DEVELOPMENT OF A PROBABILISTIC MODEL FOR THE PREDICTION OF FATIGUE LIFE IN THE VERY HIGH CYCLE FATIGUE (VHCF) RANGE BASED ON MICROSTRUCTURAL PROPERTIES

摘要

Numerous investigations indicate a strong influence of microstructural inhomogeneities on the fatigue life of components subjected to more than 10 Mio loading cycles. This influence has a probabilistic character and increases the scatter band width up to three decades for the SN-curve. Thus, the application of a reliable fatigue life prediction concept by means of traditional statistical approaches is impeded and a detailed investigation of the relevant fatigue damage mechanisms is required. The aim of the present work is the development of a prediction model taking microstructural properties of failure-relevant defects or inhomogeneities and corresponding fatigue behaviour into account. For this purpose the fatigue behaviour of different metallic materials showing crack initiation at different defect types was investigated and modelled based on metallographic observations. In case of the nickel-based superalloy Nimonic 80A twin boundaries as well as regular grain boundaries with high misorientation factors were identified as crack initiation sites. In the case of the metastable austenitic stainless steel AISI 304 cracks initiated primarily from non-metallic inclusions provided the deformation-induced martensite volume fraction exceeded 30%. Based on a comprehensive experimental database, the observed correlation between failure-relevant parameters and corresponding numbers of cycles until failure or crack initiation was modelled.