Search results for "FRICTION"
showing 10 items of 352 documents
Approach to rotor-shaft hysteretic whirl using Krylov-Bogoliubov techniques
2013
The internal friction associated with the shaft hysteresis or with the possible release of some shrink-fit coupling exerts a destabilizing effect on the over-critical rotor running, but may be efficiently counteracted by other external dissipative sources or by a proper anisotropic configuration of the support stiffness. The present analysis considers a symmetric rotor-shaft system on viscous-flexible supports with different stiffness on two orthogonal planes containing the bearing axis. The internal friction of the shaft is described either by a linear hysteretic model or by a nonlinear Coulombian force contrasting the rotor motion relative to the shaft ends. The nonlinear equations of mo…
Coupling of heat flux and vortex polarization in superfluid helium
2020
We consider a macroscopic description of the mutual influence between heat flux and vortex polarization in superfluid helium, in which the vortices produce a lateral deviation of the heat flux, and the heat flux produces a lateral drift of vortices. This coupling is a consequence of a microscopic Magnus force and mutual friction force between the vortices and the flow of excitations carrying the heat. We keep track of these effects with simplified macroscopic equations, and we apply them to second sound propagation between rotating concentric cylinders and to spatial distribution of polarization across a rectangular channel with vortices polarized orthogonally to the channel in the presence…
Non-equilibrium thermodynamics analysis of rotating counterflow superfluid turbulence
2010
In two previous papers two evolution equations for the vortex line density $L$, proposed by Vinen, were generalized to rotating superfluid turbulence and compared with each other. Here, the already generalized alternative Vinen equation is extended to the case in which counterflow and rotation are not collinear. Then, the obtained equation is considered from the viewpoint of non-equilibrium thermodynamics. According with this formalism, the compatibility between this evolution equation for $L$ and that one for the velocity of the superfluid component is studied. The compatibility condition requires the presence of a new term dependent on the anisotropy of the tangle, which indicates how the…
Cohesive-frictional interface in an equilibrium based finite element formulation
2020
The Hybrid Equilibrium Element (HEE) formulation, with quadratic stress field is defined in the class of statically admissible solutions, which implicitly satisfy the homogeneous equilibrium equations. The inter-element equilibrium condition and the boundary equilibrium condition are exactly imposed by considering a quadratic displacement fields at the element sides, as an interfacial Lagrangian variable, in a classical hybrid formulation. The displacement degrees of freedom are independently defined for each element side, where a cohesive-frictional interface can be embedded. The embedded interface is defined by the same stress fields of the hybrid equilibrium element and it does not requi…
Simple Microscopic Theory of Amontons' Laws for Static Friction
2001
A microscopic theory for the ubiquitous phenomenon of static friction is presented. Interactions between two surfaces are modeled by an energy penalty that increases exponentially with the degree of surface overlap. The resulting static friction is proportional to load, in accordance with Amontons' laws. However the friction coefficient between bare surfaces vanishes as the area of individual contacts grows, except in the rare case of commensurate surfaces. An area independent friction coefficient is obtained for any surface geometry when an adsorbed layer of mobile atoms is introduced between the surfaces. The predictions from our simple analytic model are confirmed by atomistically detail…
Atomistic Simulations of Solid Friction
2002
Friction between two solid bodies in relative sliding motion takes place on a large spectrum of length and time scales: From the nanometer/second scale in an atomic force microscope up to the extremely macroscopic scales of tectonic motion. Despite our familiarity with the effects of friction, fundamental questions remain unanswered. The atomistic origins of well-established phenomenological friction laws are controversial. Many explanations, seemingly well-established, have recently been called into question by new experimental results. Computer simulations have also revealed flaws in previous theoretical approaches and led to new insights into the atomistic processes responsible for frict…
A Study on the Measurement Instrumentation for a Custom-Made Bipendulum Impact Testing Machine
2008
Experimental investigations of structures under impact loads are normally carried out by means of drop-weight testing rigs, which are often tailor made, and require sensors for fast transient measurement of mechanical quantities (namely, force, displacement, velocity, and absorbed energy). Since these tests are quite expensive, time consuming, and, often, not repeatable, the sensors must provide reliable and interpretable results in the first trial. In this paper, a thorough study on the measurement instrumentation suitable to carry out reliable low-velocity impact tests by means of a custom-made bipendulum impact testing machine is presented. Attention has been focused on the choice of the…
Influence of distinct tool pin geometries on aluminum 8090 FSW joint properties
2023
Abstract. Aluminum Lithium alloys are recuperating substantial interest from automotive and aerospace industries owing to their extraordinary specific strength as compared to conventional aluminum (2xxx, 6xxx, and 7xxx) alloys. The goal of the present investigation is to study AA 8090 joints produced with the unique solid-state welding technique friction stir welding (FSW). Tool pin profile induces remarkable influence on friction and further plastic deformation during FSW. Therefrom, the influences of three distinct but constant dynamic area conditioned tool pin geometries namely, square trapezoidal, hexagonal trapezoidal, and threaded taper on the resulting material flow patterns, mechani…
Finite Element Studies On Friction Stir Welding Processes of Tailored Blanks
2008
Although friction stir welding (FSW) has been successfully used to join materials that are difficult-to-weld or unweldable by fusion welding methods, it is still in its early development stage and, therefore, its potential has not been fully exploited yet. FSW appears to be a very promising process for tailor-welded blanks to overcome the difficulties encountered in traditional fusion welding processes for obtaining a sound welding with good nugget integrity. In this paper the friction stir welding process for tailored blanks of aluminum alloy is investigated through a FEM developed by the authors. In particular FSW for sheets with different thicknesses is studied. For each setup a differen…
Influence of material characteristics on plastomechanics of the fsw process for T-joints
2009
Abstract The potential of friction stir welding (FSW) has thoroughly been investigated by several authors, however their focus has primarily been on butt joints. T-joints are of paramount interest for transportation industries especially due to their capacity to straighten panels. Welding of T-joints is very challenging due to thin walls, poor location of the rib–web interface and the requirements for corner-fillets. This paper investigates FSW of T-joints of two popular aluminum alloys, i.e. 2024-T4 and 6082-T6, and the role played by the material characteristics on joining. First, an experimental study is carried out with specially designed fixture to determine the effect of process condi…