Analysis of Three-Dimensional Cracks in Submodels

Sammanfattning: A common technique to evaluate load paths in complex structures is to perform FE-calculations with relative large elements. This procedure gives no information regarding stress concentrations at e.g. holes or radius but this phenomenon can later on be investigated in details with local individual submodels. Displacements is taken from the global model and used to analyse stress concentrations and crack driving parameters in the submodel. Today, the crack controlling stress intensity factors are in general cases obtained from handbook solutions of elementary cases. This method requires engineering judgements in a conservative manner and one way to improve the solution is to model the crack in its correct surroundings in a local three-dimensional submodel. This master thesis is focused on the development of an automated support for analysing three-dimensional cracks in submodels. The results from a global Nastran model can be imported to Trinitas and used for a more accurate stress and fatigue life analysis in a local model. Here a three-dimensional crack tip subdomain can be generated inside an eight point brick volume. The crack tip subdomain is specially designed and adjusted for accurate determination of stress intensity factors along the crack front. For example, all points are adjusted with respect to the brick volume and the crack size, triangular wedge elements are applied around the crack tip, the midpoints for these elements are moved to quarter points and the crack front is curved. The crack tip subdomain is validated against several reference cases and shows sufficiently good results with respect to the stress intensity factor. Finally, the automated crack tip subdomain generation is applied to a geometrically complex part of a main wing carry-through bulkhead of a fighter aircraft in order to show the applicability of the procedure in an industrial environment.