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Fatigue crack growth was studied at 650°C as a function of frequency for several ratios of minimum-to-maximum stress intensity (R) and for two values of maximum stress-intensity factor (Kmax), for a nickel-base superalloy, Inconel 718. All the tests were conducted under computer control at constant Kmax. Crack lengths were monitored at low frequencies from compliance calculations based on crack-mouth opening displacement measurements. At higher frequencies, crack length was measured using a d-c electric potential system. It was found that fatigue crack growth rate can be characterized in three distinct frequency regions. These three regions represent fully cycle-dependent, mixed, and fully time-dependent crack growth behavior, and each region can be modeled by a power law function. Observation of micromechanisms support the existence of these three different modes of crack growth.
fatigue crack growth, time-dependent growth, cyclic-dependent growth, nickelbase superalloy, crack growth rate model, frequency effects, fracture mechanics
Research Scientist, University of Dayton Research Institute, Dayton, OH