Eric Felder

Mechanical analysis of the scratch test on elastic and perfectly plastic materials with the three-dimensional finite element modeling

Abstract Scratch test provides a convenient mean to study mechanical properties of thin coatings. The mechanical analysis of this test is very intricate, especially for polymers, for which a large elastic part accompanies the plastic deformation. Most existing models describe the ploughing of a rigid plastic body by a rigid indenter. This paper describes a numerical study of the behavior of elastic–plastic materials during a scratch test. Simulations have been performed with a three-dimensional finite element code, the indenter is a cone of semi-angle 70.3° and the contact is frictionless. The scratched material is elastic and perfectly plastic, with a constant flow stress σ 0 . For small Young’s modulus, a sinking-in under load and an elastic recovery at the rear face of the indenter have been observed. In order to take into account this elastic recovery, we have suggested a new model of the apparent coefficient of friction. For high Young’s modulus, the deformation is mainly plastic, the behavior was close to the behavior of a metal, frontal and lateral pile-up pads have been observed. The scratch hardness and the shape ratio have been compared with results obtained in normal indentation under the same conditions: geometry under load is similar, but the deformation level is higher for scratch than for indentation. We have found a good agreement for the shape ratio between our numerical results and scratch experiments performed by other authors with a Berkovich pyramid on elastic–plastic materials.

Identification of the viscoplastic behavior of a polycarbonate based on experiments and numerical modeling of the nano-indentation test

Indentation testing is a convenient means to study mechanical properties of thin coatings. We suggest a new method to identify the viscoplastic behavior of a polymer by using the force-penetration curves during nano-indentation testing performed with two indenter shapes. During loading, the load applied by the indenter and the penetration depth have been measured. These force-penetration curves have been compared to the load computed by using the finite element method with a two dimensional software. The viscoplastic behavior of the polymer is modeled with the Gsell-Jonas law. The main particularity of this law is the modeling of the large strain-hardening at large strains. The unknown parameters of this law have been obtained by fitting computed and experimental force-penetration curves. We have identified each parameter independently of the others by taking into account the indenter tip defect. The nano-indentation tests have been performed with three strain rates and with two indenter shapes: a Berkovich indenter and a cone with a semi angle of θ = 30° and a tip radius. In this paper, the polymer is a polycarbonate. Several authors have made rheological tests on this polymer. The true strain-true stress curve obtained with our method is in good agreement with the compression curve.

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.

Theoretical and experimental study of the ploughingof a rigid-plastic semi-infinite body by a rigid pyramidal indenter

Summary Surface ploughing of a rigid-plastic semi-infinite body by a rigid pyramidal indenter is modelled by using a velocity field calculated by minimizing the dissipated power. The theoretical predictions are compared with the results of simulation tests using a model material. Good agreement is found for the tangential force, but the prediction of the flow pattern is less satisfactory. A simplified version of this model gives good results for forces and geometry when compared with tests on a low carbon steel.

Finite-element analysis of deformation during indentation and scratch tests on elastic-perfectly plastic materials

Abstract We have studied the distribution of plastic strain around normal indentation and scratches in elastic-perfectly plastic materials. A three-dimensional finite-element analysis of a cone scratching and indenting elastic-perfectly plastic materials is presented. The indenter is the axisymmetric equivalent cone of the Berkovich indenter, with semiapical angle θ = 70.3°. The plastic behaviour of the material is modelled with the yield stress σ0. No strain hardening and no sensitivity to the strain rate occur. The elasticity of the material is modelled with Youngs modulus E, which varies from 2.79 to 2793 GPa. In fact, the behaviour of the scratched or indented material depends on the parameter X = (E/σ0) cotθ, called the rheological factor (X = 1, …, 1000). For small rheological factors, the deformation is mainly elastic; for high rheological factors, the deformation is essentially plastic, and in this case the behaviour of the material is close to the behaviour of a metal. The contact between the indenter and the mesh is frictionless. We have defined a mean representative strain in indentation and scratch tests. This value is independent of the scratch length and the penetration depth. It has been shown that the mean representative strain increases with X, and that it is larger in scratch tests than in indentation tests.

