When doing finite element analysis upon the structure of Saab’s aeroplanes a coarse global model of mainly shell elements is used to determine the load distribution for sizing the structure. At some parts of the aeroplane it is however desirable to implement a more detailed analysis. These areas are usually modelled with solid elements; the problem of connecting the fine local solid elements to the coarse global model will shell elements then arises.
This study is preformed to investigate possible Global-Local methods to use for the structural analysis on Gripen. First a literature study of current methods on the market is made, thereafter a few methods are implemented on a generic test structure and later on also tested on a real detail of Gripen VU. The methods tested in this thesis are Mesh refinement in HyperWorks, RBE3 in HyperWorks, Glue in MSC Patran/Nastran and DMIG in MSC Nastran. The software is however not evaluated in this thesis, and a further investigation is recommended to find the most fitting software for this purpose. All analysis are performed with linear assumptions.
Mesh refinement is an integrated technique where the elements are gradually decreasing in size. Per definition, this technique cannot handle gaps, but it has almost identical results to the fine reference model.
RBE3 is a type of rigid body elements with zero stiffness, and is used as an interface element. RBE3 is possible to use to connect both Shell-To-Shell and Shell-To-Solid, and can handle offsets and gaps in the boundary between the global and local model.
Glue is a contact definition and is also available in other software under other names. The global respectively the local model is defined as contact bodies and a contact table is used to control the coupling. Glue works for both Shell-To-Shell and Shell-To-Solid couplings, but has problem dealing with offsets and gaps in the boundary between the global and local model.
DMIG is a superelement technique where the global model is divided into smaller sub-models which are mathematically connected. DMIG is only possible to use when the nodes on the boundary on the local model have the same position as the nodes at the boundary of the global model. Thus, it is not possible to only use DMIG as a Global-Local method, but can advantageously be combined with other methods.
The results indicate that the preferable method to use for Global-Local analysis is RBE3. To decrease the size of the files and demand of computational power, RBE3 can be combined with a superelement technique, for example DMIG.
Finally, it is important to consider the size of the local model. There will inevitably be boundary effect when performing a Global-Local analysis of the suggested type, and it is therefore important to make the local model big enough so that the boundary effects have faded before reaching the area of interest.
Source: Linköping University
Author: Ahlbert, Gabriella
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