Residual strength-based damage limits for aerospace composite sandwich structures

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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.