【作者】 赵萍；

【导师】 何清华； 李维；

【作者基本信息】 中南大学 ， 机械工程， 2011， 博士

【摘要】 单晶高温合金具有优异的高温力学性能,是制造先进航空发动机涡轮叶片的主要材料。涡轮叶片工作在高温、高压和高转速的环境中,并且受到反复的疲劳载荷作用,疲劳断裂是引起叶片断裂的一个主要原因。本文将晶体塑性理论和非线性运动硬化规律应用于低周疲劳研究,采用理论分析、数值模拟与试验研究相结合的方法,基于国内第一代镍基单晶高温合金DD3,以航空发动机单晶叶片为研究对象,对其单轴、多轴疲劳等多种疲劳形式进行了深入系统的研究,以期探索一种合理有效的航空发动机单晶叶片低周疲劳本构模型。论文的主要工作如下：1.在Hill屈服准则基础上,考虑单晶材料剪应力分量之间的相互作用对屈服的影响,提出一个更适合单晶的屈服准则(简称SC屈服准则),SC屈服准则能更精确地预测DD3三个主要承载方向的屈服应力。以此为基础,建立了一个基于晶体塑性理论和非线性运动硬化的航空发动机单晶叶片低周疲劳本构模型(简称NLDH本构模型),并给出了NLDH本构模型中运动硬化背应力、参考剪应力、分解剪切应力的确定方法,开发了基于有限元软件ABAQUS的用户子程序,实现了SC屈服准则和NLDH本构模型数值模拟工具的开发。2.应用NLDH本构模型对DD3单晶三种晶体取向的单轴低周疲劳标准试样的应力-应变响应进行了模拟,并开展了相应的试验研究。有限元分析方法模拟的应力-应变曲线和试验结果非常吻合,证明了NLDH本构模型在单晶单轴低周疲劳预测中的应用可行性。试验结果还表明：DD3单晶合金的单轴低周疲劳具有显著的各向异性,以[111]取向的寿命最长,[001]取向次之,[011]取向的寿命最短。此外,本文还首次探讨了棘轮效应对单晶低周疲劳寿命的影响。3.应用NLDH本构模型对带切口的DD3单晶试样低周疲劳应力-应变响应进行了模拟,并开展了不同温度和应力比条件下的带切口单晶试样的低周疲劳行为的试验研究。有限元分析结果表明：试样的切口尖端都存在着应力松弛现象,应力松弛的程度受加载条件和应力集中程度的影响,应力松弛能在一定的程度上减弱疲劳裂纹的扩展速率,从而延长疲劳寿命。同时,切口尖端出现了明显的棘轮效应,当塑性变形累积增加到一定的程度,切口就会有裂纹启裂直至试样最后断裂。试验结果表明：在相同温度和应力比条件下,切口疲劳寿命受应力集中程度和加载条件的共同影响,这与有限元分析结果是一致的。因此,在分析某一部件切口位置的疲劳寿命时,必须把部件切口部位的应力集中程度和其实际受力情况进行综合考虑。此外,采用有限元计算结果预测的切口试样低周疲劳寿命与试验得到的低周疲劳寿命基本一致,证明了NLDH本构模型可以应用到多轴应力状态下的低周疲劳行为研究。4.利用正交优化试验设计方法进行了非标准DD3单晶薄壁圆筒试样拉-扭复合加载下的试验方案设计,首次进行了非标准DD3单晶薄壁圆筒试样的设计和加工。首次进行了非标准DD3单晶薄壁圆筒试样的拉-扭复合加载低周疲劳试验,并利用电子扫描显微镜观察了断口,对试验结果进行比较全面深入地分析。利用NLDH本构模型对薄壁圆筒试样-多轴应力下的低周疲劳行为进行了有限元分析和寿命预测,有限元模拟结果能在一定程度反映拉-扭加载下的低周疲劳行为特性,寿命预测也有较高的准确度,这进一步验证了本文提出的NLDH本构模型在多轴低周疲劳分析中的可行性。5.采用国外先进的XactLIFETM系统和本文提出的NLDH本构模型分别对某单晶涡轮叶片出现的裂纹故障进行了深入的分析,两系统都能较为准确的预测该涡轮叶片的断裂位置。但XactLIFETM系统分析认为蠕变是裂纹形核的主要驱动力。而NLDH本构模型预测的断裂模式是疲劳断裂,这与该单晶涡轮叶片的断裂形貌更为一致,说明本文提出的NLDH本构模型可以更好的描述镍基单晶涡轮叶片的断裂机理。更多还原

