Abstract
The variation of the fatigue limit of a metal with the diameter or length of a testpiece is still poorly understood in spite of its great importance in the design of components subjected to fatigue loading. To try to explain this size effect for martensitic steels used in ball bearings, rotating beam tests were performed on testpieces with diameters of 2 or 5 mm. Differences between the two diameters are explained in terms of an analysis of the nucleation of cracks around carbides or non-metallic inclusions. Then, the localization of the harmful defects on a great number of pieces enables the definition of a stress which is a characteristic of the behaviour of a particular defectmatrix combination. When related to the density of defects in harmful zones, this leads to a rationalization of the size effect for high-carbon martensites and permits fatigue limits to be calculated using a simple nomogram.
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