A combined Importance Sampling and Kriging reliability method for small failure probabilities with time-demanding numerical models

Analysis of the fatigue strength of a stainless steel based on the energy dissipation

Fatigue analysis of welded joints: state of development

Excellent thermal conductivity and lower density make Al–Si alloys a suitable alternative for cast iron in the fabrication of engine components. The increase in the maximum operation temperature and pressure of engines necessitates improving the thermomechanical fatigue performance of Al–Si alloys. This paper has two major parts focussing on the use of Al–Si based alloys in cylinder heads and engine blocks.In the first part, the structural stress–strain and material property requirements of cylinder heads are discussed. In addition, the physical and mechanical properties of different competing materials used in the manufacture of engine components are reviewed. The physical metallurgy, solidification sequence and thermal conductivity of Al–Si based alloys are reviewed. Also discussed is the effect of microstructural features on thermomechanical fatigue lifetime. This part also includes an overview of the strengthening mechanisms of cast Al–Si alloys, by dispersed phases and heat treatment.Demands ...

Application of cast Al–Si alloys in internal combustion engine components

Comparative assessment of dissipated energy and other fatigue criteria

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.

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Fatigue design of structures under thermomechanical loadings

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.

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Thermomechanical design in the automotive industry

Fatigue assessment of automotive components is performed at PSA using a statistical approach to predict reliability. Calculations provide a lot of information but experimental work is always necessary to determine this reliability. Results exploitation is carried out using the “Stress-Strength interference analysis” method (SSIA). The “Stresses” represent the distribution of the car owners severity, the “Strength” represents the distribution of the fatigue strength of all the components. If the determination of the Strength is easy (fatigue tests results), the application of the “Stress-Strength” Method also requires the knowledge of the owners severity. This information is accessible as statistical distribution through owner enquiries on car usage. On the one hand a “car usage” enquiry allows conditions in which cars are driven (loading, route) to be known. On the other hand an “owner behaviour” enquiry allows the determination for a given use of the distribution of the owners' severity. The fatigue assessment of an engine subframe is used to illustrate the approach.

Fatigue design and reliability in the automotive industry

— 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 hot-spots, the very local stress states are difficult to determine so that standard fatigue criteria are very difficult 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.

Fatigue design criterion for welded structures

The thermomechanical fatigue assessment in the automotive industry is discussed. The hypothesis and the principal aspects of the design strategy are presented. There are 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 to perform predictive calculations of automotive parts subjected to thermomechanical loading.

A. Bignonnet

Papers

Fatigue design of structures under thermomechanical loadings

Thermomechanical design in the automotive industry

Fatigue design and reliability in the automotive industry

Fatigue design criterion for welded structures

Thermomechanical fatigue in the automotive industry