Residual strength-based damage limits for aerospace composite sandwich structures

复合夹层结构是由夹在两片薄面板之间的轻质核心组成，用于飞机部件，如地板、机翼和机身面板。这些面板通常由于工具掉落、冰雹、跑道碎片或货物操纵的低速撞击而损坏。每架飞机的标准修理手册都根据凹痕的深度和直径规定了允许的损坏。然而，由于这些损伤限制尚未优化，并且不是针对单个面板的，因此它们可能导致过早修复或更换基于其设计负载而不是关键的面板。该研究旨在建立有效的数值模拟，以预测由于低速冲击而造成的损伤对铝蜂窝板在拉伸、压缩和弯曲中的残余强度的相对降低。最终目标是能够识别全尺寸面板的关键多位点损伤配置，从而可以进行新损伤允许度认证测试。在航空航天应用中，铝夹芯板的低速冲击损伤仅通过残余表面凹痕来量化。尽管已有研究表明，蜂窝芯损伤和面板损伤都会导致残余强度的降低，但目前还没有标准化的指标来量化蜂窝芯损伤的严重程度。提出的研究首先旨在确定表面和堆芯损伤之间的关系，同时确定如何量化堆芯损伤。对受损面板的冲击载荷进行高保真有限元模拟，以确定面板和核心损伤的不同方面如何影响剩余强度。计算效率高的有限元模拟将只考虑关键的损伤特征，如细胞壁屈曲，从而减轻了使用均匀岩心表示时所需的刚度校准。有限元分析将通过冲击载荷、静态载荷、3D激光扫描、破坏性切片和光学显微镜等形式的物理测试来支持。由于面板更换和维修的减少，这项工作最终将在航空航天领域产生更优化的损伤允许范围，节省成本并减少停机时间。了解表面损伤和残余强度之间的关系，可以在不需要破坏性或专门的无损检测技术的情况下，在现场做出更好的决策。拥有计算效率高的有限元模型，可以预测指定表面凹痕的剩余强度，而无需校准损坏的核心属性，这将允许对全尺寸面板上多个凹痕的无限配置进行评估。

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.