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...

This paper presents a global approach to the design of structures that experience thermomechanical fatigue loading, which has been applied successfully in the case of cast-iron exhaust manifolds. After a presentation of the design context in the automotive industry, the important hypotheses and choices of this approach, based on a thermal 3D computation, an elastoviscoplastic constitutive law and the dissipated energy per cycle as a damage indicator associated with a failure criterion, are first pointed out. Two particular aspects are described in more detail: the viscoplastic constitutive models, which permit a finite element analysis of complex structures and the fatigue criterion based on the dissipated energy per cycle. The FEM results associated with this damage indicator permit the construction of a design curve independent of temperature; an agreement is observed between the predicted durability and the results of isothermal as well as non isothermal tests on specimens and thermomechanical fatigue tests on real components on an engine bench. These results show that thermomechanical fatigue design of complex structures can be performed in an industrial context

For continuously welded structures subjected to cyclic loading, the highly stressed zones where cracks initiate and lead to failure are usually located at weld toes. At these critical points, called hotspots, the very local stress states are difficult to determine so that standard fatigue criteria are very diflicult to apply for fatigue life prediction. This work presents a fatigue design criterion for continuously welded thin sheet structures, based on a unique S-N curve. The approach, which refers to the hot-spot stress concept, defines the design stress S as the geometrical stress amplitude at the hot-spot.In practice, the geometrical stress state is calculated by means of the finite element method (FEM) using thin shell theory. Meshing rules for the welded connection, which can be applied methodically to any welding situation, allow the hot-spot location, and therefore the design stress of any structure, to be determined.Experimental data and FEM calculations show that a unique S-N curve can be obtained whatever the geometry of the welded structure and the loading mode. NOMENCLATURE e e = thickness of the attachment, and thickness of the plate. e!$fi$ = weld leg length on the attachment side, and on the plate side. I$), E p ) = shell element number i, perpendicular to the intersection curve of the shell element mean surfaces, on the attachment side, and on the plate side. Fi = load in the Oi direction. F,,, F-= maximum load and minimum load. @), n? = node number i, on the attachment side and on the plate side. N = number of cycles to fatigue failure. R = load ratio. = F,,,,,,/F,, S = design stress. uHs = geometrical stress at the hot spot. crL = actual stress at the hot spot (or local stress). AuHs = uHs range.

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