Quantitative Analysis of Low Cycle Fatigue Fracture Stress in Nickel-Based Single Crystal Superalloys Based on Crack Tip Plastic Zone

Abstract:

Low cycle fatigue failure is the main failure mode of tenon part of single crystal turbine blades. Due to the difference between the actual working load and the design load, the stress leading to fatigue failure often needs to be given after fatigue failure, and the fracture is a comprehensive reflection of load and temperature. Quantitative analysis of the fracture and inverse fatigue stress have important engineering application value in blade failure analysis. The unique microstructure and crystal structure of single superalloy make its fatigue fracture characteristics different from those of polycrystalline materials. The main fatigue fracture characteristics of single crystal superalloy are slip plane rather than fatigue band. A model and method for quantitative analysis of crack tip plastic zone were presented in this paper. There is a certain angle between fatigue fracture and load of single superalloy, which is a composite cracking mode rather than a type Ⅰ cracking mode. According to the cracking characteristics of single superalloy, using the test data of DD6 single-crystal high-temperature alloy under the condition of 530 ℃ and strain ratio r=0.05, the hysteresis loop of its different life intervals was analyzed. The results show that the life span is between one thousand and ten thousand times, and its hysteresis loop is very narrow; the life span is greater than ten thousand times, and its hysteresis loop is basically a straight line. in addition, DD6 single-crystal high-temperature alloy under the conditions of 530 ℃ and strain ratio r=0.05 has the small yielding characteristics. Based on this, for the low-cycle fatigue fracture, the characteristics of crack initiation and extension stage and its fracture characteristics were studied, and a quantitative analysis model of fatigue stress fracture was established by considering the composite cracking and based on r_p in the plastic zone at the crack tip. The quantitative analysis of fatigue stress fractures at different locations was carried out using a total of 12 crack locations for 3 specimens. The analysis results show that the error of fatigue initiation stress is within 1.3 times, and that of inverse extrapolation result of the first stage of extension is within 1.5 times of the dispersion band. The results provide models and methods for quantitative fracture analysis of stresses in single-crystal superalloys mainly by slip-surface cracking (non-fatigue strips).