A kinematic model for plastic indentation of a bilayer

Abstract A model is proposed, which allows the interpretation of hardness tests performed on coated materials. The substrate and layer materials are assumed to be rigid-perfectly plastic, minimization of the plastic work rate is used, and the calculation is done incrementally. The model is extended from wedge to pyramid indentation through a simple analysis, and it compares satisfactorily with experimental results obtained on two representative bilayers. Examples are given for the depth of indentation which is allowed without any significant influence of the substrate, and it is shown that it depends strongly on the layer-substrate hardness ratio and on the indenter angle.

EXPERIMENTAL STUDY AND THEORETICAL INTERPRETATION OF THE FRICTIONAL MECHANISMS IN STEEL SHEET FORMING

Abstract The success of many sheet forming operations depends critically on the friction of the sheet on the blankholder. First we review briefly the various frictional mechanisms involved and discuss mainly the occurrence of hydrodynamic effects at macroscopic and microscopic scales, especially with regard to the experimental work performed by Emmens ( Proc. 15th Bienn. Congr. IDDRG, Dearborn, MI, May 1988 , ASM International, 1988, pp. 63–70). Then we describe the results of friction measurements performed with a tribometer on four low carbon steel sheets: two high formability sheets with electro-discharge texture and laser-textured roughness, a high strength sheet and a hot dip-galvanized sheet with laser-textured roughness; friction tests were performed between two flat tools at two contact pressures p (about 10–30 MPa), two sliding speeds V (about 2–60 mm s −1 ) and along the rolling direction and the transverse direction. The friction of the galvanized sheet is rather isotropic and decreases markedly as the ratio V/p increases; in contrast, the friction of the uncoated sheets is anisotropic and its decrease as V/p increases is smaller. Observations by optical microscopy of the surface of the sheets after friction testings provide three major results: a small decrease in the friction coefficient μ corresponds generally to a more marked decrease in the final plateaux area. At very low value of the Vp ratio where hydrodynamic effects cannot occur friction is the sum of two mechanisms: at a very small scale the formation of waves, with moderate friction μ ≈ 0.12, and at the greater scale of the plateaux of the sheet surface shearing of transfer film, with higher friction μ ≈0.22; the extent of each mechanism depends on the sheet roughness texture. As the ratio V/p increases the viscosity of the lubricant promotes the formation of a very thin lubricant film (thickness 10–70 nm) on the plateaux which slightly reduces the friction coefficient and more markedly the extent of the two previous mechanisms and the associated surface damage of the sheet and tool surface. This effect is effective before the generation of pressure by viscous shearing of oil pockets proposed by Emmens and responsible of a very marked decrease in friction at high values of the V/p ratio.

Friction and wear during the hot forging of steels

Abstract This paper aims to specify the nature and importance of the friction and wear problems which occur during hot forging. Some practical insight was gained from a high speed film of the forging of simple shaped workpieces with a hammer. Characteristics values of the parameters (stresses, temperatures, contact times) and the lubrication mechanisms are presented. The principle of the ring compression test, examples of practical applications and various methods for measuring the friction for use in subsequent analysis are shown. Finally, a survey of the classification of sticking mechanisms, and how to avoid them by surface treatment of the dies, is included

Propriétés mécaniques des films minces : Problématiques et moyens de mesure

Summary The aim of this work is the characterization of the mechanical behaviour of a couple consisting of a substrate and of a ceramic coating with a few micrometer thickness. We review here the related quantities and the measurement methods with particular emphasis on the significant factors which influence these quantities. With the choosen testing techniques we discuss the various approaches and the constitutive equations of multilayer systems. Specific papers will describe in more details the results related to coatings composed of carbides or nitrides manufactured by dry routes.

Gilding of cultural heritage artefacts: an elaborated technology

Abstract Gilding has been used to decorate, one may say sublimate, the surface appearance of artworks since the beginning of gold use in art. Gold foils and leaves were used first, thanks to the high ductility of that metal, and the progress of gilding art through centuries has been linked to: (1) the evolution of the thickness of the gold film used for the process; (2) the continuous research for efficient adhesive materials adapted to the various substrates; and (3) the development of techniques of direct adhesion of the gold coating, especially on metals. This paper, based on recent laboratory results obtained through laboratory studies of various museum artworks, discusses these three aspects. First, it shall develop a mechanical modelling of gold leaf beating. Second, it shall detail the properties of the main classes of adhesive materials used for leaf gilding on various materials. Finally, the importance of the diffusion phenomena at the interface between a metal substrate and a gold coating shall be discussed, especially in the case of gilding involving a high temperature treatment.