【作者】 栾旭；

【导师】 程靳；

【作者基本信息】 哈尔滨工业大学 ， 固体力学， 2009， 博士

【摘要】 金属热防护系统具有大尺寸、可重复使用、全寿命周期成本低的突出优势,是可重复使用运载器次高温区大面积防热的首选热防护系统。金属热防护系统主要由三部分组成:金属蜂窝夹芯板、多层反射隔热结构和支架连接结构。作为金属热防护系统的重要组成部分,金属蜂窝夹芯板的性能和结构可靠性关系到金属热防护系统的使用寿命和可重复使用运载器的安全。金属蜂窝夹芯板具有质量轻、比刚度大、比强度高、隔热性能优良等优点,因此被广泛地应用于火箭、导弹、飞机、卫星等航空航天领域。蜂窝夹芯板作为可重复使用运载器的主要表面承力部件,不可避免的承受气动力、气动热、交变疲劳载荷、冲击载荷等。由于蜂窝夹芯板是复合结构,各种模拟方法都要以准确的材料参数为基础,而且金属蜂窝夹芯板在使用过程中还会受到疲劳载荷和不同程度的冲击,材料在交变载荷下的疲劳性能和在动态载荷作用下的力学性能并不清楚。另外,面板与蜂窝芯子之间的脱粘缺陷是蜂窝夹芯板最普遍也是危害性最为严重的缺陷之一,缺陷的位置位于结构内部,很难观察和检测,在损坏或断裂之前几乎没有什么先兆,其破坏具有突然性,往往对结构造成致命威胁,形成安全隐患。因此,准确定位面板内部缺陷的位置,并对含脱粘界面的蜂窝夹芯板的强度进行预报,以及分析脱粘界面裂纹的扩展过程对认识金属蜂窝夹芯板的损毁机制有很大的帮助。由于金属热防护系统的研究在国内尚处在起步阶段,蜂窝夹芯板的力学特性并不清楚,对蜂窝夹芯板失效过程的表征与评价方法比较有限。本文针对蜂窝夹芯板在使用过程中的力学性能进行了理论、实验以及数值模拟等方面的研究,研究了其在不同温度下的力学性能、疲劳性能、冲击性能,并预报了界面脱粘的强度,研究了脱粘界面裂纹的扩展问题,为蜂窝夹芯板的结构设计与开发等奠定了理论基础,对工程应用具有重要意义。在第二章中,利用实验的方法对蜂窝夹芯板的力学性能进行了测试。采用数字散斑相关技术和时间序列散斑检测技术,对蜂窝夹芯板共面拉伸进行了实验研究,测得了共面拉伸的弹性模量,与利用等刚度法计算得到的蜂窝夹芯板的等效弹性模量进行对比,验证了数字散斑相关技术的有效性和实用性。利用不同温度下的异面压缩实验和三点弯曲实验给出了蜂窝夹芯板的力学性能随温度的变化规律。随着温度的升高,异面压缩的弹性模量、平台应力以及三点弯曲强度都不同程度的降低;随着三点弯曲跨距的增大,屈服载荷减小。针对脱粘缺陷,进行了基于电子剪切散斑干涉技术的无损检测实验研究,并利用相移技术、灰度提取与二值化处理技术,得到了较为理想的结果。针对蜂窝夹芯板的三点弯曲力学性能,建立了含缺陷蜂窝夹芯板的有限元模型,基于双线性内聚力模型和B-K准则,模拟了含脱粘缺陷的蜂窝夹芯板的力学性能,通过计算表明界面层间脱粘会导致应力集中,这些由脱粘引起的应力集中是导致蜂窝夹芯板在实验过程中力学性能显著下降的根本原因,并进行了验证性实验,证明了上述模型的有效性。在疲劳实验中,发现了蜂窝夹芯板的疲劳寿命由于蜂窝芯子的方向性而不同,且高温的疲劳寿命要高于室温的疲劳寿命。在所承受载荷接近材料服役极限载荷的情况下,材料的疲劳破坏成为蜂窝夹芯板失效的主要控制因素,裂纹在低于材料屈服应力的反复载荷作用下成核,并发生亚临界扩展,当裂纹长度达到临界值,裂纹发生失稳扩展,导致整体破坏。而当材料承受的载荷远小于服役极限载荷的条件下,由于应力水平低于或接近裂纹成核的门槛值,材料的疲劳破坏很难发生,但在蜂窝夹芯板制备的过程中不可避免的存在一些微缺陷(焊缝、胶接处),这些缺陷在疲劳载荷下成为新的裂纹源,裂纹扩展导致结构连接处发生破坏,产生应力集中,从而导致整体失效。采用动态压缩实验装置霍普金森压杆和Instron试验机分别研究了蜂窝夹芯板的压缩力学性能,比较了动态载荷与准静态载荷作用下的力学性能。由于蜂窝夹芯板具有较明显的应力不均匀性,采用波形整形技术,将入射波进行平滑处理。通过比较实验结果,选择了尺寸为Φ10mm×1mm的软质材料作为整形器,实现了试件在变形过程中处于常应变率变形状态。通过霍普金森压杆实验得到了蜂窝夹芯板在2500/s-3850/s应变率变化范围内的应力应变曲线。结果表明蜂窝夹芯板是率相关材料,动态最大应力随着应变率的增大而增大。分别测试了20℃、200℃、400℃和800℃下的应力应变曲线,结果发现在800℃以下,在相同的应变率条件下,温度对动态最大应力基本没有影响。由于层间脱粘是蜂窝夹芯板力学性能降低的主要因素之一,本文研究了等效蜂窝夹芯层板的界面裂纹扩展过程,针对脱粘分层过程中出现的裂纹扩展建立了断裂动力学模型,以复变函数论为基础,应用自相似函数的方法将所讨论的问题转化为Riemann—Hilbert问题,得到了运动变载荷Pt n /x n、Px n +1 /tn分别作用下Ⅲ型非对称动态界面裂纹扩展的裂纹尖端的应力、位移、动态应力强度因子解析解的一般表达式。更多还原

【Abstract】 Metal thermal protection system has the outstanding advantages, such as large-size, reusability, low life-cycle cost, and it is the first choice of widespread passive thermal protection system of reusable launch vehicle in sub-high-temperature area. Metal thermal protection system is mainly composed of three parts: metal honeycomb panel, multilayer reflection insulation structure and connecting structure. As the main component of metal thermal protection system, the reliability of performance and structure of metal honeycomb panel structure is related with lifecycle of metal thermal protection system and safety of reusable launch vehicle.Metal honeycomb panel is widely used in the rockets, missiles, aircraft, satellites and other aerospace area for its light weight, high specific stiffness, high specific strength, and excellent heat-shielding performance. Honeycomb sandwich panel is the main surface load-bearing component of reusable launch vehicle, it has to bear the aerodynamic force, aerodynamic heat, alternating fatigue load, impact load inevitably. As the honeycomb sandwich panel is a composite structure, and there is a big difference between different forecasting methods, in order to obtain