IJTC2008-71055 CYCLIC LOADING OF AN ELASTIC-PLASTIC ADHESIVE CONTACT

ABSTRACT A numerical simulation is presented for several loadingunloading cycles of an adhesive contact between an elasticplastic sphere and a rigid flat. The main goal of the simulation is to study the plastic deformation evolution in a contact bump material -the microscopic electrode found in a MEMS microswitch for providing a good electric contact. This bump is subjected to a cyclic contact interaction with a harder substrate and cyclic plasticity of the bump material can lead to its wear and as result to a failure of the whole MEMS device

A Tight Karp-Lipton Collapse Result in Bounded Arithmetic

Cook and Krajíček [9] have obtained the following Karp-Lipton result in bounded arithmetic: if the theory proves , then collapses to , and this collapse is provable in . Here we show the converse implication, thus answering an open question from [9]. We obtain this result by formalizing in a hard/easy argument of Buhrman, Chang, and Fortnow [3]. In addition, we continue the investigation of propositional proof systems using advice, initiated by Cook and Krajíček [9]. In particular, we obtain several optimal and even p-optimal proof systems using advice. We further show that these p-optimal systems are equivalent to natural extensions of Frege systems

Modelling the criticality of silicon nitride surface imperfections under rolling and sliding contact

Ceramic rolling elements of hybrid bearings may initially include surface imperfections. In order to provide reliable operation of a bearing, the criticality of such imperfections under rolling contact fatigue is examined by defining them as Star features: intersecting semi-elliptical surface cracks. Parametric study is conducted using Finite Element Method and discussed with help of previously published experimental observations. The effects of the Star feature morphology and configuration, contact pressure and crack face friction are investigated in terms of stress intensity factors. Possible crack propagation scenarios are explained in the present study

Resistivity scaling and critical dynamics of fully frustrated Josephson-junction arrays with on-site dissipation

We study the scaling behavior and critical dynamics of the resistive transition in Josephson-junction arrays, at f=1/2 flux quantum per plaquette, by numerical simulation of an on-site dissipation model for the dynamics. The results are compared with recent simulations using the resistively-shunted-junction model. For both models, we find that the resistivity scaling and critical dynamics of the phases are well described by the same critical temperature as for the chiral (vortex-lattice) transition, with a power-law divergent correlation length. The behavior is consistent with the single transition scenario, where phase and chiral variables order at the same temperature, but with different dynamic exponents z for phase coherence and chiral order.Comment: 17 pages, 13 figures, to appear in Phys. Rev.

Finite elements based approaches for the modelling of radial crack formation upon Vickers indentation in silicon nitride ceramics

© 2019 By having superior properties silicon nitride ceramics can be considered as the state-of-the-art material in the bearing industry. Vickers indentation of this material is typically accompanied by formation of cracks visible on surface. Two Finite Elements models are developed in the current work: the first model is based on fracture mechanics and the second on cleavage stress criterion. Plastic behavior of silicon nitride is included in the modeling, and since little is known on the plasticity of this material, the Drucker-Prager model (used for non-metallic materials)along with the classical J2-plasticity are explored. The results of the fracture mechanics based model correlate well with experimental results in terms of surface crack length. The numerical results in terms of the morphology of the indented zone (including cracks and plastic zone)are provided by the stress criterion based model, and these results correlate well too, with the experimental data

Incoherent Pair Tunneling as a Probe of the Cuprate Pseudogap

We argue that incoherent pair tunneling in a cuprate superconductor junction with an optimally doped superconducting and an underdoped normal lead can be used to detect the presence of pairing correlations in the pseudogap phase of the underdoped lead. We estimate that the junction characteristics most suitable for studying the pair tunneling current are close to recently manufactured cuprate tunneling devices.Comment: ReVTeX 3.1; 4 pages, 2 EPS figures (included

Rearrangement of the vortex lattice due to instabilities of vortex flow

With increasing applied current we show that the moving vortex lattice changes its structure from a triangular one to a set of parallel vortex rows in a pinning free superconductor. This effect originates from the change of the shape of the vortex core due to non-equilibrium effects (similar to the mechanism of vortex motion instability in the Larkin-Ovchinnikov theory). The moving vortex creates a deficit of quasiparticles in front of its motion and an excess of quasiparticles behind the core of the moving vortex. This results in the appearance of a wake (region with suppressed order parameter) behind the vortex which attracts other vortices resulting in an effective direction-dependent interaction between vortices. When the vortex velocity $v$ reaches the critical value $v_c$ quasi-phase slip lines (lines with fast vortex motion) appear which may coexist with slowly moving vortices between such lines. Our results are found within the framework of the time-dependent Ginzburg-Landau equations and are strictly valid when the coherence length $\xi(T)$ is larger or comparable with the decay length $L_{in}$ of the non-equilibrium quasiparticle distribution function. We qualitatively explain experiments on the instability of vortex flow at low magnetic fields when the distance between vortices $a \gg L_{in} \gg \xi (T)$. We speculate that a similar instability of the vortex lattice should exist for $v>v_c$ even when $a<L_{in}$.Comment: 10 pages, 11 figure

Dynamic Scaling and Two-Dimensional High-Tc Superconductors

There has been ongoing debate over the critical behavior of two-dimensional superconductors; in particular for high Tc superconductors. The conventional view is that a Kosterlitz-Thouless-Berezinskii transition occurs as long as finite size effects do not obscure the transition. However, there have been recent suggestions that a different transition actually occurs which incorporates aspects of both the dynamic scaling theory of Fisher, Fisher, and Huse and the Kosterlitz-Thouless-Berezinskii transition. Of general interest is that this modified transition apparently has a universal dynamic critical exponent. Some have countered that this apparent universal behavior is rooted in a newly proposed finite-size scaling theory; one that also incorporates scaling and conventional two-dimensional theory. To investigate these issues we study DC voltage versus current data of a 12 angstrom thick YBCO film. We find that the newly proposed scaling theories have intrinsic flexibility that is relevant to the analysis of the experiments. In particular, the data scale according to the modified transition for arbitrarily defined critical temperatures between 0 K and 19.5 K, and the temperature range of a successful scaling collapse is related directly to the sensitivity of the measurement. This implies that the apparent universal exponent is due to the intrinsic flexibility rather than some real physical property. To address this intrinsic flexibility, we propose a criterion which would give conclusive evidence for phase transitions in two-dimensional superconductors. We conclude by reviewing results to see if our criterion is satisfied.Comment: 14 page

Microscopic nonequilibrium theory of double-barrier Josephson junctions

We study nonequilibrium charge transport in a double-barrier Josephson junction, including nonstationary phenomena, using the time-dependent quasiclassical Keldysh Green's function formalism. We supplement the kinetic equations by appropriate time-dependent boundary conditions and solve the time-dependent problem in a number of regimes. From the solutions, current-voltage characteristics are derived. It is understood why the quasiparticle current can show excess current as well as deficit current and how the subgap conductance behaves as function of junction parameters. A time-dependent nonequilibrium contribution to the distribution function is found to cause a non-zero averaged supercurrent even in the presence of an applied voltage. Energy relaxation due to inelastic scattering in the interlayer has a prominent role in determining the transport properties of double-barrier junctions. Actual inelastic scattering parameters are derived from experiments. It is shown as an application of the microscopic model, how the nature of the intrinsic shunt in double-barrier junctions can be explained in terms of energy relaxation and the opening of Andreev channels.Comment: Accepted for Phys. Rev.