Gap reduction vs. geometric deviation: a trade-off in composite assembly

In the assembly of large-scale composite structures, interface gaps arising from part deviations are commonly filled with shims to avoid induced stresses due to the forced contact. However, excessive shim thickness increases bending stresses in fasteners, while the process of gap minimization itself can distort the final assembly shape. This study presents a framework that integrates fastener load limits with fixture optimization to determine maximum allowable shim thickness and reduce interface gaps. Using a combination of finite element simulation, contact modeling, and numerical optimization, the method computes optimal fixture adjustments to minimize gaps while respecting structural constraints. A case study on a simplified wingbox section shows that, although local gaps are reduced by an average of 43%, global geometric deviations often increase, revealing a fundamental trade-of between gap minimization and geometric accuracy. The framework supports pre-manufactured shims and is particularly suited for digital-twin environments, enabling inspection data to inform fixture setup before assembly. Results highlight the importance of jointly considering structural and geometric objectives and suggest that multi-objective optimization and alternative shimming strategies can further enhance geometry assurance and joint integrity.