accurate material parameters, it is necessary to operate the mechanical properties test. Moreover, in use process metal honeycomb panel will be attacked in various extents and the mechanical property of materials under fatigue loads and dynamic loads is not clear, therefore, it is necessary to study the fatigue property and the impact property at different temperatures. The debonding defects between panels and honeycomb core are one of the common and most serious defects of honeycomb panel. The defects are in the internal area, so it is hard to observe and detect, there are no sign before the damage or fracture. The suddenness of its destruction often cause lethal threat to the structure and form a security risk. Thus, it is a great help to understand damage mechanism of metal honeycomb panel with the accurate positioning of internal defects and forecast the intensity of metal honeycomb panel with debonding interface, as well as the analysis of crack propagation of debonding interface.As the study of metal thermal protection system is still in its developing period, thus the failure mechanism of honeycomb sandwich panel is not clear enough, and there are relative limited characterization and evaluation methods of these failure processes. This paper illustrated the theory, experiment and numerical simulation of mechanical properties in honeycomb sandwich panel’s using process. The problems of mechanical properties, fatigue properties, impact properties, interface debonding strength prediction and crack propagation of debonding interface were illustrated in this paper as well. It provided some theoretical basis for honeycomb sandwich panel design and development, at the same time it was significant to engineering application.In chapter two, experimental method was used to test the mechanical properties of honeycomb sandwich panel. Using digital speckle correlation technology and time-series speckle detection technology, flatwise tension of honeycomb sandwich panel was studied and the tensile modulus of in-plane has been got. Compared with the honeycomb sandwich panel efficient elastic modulus calculated with the equivalent stiffness method, the efficiency and practicability of digital speckle correlation technology were verified. Using flatwise compression experiments, three-point bending experiments; it generalized the mechanical performance parameters of honeycomb sandwich panel under different temperature. With increasing temperature, elasticity modulus of flatwise compression, platform stress and three-point bending stress intensity were reduced in various degree; with the increasing span of three-point bending, yield load decreased.Aiming at debonding defects, non-destructive testing experiment based on electronic speckle shearing pattern interferometry technology was conducted. In the experiment, phase-shifting technique, gray-scale extraction and binarization processing technology were added, and more satisfactory results were achieved. In view of three-point bending mechanical properties of honeycomb sandwich panel, the finite element model of honeycomb sandwich panel with defects was established. Based on the bilinear cohesive model and B-K criterion, mechanical property of honeycomb sandwich panel with debonding defects was