Design gap specification in shimmed composite assemblies: trade-off between geometric deviation and shim thickness

In the assembly of large carbon fiber reinforced polymer (CFRP) structures, geometric deviations create interface gaps that must be filled with shims before fastening to avoid forced-fit stresses. The design gap, the interface clearance intentionally built into the nominal geometry, governs both the shimming demand and the post-assembly geometric deviation, yet it is currently specified by engineering judgment rather than analysis. If the design gap is too small, contact forces deform the parts and increase the deviation after fixture release. If it is too large, the shim thickness grows, adding weight and fastener bending stress. This paper treats the design gap as an explicit design variable in a non-rigid variation simulation and sweeps it across two populations of 100 virtual CFRP wingbox assemblies with different manufacturing signatures, using the method of influence coefficients with contact modeling. Geometric deviation decreases asymptotically with increasing design gap while shim thickness increases monotonically. For the wingbox studied, the deviation RMS falls from a baseline of 0.349 mm to a minimum of 0.247 mm at a design gap of approximately 0.3 mm, beyond which shim thickness keeps growing with no geometric benefit. The improvement depends strongly on the manufacturing signature, ranging from 29% for a population with a consistent spring-in bias to 66% for a signature-free population, while the optimal design gap remains approximately 0.3 mm in both cases. The results provide a quantitative, simulation-informed basis for design-gap specification in shimmed CFRP assemblies.