【作者】 刘斌；

【导师】 徐绯；

【作者基本信息】 西北工业大学 ， 固体力学， 2016， 博士

【摘要】 随着复合材料日渐主导航空器材料的使用,与之对应的复合材料胶接修补也得以大量应用。复合材料斜胶接修补是复材胶接修补中广泛采用的一种高效方式。现有文献对复合材料斜胶接修补结构研究不足,如理论方法精度较差、修补后的结构抗冲击性能研究欠缺等。因此,本文进行了四部分研究:基于半解析法的初步设计方法研究,有限元优化设计方法研究,面内承载能力、损伤机理及半解析法的验证研究,以及低速冲击损伤机理及冲击后承载能力研究。首先,进行了基于改进的半解析法MAM(Modified Semi-analytical Method)的初步设计方法研究。通常,采用有限元建立复合材料斜胶接结构的数值模型,计算获得不同铺层对接处胶层的应力分布。本文基于Harman最新推导出的控制方程,在复材中引入局部微元并按刚度分配原则得到微元的应力,利用有限差分法求解出胶层应力分布,得到改进的半解析法MAM,解决了Harman方法的几个不足。MAM的优点:相对Harman的方法,降低了0°层对接处胶层的剪应力峰值与FEM的误差;可得到全部胶层剪应力峰值;解决了Harman方法胶层剪应力沿斜接线分布不对称问题。第二,提出了采用优化补片铺层的方法来减少母材去除量。工程中一般采用减小斜接角度方法避免或降低应力集中,但是随着板厚的增加,需要去除的母板材料会急剧增多。本文通过Isight及Abaqus商用软件,采用试验设计的方法对斜接角度、胶层厚度、补片铺层角度各参数的敏感性和交互性进行了系统分析。并提出采用遗传算法优化补片铺层角度来降低胶层应力集中的思想。基于优化后的结果,本文提出利用蒙特卡洛描述抽样法进行了优化结果的稳健性分析,优选出了更稳健的补片铺层角度方案。针对试验件模型进行了铺层优化设计,在不改变斜接角度的前提下通过优化补片铺层使胶层应力集中显著降低。第三,针对复合材料斜胶接修补结构进行了拉伸承载能力及失效机理的试验及数值研究。首先通过复合材料斜接结构的面内拉伸载荷试验研究,获取结构的拉伸承载能力,掌握了拉伸载荷工况下的失效模式。然后通过建立有限元模型,对该结构的拉伸及压缩承载能力及损伤演化过程进行相关数值分析,发现了90°基体开裂、±45°基体开裂、分层、胶层内聚破坏及胶层粘附破坏等损伤模式出现的竞争先后顺序。最后通过试验结果进一步验证了本文提出的改进的半解析法MAM的合理性。第四,针对复合材料斜胶接修补结构进行了低速冲击损伤试验研究及相关数值模拟,考虑了不同冲击能量和冲击位置的影响。发现对于本文所研究的试样及边界条件,临界能量(23J)以下主要以复合材料层间损伤为主,临界能量以上除了层间损伤还有胶层的内聚破坏和粘附破坏,且胶层损伤由背部尖端位置沿胶层向里扩展。冲击过程分为4个阶段——阶段Ⅰ:冲击载荷线性增长无损伤阶段。阶段Ⅱ:冲击载荷突降复材损伤起始及快速扩展阶段。阶段Ⅲ:冲击载荷再次上升复材损伤继续扩展阶段。阶段Ⅳ:冲击载荷非线性下降无损伤回弹阶段。冲击能量23J以上时,冲击载荷-挠度曲线在第Ⅳ阶段伊始出现了冲击载荷突降现象,这是由于胶层损伤引起。胶接区域五个典型冲击位置中,中心点冲击损伤最大,冲击敏感性最高。第五,对复合材料斜胶接修补结构冲击后的拉伸承载能力及失效机理进行了试验和相关数值研究。发现低于临界能量(23J)时只有复合材料的损伤,冲击后承载能力无明显下降。高于临界能量(23J)时,冲击后剩余承载能力较完整修补结构的承载能力下降29%~40%,中心位置冲击后胶层损伤最大,胶接区域中心位置冲击后剩余承载能力下降最明显。冲击后拉压失效机理与原修补结构拉压失效机理不同之处是:冲击后拉伸时90°和±45°基体开裂较少;冲击后压缩时冲击产生的分层有进一步少量的扩展。最后,通过拟合冲击后拉伸承载能力的试验值,给出可供设计人员参考的复合材料斜胶接修补的经验公式。更多还原

