François Delamare

An upper bound model of ploughing by a pyramidal indenter

Abstract An upper bound method was used to study surface ploughing by a rigid pyramidal indenter. The normal and tangential forces, the geometrical parameters of the track, the strain and the strain rate of the ploughed material are calculated. The model is compared with experience and is applied to the calculation of scratch hardness.

The Incorporation of Indigo Molecules in Sepiolite Tunnels

Evidence for access of molecules the size of acetone or pyridine to the intracrystalline tunnels of nanofibre clay (sepiolite) has indicated formation of a new type of organic-inorganic nanocomposites. However, the introduction of larger molecules has been a recurring problem. It is now agreed that for indigo, the molecules are located on the external surface and at the ends of the fibres, thus blocking access to internal tunnels. We claim, however, that it is possible for indigo molecules to access the internal channels of sepiolite. FTIR and XRD analyses have provided evidence for folding of the sepiolite structure preheated at high temperature (above 350 degrees C). By comparison, we have shown that for indigo/sepiolite mixtures treated in the same conditions, no change in the crystalline structure of the sepiolite is observed, and that blue samples, related to Maya blue, with indigo molecules incorporated deeply enough into sepiolite to prevent folding of the tunnels, can be obtained. NMR, FTIR and thermal analysis confirm the interaction of indigo with the water coordinated to magnesium(II) and located inside the internal and external channels of sepiolite. Two other hypotheses are excluded; we show both that zeolitic water is not blocked in the tunnels by indigo, and that if thermal decomposition products of indigo can be formed, they are in a minority.

Transfer layer and friction in cold metal strip rolling processes

Mechanically mixed layers (MMLs) and transfer layers (TLs) are present in the metal forming industry, in rolling processes among others, just as in other fields of Materials and Mechanical science and engineering. Due to the asymmetry of mechanical properties between workpieces and tools, however, TLs are much more frequent - indeed, almost systematic - and the present paper focuses on them. A review is given of experiments performed both on an experimental rolling mill and on a tribological simulation test, the plane strain compression test (PSCT). Various metals and alloys have been investigated under a range of deformation conditions. As in other systems, the origin is abrasive wear followed by adhesion, or adhesive wear. Constant exchanges between the tool surface, the workpiece and the re-circulating lubricant lead to a steady state TL, the characteristics of which (density and size of transferred particles, hardness and roughness of the TL) depend on the tool-workpiece contact conditions. Tribological consequences, in terms of friction and formed piece surface aspect, vary with the TL properties.

An Evaluation of the Plane Strain Compression Test. II. Experimental Study of the Friction Test: Role of Transfer Layers in Boundary Lubrication

The Plane Strain Compression Test is used to measure the mean friction shear stress at the sheet-die interfaces. It is found that the shear-stress level is strongly influenced by transfer film formation on the dies. The thickness of these films, and their elementary chemical composition, have been determined by Auger Electron Spectroscopy and rugosimetry. The effect of contact mechanics and lubricant parameters on transfer film formation is discussed.

Polymorphism of crystalline phases of calcium stearate

The structures of the low temperature (T⩽400 K) phases of calcium stearate have been investigated by X-ray diffraction. Several forms are shown to exist at room temperature, depending on hydration and thermal treatment. New transitions at cryogenic temperatures are discussed. Transitions at higher temperatures have been investigated together with their reversibility. Thermogravimetry and two calorimetric techniques have been used as a complement. This data completes the already somewhat complex picture of the phase transitions of calcium stearate.

Mechanical analysis of coin striking: Application to the study of byzantine gold solidi minted in constantinople and carthage

Abstract A mechanical analysis of the forging of a coin, using an upper-bound method, is briefly presented: the geometry of the coin, simplified, consists of an axisymmetric disk with a central circular design and an outer annular legend. The energy and strain necessary to form the design are calculated and the shape of the blank then computed from the shape of the coin. This analysis is applied to numismatic problems, in the present instance considering whether mechanics is able to explain the evolution of the geometry of Byzantine gold solidi at the Mints of Carthage and Constantinople between the VIth and XIIth centuries. It is shown that, in Carthage, this evolution was aimed at saving energy, and in Constantinople, the evolution of the geometry of solidi was due to variation in the fineness of the gold.

Interface adherence after cold backward-extrusion of a bi-layered plain bearing

Abstract The manufacture of bi-layered plain bearing using cold backward-extrusion has been described previously in this journal (8(1983) 327–347). The bearing consists of an outer hard metal internally clad with a low-friction alloy, and the adherence of the two metals at the interface is of primary importance. In the present paper, the adherence is first studied experimentally, and it is observed to be stronger at the bottom of the wall, and to increase with increasing reduction. Secondly, the strain at the interface is calculated analytically using the velocity field given in the earlier publications. The variations of strain explain, qualitatively, the tendencies found experimentally. Furthermore, a good correlation is found to exist between the adherence and the strain undergone by the metal. Finally, backward extrusion is shown to be a quite interesting process for this purpose, since the strain reached during extrusion is very high.

Metallic soap lubrication in wire-drawing: A thermopseudoplastic model

Abstract Rheological experiments show that soaps used as lubricants for wiredrawing exhibit a marked pseudoplastic behaviour at temperatures above 100 °C. Using a “slab” method, a model was derived of the drawing of a pseudoplastic body by the wire into the die. The influence of some rheological and mechanical parameters on the thickness of the drawn film was investigated.

Internal surface roughness of cold pilgered zircaloy tubes

Abstract Cold pilgering imposes a mixed lubrication regime which gives a “plateaux and valleys” surface, with generally a high rate of plateaux, and related transfer, galling and scratching hazards. An investigation of the inner surface topography of zircaloy pilgered tubes is carried out. First, the evolution of topography along the pilgering process for given (reference) tooling and lubricant is illustrated. Then the final surface is studied as a function of lubricant nature and tool design. A mechanical model of pilgering (published elsewhere) and experimental roll force measurements are used to investigate the correlation between mechanics and surface topography. For a given tool, the higher the roll force (or contact stress), the higher the proportion of plateaux. Moreover, severe risks of defects (scratches, etc.) appear when the plateaux cover more than about 85% of the surface.

Geometrical optimization of bi-layered plain bearings manufactured by cold backward co-extrusion

Abstract Previous papers have reported on the study of the manufacture of bi-layered plain bearings by cold backward extrusion, the purpose of which is to obtain constant thickness of an internal anti-friction cladding on a plain bearing in one operation. Simulation by plasticine had been carried out and has led to the defining of a velocity field that is quite effective for predicting the shape of the interface after co-extrusion and also the strain maps of the extruded samples. Moreover, by experimentation, it was possible to improve the final geometry of the plasticine workpieces. In the present paper, it is shown how the above-mentioned velocity field allows the calculation a priori of the shape of the initial interface that will give a constant cladding thickness at least cost. These calculations are applied to the forming of metal workpieces, with very good agreement being found between theory and experiment. Hence, it is concluded that the proposed forming process is technically feasible.