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芯层组成，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冲击而损坏。 每架飞机的标准维修手册都规定了允许的损坏程度，包括造成凹痕的深度和直径。 然而，由于这些损坏极限尚未优化，并且不特定于单个面板，因此它们可能导致过早维修或更换基于其设计载荷并不关键的面板。 拟议的研究旨在开发有效的数值模拟预测铝蜂窝试样在拉伸，压缩和弯曲损伤由于低速冲击的剩余强度的相对减少。 最终目标是能够识别全尺寸面板的关键多地点损坏配置，可以对这些配置进行测试以认证新的允许损坏。 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。 尽管已经表明，芯损伤和表面损伤都有助于剩余强度的降低，但是没有用于量化蜂窝芯中损伤的严重性的标准化度量。 拟议的研究首先旨在确定表面和核心损害之间的关系，同时确定如何损害的核心可以量化。 对受损板进行冲击后载荷的高保真度有限元模拟将用于确定因素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，然后将开发仅纳入关键的损伤功能，如细胞壁屈曲，这是需要时，使用均匀的核心表示的刚度校准。 有限元分析将得到物理测试的支持，物理测试的形式包括冲击载荷、静态载荷、3D激光扫描、破坏性切片和光学显微镜。 这项工作将最终导致更优化的允许损坏，节省成本和减少停机时间，因为减少了面板更换和维修。 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策，而不需要破坏性或专门的非破坏性检测技术。 具有计算效率高的有限元模型，可以预测剩余强度为指定的表面凹痕，而不需要校准损坏的核心属性将允许无限的配置，多个凹痕的全尺寸面板进行评估。

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.