Relationship between critical size of inclusions responsible for fatigue failure and inclusion size distribution.

Abstract

The rotating bending fatigue test and the measurement of inclusion size distribution in a material using the quantitative metallography technique were carried out on a quenched and tempered SAE9254 steel. The relationship between the size of inclusions which act as nucleation sites for actual fatigue failures and the inclusion size distribution was discussed, and an attempt was also made to estimate the effect of the size distribution of inclusions on the scatter of fatigue lives of the steel. The results obtained were summarised as follows. (1) Almost all the sizes of inclusions, Dfs, which were observed at crack nucleation sites on the fracture surfaces are larger than the maximum size of inclusions, Dic, expected from the inclusion size distribution in the material. indicating that the inclusion which acts as a nucleation site for actual fatigue failure is the largest one in the present microstructure. (2) The scatter of fatigue life is closely related to the inclusion size distribution in the critical volume for fatigue failure and can be estimated using a new parameter of Dfs/Dic.