A B S T R A C T In this paper thermomechanical fatigue assessment in the automotive industry is discussed.The design strategy is based upon a consistent approach of the thermomechanical loading, the mechanical constitutive law of the material, the damage parameters and the fatigue strength criteria. The good understanding of these different steps allows one to perform predictive calculations of automotive parts subjected to thermomechanical loading. The main hypotheses and modelling choices are presented and results are illustrated by a series of computations on real 3D structures. Cracked area and lifetime prediction are described in the case of aluminium alloy cylinder heads subjected to transient thermal loadings.Development in numerical simulation over the past 20 years have led to increased performance of numerical tools and the development of new algorithms. Today this enables ever-quicker calculations of loads and displacements in structures. Nevertheless, whilst academic work is quite active few studies have been performed on structures subjected to multiaxial thermomechanical loading. Only a small number of components in automotive technology experience thermomechanical loading cycles of a severity capable of producing low-cycle fatigue. Most of these parts are related to the engine: cylinderheads, exhaust manifolds, crankcases, . . . . It is important to note that the low-cycle fatigue problems in such engine components are related to the start-operate-stop cycles and not to the combustion cycles. Therefore they involve large temperature changes. A previous study was dedicated to cast-iron exhaust manifolds. 1 This work showed an original comprehensive approach, which resulted in a reliable prediction of damage location and of lifetime in spite of basic assumptions like the separation of the damage and the mechanical constitutive behaviour. The paper presents an overview of this approach and the specific details of the applications in the case of the cylinder head in aluminium. The results are illustrated by a series of computations on real 3D structures. The words comprehensive approach shall denote the following series of items:Correspondence: Jean-Jacques Thomas. r thermal analysis, r elasto-viscoplastic mechanical analysis and r fatigue analysis.These three aspects are linked together; the thermal loading and the material behaviour determine the mechanical response of the structure and from that the fatigue strength of the structure can be estimated through a damage indicator and a pertinent fatigue criterion.For a design strategy, one should not concentrate separately on these four aspects of the problem. It should be a comprehensive approach to obtain an important robustness and to allow its integration in a development process 2 by the ability to answer the asked question in the appropriate time. That is to say in our case: is the fatigue strength of the component guaranteed or not before the production of tooling for a prototype? The cylinder head is made of an aluminium alloy (in this case an A...
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Cet article présente une solution alternative aux méthodes classiques (comme la méthode incrémentale) de résolution de problèmes thermomécaniques cycliques non-linéaires. Il s'agit d'une Méthode Cyclique Directe qui consiste à rechercher directement la solution asymptotique d'une structure anélastique soumise à un chargement thermomécanique périodique, sans suivre l'histoire du chargement. Elle est fondée sur le Grand Incrément de Temps et la périodicité de l'état limite et la transformation de Fourier. solides et structures / cycle limite / thermomécanique / méthode numérique Determination of the asymptotic response of a structure under cyclic thermomechanical loading
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