AbstractsBiology & Animal Science

Artificially aged aluminium-lithium alloys have recently seen renewed interest in them in the aviation industry. They have a decreased density, while keeping or improving the fatigue properties compared to other aluminium alloys. Research shows that these aluminium-lithium and other advanced aluminium alloys exhibit crack turning and crack. Currently no predictive capabilities exist for such phenomena, therefore designers cannot use the full potential of these alloys. This study aims at the development of a numerical strategy that can predict in-plane crack turning and crack branching in a crack-arrestor configuration. Additionally, fatigue tests have been performed in this study to gain insight into the effects of crack turning and crack branching on the fatigue crack life, crack paths and FCGR. The fatigue test coupons are manufactured in seven different orientations in the ST-L material plane, where the L-axis has been rotated increasingly w.r.t the expected mode-I crack growth direction. Two different aluminium alloys are used in these tests, Al 2050-T84 and Al 7010-T7451. Lastly, the main drivers are established for both in-plane crack turning and crack branching in a crack arrestor configuration. Two separate numerical methods are developed in this study, the k2-method and the Pettit-method. Both methods are based on a LEFM framework. The key hypotheses of the k2-method have been judged to be invalid. On the other hand, the key assumptions for the Pettit method have proven to be very reasonable. The Pettit-method uses the MSERR approach combined with a method to account for fatigue fracture resistance anisotropy, dependent on the orientation of the microstructure and Kmax of the primary crack. A comparison of the predictions with the crack paths observed in the fatigue tests, shows that the Pettit-method is accurate for coupon orientations in between ST00-L and ST45-L. Fractography of a branch in an Al 2050-T84 DEN(T) coupon revealed that void growth and coalescence was the primary fracture process. A recent study confirms that void growth and coalescence is the primary fracture process for crack branches in a crack arrestor configuration. A combination of shielding and amplification effects on a microstructure level is responsible for this void growth, which concentrates on grain boundaries between favourable stiff/soft grain pairings. These observations make it unlikely that crack branching in a crack arrestor configuration can be predicted with macroscopic crack growth criteria. The main driver for the crack turning and fatigue fracture resistance anisotropy, observed in the fatigue tests, likely is the orientation of the microstructure w.r.t the primary crack and loading direction. Numerical simulations reveal that turning cracks grow in near mode-I conditions, which is confirmed by fractography of such a turning crack path. The fact that such cracks grow in near mode-I conditions, makes them well predictable with macroscopic crack growth criteria, as proven by the predictions of the Pettit-method.