75 research outputs found
Forced oscillations dynamic tribometer with real-time insights of lubricated interfaces
This paper presents an innovative forced oscillations dynamic tribometer, the CHRONOS tribometer, with a lubricated ball-on-flat contact configuration fitted out with an in situ optical visualization system and a triggered high-speed camera. The CHRONOS tribometer generates controlled oscillating kinematics by means of a shaker with a range of strokes from 5 ÎĽm to 2.5 mm and an oscillation frequency which can be adjusted from 5 Hz to 250 Hz. Displacement and velocity are measured using a vibrometer. The ball-on-flat mean contact pressure is set between 200 MPa and 600 MPa. During motion, the instantaneous normal and friction forces and the interfacial film thickness distribution (in the nanometer scale) are simultaneously measured. In addition to this instantaneous approach, a more macroscopic approach is developed in terms of moving averages of friction and velocity. Another parameter, the friction-velocity tilt angle, is also introduced. This last parameter may give information on the friction-velocity dependence. Eventually, the experiments performed on the CHRONOS device lead to the representation of synchronized temporal signals of displacement/velocity, friction, and lubricant central film thickness. This superimposition of key parameters reveals time effects introduced by the periodical fluid squeeze and flow in the contact
Modèle géométrique de frottement entre matériaux composites
Dans cette étude, on propose un modèle géométrique de frottement sous faible charge normale entre deux matériaux composites. Le contact est présenté comme une multitude de microcontacts dont la nature peut être de trois types : fibre-fibre, fibre-matrice ou matrice-matrice. La force de frottement interfaciale est la somme des forces locales de frottement en ces microcontacts. Le calcul montre que le coefficient de frottement hybride ne dépend pas de l'orientation de fibres mais seulement de leur fraction surfacique. Ce modèle est testé expérimentalement
The Current State of Performance Appraisal Research and Practice: Concerns, Directions, and Implications
On the surface, it is not readily apparent how some performance appraisal research issues inform performance appraisal practice. Because performance appraisal is an applied topic, it is useful to periodically consider the current state of performance research and its relation to performance appraisal practice. This review examines the performance appraisal literature published in both academic and practitioner outlets between 1985 and 1990, briefly discusses the current state of performance appraisal practice, highlights the juxtaposition of research and practice, and suggests directions for further research
Friction between carbon fibre reinforced polymers: Experiments and modelling
Carbon fibre reinforced polymers (CFRP) are well-known for the excellent combination of mechanical and thermal properties with light weight. However, their tribological properties are still largely uncovered. In this work an experimental study of friction between two CFRP at weak normal load (inferior to 20 N) was performed. Two effects were scrutinuously studied during the experiments: fibre volume friction and fibre orientation. In addition to this experimental work, a modelling of a contact between two FRP was realized. It is supposed that the real area of contact consists of a multitude of microcontacts of three types: fibre-fibre, fibre-matrix and matrix-matrix. The experimental work has shown a small rise in friction coefficient with the change of fibre orientation of two composites from parallel to perpendicular relative to the sliding direction. In parallel, the proposed analytical model predicts a independence of this angle. Regarding the influence of the fibre volume fraction, Vf, the experiments reveal a decrease in friction coefficient of 50% with a change of Vf from 0% to 62%. This observation corresponds to the qualitative dependence depicted with the model. © 2012 EDP Sciences
Analytical model and experimental validation of friction laws for composites under low loads
In order to account for interfacial friction of composite materials, an analytical model based on contact geometry and local friction is proposed. A contact area includes several types of microcontacts depending on reinforcement materials and their shape. A proportion between these areas is defined by in-plane contact geometry. The model applied to a fibre-reinforced composite results in the dependence of friction on surface fibre fraction and local friction coefficients. To validate this analytical model, an experimental study on carbon fibrereinforced epoxy composites under low normal pressure was performed. The effects of fibre volume fraction and fibre orientation were studied, discussed and compared with analytical model results. © Springer Science+Business Media, LLC 2012
Frottement des matériaux composites polymères à renfort fibre de carbone : expériences et modélisation
Les matériaux composites polymères renforcés avec des fibres de carbone (PRFC) sont
connus pour l’excellente combinaison de leurs propriétés mécaniques et thermiques avec
leur faible masse. Cependant, leurs propriétés tribologiques restent largement méconnues.
Dans le cadre de ce travail, l’étude expérimentale du frottement entre deux PRFC
composites sous faible charge normale (jusqu’à 20 N) a été réalisée. Deux effets ont été
soigneusement étudiés pendant l’expérience : la fraction volumique et l’orientation des
fibres. En complément de ce travail expérimental, une modélisation du contact entre deux
PRFC a été réalisée. On suppose que l’aire réelle de contact est constituée d’une
multitude de microcontacts de trois types : fibre-fibre, fibre-matrice et matrice-matrice.
L’étude expérimentale a montré une légère augmentation de coefficient de frottement avec
le changement d’orientation de fibre de deux composites de parallèle à perpendiculaire par
rapport au sens de glissement, alors que le modèle analytique proposé prédit une
indépendance de cet angle. Quant à l’influence de la fraction volumique de fibres,
Vf, les essais soulignent une diminution de
coefficient de frottement de moitié avec une augmentation de
Vf de 0 % Ă 62 %, ce qui correspond Ă la
dépendance qualitative trouvée dans le cadre du modèle
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