P ierre Osmond

CETIM, Senlis, France



Gas and shrinkage pores in cast alloys are known to have a strong detrimental effect on the high cycle fatigue strength of materials, as stress-raisers that lead to fatigue crack initiation. Size and location of defect have been identified as parameters of tremendous importance to investigate defect severity. The effect of defect on fatigue strength have been largely documented for cast alloys, especially for surface defects with size less than 1mm. Few studies are available for defects larger than 1mm and the literature related to “large” internal defect is even more limited. However, many of the issues met in casting production are related to this last category of defect, justifying further investigations. This is one of the objectives of the DEFOND project launched by CETIM where samples having internal controlled defects, with dimensions appropriate for studying “large” defects under small scale yielding conditions, are used to promote initiation and propagation from internal defects. The present contribution aims to present the first results of this project, focusing on the link between features of internal defects responsible for crack initiation and fatigue strength of G20Mn5 QT cast steel alloy. After analysis of damage mechanisms, the relationship between internal defects features, with particular attention on defect size and morphology, will be assessed through classical Kitagawa-Takahashi approach. As a first phase for further applications to fatigue design, the performance of the main models available in the literature to account for “large” internal defects on fatigue strength is evaluated.


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