41 research outputs found

    Multi-black holes and instantons in effective string theory

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    The effective action for string theory which takes into account non-minimal coupling of moduli admits multi-black hole solutions. The euclidean continuation of these solutions can be interpreted as an instanton mediating the splitting and recombination of the throat of extremal magnetically charged black holes.Comment: 10 pages, plain Te

    Effetto delle dimensioni del cordone di saldatura sulla resistenza a fatica dei giunti a croce

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    La scelta della dimensione da assegnare al cordone di saldatura nel caso di giunzioni saldate a croce può presentarsi problematica, specie quando gli spessori delle lamiere che formano il giunto sono differenti tra loro. Le normative di tipo tecnologico suggeriscono in genere di prevedere uno spessore del cordone di saldatura inferiore al minimo spessore della lamiera da collegare, mentre le normative di tipo strutturale non prevedono una dipendenza della resistenza del giunto, sia statica che a fatica, dalle dimensioni del cordone di saldatura, a meno che queste non siano tanto ridotte da portare a rotture che si inneschino e si propaghino non più nella lamiera base ma nel cordone di saldatura. Scopo del presente lavoro è la verifica, teorica e sperimentale, della variazione della resistenza a fatica al variare del rapporto tra lo spessore del cordone di saldatura e quello minimo delle lamiere da saldare

    The electrical/thermal conductance of rough surfaces - the Weierstrass - Archard multiscale model

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    Rough surfaces show a load-dependent electrical constriction resistance. Here, using a recent analogy due to Barber [Proc. R. Soc. London A 459 (2003) 53] between the incremental stiffness and the conductance in the elastic regime, and the Archard hypothesis to solve the multiscale contact problem, the conductance is found for the profile defined by the Weierstrass series. The analysis is also approximately valid, neglecting thermal effects on the contact area, for the thermal conductance

    A re-examination of rolling contact fatigue experiments by Clayton and Su with suggestions for surface durability calculations

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    A re-interpretation of recent RCF experiments by Clayton and Su (C&S) [Wear 200 (1996) 63] under water lubricated rolling/sliding conditions, with careful measurements of ratchetting strains, and their comparisons with experimentally observed lives, seems to confirm the validity of ratchetting failure (RF) mechanism and Kapoor’s "critical ratchet strain" as a material property. However, the complexity of modelling the ratchetting phenomenon and the uncertainties on the material’s critical ratchet strain, suggests that perhaps a more realistic alternative is the use of empirical Wöhler-like life curves similarly to currently used for the contact fatigue evaluation in gears design and standards. In particular, it is found that the "pitting" fatigue limit at 107 cycles suggested by the gears standard is reasonably accurate also for the C&S experiments on various typical rail steels. Since the gears life factor suggested for gears turns out quite conservative at shorter lives, it seems a single new life factor could be suggested, at least for all pearlitic and bainitic steels tested by C&S under water lubrication

    The influence of specimen size on the surface stress distribution induced by indentation testing

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    The stress state in an elastic cylindrical coupon of finite dimensions on a rigid, frictionless foundation under axisymmetric contact loading is studied. The axisymmetric stress analysis is devoted to an investigation of the effects of the free surfaces of the plate on the surface radial tensile stress field, which is particularly important for an assessment of crack initiation in brittle materials. Attention is focused on cases where the ratio of plate thickness (h) and plate radius (W) to contact radius (a) fall in the range 0.5 < h/a < 10, and 1 < w/a < 5, respectively, which is the region of both experimental and theoretical interest

    The general 3D Hertzian contact problem for anisotropic materials

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    This paper presents a general method for solving the 3D frictionless contact problem between generally anisotropic materials with any second order surface geometry. The method uses the Stroh formalism to find the Green's Functions (GF) of the materials with an efficient numerical integration process. The GFs are then expanded in Fourier series in order to solve the Hertzian contact problem between the two bodies as a perturbation to the first order, 2equivante isotropic2, solution to the problem. The latter permits to define an 2equivalent indentation modulus of the contact" which is a single parameter computed from the first terms of the Fourier expansion of the two GFs (ie the average values) and permits to use the standard Hertzian solution: this gives a good approximation to the contact area (at most elliptical in any case) which is approximated as a circle for axi-symmetrical geometry, and for the approach of remote points in the two bodies. The "equivalent indentation modulus", which depends on materials and orientation, is computed for a set of composite materials of practical interest

