【作者】 栾新刚；

【导师】 成来飞；

【作者基本信息】 西北工业大学 ， 复合材料， 2007， 博士

【摘要】 本文以航空发动机热端服役环境为应用背景建立环境试验模拟平台，对3DC／SiC在复杂耦合条件下的环境性能进行系统测试。然后基于试验过程中3DC／SiC的伸长、电阻变化等损伤过程信息和试验后复合材料在弯曲和拉伸试验中的载荷-位移曲线、微结构照片等损伤终态信息，研究3D C／SiC在航空发动机等效模拟环境、风洞模拟环境和热震模拟环境中的损伤过程、损伤模式、损伤机理及其影响因素。在此基础上建立3D C／SiC应力氧化损伤模型，并实现寿命预测。基于相似理论，采用分步模拟、逐步逼近的方法建立了航空发动机热结构材料环境性能试验模拟平台。该试验模拟平台由等效模拟系统和风洞模拟系统两部分组成，实现了对燃气温度、燃气流速、氧分压、水分压和熔盐分压等热物理化学环境以及蠕变、疲劳和热震疲劳等复杂应力环境的耦合模拟，并可以在线监测材料长度、电阻等多种过程演变信息，可以有效地模拟C／SiC在航空发动机热端环境中的失效机理，具有科学、简易和低成本等优点。研究了3D C／SiC在等效模拟环境中的应力氧化损伤过程及其机理，并分析了热物理化学环境参数、复杂应力参数对应力氧化损伤的影响。研究表明，3DC／SiC的应力损伤过程具有继承性和周期性。提出3D C／SiC分区承载、分区损伤的应力损伤模式，该模式不受应力类型影响，承载区域的大小取决于受力历史中的最大值。氧气是导致3D C／SiC损伤的主要原因，水蒸气和硫酸钠熔盐蒸气加速3D C／SiC的氧化损伤，应力参数影响复合材料损伤的速率。3D C／SiC界面层较薄时，适当的界面损伤提高复合材料强度。研究了3D C／SiC在航空发动机风洞模拟环境中的损伤过程及其机理，并分析了燃气温度、界面层厚度、纤维预制体结构和纤维类型对复合材料应力损伤的影响。研究表明，3D C／SiC的氧化机理随归一化应力不同而不同：归一化应力大于0．4时，氧化由C相反应控制；归一化应力小于0．4且大于0．25时，氧化主要由气体通过裂纹的扩散控制；归一化应力小于0．25时，氧化主要由气体通过尺寸小于裂纹的制备缺陷的扩散控制。3D C／SiC的强度下降速率随纤维预制体的编织角的增大而增大，随纤维抗氧化性能的提高而减小。燃气流速对3D C／SiC应力氧化损伤的加速与复合材料的氧化机理有关。氧化由气体扩散控制时，加速作用很剧烈；氧化由C相反应控制时，加速作用也很显著。研究了界面层较薄的3D C／SiC在热震模拟环境中的氧化损伤机理，并探讨了应力氧化对该复合材料热震损伤的影响。研究表明，700-1200℃热震时界面层较薄的3D C／SiC存在损伤饱和现象，且在热震60次后出现。热震是导致复合材料损伤的主要原因，应力氧化加速热震过程中的界面损伤。热震加速界面层较薄3D C／SiC复合材料的应力氧化损伤，加速程度取决于复合材料界面结合强度。基于3D C／SiC在等效模拟环境、风洞模拟环境和热震模拟环境中相似的损伤模式，建立了包括已承载区域、承载区域和未承载区域的统一的分区域应力氧化模型。基于分区域应力氧化模型，推导出了3D C／SiC在等效模拟环境中的寿命预测公式。该公式不仅与环境温度、环境总压、氧分压和应力有关系，还与材料的纤维特性、预制体结构和试样尺寸有关。另外还基于分区域应力氧化模型，推导出了3D C／SiC在风洞模拟环境中的寿命预测公式。该公式不仅包含了燃气流速、燃气组分、氧分压以及应力对复合材料寿命的影响，还包含了材料的纤维特性、预制体结构和试样尺寸等结构参数对复合材料寿命的影响，而温度、应力、总压及氧分压等的影响则通过裂纹宽度引入到公式中。计算表明预测公式在适用范围内具有较好的预测精度，预测结果与试验结果具有相同数量级。更多还原

