Comparison of finite element modelling techniques applied on Gripen structure

Sammanfattning: The goal of the thesis was to investigate and compare internal loads and bolt loads from different modelling methods using finite element method, from global to detailed, applied to a topologically correct air brake of a JAS 39 Gripen. The finite element method is a common used method for analysing strength properties of different structures. The finite element models used in finite element analyses can be modelled with different levels of details. The different finite element modelling methods were initially tested on a simplified structure consisting of an I-beam bolted to a plate. To further reinforce the structure, an additional I-beam was placed beneath the plate perpendicular to the upper one. Five different approaches to model the structure was made and internal loads, as well as axial bolt loads, were investigated. The results from the simplified structure was later used to plan and understand the modelling methods used for a more complex structure. This complex structure, representing the air brake, consisting of a plate attached to an I-beam and an U-beam with two additional I-beams positioned perpendicularly below the plate, was modelled in six different ways to further investigate the modelling methods. The examined component was the upper I-beam experiencing high torsion as well as bending. Internal loads, axial bolt loads and shear forces on the bolts was investigated for this structure.  The conclusion of the complex structure was that the simplest and most global FE-models can be used to extract the internal loads of a structure with fairly similar precision as more detailed models. However, the bolt loads gave a large discrepancy depending on which model used and how the data was extracted. The choice of modelling method depends on the specific situation and the purpose of the model, making the planning of the finite element analysis a critical step.