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Based on the assumption that normalized Kitagawa-Takahashi diagrams for different materials are the same, a unified model for microstructurally small fatigue crack and physically small fatigue crack growth rates was developed to describe their behaviour under different fatigue stress ranges. The stress-sensitive blocking effect of microstructural barriers to small fatigue crack growth is satisfactorily simulated by the model. Incorporated with the materials fatigue limit and microstructural barrier spacing, this model can be easily used in the prediction of small fatigue crack lifetime. Small fatigue crack growth rates of previous experimental studies in 7075-T6 Aluminium alloy and HT60 steel under different stress ranges are in an envelope between two boundary prediction curves corresponding to the largest and smallest stress ranges applied in the experiments. Problems concerning model accuracy and model application are also discussed in the present paper. NOMENCLATUREA , C, m, n = constants c = half surface crack length dc/dn = fatigue crack growth rate D = spacing between microstructural barrier f= barrier blocking factor in small fatigue crack growth model AK = stress intensity factor range do = fatigue stress range Auf = fatigue limit stress range As, =threshold stress range in small fatigue crack regime

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