On a simplified inelastic analysis of structures

In this paper two main problems are considered: the derivation of cyclic constitutive relations during inelastic regime where hardening, softening and creep can occur, and the development of the eventual periodical state in the structure during cyclic thermodynamical loadings.We give a very simple and practical framework to solve these problems in one unique manner.Its essential feature consists in the introduction of a family of internal parameters which characterize local inelastic mechanisms and the family of transformed internal parameters which are linearly linked to the previous ones through a symmetrical non-negative matrix and are indeed the opposite of the associated residual stresses. Thanks to that, the treatment of the local plastic or viscoplastic yield conditions can be easily made from only the classical simple purely elastic (or viscoelastic) analysis.This property allows important results during cyclic loadings: conditions for elastic shakedown, plastic shakedown, ratcheting and bounds for the limiting state.Several examples are given in the text.     

Contact fatigue of automotive gears: evolution and effects of residual stresses introduced by surface treatments

Helical gears from an automotive gearbox, previously subjected to the surface treatments of carbo‐nitriding and shot‐peening, were submitted to contact fatigue tests. The X‐ray diffraction technique was used to characterize the evolution of different mechanical and metallurgical parameters as a function of gear damage. Particular attention was paid to residual stress relief. A numerical model was developed to predict residual stress relaxation and estimate the most likely localization of contact fatigue crack initiation. The stress–strain laws of the surface‐treated layers were determined by means of two separate experimental methods, based on locally measured parameters. The Dang Van multiaxial fatigue criterion was used to analyse the failure of the gears, taking into account the effects of friction and roughness.     

Instabilités d'un monocristal parfait applications au maclage et au changement de phase

The link between elastic instabilities in a perfect single crystal and the different plastic modes (slip, twinning, change of phase) is studied. At first, a constitutive law in large strain elasticity which may be available for a real crystal is assumed. A bifurcations research is then realized on a simply loaded single crystal. The occuring of homogeneous strained zones, characteristic of plastic modes, is the main feature.