is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. AbstractThis work investigates the effect of natural and artificial surface defects and quenching on the fatigue strength of a Boron Steel (22MnB5). A vast experimental campaign has been undertaken to study the high cycle fatigue behaviour and more specifically the fatigue damage mechanisms observed in quenched and untreated materials, under different loading conditions and with differents artificial defects sizes (from 25µm to 370 µm radius). In order to test the sheet metal in shear an original test apparatus is used. The critical defect size is determined to be 100±50µm. This critical size does not appear to depend on the loading type or the microstructure of the material (i.e. ferritic-perlitic or martensitic). However, for large defects, the quenched material is more sensitive to the defect size than the untreated material. For a defect size range of 100µm to 300µm the slope of the Kitagawa-Takahashi diagram is approximately -1/3 and -1/6 for the quenched and untreated materials respectively. A probabilistic approach that leads naturally to a probabilistic Kitagawa type diagram is developed. This methodology can be used to explain the relationship between the influence of the heat treatment and the defect size on the fatigue behaviour of this steel.

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