Decision Framework for Selecting Shell and Solid Finite Element Models in Modal Analysis of Rib-Reinforced Vibration Test Fixtures

Modeling choice is key to modal analysis of rib-reinforced vibration test fixtures; however, systematic criteria for selecting shell or solid finite element (FE) models remain limited. This study proposes a decision framework that translates comparisons between shell and solid models into selection criteria. Five headlamp vibration test fixtures were modeled using shell and solid elements under identical material properties, boundary conditions, bonded contacts, and mounted point mass conditions. Natural frequency differences ranged from 1.81% to 11.21% for the first three modes, averaging 6.52%. The shell models reproduced the overall lower mode deformation trends of the solid models, particularly global bending and torsional modes. The dominant effective mass direction was consistent in most individual mode comparisons. Shell models required fewer nodes and elements, with analysis times of only 3.99% to 11.97% of those for solid models. Shell models are suitable for preliminary and iterative modal assessment, whereas solid models are recommended when the margin for resonance avoidance is small, dominant effective mass directions differ, or the representation of local three-dimensional stiffness is important. These results indicate that the proposed framework provides practical criteria for selecting shell or solid FE models for individual modes according to the analysis objective and design stage.