【作者】 王遵；

【导师】 肖加余；

【作者基本信息】 国防科学技术大学 ， 材料科学与工程， 2007， 博士

【摘要】 复合材料胶接修复是一种新型、高效、低成本的飞机损伤构件延寿技术,已成功地在多种军用和民用飞机上获得应用。其中,采用复合材料单面补强含裂纹损伤构件是该项技术的重要研究内容,主要用于修复难以或无法拆卸的飞机损伤金属构件（如机身或机翼蒙皮等部位）。经高温胶接修复后,复合材料单面补强构件在常温下会因复合材料补片与金属母体之间的热失配而产生显著的残余热应力。复合材料单面补强试件为不对称结构,在残余热应力或外加载荷的作用下试件会因中性轴的偏移而发生弯曲。在残余热应力和弯矩的共同作用下,复合材料单面补强构件的力学行为研究就变得非常复杂。本文采用实验研究和理论分析相结合的方法,开展了用高性能碳/环氧复合材料单面补强含裂纹铝合金薄板的相关研究。着重考察了复合材料单面补强铝合金薄板试件中的残余热应变,并采用解析法和数值计算分别预测了单面补强试件中的残余热应力/应变分布,最后考察了残余热应力与弯矩共同作用对复合材料单面补强试件准静态拉伸性能和疲劳行为的影响。本文的成果包括:（1）复合材料单面补强铝合金薄板残余热应力/应变的实验研究与理论分析考察了固化制度及加载历史对复合材料单面补强试件应力释放温度的影响,并在胶粘剂固化动力学分析与Maxwell粘弹性模型的基础上,建立了预测多步法固化制度下复合材料单面补强试件应力释放温度的模型。研究结果表明,随着固化温度的升高,应力释放温度与固化温度之差逐渐增加;加载历史对应力释放温度的影响较小,应力释放温度的降低主要集中在疲劳载荷作用的初始阶段;应力释放温度预测模型在预测两步法固化制度对应的应力释放温度时,能够将偏差控制在7.4%以内。通过实验和数值分析较全面、系统地考察了固化制度、补片设计、裂纹长度及界面脱粘等因素对复合材料单面补强试件热变形挠度和残余热应力/应变的影响。实验及三维有限元计算结果均表明,复合材料胶接修复在被修复构件和补片中引入显著的残余热应力/应变;采用两步法固化制度可以有效降低残余热应力/应变;铝合金板中残余热应力/应变集中现象会随着补片宽度的增加而得到改善;补片铺层设计能够有效减小试件中的残余热应力/应变;除裂纹及其尖端附近外,裂纹对复合材料单面补强试件中的残余应力/应变场影响较小;界面脱粘可显著减小铝合金板胶接界面在脱粘区域内的残余热应力。有限元分析还发现,增加胶层厚度或补片与铝合金板的刚度比能够降低铝合金板胶接界面上的残余热应力。改进了Hart-Smith双面搭接接头模型,引入了弯矩的影响,使之适用于复合材料单面补强构件的分析。在计算残余热应力时,双金属片模型和改进的Hart-Smith模型计算结果与实验值出现一定偏差,但解析模型能够有效预测不同固化制度下,复合材料单面补强试件中残余热应力及其分布的变化趋势。（2）残余热应力与弯矩共同作用对复合材料单面补强含裂纹铝合金薄板力学性能的影响采用Rose模型和Wang-Rose模型考察复合材料单面补强半无限板和有限宽板时均发现,复合材料单面补强能够有效降低裂纹尖端的应力强度因子。修正了Wang-Rose模型,使应力强度因子K包括了残余热应力和弯矩的影响。研究发现,当应力释放温度为36.2℃时,残余热应力对应力强度因子没有贡献;当应力释放温度大于36.2℃时,残余热应力对应力强度因子的影响随着外加载荷的增加而减小。经过单向碳/环氧复合材料单面补强后,含裂纹铝合金板的准静态拉伸断裂载荷可恢复到完好铝合金板的90%以上,残余热应力的存在降低了复合材料单面补强试件的承载能力,但降低幅度较小。提出了等效模量的概念,并采用其表征复合材料单面补强试件的刚度恢复效果,完善了复合材料修复构件的评价标准。在拉伸初始阶段,复合材料单面补强试件的弯曲变形逐渐回复,补强区域铝合金板表面应变随着应力的增加而快速增加;随着载荷的增加,未修复的含裂纹铝合金板的等效弹性模量不断减小,而复合材料单面补强试件补强区域的等效模量逐渐增加。采用等效模量还可以表征不同固化制度对修复结构刚度特性的影响规律。确定了含裂纹铝合金板的Paris公式的材料常数,材料常数C和m分别为2.55×10^-10及2.85。复合材料单面补强能够显著提高含裂纹铝合金板的疲劳性能,残余热应力显著缩短了复合材料单面补强试件的疲劳寿命。实验结果表明,单向碳/环氧复合材料单面补强能够使含裂纹铝合金板的疲劳寿命延长11倍以上。采用修正的Wang-Rose模型及Paris公式对复合材料单面补强含裂纹试件的疲劳寿命进行预测,理论计算值与实验测量值吻合较好。还发现,在疲劳应力比较低时,残余热应力使试件的疲劳寿命显著缩短;在疲劳应力比较高时,残余热应力对试件疲劳寿命的影响较小。更多还原

