A size effect oil the fatigue crack growth rate threshold of alloy 718

Fatigue crack growth rate (FCGR) tests were conducted on Nickel-based super alloy 718 in the solution annealed and aged condition at room temperature. In each test, the FCGR threshold was measured using the K-decreasing consta nt R method. Initial testing was at two facilities, one of which used C(T) specimens with W = 127 mm. Previous data at the other facility had been obt ained with specimens with W = 50.8 mm. A comparison of test results at R = 0.1 showed that the threshold for the 127 mm specimen was considerably high er than that of the 50.8 mm specimen. A check showed that this difference w as not due to a heat-to-heat or lab-to-lab variation. Additional tests were conducted on specimens with W = 25.4 mm and at other R values. Data for th e various specimens is presented along with parameters usually used to desc ribe threshold behavior.