A progressive crack-tip strain approach to fatigue crack propagation is presented. Localstrain damage accumulation curve for crack initiation is used as a "cyclic damage accumulation" and "fracture criterion" in the heavily deformed region, Rh, just adjacent to the crack-tip. The changes of cyclic strain ahead of the crack-tip is experimentally measured by the real-time finegrid-method. Using the average strain range in Rh and linear cumulative damage law of local strain, the number of cycles N* necessary for the material failure over the distance, Rh is calculated from local-strain damage accumulation curve. The average crack propagation rate, da/dN, in Rh is estimated as the ratio Rh/N*. As a result, very good agreements between the measured and predicted da/dN are obtained in cases of two kinds of steels, MANTEN and RQC-100. This fact suggests the possibility that we can estimate the fatigue crack propagation rate by combining the cyclic strain ahead of the crack-tip with local-strain damage accumulation curve, and that, we can unify the two fatigue stages, crack initiation and propagation, from the view point of the local crack-tip strain.
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