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International audience; Chromium-based alloys have been developed to form chromia layers in oxidizing environments for high temperature applications, providing a good protection against further oxidation. The mechanical behavior and oxide layer growth of yttria-coated or zirconium-doped Ni28Cr alloy are presently studied under high temperature oxidation conditions. These kind of reactive elements can decrease oxidation kinetics and significantly increase the adhesion of chromia layer on the metallic substrate. However, during oxidation, a compressive stress generally occurs in thermally grown oxide (TGO) due to the oxide layer and its mechanical interaction with the metallic substrate. These growth stresses, combined with the thermal stresses that develop during temperature changes, may initiate and cause layer cracking or spallation, which limits the lifetime of alloys in such a challenging environment. For prediction of this lifetime, it is important to know with accuracy the thermomechanical behavior of such a system. Therefore, a thermomechanical modelling is proposed and applied to optimize numerically the unknown parameters, especially viscoplastic ones. For the raw material and modified materials by reactive elements, the creep parameter decreases with a lower temperature, which means that the viscoplastic strain decreases with a decreasing temperature. For the yttria-coated or zirconium-doped material, the main result is an increase of the activation energy of this creep parameter with the quantity of reactive element.

International audience; To say that a constitutive model has to verify “the principle of material objectivity” to ensure its frame-indifference has become a common wisdom. Objective transports are thus defined to serve as tensor rates. These operators are in particular applied to the Cauchy stress tensor. They are used as time derivatives to describe non-linear or dissipative phenomena observed during the finite transformations of a material continuum. Because an infinite number of such transports may be constructed and shown to be objective, the selection of the appropriate transport and its validity still constitutes an open and debatable question.Differential geometry, within its four-dimensional formalism, has proven its ability to describe physical fields and their variations in space and time while ensuring the covariance of any physical law. This description is here applied to the motion of a material continuum within the classical hypotheses of Newtonian physics. In this context, we show that the rate of a tensor as seen by a point of space–time is uniquely defined by the covariant rate; this quantity is not invariant with respect to superposed rigid body motions. The rate of a tensor as seen by a moving particle of matter is uniquely defined by the Lie derivative of the tensor. This operator is invariant with respect to superposed rigid body motions. Both, the covariant rate and the Lie derivative are independent of the observer and could thus be used in a constitutive model within a four-dimensional formalism. We show next that the projection of the Lie derivative of the Cauchy stress tensor within an inertial 3D Cartesian frame corresponds to Truesdell’s transport and that the other 3D objective stress transports, if they have the dimension of a rate, do not correspond to a time derivative of this tensor. The Truesdell transport is thus the only objective transport that represents a frame-indifferent time derivative of the Cauchy stress tensor.

International audience; In the present work, mechanical features of different thermal oxide films growing on metals have been investigated at different temperatures. An adapted model is performed to describe the stresses evolution within the oxide films. These predictions are compared with experimental results obtained by different experimental techniques. In order to obtain some accurate mechanical characteristic parameters of the system, both an asymptotic approach and an optimization procedure have been developed and presented through a general methodology. In respect to the chosen assumptions, the present model enables to give a precise mechanical description of each considered layer.© 2013 Elsevier B.V. All rights reserved.

Ductile damage is a consequence of large strains more or less localized inside bands. Taking into account damage in constitutive behaviour of metallic materials is necessary to model various engineering problems involved in forming processes (stamping, punching, shearing…). Damage can be described at macroscopic level with continuum mechanics theories but introducing microstructural features can lead to more accurate predictions. In the present study, two polycrystalline plasticity models including damage effects in the framework of scale transition methods are investigated. These models are based on different approaches with direct application to duplex stainless steel. The first one is a variant of the Lipinski–Berveiller model in which ductile damage effects have been introduced. The second one is a generalized Cailletaud model taking into account ductile damage. Because of the microstructural complexity of the chosen materials, some particular developments of the micro-mechanical approaches are considered. Moreover, continuous damage mechanics is used at grains scale including its coupling with plastic or elastic–plastic flow. The modelling is justified from previous experimental results obtained by neutrons diffraction on duplex stainless steels. The developed models allow then deducing from the grains behaviour the macroscopic behaviour of the aggregate with damage effects.

Mechanical features have been investigated in the case of α - Cr 2 O 3 thermal oxide films growing on NiCr 30 alloy in the temperature range 700–900 °C. From previous experimental results obtained by Raman spectroscopy and giving the growth stress evolution in α - Cr 2 O 3 films, and based on a modelling of the system, both asymptotic forms and an optimization procedure have been developed which give values for the mechanical characteristic parameters of the system and also its evolution with the oxidation temperature.