Influence of microstructure on fatigue process in a low carbon steel. Analysis and modelling

Fatigue in a low-carbon steel is investigated through observation on surface crack propagation and on growth of cracks in preliminary notched specimens. Testing uses three groups of specimens. For surface crack observation there are two groups of samples consisting of cylindrical specimens subjected to tension-tension and rotating-bending fatigue; in this case surface microstructurally-short crack propagation is monitored by acetate-foil replica technique. For crack growth observation (in situ) in notched specimens there is a third group of samples including flat specimens preliminary notched by FIB-technique and then subjected to pure-bending fatigue. Here microstructurally-short crack propagation is examined at interruptions of each test at a given equal number of cycles for detailed observation of specimen surface by optical- and SEM-microscopy. The study is focused on examining of crack paths in terms of interaction between the propagating short cracks and the microstructure, and on a suitable mathematical description of crack growth in the investigated microstructure. The obtained data for pure-bending fatigue show higher crack growth rates (dominated by the interaction with ferrite and pearlite grain boundaries and interfaces, ferrite grains, pearlite colonies and non-metal inclusions) and shorter fatigue lifetimes than those found for rotating-bending fatigue. In comparison, the registered tension-tension fatigue data present the lowest crack growth rates, due to much lesser loading than that applied at rotating-bending and pure-bending fatigue. Based on data obtained, a Parabolic-linear model “Crack growth rate - Crack length” is used for describing and predicting adequately short crack propagation under the specified three types of fatigue. The model is supported by a comparison between the predicted and the actual fatigue lifetimes.