Overload Effects in Sustained Load Crack Growth in Inconel 718

摘要

Crack-growth-rate experiments were conducterd on CT specimens of Inconel 718 at 649 C. The loading spectrum consisted of a single 1 Hz cycle at R = 0.1 and a hold time. The ratio of the amplitude of the hold time to the maximum amplitude of the fatigue cycle, R sub in, was 1.0, 0.9, 0.8 and 0.5 with hold times 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. the square root of m. Data show that for R sub in = 1.0, a linear summation model works well, while at R sub in = 0.9 there is a measurable retardation effect on the crack growth during the hold time. For values of R sub in less than 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. (Author)