Predictive Modeling of Bolted Assemblies with Surface Irregularities

Abstract

Bolted interfaces are a major source of uncertainty in the dynamic behavior of built-up assemblies. Contact pressure distribution from a bolt’s preload governs the stiffness of the interface. These quantities are sensitive to the true curvature, or flatness, of the surface geometries and thus limit the predictive capability of models based on nominal drawing tolerances. Fabricated components inevitably deviate from their idealized geometry; nominally flat surfaces, for example, exhibit measurable variation about the desired level plane. This study aims to develop a predictive, high-fidelity finite element model of a bolted beam assembly to determine the modal characteristics of the preloaded assembly designed with nominally flat surfaces. The surface geometries of the beam interface are measured with an optical interferometer to reveal the amount of deviation from the nominally flat surface. These measurements are used to perturb the interface nodes in the finite element mesh to account for the true interface geometry. A nonlinear quasi-static preload analysis determines the contact area when the bolts are preloaded, and the model is linearized about this equilibrium state to estimate the modal characteristics of the assembly. The linearization assumes that nodes/faces in contact do not move relative to each other and are enforced through multi-point constraints. The structure’s natural frequencies and mode shapes predicted by the model are validated by experimental measurements of the actual structure.