combinations of elastic parameters. Following these lines, it is shown that there is no possibility of cyclic plasticity under remote shear; there is a limited area of the Dundurs plane for tension, including the hole case; finally, in the equibiaxial limiting case, cyclic plasticity is always possible for any combination of elastic properties
The influence of the indenter tip-radius on indentation testing of brittle materials
Full text linkIndentation testing of a brittle material using a notionally ‘sharp’ indenter may reveal several important physical properties, including fracture toughness, surface finish information and the residual stress state. In the case of shallow cone indenters, the contact and fracture mechanics is well defined and closed-form solutions exist in elasticity theory. However, no real indenter is atomically sharp, and the scope of the present article is to quantify how a finite apex radius may modify the stress state induced by a conical indenter. In particular, implications for the load-displacement relation, occurrence of yielding and maximum contact pressure induced are found. A brief discussion of the influence of edge radius on the flat-ended indenter, once used to induce Hertzian type ring cracks, is also included, as this may be treated by a similar procedur
Brief note: Some observations on oscillating tangential forces and wear in general plane contacts
No full textFor general plane contact of elastically similar materials, including cases where there are multiple regions of contact, general properties of the partial slip solution for conditions of constant normal force and monotonically increasing shearing force have been found recently by the first author. An extension is given here to cover the unloading and cyclic loading cases. Further, it is shown that, if the tangential load varies between two fixed limits, the region of stick does not change, even if relative microslip causes wear, changing continuously the profile of the indenter. The contact area will change, but wear will not enter the original region of adhesion. The theoretical limit to which wear will eventually, asymptotically proceed is established, viz. almost complete contact over what is the initial stick zone, although it may, in practice, take a long time to reach this stat
An approximate JKR solution for a general contact, including rough contacts
No full textIn the present note, we suggest a simple closed form approximate solution to the adhesive contact problem under the so-called JKR regime. The derivation is based on generalizing the original JKR energetic derivation assuming calculation of the strain energy in adhesiveless contact, and unloading at constant contact area. The underlying assumption is that the contact area distributions are the same as under adhesiveless conditions (for an appropriately increased normal load), so that in general the stress intensity factors will not be exactly equal at all contact edges. The solution is simply that the indentation is δ=δ1−2wA′/P′′ where w is surface energy, δ1 is the adhesiveless indentation, A′ is the first derivative of contact area and P′′ the second derivative of the load with respect to δ1. The solution only requires macroscopic quantities, and not very elaborate local distributions, and is exact in many configurations like axisymmetric contacts, but also sinusoidal waves contact and correctly predicts some features of an ideal asperity model used as a test case and not as a real description of a rough contact problem. The solution permits therefore an estimate of the full solution for elastic rough solids with Gaussian multiple scales of roughness, which so far was lacking, using known adhesiveless simple results. The result turns out to depend only on rms amplitude and slopes of the surface, and as in the fractal limit, slopes would grow without limit, tends to the adhesiveless result – although in this limit the JKR model is inappropriate. The solution would also go to adhesiveless result for large rms amplitude of roughness hrms, irrespective of the small scale details, and in agreement with common sense, well known experiments and previous models by the author
Closure to “Discussion of ‘Tangential Loading of General Three-Dimensional Contacts’” (1999, ASME J. Appl. Mech., 66, pp. 1048)
No full textA note on the possibility of roughness enhancement of adhesion in Persson's theory
No full textIn an attempt to model the observed enhancement of adhesion in some classical experiments in the 1970-1980's, Persson introduced in his theory of adhesion between rough solids an "effective adhesion energy" term which is increased due to roughness-induced area increase. In the old experiments, the adhesion enhancement was measured to be up to one order of magnitude in terms of rolling resistance (and hence adhesion hysteresis), whereas it is generally smaller (up to 30-40%) for pull-off force. Here, an estimate the area increase in those experiments shows Persson's postulate is not supported since a discrepancy of at least one order of magnitude is found. Other explanations of adhesion enhancement come from more recent studies involving special axisymmetric indenters confirmed by experimental findings with negligible area increase, or by a model with well separated gaps in a fully random surface contact. However, even these models are very limited by their assumptions, and in the most general case of adhesion of random rough surfaces a comprehensive model remains elusive, and many questions remain open, requiring a detailed understanding of loading and unloading processes, of the possible effects of range of adhesive forces, of effects of roughness anisotropy, and so on
Comments on old and recent theories and experiments of adhesion of a soft solid to a rough hard surface
No full textThe 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 qualitatively 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
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