【Abstract】 As composite material plays a leading role in aircraft,composite bonding repair has extensive application.Among composite bonding repairs,the scarf bonding repair is widely adopted and has high repair efficiency.Available literatures related to composite bonding repair have the shortages that existed theoretical methods have relative low accuracy and the research on the impact resistance for repair structure is defective.This thesis has four researching contents as follows: preliminary design method based on semi-analytical method;FEM optimization design method;in-plane load-bearing capacity,damage mechanisms and verifications of semi-analytical method based on experiments and simulations;and the damage mechanism of low velocity impact and tensile carrying capacity after impact.Firstly,preliminary design method based on MAM(Modified Semi-analytical Method)was studied.For composite scarf bonding structures,most reseachers use FEM to simulate stress concentration of adhesive,but seldom using analytical method and semi-analytical method.Based on Harman’s new governing equations,this thesis introduced local micro units that get stress by stiffness allocation critiron,finite difference method,and gave MAM method which could solve Harman’s shortages.Comparing to Harman method,MAM had some advantages of reducing the error of shear stress peak of 0°plies with FEM,being able to obtain all of the shear stress peak,and solvinging the problem of shear stress distribution asymmetry caused by Harman method.Secondly,optimizing plies angles were proposed to reduce parent material removing.Usually,decreasing scarf angle would be adopted to reduce stress concentration in engineering.But this method could lead to remove more material as the composite plate thickness increases.Sensibilities and interactions of scarf angle,adhesive thickness and patch plies angles were analyzed by Isight and Abaqus.Genetic algorithm was adopted to optimize patch plies angles to reduce the stress concentration of adhesive.Based on the results of optimizations,robustness analysis was carried out by using Mont-Calos descriptive sampling method,which got the optimized solution of plies angles.Keeping the scarf angle constant,the specimen was optimized and adhesive stress concentration was reduced obviously.Thirdly,carrying capacity and damage mechanisms of composite scarf structures were studied by experiments and simulations.The damage mode,damage mechanisms and benchmark of carrying capacity were investigated by experiments of in-plane tensile load.Then the damage propagations and carrying capacity under tensile and compressional loads were studied by simulations.Competing sequences of all kinds of materials damage such as 90°,±45 ° matrix crack,delamination,adhesive failure in structures were found from the simulations.Furthermore,Modified Semi-analytical Method(MAM)proposed in this thesis was validated by experimental results.Fourthly,experiments of low velocity impact and related simulations were investigated for scarf bonding repair of composite structures considering different impact energy and locations.It was found that smaller than the critical energy(23J)the major damage mode was delamination,and more damage modes appear upon critical energy including adhesive cohesive damage and adhesion damage which propagated from back tip to inside of adhesive.Impact procedure of composite scarf repair can be divided into 4 phases.In phase Ⅰ,impact load increased linearly and structure has no damage.In phase Ⅱ,impact load decreased steeply and the composite damage propagated rapidly.In phase Ⅲ,impact load increased again and the composite damage propagated sequentially.In phase Ⅳ,impact decreased nolinearly and no damge occurred.Greater than 23 J impact energy,impact load decreased steeply at the beginning of phase Ⅳ in the graph of impact load and deflection,which was caused by adhesive damage.In five typical impact locations,impact damage upon the center location was biggest,and impact sensitivity of center location was highest.Fifthly,carrying capacity and failure mechanism of tension after impact(TAI)were investigated by experiments and related simulations.Smaller than critical impact energy(23J),composite damage exists only,and TAI doesn’t decrease obviously.Upon the critical impact energy,TAI decreased about 29% to 40% comparing with undamaged composite scarf repairs,and adhesive damage area is biggest.From the research of impact locations,it was found that the most sensitive location is central point of adhesive bonding area.The difference of tensile and compressional mechanism between TAI and no damage specimen is that,90°,±45° matrix crack of TAI reduce;Compression after impact(CAI)causes a little delamination propagation of impact.Furthermore,some empirical formulas were given as design reference from the experimental carrying capacity of the tension after impact.更多还原