Michel Barquins

Fracture mechanics and the adherence of viscoelastic bodies

The strain energy release rate G and its derivative delta G/ delta A are used to provide a general picture of the adherence of viscoelastic bodies. Two bodies in contact on an area A are in equilibrium if G=w, where w is the thermodynamic (or Dupres) work of adhesion. The quasistatic force of adherence is the load corresponding to delta G/ delta A=0. When G>w, the separation of the two bodies starts, and can be seen as the propagation of a crack in mode I. Three geometries are investigated: adherence of spheres, adherence of punches, and peeling. The variation of energies with the area of contact is given, and the kinetics of crack propagation are studied. The theory is supported by experiments on the adherence of polyurethane to glass.

Adherence, Friction and Wear of Rubber-Like Materials

Abstract The underlying mechanisms of the adherence and of the sliding and rolling friction of rubber-like materials are studied on a microscale level, by scrutinizing phenomena occurring inside both static and rolling or sliding contact areas between a single hard transparent spherical or cylindrical asperity and the smooth and flat surface of a soft elastomer sample. Various mechanisms have been studied such as equilibrium conditions and kinetics of adherence of different rigid punches, contact geometry and rolling frictional force of a rigid cylinder, static friction, deformation and interfacial phenomena in sliding friction of a spherical asperity, influence of the radius of curvature of a blunt rigid asperity in sliding contact, stages of formation of reattachment folds, conditions for the appearance of detachment waves, friction of silica-filled compounds and the onset of wear.

Adhesive contact of sectionally smooth-ended punches on elastic half-spaces: theory and experiment

Attractive molecular forces give rise to singular tensile stresses and to a discontinuity of displacement at the edge of contacts. These boundary conditions are those of fracture mechanics and correspond to adding a rigid body displacement to the classical elastic solution. The adhesive contact of sectionally smooth-ended punches (sphere or cone) is studied. Stress distribution, shape of the deformed surface, and relation between load and penetration are given either for equilibrium or kinetic conditions. For a flat-ended sphere, the adherence of a flat punch and the JKR theory for sphere are obtained as particular cases; for zero surface energy systems, the solution reduces to that of Ejike. The case of a sphere with a spherical cap of different radius and that of a rounded cone are also examined. Experimental verification has been performed with a glass ball (R=2.19 mm and flats between 100 and 240 mu m radius) on polyurethane. Radii of contact, penetration and kinetics of detachment agree with the theory within 2%.

Rubber friction variation with rate and temperature: some new observations

Friction measurements are reported for the sliding of rubber over individual glass lenses, wavy-surface glass and abrasive paper. Two different apparatuses were employed to obtain friction data. All the experiments were made with the same vulcanised rubber compound moulded with smooth test surfaces to facilitate contact area observations. The measurements covered large ranges of sliding speed (0.001 to 10 mm s-1), ambient temperature (-60 to +90 degrees C), normal load (0.01-57 N) and contact geometry (R=1-18 mm). Some results are given as a function of the sliding velocity by use of the WLF transform. It was found that, particularly for smooth surfaces, the friction varied only slightly with rate (WLF parameter). For the test arrangements employed the viscoelastic response of the rubber compound appeared to be of less importance than the geometry of contact, except when operating conditions were near to the glassy region of the rubber. A simple plot of friction coefficient as a function of the Hertz contact area brought together all the diverse measurements. In the course of experimentation the influence of the method of surface cleaning upon friction was investigated. New insight was also obtained concerning the nature and magnitude of the static friction of rubber.

Friction and wear of rubber-like materials

Abstract In this paper, the author describes his own investigations carried out at microscale level in order to scrutinize fundamental phenomena occurring inside both static and sliding contact areas between a single, hard, transparent, hemispherical asperity and a flat, smooth surface of a soft elastomer. Various mechanisms have been studied, such as adhesive contact, first stage of friction (preliminary displacement), friction at increasing sliding speed, stages of formation of reattachment folds (effect of the radius of curvature of the rigid slider), conditions for the appearance of Schallamachs waves and the onset of adhesive and abrasive wears, with rubber ligaments or engraved ridges formation, of various unfilled and filled rubber compounds.

