Correlation Between Fatigue Crack Propagation and Low Cycle Fatigue Properties

Fatigue crack propagation is treated as a succession of fatigue crack initiation events. The cyclic stress and strain experienced by elements ahead of the crack tip are computed by means of an elastic-plastic analysis and the cumulative fatigue damage is evaluated from low cycle fatigue properties of the metal. If it is assumed that the appropriate element width is related to the microstructure size of the metal, a significant deviation from the log-log linear relationship usually assumed to exist between the crack propagation rate, da/dN, and the stress intensity range, ΔK, is predicted. The analysis indicates that at small values of ΔK, da/dN rapidly decreases until it approaches zero at a “threshold” value of ΔK.

Fatigue crack propagation data for eight steels from Barsom's work are compared with predictions from the analysis using low cycle fatigue properties and values of the microstructure size based on Gurland's correlation between yield strength and the dimensions of the microstructural units responsible for strengthening in steels. In all cases the estimate of da/dN for a given ΔK was within a factor of two of the measured values.