A Proposal on Quantitative Treatment of Multiple Cracks Nucleating in Single Crystal Superalloys

Abstract

Multiple crack propagation tests were performed at room temperature using four point bending specimens which contained nineteen cracks and were made of Ni-based single crystal superalloy CMSX-4. Two types of specimen orientations were machined. One’s longitudinal direction was [010] and machined notch direction was [100], and another’s was [010] and [101]. In each specimen, cracks propagated along {111} slip planes making ridges which inclined to the thickness direction. The cracks of the former also inclined to the longitudinal direction at an angle of 45°, while those of the latter propagated along the machined notch direction. To evaluate such crack propagation behavior, mode I III stress intensity factors were calculated under three conditions. First, cracks were treated as single edge crack without considering crack interaction. Secondly, cracks were treated as multiple parallel edge cracks with same lengths. Third, cracks were treated as multiple parallel edge cracks with alternately different lengths. Since stage-I cracking was evaluated by resolved shear stress intensity factor range on {111} plane, resolved shear stress intensity factor was calculated from mode I - III stress intensity factors obtained by the above three conditions. Then, crack propagation simulations were performed. In case of the [010][101] orientation specimen, simulated crack propagation rates and behavior calculated under the third condition agreed well with the experimental ones. On the other hand, in case of the [010][100] specimen, those calculated by the first condition agreed better with the experiments than those simulated by the other conditions.