Nickel-based superalloys are typically used as blades and discs in the hot section of gas turbine engines, which are subjected to cyclic loading at high temperature during service. Understanding fatigue crack deformation and growth in these alloys at high temperature is crucial for ensuring structural integrity of gas turbines. Experimental studies of crack growth were carried out for a three-point bending specimen subjected to fatigue at 725°C. In order to remove the influence of oxidation which can be considerable at elevated temperature, crack growth was particularly tested in a vacuum environment with a focus on dwell effects. For simulation, the material behaviour was described by a cyclic viscoplastic model with nonlinear kinematic and isotropic hardening rules, calibrated against test data. In combination with the extended finite element method (XFEM), the viscoplasticity model was further applied to predict crack growth under dwell fatigue. The crack was assumed to grow when the accumulated plastic strain ahead of the crack tip reached a critical value which was back calculated from crack growth test data in vacuum. Computational analyses of a stationary crack showed the progressive accumulation of strain near the crack tip under fatigue, which justified the strain accumulation criterion used in XFEM prediction of fatigue crack growth. During simulation, the crack length was recorded against the number of loading cycles, and the results were in good agreement with the experimental data. It was also shown, both experimentally and numerically, that an increase of dwell period leads to an increase of crack growth rate due to the increased creep deformation near the crack tip, but this effect is marginal when compared to the dwell effects under fatigue-oxidation conditions. The strain accumulation criterion was successful in predicting both the path and the rate of crack growth under dwell fatigue. This work proved the capability of XFEM, in conjunction with advanced cyclic viscoplasticity model, for predicting crack growth in nickel alloys at elevated temperature, which has significant implication to gas turbine industries in terms of “damage tolerance” assessment of critical turbine discs and blades.

中文翻译：

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镍基高温合金在高温下的疲劳裂纹扩展-实验研究，粘塑性模型和XFEM预测

镍基高温合金通常用作燃气轮机发动机热段中的叶片和盘片，在维修过程中会经受高温下的循环载荷。了解这些合金在高温下的疲劳裂纹变形和生长对于确保燃气轮机的结构完整性至关重要。对于在725°C疲劳的三点弯曲试样进行了裂纹扩展的实验研究。为了消除在高温下相当大的氧化影响，特别在真空环境下测试了裂纹扩展，重点是保压效果。为了进行仿真，通过循环粘塑性模型描述材料的行为，该模型具有非线性运动学和各向同性的硬化规则，并根据测试数据进行了校准。结合扩展有限元方法（XFEM），进一步将粘塑性模型用于预测保压疲劳下的裂纹扩展。当裂纹尖端之前的累积塑性应变达到临界值时，假定裂纹在扩展，该临界值是根据真空中的裂纹扩展测试数据计算得出的。对固定裂纹的计算分析表明，在疲劳下裂纹尖端附近逐渐累积了应变，这证明了在XFEM预测疲劳裂纹扩展中使用的应变累积准则是合理的。在模拟过程中，记录裂纹长度与载荷循环次数的关系，结果与实验数据吻合良好。从实验和数值上也表明了这一点，由于在裂纹尖端附近蠕变变形的增加，保压时间的增加导致裂纹扩展速率的增加，但是与疲劳氧化条件下的保压效果相比，这种影响是微不足道的。应变累积准则成功地预测了驻留疲劳下的裂纹扩展路径和速率。这项工作证明了XFEM与先进的循环粘塑性模型相结合的能力，能够预测高温下镍合金中的裂纹扩展，这对燃气轮机行业的关键涡轮盘和叶片的“损伤容限”评估具有重要意义。应变累积准则成功地预测了驻留疲劳下的裂纹扩展路径和速率。这项工作证明了XFEM与先进的循环粘塑性模型相结合的能力，能够预测高温下镍合金中的裂纹扩展，这对燃气轮机行业的关键涡轮盘和叶片的“损伤容限”评估具有重要意义。应变累积准则可以成功预测停留疲劳下的裂纹扩展路径和速率。这项工作证明了XFEM与先进的循环粘塑性模型相结合的能力，能够预测高温下镍合金中的裂纹扩展，这对燃气轮机行业的关键涡轮盘和叶片的“损伤容限”评估具有重要意义。