In steels, a cyclic loading below the yield strength can lead to failure. Cracks are formed on the surface or in the volume of components, propagate, and finally lead to the fracture of the specimen. In case of internal failures non-metallic inclusions often act as crack initiating sites. In this presentation a calculation approach for the fatigue strength of high strength steels based on non-metallic inclusions determined in microsections is introduced. The calculation approach consists of two parts. In the first step, the failure-critical defect is determined for the respective specimen or component geometry. In the second step, the fatigue strength of the specimen is calculated in dependence of the loading condition, steel grade and expected inclusion size. The determination of the inclusion size is based on metallographic investigations. Based on the ASTM 2283 standard, the distribution of the largest inclusions per metallographic section is determined. Various distribution functions are used to mathematically describe the distribution of the inclusions sizes. Depending on the specimen geometry and the loading condition, the failure-critical inclusion size is determined with aid of the distribution functions.
The fatigue strength prediction is based on a fracture mechanical approach. As described by Murakami , inclusions are regarded as cracks of the size of their cross sectional area perpendicular to the maximum principal stress. The calculation method takes into account the hardness of the steel, the inclusion size, local residual stresses, multi-axial load stresses and mean stresses. Various parameters are required to perform the calculation. For example, parameters which describe the threshold stress intensity factor range of small defects or material dependent parameters of fatigue criteria are required. Case hardened, quenched and tempered as well as bearing steels in different heat treatment conditions, which have been investigated in several research projects over the last two decades, are used as data basis for the parameter determination. By combining the inclusion size prediction with the fatigue strength calculation, it is possible to estimate the fatigue strength of different components on the basis of metallographic investigations.