【作者】 孟黎清；

【导师】 树学峰；

【作者基本信息】 太原理工大学 ， 固体力学， 2011， 博士

【摘要】 复合材料由于有重量轻、比强度高、比刚度大,可设计并耐腐蚀、抗疲劳的优良性能,蜂窝夹层复合材料具有明显的冲击敏感性,飞机蜂窝夹层结构缺陷与损伤直接影响飞机结构安全,其结构的冲击损伤形式是与冲击的能量水平密切相关的,冲击损伤是在持续时间短暂的强载荷作用下,夹层复合材料发生的变形和破坏,材料的损伤物理尺寸、材料的变形与其承受的应力及变形中的应变率和内能等变量之间出现复杂的关系。在冲击载荷作用下材料的存在多种动态破坏形式,冲击载荷在材料中引起的微观组织的变化有些是不可逆,对材料的力学性能有着明显的影响。基于均匀化理论,利用蜂窝芯材的表征单元,可以推导出蜂窝芯材的等效密度和面外等效弹性模量的表达式。蜂窝芯层压缩破坏的过程是以芯层单层斜壁的失稳开始,随着面外压缩载荷的增加,蜂窝芯层结构的破坏以双层壁达到最大承载能力失稳开始,并且按一定波长折叠,形成多层折叠结构。面外压溃实验用来研究不同材质及不同蜂窝尺寸的蜂窝芯层和蜂窝夹芯板,并对变形失效机理进行了对比。蜂窝芯层材料的压溃失效特征依赖于蜂窝基体材料特性以及蜂窝尺寸。蜂窝夹层梁蒙皮可以看做工字梁的翼缘,因蜂窝芯层在而内的拉伸刚度很小,结构的弯矩、弯曲正应力,面内压缩应力及扭矩主要由蒙皮材料承担,蜂窝芯层类似于工字梁的腹板,只承担剪力,对蜂窝夹层结构的蒙皮起支撑作用并使结构的弯曲刚度大幅度增加。复合材料蒙皮蜂窝夹层梁准静态三点弯曲实验,得到复合材料蜂窝夹层梁的载荷与跨中点挠度的理论关系,能够与实验结果吻合较好,并可预测出蜂窝夹层梁在三点弯曲状态下能够承受的最大的载荷。金属蒙皮蜂窝夹层梁在冲击载荷作用下的动力学特征和动态响应问题,可以通过冲击物的动力特性和蜂窝夹层梁的冲击动力特性共同描述。在弹道冲击和落锤冲击实验中,蜂窝夹层梁的厚度不同而导致冲击损伤与失效模式出现不同。通常情况下,结构承受动载荷的能力常常以结构永久变形的程度作为研究结构抗冲击性能的主要参数,但对于蜂窝夹层结构,其损伤及失效模式的复合性及复杂性,很难从外部损伤状况去了解结构强度退化的程度。蒙皮厚度不同的蜂窝夹层梁在不同冲击载荷作用下的动态响应结果可以发现,厚蒙皮蜂窝夹层梁在冲击损伤后结构完整性退化较为严重。复合材料蜂窝夹层结构在冲击载荷作用下的损伤模式主要包括蒙皮基体损伤、蒙皮纤维断裂、分层损伤和蜂窝芯层压溃失效损伤。准静态刻痕实验的目的是研究复合材料蒙皮蜂窝夹层结构板在压缩载荷作用下的局部刻痕失效模式,其失效模式依赖于蒙皮的厚度以及边界条件,蜂窝结构底面刚性支撑条件是影响失效破坏的主要因素。在落锤冲击载荷作用下,凹坑深度与冲击能量之间的关系,能够反映出随着冲击能量的增加损伤程度变化的对应性和规律性。对于厚蒙皮蜂窝夹层板,冲击损伤阻抗在蒙皮穿透之前较强,抗冲击性能良好,但在蒙皮断裂穿透之后抗冲击能力减弱,凹坑深度与冲击能量之间的关系可以在拟合实验数据的曲线中预测不同冲击能量载荷作用下蜂窝夹层结构的损伤凹坑深度。蜂窝芯材是良好的吸能材料,这与蜂窝芯材在面外压缩载荷作用下的应力应变曲线中很宽的屈服平台有关,在蜂窝芯材塑性压溃屈服变形过程中,蜂窝壁有规律地折叠褶皱变形过程中通过破坏吸收大量的压缩能。金属蒙皮蜂窝夹层梁在低速冲击载荷作用下吸收的冲击能量大部分耗散于结构材料的失效和破坏,只有很小部分耗散于结构的整体弹性变形。吸收能量分割与能量耗散方法为受冲击而发生变形和破坏的蜂窝夹层梁提供了量化的计算方法,这样的分析方法与结论对设计适当蒙皮厚度的蜂窝夹层结构起到指导作用。更多还原

