تأثیر تخلخل بر استحکام
Abstract
1-Introduction
2-Materials & experimental conditions
3-Assessment of average critical defect size
4-Towards a fatigue design procedure for cylinder heads accounting for the size effect
5-Conclusion
Acknowledgement
References
Abstract
The fatigue strength of cast aluminium alloys is known to be greatly affected by different defect types related to the manufacturing process, particularly microshrinkage pores created during the solidification phase of the casting process. Even if certain classification procedures are given in the standard ASTM E155-15 [1], the presence of defects is not readily related to the capacity of a component or a structure to meet the requirements of the mechanical technical specifications. The present study aims at establishing a clear link between certain microstructural features and the average fatigue strength. This is possible by looking for the average size of critical defects and using a relevant statistical analysis. More exactly, the Murakami approach based on the statistics of extremes is employed. The main originality of this work lies in the application of this approach to the case of a real structure submitted to high cycle fatigue damage: engine cylinder heads, used in the automotive industry. Indeed, both fatigue tests and microstructural characterizations are carried out on cylindrical specimens and real structures. The specimens are subjected to uniaxial and multiaxial loading conditions [2]. Original fatigue tests, developed by PSA to load in-service critical regions, are carried out on cylinder heads. Systematic analyses of fatigue failure surface are conducted to obtain the statistics of critical defects at the origin of the failures for both specimens and structures. In parallel, critical regions and the associated local loading mode in the structure are characterized by an appropriate high cycle fatigue analysis. The latter, combined with the fatigue test data and the statistical analysis of the critical defects, leads to a discussion about the size effect and an approach is proposed for a relevant fatigue design procedure.
Introduction
The cast aluminium alloys are widely used in automotive industry due to their relative good strength combined with a low density and an excellent conductivity. These alloys are commonly employed for chassis and engine components which are, for the most part, subjected to mechanical cyclic loads and present a serious risk of mechanical fatigue. In order to avoid fatigue crack initiation and ensure the component resistance, appropriate design methodologies have to be developed. They are mostly based on an appropriate fatigue criterion, whose identification is carried out by fatigue tests performed on test specimens, who’s size is generally quite different to the size of the fatigue critical zones found on the real manufactured components. The fatigue strength of cast aluminium alloys is known to be greatly affected by different types of microstructural heterogeneities due to the manufacturing process. According to this process and subsequent treatment, the microstructural features can be significantly modified especially in terms of eutectic components (silicon particles and iron-based intermetallics) and casting defects, notably micro-shrinkage pores and gas porosity. Despite the significant effort to characterize and understand the influence of these microstructural heterogeneities on the High Cycle Fatigue (HCF) behaviour of these alloys [2][7], the development of design methodologies taking into account the effect of defects on the fatigue life prediction of structures is still under progress [8][10].