| The aging of aircraft fleets has increased the concern about aircraft structural integrity. As an aircraft is being used beyond its original design life, small fatigue cracks accumulate at critical locations such as highly loaded riveted joints. This type of fatigue cracking is often referred to as multiple site damage (MSD). The presence of MSD may reduce the ability of aircraft structure to carry the design load. Conventional linear elastic fracture mechanics LEFM, fails to predict the reduction of strength caused by MSD, especially for more ductile materials such as 2024-T3 aluminum. An analytical model, referred to as the linkup model or the plastic-zone-touch model, has been used to predict the residual strength of structures with MSD. However, the linkup model does not accurately predict the magnitude of the loss of strength for some geometric configurations. Therefore, there has been a need to improve it. The approach that has been taken is to develop a modification of the linkup model based on the empirical analyses of test data of simplified panel configurations then to validate the modified model with test data from panels with more geometric complexity. Test results from 40 flat unstiffened 2024-T3 aluminum panels where used to develop a modification of the linkup model. For the purpose of validating the modified linkup model, 21 configurations of a one-bay stiffened panel, and 15 configurations of a two-bay stiffened panel with a severed middle stiffener were tested. Finally, 36 crack configurations of a single lap joint panel where tested to complete the validation process of the modified linkup model for 2024-T3 aluminum. Predicting the residual strength of 7075-T6 aluminum sheets with MSD cracks had been somewhat ignored, since it was assumed to be a brittle fracture phenomenon. However, this assumption became questionable, and 12 flat unstiffened open-hole panels of 7075-T6 were tested, and the results showed that neither the brittle fracture model nor the linkup model was accurate. Therefore, the data were used in an empirical analysis to develop a modified brittle fracture model, as well as a modified linkup model. Furthermore, the 36 configurations of the bolted lap joint panels were also analyzed with FRANC3D/STAGS using the crack tip opening angle CTOA fracture criterion to predict residual strength, and the predictions where compared to those of the modified linkup model. |
