【作者】 廖晓玲；

【导师】 李贺军；

【作者基本信息】 西北工业大学 ， 材料学， 2006， 博士

【摘要】 C／C复合材料综合了碳材料的高温性能和复合材料优异的力学性能，具有比重轻、高比强度和比刚度、优异的烧蚀性能和摩擦性能、良好的抗热震性能、低蠕变、高温下强度保持率高以及生物相容性好等一系列优异的性能，它既可以作为功能材料、又可以作为高温结构材料使用，是目前唯一可用于2800℃以上高温的复合材料。其中，以碳纤维三维编织物为增强体制得的C／C复合材料，由于其一次编织成型、不需要缝合和机械加工，纤维贯穿材料的长、宽、高三方向形成三维整体网状结构，可更有效地提高厚度方向的强度和抗冲击损伤的性能，从而克服了一维和二维增强体的结构缺陷，因此已成为航天、航空等高科技领域的重要新型材料。作为理想的高温结构材料，三维整体编织C／C复合材料在服役过程中不可避免地涉及疲劳加载的情况，而疲劳损伤的逐步积累会在某一循环次数下导致材料的突然断裂，这种断裂往往无明显征兆，危害性极大，因此对其疲劳行为进行研究具有十分重要的意义。在本论文中，以C／C复合材料疲劳行为及损伤扩展为核心内容，主要研究分为六部分：弯弯疲劳行为研究、拉拉疲劳行为研究、疲劳损伤演变及强化机制研究、切口试件疲劳行为研究、疲劳加载对断裂韧性的影响、材料热膨胀规律及高温强化机制的研究。研究了三维整体编织C／C复合材料的弯弯疲劳行为，测定了三维整体编织C／C复合材料的弯弯疲劳寿命(S-N)曲线以及疲劳加载过程中的载荷-挠度回滞曲线，通过试件实物照片和SEM疲劳断口分析，探讨了在不同应力水平下，材料的损伤模式。揭示了纤维与基体界面的滑动磨损在疲劳失效中起重要的作用，应力水平的高低控制着这种滑动磨损的程度和速度。通过夭折实验(实验进行到一定周次中止)以及SEM疲劳断口分析，研究了拉拉疲劳加载对三维整体编织C／C复合材料性能的影响。测定了三维整体编织C／C复合材料的疲劳寿命(S-N)曲线。发现拉拉疲劳加载不但使材料的拉伸强度增强，使其弯曲强度也得以强化，但疲劳强化存在有极限水平。并且，随疲劳循环次数的增加，材料的断裂也由脆性转化为假塑性模式。根据三维整体编织C／C复合材料自身的结构特点，选择X-ray衍射以及电阻检测作为无损检测手段，辅以SEM扫描电镜观察，对该材料在疲劳加载过程中的显微结构演化进行了的全面、深入的研究。发现了疲劳加载不但使纤维／基体之间的界面弱化，同时还具有增加碳层面间距、表观微晶尺寸减小以及消除界面残余热应力的功效，这些微观结构的变化都将使C／C复合材料的性能得以提高。提出了低应力作用下的疲劳加载实际上是C／C复合材料“疲劳训练”过程的观点。考察了切口三维整体编织C／C复合材料的静态力学性能以及疲劳行为。发现三维整体编织C／C复合材料在静载以及疲劳加载条件下对切口均无敏感性，而疲劳载荷则会进一步降低材料对切口的敏感性，在断裂过程中，会不断出现应力松弛和应力提高的交替现象。切口材料在静态弯曲时的断裂模式以复合材料的“裂纹扩展模式”为主，虽然在断裂时，材料也表现出界面损伤、纤维拔出的现象，但主要还是由垂直于加载方向的单一裂纹主导的断裂模式，表现出脆性断裂的特征。而切口试件的疲劳断裂，却是表现出多种损伤形式：裂纹扩展方向偏转，基体的松散、基体中的分层裂纹、裂纹尖端的塑性变形等等，材料的断裂模式以复合材料的“总体损伤模式”为主。研究了疲劳加载对C／C复合材料断裂韧性的影响。试验结果发现疲劳加载使C／C复合材料断裂韧性显著提高，提高幅度达4.5倍。全面的考察了三维整体编织C／C复合材料从室温到1300℃温度范围内的热膨胀行为，发现三维整体编织C／C复合材料从室温到100℃的温度范围内，CTE受材料密度和孔隙的制约，呈负膨胀状态；在高于100℃以后的温度范围内，材料的热膨胀变化规律则由碳纤维和碳基体的热膨胀行为以及界面热应力共同决定的，是纤维和基体相互限制、相互竞争的结果。与常温相比，三维整体编织C／C复合材料在高温下不但材料的承载能力显著提高了，材料的应变能力也有大的增加，但并不会随温度的升高而无限升高，在基体裂纹达到愈合状态时，强度将达到极限值。高温时由于材料内部摩擦阻尼和碳本身的内摩擦行为的共同作用，材料的内摩擦随温度上升而增加，使界面承载能力加大。更多还原

