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Prediction of fatigue damage progression in bonded composite repairs to aluminum aircraft structures Clark, Randal John

Abstract

The focus of this thesis is the development of a predictive model for fatigue damage progression in unidirectional bonded composite repairs of cracked isotropic plates. The principal use of this technology is in the design of repairs for aluminum aircraft structures. The ability to predict the rate of fatigue damage is critical to damage tolerance analysis of a repair. A damage tolerance analysis will allow designers to assess the design life, assign inspection intervals, and determine likely failure modes for a repair, and will be required for airworthiness certification for a long or indefinite period of operation. In this thesis, classical methods of bonded joint analysis are presented, and extended to the case of reversed plasticity of an internally pressurized lap joint. The crack bridging effect of the repair is examined using a boundary element method employing the Green's functions for a point load applied to a center-cracked plate. This results in a system of linear equations solvable by Gauss- Seidell iteration. The boundary element method allows calculation of the stress intensity and adhesive shear stresses under the bonded patch. These parameters govern the fatigue and static strength of the repair. The boundary element model combines engineering fracture mechanics and bonded joint analysis techniques in a very direct and straightforward manner. Results from the boundary-element model are compared to approximate analytical methods for a disbonding patch, and an improved analytical model employing correction factors is presented. Power law methods are then used to predict crack and disbond growth rates, which are compared to experimental results. The influence on patch life of various secondary effects, such as adhesive plasticity, process-induced thermal residual stresses, patch bending, shear deformation of the patch, and cracked-plate geometry are investigated. Based on this work, conclusions are drawn regarding patch behavior, limitations of modeling techniques, and experimental results still necessary to validate patch mechanics models. The techniques developed are also of interest in the study of cracking in fiber-reinforced metal laminates (FRML).

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