Inspections carried out during routine servicing of the Royal Australian Air Force Caribou aircraft have found cracking in the horizontal stabilizer outriggers on a number of recent occasions. The cracks are propagating from blind holes, which are used to fasten bump stop rubbers, on the ends of the outriggers. The ultimate aim of this thesis is to develop a cost effective solution to the cracking problem, in order to avoid the high expenses that are associated with the component replacement.
A finite element model of the outrigger under the critical rearward gust load case was constructed. The results from this model did not indicate any reason for the crack propagation. Further research indicated stress corrosion cracking (SCC), which causes materials to fail far below their yield stress, to be the most likely cause of the problem. The sustained tensile stress that is necessary for SCC to occur is most likely a result of the residual stress in the material after manufacture and fitting of the bump stop fastener.
The proposed solution involves introducing compressive residual stress into the critical cracking region, thereby eliminating the tensile surface stress necessary for SCC. The solution process utilizes conventional techniques in the aviation industry, namely: cold expansion of the fastener hole, surface treatment of the surrounding area and installation of a controlled interference fit bushing. In addition to being a preventative solution, the process may also be used as a repair for small cracks.
In support of the proposed solution, a finite element model was constructed of the cold expansion process. As expected, the model predicted a zone of compressive residual hoop stress around the hole, which will resist the nucleation and growth process of SCC at the hole edge. Accounting for errors in the model, we can be reasonably confident that the proposed solution will prevent the cracking problem. Furthermore, the aim of a cost effective solution has been met, as the techniques are relatively inexpensive and the process is to be undertaken while the outrigger is still installed in the horizontal stabilizer.