Implementation of modelled surface roughness in the accelerated lifetime prediction of a 20MnMoNi5-5 steel

Abstract

Fatigue of metals is a complex, multi-parametric phenomenon with a high amount of influencing factors. In order to ensure a reasonable lifetime prediction of specimens, structures or components, the impact of these factors need to be understood in detail. Since fatigue cracks mostly initiate at the surface, especially in the high cycle fatigue regime, the surface condition is a very important factor, which has to be taken into account. The presented research proposes a method to model the surface roughness using a Fourier transformation to calculate a fatigue notch factor, which can be included in the accelerated lifetime prediction method MiDAcLife. Therefore, it is possible to generate virtual S-N curves for different surface conditions based on only one fatigue test. The investigated material is a ferritic-bainitic 20MnMoNi5-5 steel, which is mainly used in spray-lines of German nuclear power plants. In addition, a method for calculating the fatigue strength of various materials is presented in which a newly defined parameter, the specific hardness, is introduced.