Use of a statistical approach to verify the cumulative damage laws in fatigue

Abstract

This paper describes a new methodology for fatigue tests of materials, developed to verify the cumulative damage laws. The aim is to overcome the difficulties due to the widely disparate lives and to the complexity of their distribution. The method of ‘usable classes’ was tested on a specimen of 35CD4 steel, in rotative bending, for two neighbouring stress levels, where the distribution of cycle numbers is lognormal. The separation into usable classes rests on the fact that the specimens which are most resistant to one level are the same as those which are most resistant to another. The application to two widely spaced levels (the lower level being close to the endurance limit) permits us, on the one hand, to take into account the non-lognormality of the life disparity, on the lowest level. On the other hand, it brings to light the fact that there is not just one curve of cumulation for a specimen, but several (one per class), in connection with the endurance limit of the class.