【Abstract】 Adhesively bonded composite repair, a novel, efficient and cost-effective method to extend the service life of the damaged aircraft component, has been widely used in the military and commercial aircrafts. Reinforcing the cracked component by single-sided composite patching technology is gaining more and more concern when it is difficult or not possible to access both sides of a component, such as the aircraft fuselage or wing skin. Composite repair method has been shown to be very promising owing to the light weight, high strength and stiffness of the composites. While significant thermal residual stresses, resulting from the mismatch in the thermal expansion coefficients between the composite patch and the metallic substrate, arise in the single-sided composite patched components after cured at an elevated temperature. The single-sided composite patched specimen （SSCPS） may experience a considerable out-of-plane bending induced from thermal residual stresses or external loadings due to the load-path eccentricity. Therefore, thermal residual stress and bending are key features for the design of single-sided composite patching. Interactions between thermal residual stress and bending present a great challenge in the study of the mechanical behaviors of the SSCPS.In this paper, studies on the properties of the cracked thin aluminum alloy plate with a single-sided repair by high-performance carbon/epoxy composite patch is carried out experimentally and theoretically. Measurement of the thermal residual strains in the SSCPS is focused on, and the thermal residual stresses/strains in the SSCPS are predicted by analytical and numerical methods, respectively. Effects of the interactions between thermal residual stress and bending on the quasi-static tensile properties and fatigue behaviors of the SSCPS are also discussed.（1） Experimental and theoretical studies on the thermal residual stress/strain in the SSCPSEffects of cure cycle and loading history on the stress free temperature （SFT） of the SSCPS is discussed, and a prediction model for the SFT of the SSCPS after multi-step cure cycle, based on the adhesive cure kinetics and Maxwell viscoelastic model, is developed. The results show that the difference between the stress free temperature and the cure temperature rises as the cure temperature increases, and loading history slightly affects the SFT. The contributions of the loading history to the decrease of the SFT are mainly within the early stage of the fatigue loading. When predicting the SFT of the SSCPS prepared under the two-step cure cycle, the SFT prediction model can control the error within 7.4%, compared with the experimental data.Effects of some important parameters, including cure cycle, patch design, crack length and interface disbond, on the specimen deflcction and the thermal residual stress/strain are comprehensively investigated, experimentally and numerically. Experimental data and three-dimensional finite element calculation results indicate that significant thermal residual stresses/strains are introduced into the SSCPS by the temperature difference between the SFT and ambient temperature. Two-step cure cycle and patch design are efficient ways to decrease the thermal residual stress/strain. Thermal residual stress/strain concentration can be reduced with the increase of patch width, therefore a full-width composite repair is recommended. Crack in the aluminum substrate greatly affects the stress/strain field around the crack and its tip, while the stress/strain field in the other region of the SSCPS is less sensitive to the crack. Interface disbond can remarkably decrease the thermal residual stress/strain in the aluminum substrate near the adhesion interface within the disbond region. It is also found that the thermal residual stress can be decreased with the increase of the adhesive layer thickness or the patch to substrate stiffness ratio.Hart-Smith double-lap joint model is improved to describe the thermal residual stress distribution in the SSCPS, and the improved Hart-Smith model and the bi-metallic strip model are used to predict the thermal residual stress in the SSCPS. The comparison between the predicted results and the experimental data shows a relatively large difference. While the analytical models are efficient to predict the variational tendency of thermal residual stress distributions at various cure cycles.（2） Effects of the interactions between thermal residual stress and bending on the mechanical properties of the SSCPSRose model and Wang-Rose model are adopted to investigate the stress intensity factors （SIF） in the single-sided composite patched semi-infinite and finite-width aluminum plate. The calculated results show that single-sided composite patching can significantly reduce the SIF at the crack tip.Wang-Rose model is modified to involve the effects of thermal residual stress and bending on the SIF. It’s found that thermal residual stress makes no contributions to the SIF when the SFT reaches 36.2℃, however the effect of thermal residual stress on the SIF diminishes with the increasing external loadings when the SFT exceeds 36.2℃. The quasi-static tensile strength of the cracked aluminum plate can recover over 90% of the fracture strength of the perfect aluminum plate after the single-sided reinforcement by unidirectional carbon/epoxy composite patch. The tensile strength of the repaired specimen was decreased by the thermal residual stress.Equivalent modulus, which perfects the evaluation criteria for the composite repaired components, is defined to characterise the stiffness-recovery capability of the SSCPS. During the early stage of the tensile loading, the strains on the aluminum plate surface within the bonded region increase rapidly with the increase of the loading due to the recovery of the specimen warpage. With the further increase of external loading, the equivalent modulus of the unpatched cracked aluminum decreases, however the equivalent modulus of the composite patch reinforced region increases. It is also found that equivalent modulus is efficient to describe the effects of cure cycle on the stiffness of the SSCPS.The material constants of Paris law, C and m, are determined, and they are 2.55×10^-10 and 2.85, respectively.Single-sided composite patching can significantly improve the fatigue properties of the cracked aluminum plate, and the thermal-mechanical coupling shows adverse influence on the fatigue life of the SSCPS. The experimental results indicate that single-sided composite patching can extend the fatigue life of the cracked aluminum plate over 11 times. Modified Wang-Rose model and the Paris law are efficient to investigate the effects of thermal residual stress on the fatigue life. It is found that the theoretical predictions of the fatigue life agree well with the experimental measurement results. The fatigue life can be significantly shortened due to the thermal residual stress when the stress ratio is low. Effects of the thermal residual stress on the fatigue life of the SSCPS become small when the stress ratio is high.更多还原