Stick-Slip and Peeling of Adhesive Tapes

The everyday experience shows that in a given range of temperatures and velocities, the peeling of an adhesive tape is jerky with emission of a characteristic noise. This phenomenon of self sustained oscillations (stick-slip) often described for peeling [1–10] is also observed in other fracture mechanics geometries such as tearing [11–13], wedge loaded double cantilever beams (DCB) [14–22], double torsion [23–29] and blister pressurized by incompressible liquid [30]. All these tests are characterized by the fact that the strain energy release rate G does not increase with the crack length, otherwise a single velocity jumps would occur [31]. The main experimental results are the following: 1/ When the imposed cross-head velocity increases, stick-slip appears abruptly with a large amplitude (defined by the difference between Gi for crack initiation and Ga for crack arrest) at a given velocity (depending on temperature), and its amplitude decreases as the cross-head velocity is further increased, until stable propagation is observed anew [1,7,24,26,27]. Generally it is the peak value Gi which decreases, Ga remaining more or less constant or increasing slowly.

Effect of surface cavities on static and dynamic adhesion to an elastomer

Abstract The analysis of contact between two spherical surfaces introduced in the 19th century by Hertz was modified some 30 years ago by Johnson, Kendall and Roberts (JKR) to allow for adhesion between the two solids. Since then, the technique has been much used with various systems employing sphere/sphere and sphere/flat solid (sphere of infinite radius) geometry. We consider here the geometry in which one solid has a negative radius of curvature: a spherical solid contacts the second solid in a shallow spherical cavity. It is thus shown that the contact area of a rigid sphere on the smooth surface of an elastomer depends markedly on the flatness of the latter. Any neglect of cavities of large radius of curvature leads to an overestimate of the value of the intrinsic adhesion of Dupre, W0, and falsifies interpretation of separation kinetics under (variable) applied load. By allowing for the negaive radius of curvature of the cavities in the rubber, correction can be made leading to coherent values of W0 and debonding kinetics. The analysis may be of use for the assessment of flatness of surfaces and the increase in contact radius may prove beneficial for improving the precision of static adhesion tests.

Path and kinetics of branching from defects under uniaxial and biaxial compressive loading

In order to understand the physical significance of branching features, especially path and kinetics, formed from defects in confined geological or mining conditions, a set of branch fractures obtained under uniaxial and biaxial loading at the tip of an isolated pre-existing oblique open slot was studied in PMMA (polymethylmethacrylate) plates. It was found that the branching always initiated perpendicular to the local plane tangent to the slot edge at the branch crack root while the branch crack root-crack tip distance increased with the slot-loading axis angle; the influence of biaxial loading is also discussed. The study of the stress field linked to an elliptical slot under shear conditions confirms these experimental observations and predicts the influence of the radius of curvature at the crack tip on the branching distance. The conditions of propagation are studied in terms of strain energy release rate along the stress trajectory starting from the point of maximum tensile stress at the slot edge, taking into account the presence of microcracks stemming from the slot tip induced by the sawing method. This allows one to describe the three successive régimes of propagation of the branch crack: spontaneous, catastrophic and controlled ruptures, according to the intensity of the applied compressive uniaxial stress and the size of pre-existing microcracks at the edge of the open slot. The computed variation of the branch crack length versus time agrees well with our experimental observations.

Three-dimensional quantification of wear tracks on amorphous NiB coatings

Abstract The early stage of the wear process can be best investigated by means of three-dimensional profilometry. This allows not only maps to be made but also wear rates to be quantified. Linear, surface and volumetric parameters are given. An application to the wear resistance of amorphous NiB coatings (thickness, 10 μm; 26 at.% B) on iron substrates was performed. Dry sliding friction tests are carried out using a pin-on-disc system. Two specific types of behaviour (viscoplastic and plastic) are exhibited before seizure occurs.

Energy dissipation in schallamach waves

Abstract It is shown that the energy dissipated by friction with Schallamach waves is closely related to the speed of these waves and varies with the speed to the power 1.6 for the polyurethane surface tested, in accordance with the theoretical prediction. It is found that the problem of friction cannot be completely explained by a simple peeling mechanism which does not take into account compressive tangential stresses induced by the motion of the slider.