Aurélien Saulot

Self-lubricating composite bearings: Effect of fibre length on its tribological properties by DEM modelling

Self-lubricating polymer-based composites are used in space and in aircraft mechanisms as materials for solid lubricated systems. Such composites mostly consist of a polymeric matrix and fillers of two kinds: hard fillers (fibres made of glass, or of minerals) and solid lubricating particles (made of MoS 2). Their advantages are that they provide their own lubrication, and they can be used in both very high and very low temperatures (from −40 up to ~200 F). Precision ball bearings with these composites are manufactured since the 60s in these bearings the retainer material itself provides the lubrication. From the experimental analyses implemented (X-ray tomography, SEM observations, and experiences in a tribometer); it is possible to observe that the geometry of the fillers has a strong influence on the third body rheology. Nevertheless, the confined nature of the contact does not allow in-situ observation. To overcome this difficulty a combined numerical/experimental approach is carried out. To be able to reproduce the evolution of third-body particles within the contact, Discrete Element Methods (DEM) is used. Such an approach allows to represent wear: by the construction of an equivalent continuous medium resulting from the incorporation of interaction laws between the discrete particles. The motivation to this work is the understanding of the impact of filler geometry o tribological behaviour of these materials. More specifically, the goal is to study the influence of the fibre length in the tribological behaviour of self-lubricating composites by Discrete Element Methods (DEM).

Design of a tribometer for investigating tactile perception

The understanding of the tactile perception mechanism implies the reproduction and measurement of friction forces and vibrations induced by the contact between the skin of human fingers and object surfaces. When a finger moves to scan the surface of an object, it activates the receptors located under the skin allowing the brain to identify surfaces and information about their properties. The information concerning the object surface is affected by the forces and vibrations induced by the friction between the skin and the rubbed object. The vibrations propagate in the finger skin and are converted into electric impulses sent to the brain by the mechanoreceptors. Because of the low amplitude of the induced vibrations, it results quite hard to reproduce the tactile surface scanning and measuring it without affecting measurements by external noise coming from the experimental test-bench. In fact the reproduction of the sliding contact between two surfaces implies the relative motion between them, which is obtained by appropriate mechanisms having a more or less complicated kinematics and including several sliding surfaces (bearings, sliders, etc.). It results quite difficult to distinguish between the vibrations coming from the reproduced sliding and the parasitic noise coming from the other sliding contact pairs. This paper presents the design and validation of a tribometer, named TRIBOTOUCH, allowing for reproducing and measuring friction forces and friction induced vibrations that are basilar for a clear understanding of the mechanisms of the tactile sense.

First-Body Versus Third-Body: Dialogue Between an Experiment and a Combined Discrete and Finite Element Approach

The present paper proposes to analyze relations between the behavior of two bodies in contact (local stress, vibration modes) and the rheology of third-body particles. Experiments are performed on a system composed of a polycarbonate disk in contact with a steel cylinder, where birefregent property of polycarbonate allows to observe shear stress isovalues. Multi-scale numerical simulations involve the coupling between finite elements and discrete elements to model simultaneously non-homogeneous third-body flows within a confined contact and dynamical behavior of the bodies in contact. Comparisons between experiments and simulations are performed on the dynamic response of the system, the stress distribution as well as the evolution of third-body particles within the contact. Such comparisons exhibit not only qualitative results but also quantitative ones and suggest a new approach to study in deeper third-body rheology.

Identification of Contact Area From Full Field Displacement Surface Measurements

One of the most common failure modes for turbomachinery wheels is associated to high-cycle fatigue of blades. A practical way to extend working life is obtained through the introduction of specific devices that allow a reduction in vibrational magnitudes during resonance. Different kinds of components are used such as shrouds and wires for power industries and under platform dampers for aeronautics. The dry friction phenomenon between those devices and the blades induces nonlinear behaviors and flatten associated frequency response functions. This phenomenon is now well known and different modeling techniques of contact are available within numerical simulation softwares. Nevertheless, it is always difficult to estimate or to measure with sufficient precision the actual contact characteristics needed to run those softwares. Due to practical experimental capabilities, measurements are only possible quite far from the contact zone and major quantities such as the transverse loading for example are often unreachable directly. n nIn this paper, a new inverse methodology is presented. This method uses surface displacement measurements (obtained usually experimentally from conventional accelerometer and fast camera) in order to identify the characteristics of contact zones within elastic body assemblies. The new methodology is validated and illustrated by a numerical approach based on an academic set up