【Abstract】 For the superior mechanical properties, such as light weight, high stiffness to weight and strength to weight ratios, fatigue and corrosion resistance, better dimensional stability, during the sandwich structures service life, impact damage is the primary reason to degenerate the structure loading carring capacity, such as the bird strike, severe hail storm, dropped tools, collision with the ground equipment and etc. Defects and damages of aircraft sandwich structure directly influence the security. Impact damage mode closely relate to the impact energy, sandwich structure deformation and the material physical damage size appear complicated relationship with the stress, the strain rate and the variable of internal energy. Under impact loading, material existence a variety of dynamic failure modes and cause the micro-structure irreversible changes.Based the homogenization theory can deduce the equivalent density and elastic modulus by the honeycomb core representative unit cell. Under the compressive loading, the core single-layer begin to instability till the double-layer wall achieve its maximal load-carrying capability, core wall start folding with certain wavelengths from the upper side to the lower side. The purpose of quasi-static out-of-plane compressive test is to studying the honeycomb core failure mechanism with different materials and different cellular size, results show honeycomb core failure feature depends on cellular matrix material properties and cellular size.Honeycomb sandwich beam skins can be regarded as I-beam flange cause honeycomb core in-plane tensile stiffness is very small, all the bending moment, bending normal stress, in-plane compression stress and torque moment carried by the skins of sandwich beam. Honeycomb core between the skins is similar to the I-beam web carrying only shear stress but increasing the structure bend stiffness substantially. Three-point bending experiment obtains the sandwich beam relationship between the load and the deflection of the beam mid-point, the theory results can fit the experiment results well. Projectile impact and drop impact test show the sandwich beam with aluminium skins dynamic response and the capacity of absorbed kinetic energy. Honeycomb sandwich beam with different thickness skins can cause different failure mode, for the complexity failure mechanism, it is difficult to understand the structural strength degradation form the external damage. The dynamic response and failure comparison analysis results show the sandwich beam with thicker skins in the test the structural integrity degenerate seriously.The failure modes of sandwich structure with composite skins mainly include matrix cracking, fiber breakage, delamination and honeycomb core crushing damage. Quasi-static indentation experiment is designed to study composite sandwich panel failure mechanism and find its failure modes dependent on skins thickness and boundary conditions, the bottom rigid support condition is the main factors affect the failure. In the drop impact experiment, impact dent depth variable can reflect the increasing of impact energy, the curve fitting the experiment results can predict the impact dent depth by the impact energy.Honeycomb core material is well energy absorption material. Under the out-of-plane compressive loading, wide yield platform in the stress-strain curve illustrate honeycomb core can absorb more energy by cellular wall regularly folding deformation during the plastic crushing yield process. The dynamic impact response of sandwich beam with aluminium alloy skins and Nomex honeycomb core has been studied using a aerodynamic gun projectile impact test under different velocity interval. An analytic model based on the partition energy method and experimental data is proposed to deduce the energy distribution of the the absorption energy from impact projectile initial kinetic energy Eabs via elastic bending deformation Ebend and irreversible damage consumed energy Edam. The results show that ratio Ebend/Eabsof the sandwich beam with thicker skins are lower than the beams with thinner skins. The essence of the difference is varing certain degrees undermine the structural integrity under the impact loading in the sandwich beams with different thickness skins. The similar results also present the irregular of the strain-time history curve account for the destroy of the structural integrity due to blocking the impact loading wave propagating in the structure.更多还原