【Abstract】 For the applications in high temperature aero-engine environments, the stressed oxidation of 3D C/SiC composite was investigated in simulating environments following four steps. Firstly, an experimental system simulating high temperature aero-engine environments was set up. Secondly, the degradation process, modes and mechanisms of 3D C/SiC composite were investigated by the change of length and electric resistance recorded during the stressed oxidation test, by the load-displacement curves recorded during tensile and bend test, and by the microstructural observation. Thirdly, the degradation model of stressed oxidation was suggested. Based on the model, the life prediction was achieved finally.An experimental testing system simulating environments of aero-engines was developed by a multiple-step approximated method on basis of similarity theory to simulate the hot section environments of aero-engines. The system included two subsystems: equivalent experiment simulation subsystem and wind tunnel experiment simulation subsystem. Using the simulating system, the materials were tested in the environments with various atmospheres, gas velocity, loads and temperatures or thermal shock parameters. The partial pressure of oxygen, water vapor and molten salt vapor were offered and controlled. Creep, low frequency fatigue and the interaction of them were conducted by a hydraulic servo frame. The evolution of material properties such as length and electric resistance were recorded during the testing. The degradation process of C/SiC composite in high temperature aero-engine environments was recurred effectively by the system.The degradation process of 3D C/SiC was studied in the equivalent experiment simulation subsystem, the degradation mechanisms and the effects of atmospheres and loads were discussed. The continuity and periodicity of the degradation process because of the capability of the composite in remembering the damage were found. The degradation mode of divisional load-bearing and divisional damage is suggested for the first time. The divisional area depended not on the kinds of load but on the max value of load history. Oxygen and water vapor were the major factors to induce the degradation of 3D C/SiC, Sodium sulfate vapor accelerated the degradation by facilitating the oxidation of SiC which resulted in the increase of interface strength. The interface strength affected not only the strength and the toughness of the composite, but also the degradation process. When the interface strength was higher than the appropriate value, the strength of 3D C/SiC will be improved because of the slight degradation of interface. The effect of kinds of load on the degradation speed of 3D C/SiC is carried out by changing the number and distribute of cracks.The degradation process of 3D C/SiC was studied in the wind tunnel, the degradation mechanisms and the effect of temperature, interlayer thickness, structures of fiber preform and kinds of fiber were discussed. It was found that the stressed oxidation mechanisms change with the stress. The oxidation was controlled by the reaction of C/O₂ when the normalization peak strength was higher than 0.4; the oxidation controlled by the gas diffusion through cracks when the normalization peak strength was between 0.25 and 0.4; the oxidation controlled by the gas diffusion through the defects which came from the fabrication and smaller than the cracks when the normalization peak strength less than 0.25. The degradation speed of 3D C/SiC increased with the increasing braiding angle of fiber preforms and decreasing with the increasing oxidation resistance of fibers. The effect of gas velocity on the degradation of 3D C/SiC depended on the oxidation mechanism of the composite. The degradation acceleration of gas velocity was violent when the oxidation was controlled by gas diffusion. The effect of gas flowing velocity was obvious when the oxidation was controlled by the reaction of C/O₂.The degradation process of 3D C/SiC was studied in thermal shock environment, the degradation mechanisms and the effect of load were discussed. The damage saturation occurred after the composite with less interlayer thickness shocked between 600℃and 1200℃for 60 times. The major factor resulting in degradation of the composite was thermal shock, the degradation was accelerated by the oxidation. The oxidation of the composite was accelerated by the load, however, the thermal shock resistance of the composite was improved by the degradation. The acceleration of thermal shock on the degradation of the composite with less interlayer thickness depended on the interface strength.An uniform stressed oxidation degradation model was established on the basis of the similar degradation mode of 3D C/SiC composite in equivalent simulation environments, wind tunnel simulation environments and thermal shock simulation environments. The life prediction equation for equivalent environments was deduced based on the uniform model, in which the parameters included temperature, atmosphere pressure, partial pressure of oxygen, kinds of load, normalization peak strength, properties of fiber, braiding angle of fiber preform and thickness of sample, et al.. The life prediction of wind tunnel environments was also established on the basis of the uniform model, in which the parameters included gas flowing velocity, gas components, partial pressure of oxygen, load, properties of fiber, braiding angle of fiber preform and thickness of sample, et al.. The effect of temperature, load, atmosphere pressure and partial pressure of oxygen were also introduced to the equation by the width of crack. It was proved by the experimental results that the prediction accuracy of the proposed equation was good in the researched environments.更多还原