Sensitivity Analysis of Geometric Imperfections in Aerospace structures

Launch vehicle structures are designed to withstand flight loads ensuring design adequacy after thorough structural assessment. In general, Preliminary FE model assumes perfect geometry of structures for analysis without considering profile imperfections arising during course of fabrication. These deviations, occurring during manufacturing, can significantly reduce the structure's load capacity. Traditional analysis often relies on simplified or theoretical imperfection models, which are different from "fabricated" hardware, leading to either underestimation or over estimation of margin for structural design loads. This paper presents a robust methodology for a cylindrical structure by directly incorporating measured manufacturing deviations into the finite element (FE) model. Thin-walled cylindrical structures, particularly those with complex stiffening patterns like isogrid panels, are sensitive to geometric imperfections. The subject hardware exhibited significant non-conformances of end-ring ovality and high profile deviations on the shell. The deviated geometry was generated using coordinate data of measured geometric profile across the surface of the hardware. This data was then used within the Abaqus© CAE environment to construct the true imperfect geometry of subject hardware. FE mesh of theoretical structure was projected onto this deviated surface, effectively transferring the real-world imperfections into the analysis model. Finally, a nonlinear static analysis using the Riks method was performed to predict the critical buckling load and assess the structural margin. This integrated approach provides a powerful framework for performing realistic structural integrity assessments, thus enabling informed decisions on the disposition of hardware with manufacturing non-conformances and reducing the uncertainty inherent in conventional analysis techniques.