The influence of stress intensity and microstructure on fatigue crack propagation in ferritic materials

Abstract New and published fatigue crack growth data for a wide range of steels have been categorized in terms of different growth mechanisms, namely striation formation, microcleavage, void coalescence and intergranular separation. General principles emerged concerning the influence of mean stress, specimen thickness, flow stress and toughness on rates of fatigue crack propagation through their effect on growth mechanism. Crack propagation rates associated with striation formation were insensitive to changes in mean stress (except at very low stress intensities) and specimen thickness. Increase in flow stress resulted in a small decrease in growth rate, although the path of a crack through complex structures like welds was, nevertheless, strongly influenced by plastic relaxation. Crack propagation rates increased when deformation led to net-section yielding (general yielding) and the increase was related to specimen thickness and geometry. It has been shown that simple relationships between the rate of propagation and alternating stress intensity are adequate for describing fatigue crack growth by the striation mechanism. Departures from exclusively striation formation to include micro-cleavage, void coalescence or intergranular separation were found to result in accelerated growth rates. Where growth occurred by combined striation formation and microcleavage, the increase in fatigue crack growth rate was dependent on the maximum tensile stress and hence on the mean stress and specimen thickness. Similarly, when fatigue involved the void coalescence mechanism the rate was increased by raising the mean stress. The role of microstructure and fracture toughness in promoting the different growth mechanisms is discussed. Modification of the simple growth law is necessary in order to describe the observed results.