simulated. Through calculations, it is showed that the interface debonding will lead to stress concentration, which is the main reason of significant drop of mechanical properties in the honeycomb sandwich panel test. And this coincides with the experimental results, it verify the validity of the model. In the fatigue experiments, it was found that the fatigue life of honeycomb sandwich panel was different due to the different direction of honeycomb core, and the fatigue life in high temperature was higher than in room temperature. When the bearing load was near to the limit load, fatigue failure of materials was the main control factors for the failure of honeycomb sandwich. The crack became nucleus under the repeat function of loads below material yield stress, and caused subcritical expansion. When the length of crack reached critical value, the crack propagated unstably which led to the overall damage. When the bearing load was smaller than the limit load, the material fatigue damage was difficult to occur because the stress was lower or near to the threshold of crack nucleation. While there were inevitable micro-defects (weld, bonding locations) in the preparation process of honeycomb sandwich panel, these defects would become new source of crack under the fatigue load. The crack propagation would lead some damage in the connecting area, result in stress concentration, and lead to an overall failure.Using dynamic compression experimental device Split Hopkinson Pressure Bar and the Instron testing machine to study the compression mechanical properties of honeycomb sandwich panel, and compare the mechanical properties under the effect of the dynamic load and quasi-static load. As the honeycomb sandwich panel with a more pronounced heterogeneity of the stress, using pulse shaper technology and smoothing the incident wave, by comparing the experimental results, selected the size of theΦ10mm×1mm soft materials as pulse shaper, and realize deformation of constant strain rate of the specimen during the deformation process. Using Split Hopkinson Pressure Bar experiment, the stress strain curve of honeycomb sandwich panel within the 2500/s-3850/s strain rate was obtained. The results showed that the honeycomb sandwich panel is rate related material, and strain rate had a significant effect on dynamic maximum stress of honeycomb sandwich panel. Dynamic maximum stress increased with the strain rate. The test was operated under 20℃, 200℃, 400℃and 800℃and examined the stress strain curve, it was found that at 800℃and the same condition of strain rate, the temperature did not affect the dynamic maximum stress.Interface debonding is one of the reasons of honeycomb sandwich panel the mechanical properties reduction. This paper studied the crack propagation process of honeycomb sandwich panel interface, and established the crack dynamic model to crack propagation in the process of debonding. Based on complex analysis, using self-similar function, the problem is transformed into Riemann—Hilbert problem, the general expression of the analytical solution of the crack tip stress, displacement and dynamic stress intensity factor of typeⅢasymmetric dynamic interface crack propagation under the effect of movement varying load Pt n /x n、Px n +1 /tn was got.更多还原