Fasteners Modeling for MSC.Nastran Finite Element Analysis 2000-01-5585
The distribution of loads between the components of a structural assembly depends not only on their dimensions and material properties but also on the stiffness of fasteners connecting the components. So, the accuracy of the finite element analysis is influenced much by the fastener representation in the model.
This paper describes an approach designed specifically for joints with connected plates modeled by shell elements located at plates mid planes. The procedure is based on definition of independent components of a fastener joint flexibility, analysis of each component, and their assembly to represent a complete plate-fastener system of the joint.
The proposed modeling technique differs from the traditional approach where all the connected plates are modeled coplanar. The traditional approach is based on calculating a single spring rate for a particular combination of fastener and plate properties. The application of this approach is limited by single shear joint of two plates or symmetric double shear joint of three plates. It cannot be used for other joint configurations and for joints with larger number of connected plates. The proposed procedure is free of those limitations.
Considering each fastener requires the creation of additional nodes and elements, it is obvious the manual use of this procedure is practically impossible for large models of aircraft structures that could have thousands of fasteners. A new MSC.Patran utility that automates the fasteners modeling was written and is described in the paper. It takes advantage of the CBUSH element formulation in MSC.Nastran and provides a user friendly and efficient tool that creates fasteners connecting a selected group of nodes.