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Crack-growth-rate experiments were conducted on compact tension (CT) specimens of Inconel 718 in air at 649°C. The loading spectrum consisted of a single 1 Hz cycle at R = 0.1, where R is the ratio of minimum to maximum stress-intensity factors, and a hold time which was applied at various levels of the fatigue cycle. The ratio of the hold-time amplitude to the maximum amplitude of the fatigue cycle, Rin, was kept at 1.0, 0.9, 0.8, or 0.5. Hold times were varied from 0 to 200 s. Tests were performed under computer-controlled constant-K conditions using values of the maximum of the fatigue cycle of 40 and 50 MPa m1/2. Data show that for Rin = 1.0, a linear summation model works well, while at Rin = 0.9 there is a measurable retardation effect on the crack growth during the hold time. For values of Rin less than or equal to 0.8, the sustained-load crack growth is almost completely retarded. A simple retardation model is proposed which can fit the experimental data and is based on the concept of an overload plastic zone being produced by the fatigue cycles. It is concluded that hold times do not contribute to crack growth in this material unless their amplitude is at or near the maximum amplitude of the adjacent fatigue cycles.
crack growth, fatigue, overloads, crack growth retardation, nickel-base superalloy, elevated temperature
Research scientist, University of Dayton Research Institute, Dayton,
Senior scientist, (MLLN) Wright-Patterson Air Force Base, OH