Residual strength-based damage limits for aerospace composite sandwich structures

复合材料夹层结构由夹在两个薄面板之间的轻质核心组成，用于飞机部件，如地板、机翼和机身壁板。这些面板通常是由于工具跌落、冰雹、跑道碎片或货物操纵造成的低速撞击而损坏。每架飞机的标准维修手册规定了由此产生的凹痕的深度和直径所允许的损坏。但是，由于这些损坏限制尚未优化，并且不是特定于单个配电盘，因此可能会导致根据设计载荷对配电盘进行不重要的过早维修或更换。这项研究旨在开发有效的数值模拟来预测低速冲击对铝蜂窝试件的拉伸、压缩和弯曲损伤造成的残余强度的相对降低。最终目标是能够识别全尺寸电池板的关键多点损坏配置，这些配置可以进行测试，以验证新的允许损坏。*在航空航天应用中，低速冲击对铝夹层板的损伤仅用剩余表面凹陷来量化。尽管已经表明芯层损伤和面板损伤都会导致残余强度的降低，但对于蜂窝芯层中损伤的严重程度，没有一个标准的衡量标准。拟议的研究首先旨在确定表面和岩心损伤之间的关系，同时确定如何量化对岩心的损伤。高保真有限元模拟后冲击载荷的受损壁板将被用来确定不同方面的面板和核心损伤如何影响剩余强度。然后将开发计算高效的有限元模拟，只包括关键的损伤特征，如细胞壁屈曲，这减轻了当使用同质岩心表示时所需的刚度校准。有限元分析将得到物理测试的支持，包括冲击加载、静态加载、3D激光扫描、破坏性切片和光学显微镜。*这项工作最终将带来更优化的损坏容许量、成本节约和航空航天部门更少的停机时间，因为更换和维修的面板更少。了解表面损伤和剩余强度之间的关系将使现场能够做出更好的决定，而不需要破坏性或专门的非破坏性检查技术。拥有计算效率高的有限元模型，可以预测指定表面凹坑的剩余强度，而无需校准损坏的核心性能，将允许评估全尺寸面板上的多个凹坑的无限配置。********

Composite sandwich structures are composed of a lightweight core sandwiched between two thin face sheets and are used for aircraft components such as floors, wings and body panels. These panels are commonly damaged due to low velocity impacts from tool drops, hail, runway debris, or maneuvering of cargo. The standard repair manual for each aircraft specifies the damage that is allowed in terms of the depth and diameter of the resulting dents. However, since these damage limits have not been optimized and are not specific to individual panels, they may result in premature repair or replacement for panels that are not critical based on their design loads. The proposed research aims to develop efficient numerical simulations for predicting the relative reduction in the residual strength of aluminum honeycomb coupons in tension, compression and bending from damage due to low-velocity impact. The ultimate goal is the ability to identify critical multi-site damage configurations for full-sized panels that could be tested for certification of new damage allowables. ***In aerospace applications, low-velocity impact damage to aluminum sandwich panels is quantified solely by the residual surface dent. Even though it has been shown that both the core damage and the facesheet damage contribute to reductions in residual strength, there are no standardized metrics for quantifying the severity of the damage in the honeycomb core. The proposed research first aims to identify relationships between surface and core damage while determining how damage to the core can be quantified. High fidelity finite element simulations of post impact loading on damaged panels will be used to determine how different aspects of the facesheet and core damage influence the residual strength. Computationally efficient finite element simulations will then be developed by incorporating only the critical damage features such as cell wall buckling, which alleviates the stiffness calibration that is required when homogeneous core representations are used. Finite element analysis will be supported by physical testing in the form of impact loading, static loading, 3D laser scanning, destructive sectioning and optical microscopy. ***This work will ultimately result in more optimized damage allowables, cost savings and less downtime in the aerospace sector due to less panel replacement and repair. Knowing the relationship between surface damage and residual strength will enable better decisions to be made in the field without the need for destructive or specialized non-destructive inspection techniques. Having computationally efficient finite element models that can predict the residual strength for a specified surface dent without needing to calibrate the damaged core properties will allow for unlimited configurations of multiple dents on a full-sized panel to be evaluated. ********