Study of the Fatigue Behavior of Short Cracks in Nickel-Based Superalloys

摘要

Fatigue behavior of short cracks was studied in Inconel X-750, Inconel 718, Waspaloy and PM-Rene 95. Crack growth rates were measured for crack lengths from 50 microns to 2mm. Three regimes of behavior were generally observed: 1) an initiation regime in which crack propagation rates are nearly zero; 2) a short crack regime in which crack propagation rates increase slowly and variably with crack length but with propagation rates higher than would be predicted by LEFM in the near-threshold regime; and 3) a long crack regime in which conventional fracture mechanics is applicable. The threshold criterion and crack propagation rates for short cracks are shown to be strongly dependent on the stress ratio. Negative stress ratios promotes rapid crack initiation. This behavior is confirmed theoretically by a Dugdale model which establishes a criterion for crack extension based on the accumulation of plastic work in the crack tip plastic zone. Fractographic investigations show that short cracks propagate in a transgranular-crystallographic mode following a zig-zag path which is macroscopically perpendicular to the applied stress. An experimental technique was developed to generate small eliptical crack initiation sites using a pulsed Nd-YAG laser. An AC potential drop system for continuous and automated measurements of crack length at elevated temperature has been assembled.