FATIGUE CRACK DEVELOPMENT IN A LOW-CARBON STEEL. MICROSTRUCTURE INFLUENCE. MODELLING

Abstract Fatigue in a low-carbon steel with ferrite and pearlite microstructure is investigated through testing of three groups of specimens. Two of the groups consist 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. The third group of specimens includes flat specimens preliminary notched by FIB-technique and then subjected to pure-bending fatigue. The study is focused on examining of crack paths in terms of interaction between the propagating short cracks and the microstructure. The obtained data for pure-bending fatigue show higher crack growth rates 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.