Pierre Montmitonnet

Finite Element Analysis of Elastoplastic Indentation: Part II—Application to Hard Coatings

A finite element model previously described is used to analyze indentation of hard - face materials. The materials (substrate and coating) are supposed elastoplastic, the spherical indenter is elastic. The state of stress during indentation (including unloading) of hard-face materials (chromium layer on steel) is investigated in details. The tensile stresses which might lead to failure are especially discussed, and related with experiments showing fracture or delamination of the coating. The model also shows the influence of film thickness on surface and interface stresses

A plane-strain elastoplastic finite-element model for cold rolling of thin strip

Abstract In cold rolling of thin strip, elastic roll deformation is a prominent phenomenon which may indeed govern the whole process. Analysis of the literature suggests a number of methods to solve this coupled problem; for the most severe operations, the coupling technique is more important than the precision of the computation of stress and strain. To perform as general an analysis as possible, a completely coupled finite element model is formulated, meshing a global strip-roll system with internal interface with sliding and friction. The model is two-dimensional and only analyzes roll flattening. The basic equations and numerical formulation are described. Application to several kinds of rolling passes is examined (temper rolling, thin foil rolling) with special emphasis on roll deformed shape and behaviour of metal in the roll gap (sliding/sticking zones, elastic/plastic zones).

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.

3D elastic-plastic finite element simulation of cold pilgering of zircaloy tubes

In cold pilgering of tubes, a material element undergoes a series of small incremental deformations (≈100 strokes), alternatively under tensile and compressive stresses. This complex history sometimes results in surface damage, seemingly by low-cycle fatigue. Prior to studying the resistance of diverse potential materials to this kind of complex, non-proportional multi-axial, and non-periodic cycling, a thorough mechanical analysis of the stress states is necessary: the finite element method (FEM) software Forge3(®) has been used, with updated Lagrangian formulation due to the transient character of strains and stresses. The process is periodical, except for the ends of a given preform, which are cut off afterwards. One stroke only should thus be sufficient to analyse the whole process, provided the correct initialisations are done in terms of shape, strains and stresses, but these are parts of the unknown of the problem. This point will be particularly addressed in the following, where it is shown that in the non-work hardening case at least, simulating three strokes leads to an invariant geometry and state of stress, starting from a reasonable estimate of the geometry. Strains and stresses thus obtained will be discussed in detail, together with their probable consequences on the damage and fatigue of the material, to be later correlated with defects.

A three-dimensional semi-analytical model of rolling stand deformation with finite element validation

Abstract Elastic deflection and flattening of the work roll are a major concern in the strip rolling industry as they result in non uniform strip thicknesses and flatness defects. An accurate semi-analytical model of 2-, 4- and 6-high rolling stands deformation is presented in this paper; it is designed to be coupled to a Finite Element Method for the strip deformation. Predictions of this semi-analytical stand deformation model are compared to elastic Finite Element simulations, which show that the analytical model allows an excellent accuracy while requiring a much lower computational time.

A steady state thermo-elastoviscoplastic finite element model of rolling with coupled thermo-elastic roll deformation

Abstract A coupled model of thermo-elastoviscoplastic strip deformation and thermo-elastic roll deformation is presented. It is based on the 3D FEM. Its purpose is the prediction of profile defects, strain and stress maps, including residual stresses, in a hot or cold rolled strip. The emphasis of the present paper is on the development of a pseudo-steady-state version of the model, the only way to allow fine enough 3D meshes to be used with acceptable computing times. Non-uniform interstand tension fields can be predicted. It is shown that this non-uniformity may have a large influence on the computed contact stress maps (3D friction hills) and therefore on computed roll deformation and strip profile. The influence of material behaviour is examined: elastic-plastic or elastic-viscoplastic laws result in very different interstand stress patterns, just as the presence or absence of a yield stress.

A fully 3D finite element simulation of cold pilgering

Abstract Cold pilgering is a quite complex tube forming operation, highly non steady state, where the metal undergoes a long series of small incremental deformations resulting in both diameter and thickness reduction. A simplified model, based on the slab method, has been built and used in the last few years [1]. In the present paper, we apply our 3D finite element software Forge3 to analyse one forward stroke (the elementary part of the process) of a cold pilgering operation. The main purpose of the present study is to check the hypotheses and the results of the simplified model, which is used routinely. Strain curves compare well, which is important: the simplified model is based upon an estimation of incremental strain components. Comparison of force-displacement curves show some discrepancies partly due to too coarse a discretisation. Torque, which could never be obtained in our previous studies, either experimentally of by the simplified model, is computed here. Stress maps at various locations are presented. For some of theses data, the effect of varying the difference of friction between the tube/mandrel and tube/die interfaces is described.

Finite Element Analysis of Elastoplastic Indentation: Part I—Homogeneous Media

Indentation analysis is performed using a finite element model. The material is supposed elastoplastic, the spherical indenter is elastic. Elastic indentation with friction is first analyzed. It leads to a discussion of the formulation of the friction law and the effect of a sliding/sticking threshold. Elastoplastic analysis of the indentation of a homogeneous medium is conducted till unloading, suggesting the occurrence of replastification during unloading

Hardness measurement as a means of determining simultaneously the elastic modulus and yield stress of polymers as a function of temperature

Abstract A method has been perfected for deriving intrinsic mechanical properties, such as yield stress Y and elastic modulus E, from measurement of hardness versus temperature. It has been applied to five commercial polymers: polyethylene, polypropylene, 66 polyamid, polyoxymethylene and polycarbonate. The results have been verified by (a) comparing the transition temperatures on the curves E(T) and Y(T) with those found by DSC, and (b) comparing results obtained by the new method with values obtained by classical mechanical tests, such as a traction test and dynamic tests. The results of these different methods are perfectly consistent. Discrepancies with the other mechanical tests are shown to be explained by different mechanical conditions (pressure, strain). Hence, this simple method gives reliable results that are very sensitive to structural changes.

Experimental and numerical study of the ironing of stainless steel cups

Abstract An experimental study of the ironing of austenitic stainless steel cups has been undertaken to (i) quantify the discrepancy (due to tool deformation) between nominal die–punch gap and real final wall thickness; and (ii) to determine the maximum reduction for safe operation of the process. An analytical model has been derived in which the stresses based on the slab method, known to be very efficient for thin products, are augmented for shearing energy at the entry and exit of the deformation zone. A friction-factor model has been chosen to represent friction due to the rather large normal stresses involved, and work-hardening has been included in a very simple yet efficient way. The comparison of predicted forces with experimental forces is quite satisfactory. Moreover, finite-element modelling has been used to check the analytical model giving excellent agreement, proving that the remaining discrepancies between theory and experiment are mainly due to the friction factor and/or constitutive behaviour used. From the FEM results, the states of stress in the cup wall during and after drawing are analyzed and support the hypotheses of the analytical model. The maximum reduction is discussed in terms of necking and fracture criteria (the Swift and the Hill criteria) and compared with experimental findings.