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

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 rp 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).

低周疲劳失效（Low Cycle Fatigue Failure）是单晶涡轮叶片榫头部件的主要失效模式。由于实际工作载荷与设计载荷存在偏差，导致疲劳失效的应力往往需在失效发生后才能确定，而断口是载荷与温度共同作用的综合反映。对断口与疲劳反推应力开展定量分析，在叶片失效分析中具有重要的工程应用价值。单晶高温合金独特的微观组织与晶体结构，使其疲劳断口特征不同于多晶材料。单晶高温合金的主要疲劳断口特征为滑移面，而非疲劳条带。本文提出了一种裂纹尖端塑性区的定量分析模型与方法。单晶高温合金的疲劳断口与载荷间存在特定夹角，属于复合型开裂模式，而非Ⅰ型开裂模式。针对单晶高温合金的开裂特征，本文以DD6单晶高温合金在530℃、应变比r=0.05条件下的试验数据为基础，对其不同寿命区间的滞后回线开展了分析。结果表明：当寿命处于1000~10000次区间时，其滞后回线较为窄细；当寿命大于10000次时，滞后回线基本呈直线形态。此外，在530℃、应变比r=0.05条件下的DD6单晶高温合金表现出小屈服特性。基于此，本文针对低周疲劳断口，研究了裂纹萌生与扩展阶段的特征及其断口特性，并结合裂纹尖端塑性区rp与复合型开裂模式，建立了疲劳应力断口的定量分析模型。本文选取3个试样的共计12处裂纹位置，对不同位置的疲劳应力断口开展了定量分析。分析结果显示：疲劳萌生应力的误差处于1.3倍以内，第一阶段扩展的反推外推结果的误差处于离散带的1.5倍以内。本研究成果可为以滑移面开裂（非疲劳条带）为主的单晶高温合金应力断口定量分析提供模型与方法。