Computational Modal Analysis of Prestressed Half Scale Generic Business Jet Substructures
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Computational modal analysis via finite element analysis is a well-established method for modal parameter estimation. The resultant modal parameters are crucial inputs for vibro-acoustic finite element analyses and require accurate modal models. The work presented here aims to provide insight into the techniques necessary to model business jet fuselages with prestressed skin panels through computational modal analysis of two substructures of a generic business jet design. The two substructures were assembled under manufacturing limitations which resulted in bending pre-stress within skin panels. The effect of bending pre-stress in skin panels was empirically determined through the use of experimental modal analysis of two geometrically similar assemblies with differing levels of bending pre-stress. This study found bending pre-stress in flat plates to increase natural frequencies while having a negligible effect on mode shapes. Preliminary computational modal analysis models of the substructures were developed without accounting for pre-stress to determine the significance of internal loading. These results revealed that pre-stress was only significant in one of the substructures. Furthermore, pre-stress effects were found to only be significant when skin resonant modes were present. A unique model updating technique was introduced to provide the same effects as bending pre-stress. The robustness of this technique to various design changes was tested via a sensitivity analysis. This test confirmed that the technique was effective in providing a consistent increase in skin natural frequencies without changing mode shapes. This test also revealed that the technique is limited in bandwidth depending on the design. The updating technique was applied to the substructure model that was previously confirmed to suffer from significant pre-stress effects. The updated model improved the natural frequency and mode shape correlation across all metrics. A frequency response analysis was performed on the updated model to provide further validation of the technique and model. The computational frequency response analysis provided sufficient proof that the updated model produced improved frequency response plots when compared to the preliminary model.