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The effect of stress frequency on the fatigue crack propagation rate in 99.5 percent pure titanium, which has a strong strain rate dependency in the plastic region, was studied. Fatigue crack propagation tests were carried out under three stress frequencies (0.02Hz, 0.2Hz, 20Hz). An elasto/visco-plastic analysis of fatigue crack propagation was performed by the finite element method (FEM), and an analysis of the dependency of crack propagation rate on frequency and strain behavior at the crack tip. The results obtained in this study are summarized as follows: (1) It was found from the experiments that the crack propagation rate, dl/dN, was approximately in inverse proportion to fn (n>0), where f is the frequency. (2) A parameter closely related to the fatigue crack growth rate is the strain range, Δεy, or visco-plastic strain range, Δεyνp at the crack tip. (3) The effect of stress frequency on the fatigue crack propagation rate may be explained by variations of Δεy or Δεyνp based on the strain rate dependency of the material.