基于裂纹尖端塑性区的镍基单晶高温合金低周疲劳应力断口定量分析

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

摘要:

低周疲劳失效是单晶涡轮叶片榫头部位主要的失效模式，由于实际工作载荷与设计载荷之间的差异，疲劳失效后往往需要给出导致疲劳失效的应力，而断口是载荷、温度的综合体现，对断口进行定量分析反推疲劳应力，在叶片失效分析中具有重要的工程应用价值。单晶高温合金特有的组织结构和晶体结构，使其疲劳断裂特征不同于多晶材料，单晶高温合金主要的疲劳断裂特征为滑移面，而非疲劳条带。本研究提出了利用裂纹尖端塑性区进行定量分析的模型与方法。单晶高温合金疲劳断口与载荷之间有一定的角度，为复合型开裂模式，而非Ⅰ型开裂模式。针对单晶高温合金的开裂特点，利用DD6单晶高温合金在530 ℃、应变比r=0.05条件下的试验数据，分析其不同寿命区间的迟滞回线，结果表明：寿命在千次到万次之间，其滞后环非常狭窄；寿命大于万次，其滞后环基本为直线；表明DD6单晶高温合金在530 ℃，应变比r=0.05条件下具有小屈服的特点。基于此，针对低周疲劳断口，研究了裂纹萌生、扩展阶段特性及其断裂特征，建立了考虑复合型开裂和基于裂纹尖端塑性区r_p的疲劳应力断口定量分析模型。利用3个试样的共12个裂纹位置处r_p进行了疲劳应力断口定量分析，分析结果表明：疲劳萌生应力误差在1.3倍内、扩展第1阶段应力反推结果误差在1.5倍分散带内。研究结果对单晶高温合金主要以滑移面开裂（非疲劳条带）进行断口定量分析应力提供了模型和方法。

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).