【Abstract】 With superior high-temperature mechanical properties, single crystal blades have been introduced into most of the advanced aero-engines. Fatigue fracture is a pervasive problem because of elevated temperature, pressure and rotating speed and iterative fatigue loading. Taking aero-engine single crystal blade made by nickel-based single crystal superalloy DD3 as its objective, some main research on low cycle fatigue (LCF) in this paper is performed under the crystal plastic theory and non-linear dynamic hardening rule, by methods of theoretical analysis and numerical simulation combing with experiment research. After deep and systematic research on kinds of fatigues such as un-axial fatigue, notch fatigue and multi-axial fatigue, the writer tries to seek after a reasonable and effective aero-engine single crystal blade LCF constitutive model. The main works described in the paper are:(1) A more fitting yield rule (SC rule) was put forth to estimate yield stress along three major bearing orientations for single crystal based on Hill’s yield rule. An aero-engine single crystal blad LCF constitutive model (NLDH model) was set up by crystal plastic theory and non-linear dynamic hardening rule, and the calculating methods of dynamic hardening back stress, reference shear stress and resolved shear stress in the model were given. The user subroutine UMAT based on the finite element program ABAQUS was compiled, and the numerical simulation tool about SC rule and NLDH model was empoldered.(2) The NLDH model was used to predict the LCF behavior along DD3 three crystal orientations with finite element (FE) method. And then relevant tests were performed. The FE result was in good agreement with that of the experiment, which indicated that the NLDH model was feasible in nickel-based SC un-axial LCF. Furthermore, the experiment results showed that DD3 had obviously anisotropic for un-axial LCF, [111] had the longest fatigue life, [001] took second place and [011] was the shortest.The effect of ratcheting on LCF of SC was first discussed.(3) The NLDH model was used to predict the LCF behavior of SC notched specimens under different conditions. Stress relaxation and ratcheting at notch tip was observed from the FE simulation under fatigue loading. Stress relaxation could decrease the rate of crack propagation and prolong fatigue life. The presence of ratcheting showed that crack at notch tip would grow till rupture when plastic distortion cumulating to some degree. And then relevant experiments were carried out, and the scanning electron microscopy (SEM) was employed to investigate the fracture mechanism. The experimental results showed that both stress concentration factor and loading conditions affected notched specimens LCF life under the same temperature and stress ratio, which was in good agreement with the results of FE. Thus, stress concentration and loading conditions should be considered together in the analysis of fatigue life at some especial position.The predicted life with simulation results was true. All of these proved the feasibility of developed model applying to multi-axial LCF for SC.(4) The paper presented orthogonal experimental design (OED) method for the thin-walled cylindrical tensile-torque experiment schemes of DD3 SC. It was the first time that the design and manufacture of non-standard thin-wall cylindrical specimens were performed. The tensile-torque experiments at elevated temperature on DD3 SC thin-wall cylinder were successfully completed for the first time, and the SEM was employed to investigate the fracture mechanism too. All experimental results were studied entirely.The NLDH model was used to predict the LCF behavior of thin-wall cylindrical specimens, and the simulation results reflected the LCF characteristics under tensile-torque loading to some degree, the life prediction was in good agreement with the test results, which further demonstrated the feasibility of NLDH model in multi-axial LCF.(5) An in-depth analysis of the blade crack was undertaken using some advanced XactLIFETM system and the NLDH model respectively. The two methods both predicted the exact fracture position at turbine blade, but the fracture modes were different:creep is the major driver of crack with XactLIFETM system, and LCF rupture with the NLDH model. The later agreed with the fracture appearance, which showed the NLDH model could depict the fracture mechanism of SC turbine blade more exactly.更多还原