    The design of hydrodynamically lubricated journal bearings against yield

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    The stress field induced by the half-Sommerfeld pressure distribution in an infinitely elongated bearing is studied in detail. A complex potential formulation for the stress field is employed to solve the elasticity problem, with the intention to compute the required strength according to the classical von Mises criterion. Example contour plots of the yield parameter (radicJ2)/pm are given and the position and magnitude of the maximum normalized von Mises parameter are determined for a range of working conditions, analytically when they are on the surface, i.e. for eccentricity ratios epsiv < 0.6, and semi-analytically for the cases where they are located subsurface, i.e. epsiv > 0.6. Surprisingly simple results are obtained for eccentricity ratios lower than about 0.7, namely a maximum of the von Mises parameter proportional to the mean pressure, permitting a simple rule to be developed for the design of bearings against yielding: if the bearing works with eccentricity ratios smaller than 0.7, and the average pressure is smaller than 1.22k, where k is the yield stress of the material in pure shear, then yielding is avoided. When bearings are used in the range of very high eccentricity ratios, a more refined calculation is needed, taking into account the actual value of the maximum von Mises parameter and the paper provides design charts for this purpos

    On stress concentration on nearly flat contacts

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    Fretting fatigue can severely damage components subjected to oscillatory tangential loads, leading to a dramatic reduction in fatigue life and causing catastrophic ruptures. A conservative approach that can be used when considering the effect of stress concentration induced by fretting is to ensure that the peak stress is smaller than the fatigue limit of the material. However, this depends on details of the geometry as well as loading conditions. In the present work, the contact problem of a flat rounded punch in contact with a half-plane is considered, where a dovetail joint contact geometry is approximated and the classical Hertzian contact is retrieved in the limit. Developing the analytical results given by Ciavarella, Hills and Monno, an approximate Hertzian equivalent solution using Cattaneo superposition is obtained, leading to a simple formula to estimate the maximum tangential stress as a function of the load parameter Q /(f P) and geometric parameter a / b. The accuracy of the formula is checked numerically. The proposed formula gives a maximum error as low as 4 per cent in the case of zero bulk loads. For non-zero bulk loads an analytical solution is possible for the Hertzian case for moderate bulk. This leads to a second general formula containing the three dependencies (geometry, tangential load and bulk stress), which also gives a very good approximation for rounded flat and larger bulk loads, the error being generally well below 10 per cent

    On the extraction of notch stress intensity factors by the post-processing of stress data on the free edges of the notch

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    Following on the lines of a previous paper dedicated to cracked components by Ciavarella et al., here the case of a notch of semi-angle alpha is considered. Contrary to the crack case (alpha = 180°), the free edges of the notch are easily accessible to experimental analysis; moreover they provide information about all the terms of the Williams series expansion of the stress field about the notch apex, including the most important, i.e. the symmetric and antisymmetric singular term notch stress intensity factors (N-SIFs), whereas for the crack case the mode I N-SIFs cannot be extracted from those stresses. Another important different feature is that symmetric and antisymmetric N-SIFs have different singularities, and in several cases they are so close that their contributions tend to overlap. Therefore, a simple procedure is here proposed to use radial stresses, to separate their symmetric and antisymmetric contributions a priori by computing the sum and difference of the stresses on the two edges, to post-process these quantities in the 'asymptotic region' with standard least-squares techniques and to extract the N-SIFs. The method is applied to a simple case known in the literature and solved by means of a boundary element code, and the results are almost coincident with previous results, even with quite coarse mesh discretizations
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