【Abstract】 Carbon/carbon composites(C/C composites) are the only material that can maintain high strength and toughness up to high temperatures of more than 3000 K. Thus, it has been considered as the most promising candidate for high temperature structural material for various purposes. As preferred high-temperature structural materials, the potential applications for C/C composites are bound to involve fatigue loads. Therefore, a clear understanding of the behavior of these composites under fatigue loads is necessary. In the recent years, research studies on the fatigue behavior of C/C composites have been performed; however, the results of these studies were still insufficient to establish the design criteria for load-bearing structures. In particular, the influence of fatigue loading on mechanical properties of C/C composites is not well understood. Thus, the usage of C/C composites is still limited to heat resistant components of spacecraft, rocket nozzles and nuclear reactors, etc. Therefore, in this present study, attention was mainly focused on the fatigue behaviors and damage evolution of 3D integral braided C/C composites, and some new contributions are summarized as follows:In the 3D reinforcement performs, Carbon fibers were braided into a rectangular strip(with width of 6.5mm and thickness of 3.5mm) by a four-step method using 1K T-300 carbon fibers, in which the braided angle was 22°and the fiber volume fraction was 45％. In the 3D reinforcement performs, Carbon fibers were braided into a rectangular strip(with width of 6.5mm and thickness of 3.5mm) by a four-step method using 1K T-300 carbon fibers, in which the braided angle was 22°and the fiber volume fraction was 45％. 3D integral braided C/C composites have been prepared by an isothermal chemical vapor infiltration(CVI) process, and their flexural and tensile fatigue behaviors were examined under load control at a sinusoidal frequency of 10 Hz.In the present study, the stress-fracture cycles(S-N) relationships of these composites in bend and tensile fatigue mode were obtained. The load-displacement curves and the fracture surfaces of specimens at various fatigue cycles were observed in order to identify the micro-damage induced during fatigue loading. Finally, the mechanisms responsible for strength enhancement after fatigue loading was discussed based on the experimental evidence.For the flexural fatigue behavior study, attention was focused on the effect of applied stress on damage mode of 3D integral braided C/C composites under bend-bend fatigue loading. The flexural fatigue limit of the C/C was examined. The fracture surfaces and cross-sections of the original and fatigued C/C composites at various level of applied stress were observed. It is revealed that the interfacial sliding abrasion play an important role in the fatigue failure process, and the extent and speed of sliding abrasion were controlled by the level of applied stress; for the tensile fatigue behavior study, the results show that the strength of specimens was enhanced with increase in tensile fatigue cycles and applied stress, but the fatigue enhancement was not boundless and would decline gradually.With the help of modern nondestructive testing (NDT) technology, the fatigue damage and fatigue enhancement have been investigated deeply and entirely. Considering the structure character of 3D integral braided C/C composites, X-ray diffraction method (XRD) and electrical resistance monitoring, assisted with scanning electron microscopy (SEM), is selected from several available NDT means after practical trials. It is testified experimentally that X-ray diffraction method (XRD) and electrical resistance monitoring is suitable for non-microcosmic defect detection during fatigue loading process of C/C composites. With these NDT technology, it has found that the augment of interlaminar distance, the decline of graphitization degree and microcrystalline stack and the release of remain thermal stress have contributed to the enhancement of strength of C/C composites under fatigue loading.The flexural performances of un-notch and notch C/C composites have been experimentally studied before and after fatigue loading. The sensitivity of notch and the domino offect of notch have been analyzed.In addition, the effect of fatigue loading on the fracture toughness and fracture behaviors of a 2.5 dimensional tightly woven C/C composites (2.5D C/C composites) was investigated by using the compact tension (CT) test. The fracture surfaces of CT specimens were observed using a scanning electron microscope (SEM) and a digital camera respectively. The compliance curves and the crack growth resistance curves (R-curves) showed that the fracture toughness of C/C composites was increased evidently after fatigue loading. The fiber-matrix interface was weakened during fatigue loading by analyzing the fracture morphology of the specimen. The weakened interfaces relaxed the stress concentration of the tip of cracks, increased the resistances of crack expansion process effectively, improved the fracture energy remarkably, prolonged the destroying time, and reinforced the damage capacity of the material such that the fracture toughness of the composites was enhanced by fatigue loading.Meanwhile, the flexural properties and damage modes have been investigated at 1000℃, 1300℃, 1700℃and room temperature respectively, and the strength improvement mechanism at the high-temperature was discussed according to the character of the thermal expansion rules and load-displacement curves as well as the fracture morphology of C/C composites. The results show that flexural properties at high temperature are super!or greatly to these at room temperature. The enhancement of internal" friction is the main reason for the increasing strength of C/C at high temperature.更多还原