Prediction of the shape of fatigue cracks propagating in thick monolithic aluminium structures repaired using composite bonded doublers

Developed in the 1970's, the composite bonded repair technology has been proven to be an efficient and reliable method for use on metallic aircraft structures. The purpose of this method is to slow or arrest crack growth, and thus, prolong the life of the structure. The technology has been successfully used in repair applications on both military and civil aircraft for fatigue enhancement and crack patching.;However, the installation of composite bonded repairs on aircraft must satisfy their damage tolerance requirements to ensure the safe operation of the aircraft. To meet these requirements, reliable crack growth prediction models are one of the tools required to assess the safety of the repaired structures without the need for long and expensive test programs. Thus, accurate prediction tools are the key that will help this technology to be integrated in structural integrity programs by the aeronautical industry.;In 2001, Martec Limited performed a research study intended to survey the available crack initiation and crack growth prediction tools for fatigue life evaluation of thick aluminium structures repaired using composite bonded doublers. The approach developed by Martec provided accurate crack growth predictions for the case of double-sided repairs, but did not provided the same level of accuracy for the single-sided repair configurations. It was therefore recommended by Martec to investigate the influence of crack shape on the crack growth predictions to achieve more accurate predictions for single-sided repair configurations.;Consequently, a new systematic crack growth prediction methodology, based on 3D finite element methods, was developed for thick structures repaired using composite bonded doublers. The methodology considers effects specific to bonded composite repairs, such as the effect of thermal residual strains caused by the curing process, the effect of out-of-plane bending, and the crack shape. The methodology was tested using three different experimental tests found in literature. The experimental crack shapes and crack growth data were compared with numerical results.;Good agreement between experimental and numerical crack growth predictions was obtained and the gain in accuracy acquired by including the shape of the crack was demonstrated.;As part of a collaborative program on fatigue prediction of bonded repairs, Martec Limited has initiated this research project in collaboration with the Department of National Defence (DND), the Chief of Research And Development (CRAD), and Sherbrooke University. This project was conducted at Martec, under the auspices of the partnership program for masters and doctoral studies in the workplace of Sherbrooke University.