【作者】 安晶；

【导师】 孟松鹤；

【作者基本信息】 哈尔滨工业大学 ， 工程力学， 2010， 硕士

【摘要】 超高温陶瓷材料(UHTC)具有高熔点、高热导率、高强度和化学稳定性,使其成为高温环境下和反应气氛环境中的导弹和高超声速飞行器最具前景的候选材料之一,具有在如超声速飞行,重返大气层及火箭推进等极端环境下服役的潜力,能够应用于包括飞行器鼻锥、机翼前缘、发动机热端等各种关键部位和部件。然而,在制备、加工、装配过程中会在材料表面和内部形成缺陷,主要以表面裂纹的形式出现,将直接影响材料的应用范围和可靠性,研究表面裂纹对材料性能的影响,尤其是对材料抗热冲击性能,热冲击损伤行为的影响具有重要的意义。基于此,本文围绕含有表面裂纹的ZrB2+20%SiC+10%AlN(ZS10A),研究其热冲击损伤行为的影响因素及损伤机制,主要从以下几个方面进行:搭建UHTC局部快速升温试验平台,实现简单易行的材料局部快速热冲击方法。利用该设备对含表面裂纹缺陷的ZrB2+20%SiC+10%AlN超高温陶瓷材料进行局部热冲击试验。结果表明:在快速热冲击条件下,热冲击失效行为与升温速率有关,通过改变输入电流以获得不同的升温速率200℃/s ,500℃/s,800℃/s,得到临界破坏点温度与升温速率之间的线性关系;通过Abaqus有限元软件模拟分析,得到热冲击过程中试样的温度场及应力场分布,进一步分析试样失效机制,为在实际应用中材料性能的评估提供了依据。UHTC的热冲击损伤行为主要通过表面裂纹扩展程度和热冲击后剩余强度两个方面衡量。因此选用Vickers硬度仪预制表面裂纹表征材料表面缺陷,采用淬火法模拟瞬态热冲击过程,通过比较热冲击前后表面裂纹长度变化趋势、淬火-弯曲试验后材料剩余强度两方面表征热冲击损伤行为,主要从UHTC材料尺寸效应和热冲击环境参数两个方面研究影响UHTC热冲击损伤和抗热冲击性能的因素及规律。试验结果表明,裂纹扩展趋势随着试样厚度,初始裂纹长度的增加而增加,淬火温差,不同温度下重复淬火及同一温度循环淬火都将影响裂纹扩展趋势。材料临界淬火温差、剩余强度随着试样厚度,初始裂纹长度的增加而减小,揭示了材料热冲击损伤行为的影响因素。根据经典热传导方程,建立淬火过程准静态热应力模型,同时考虑应变速率对热冲击过程中热应力的影响,建立淬火动态热应力模型。针对表面裂纹特性,引入表面半椭圆形裂纹模型,结合动态热应力模型,得到动态热应力裂纹强度因子计算公式。针对ZrB2+20%SiC+10%AlN材料,进行淬火动态热应力、动态热应力强度因子以及应用包络线法计算裂纹扩展阻力。根据计算结果,淬火过程中的动态热应力远大于准静态热应力,因此淬火过程中需要考虑动态热应力的影响;应力强度因子为裂纹扩展提供趋动力,判断其与裂纹扩展阻力的关系是预测裂纹是否扩展的关键,计算结果与试验结果基本吻合,同时考虑材料的尺寸效应,通过计算得出试样厚度与裂纹扩展的关系,用于预测材料热冲击损伤行为。更多还原

【Abstract】 As ultra high temperature ceramic (UHTC) with high temperature resistance, thermal conductivity, strength and chemically stable at high temperature, it becomes one of the most prospective materials for application in ultra high temperature and reactive atmosphere, spaceflight, which has the potential to be used in extreme environments including those associated with hypersonic flight, atmospheric re-entry, rocket propulsion, key parts including the nose cones, shape leading edges and warm end of the engine. However, the preparation, processing and assembly will result in a lot of material internal or surface defects, mainly in the form of surface crack defects, will directly influence the reliability and the application of such materials. It is necessary to research on the material surface crack defects, especially for the thermal shock performance of material. Under this background, this work is focus on the influencing factors of material thermal shock damage behavior and damage mechanism with ZrB2+20%SiC+10%AlN (ZS10A)with Vickers indention, several aspects of problem for thermal shock are investigated as following:Equipment by fast local heating of UHTC was established, and it provides a simple and easy method for characterizing heating-up thermal shock property of UHTC. ZrB2+20%SiC+10%AlN were characterized by the new equipment. Results show that in the rapid thermal shock conditions, thermal shock failure behavior is related to the heating rate. We change the input current for different heating rate, 200℃/s ,500℃/s,800℃/s,and find there is a linear relationship between the heating rate and the temperature of critical damage. In addition, the thermal shock temperature and stress distribution of specimens during the process is obtained by Abaqus, in order to further characterization of sample failure mechanisms, and provide reliable data for performance assessment in practical application.Thermal shock damage behavior is mainly investigated by studying the surface crack extension and residual strength of the material under thermal shock. The surface crack is characterized by Vickers indention, and the transient thermal shock process is simulated with the quenching method. Thermal shock property of UHTC was investigated by comparison of surface crack length changes before and after thermal shock and residual strength of the material after quenching-bending test. Result shows that the crack growth increases with sample thickness and initial crack length, and the trend of crack growth will be influenced by quenching temperature difference, re-heating in different temperature and the same temperature. The critical temperature and residual strength of the material are decreased with the increase of sample thickness and initial crack length. The study reveals those factors influencing the thermal shock behavior of the material. According to the classical heat conduction equations, establish quasi-static stress model of quenching process. Then thermal shock model considering dynamical behavior and crack propagation model were established. According to the features of surface defects, introduce oval surface crack model and dynamical thermal stress model, calculation formula of dynamic thermal stress intensity factor of crack is obtained. Crack resistance of UHTC can be calculated by dynamical thermal stress, the dynamic thermal stress intensity factor and equation of ZrB2+20%SiC+10%AlN. Results show that dynamical thermal stress is much higher than quasi-static one, so effect of dynamical thermal stress should be considered during quenching test. The stress intensity factor provides driving force for crack propagation, the relationship between it and crack resistance is the key to predict whether crack extension, the calculation results and experimental results are basically identical, considering the effects of the size of material, the relationship between the specimen thickness and thermal shock damage is calculated to predict crack propagation.更多还原