Fatigue Crack Propagation in a New Generation Al-Cu-Li Alloy

In view of reducing the fuel consumption, low-density third generation aluminum‑lithium alloys are good candidates for substituting to conventional aluminum alloys widely used in the aeronautical industry. However, for in-service application, a better knowledge of their damage tolerance properties is required. To contribute in answering this question, a study of fatigue crack growth behavior of a 2050 new aluminium-lithium alloy in T8 temper was undertaken. Fatigue crack propagation tests were performed at 35Hz on CT specimens W=50mm and 6mm thick. Constant amplitude loading tests were performed with a load ratio of 0.1 and 0.7. Crack closure was systematically evaluated by mean of a strain gauge stuck at the back face of the specimens. The stress intensity factor level for crack opening, K_op, was detected by mean of the compliance method. Both the mid DK regime and near-threshold domain were explored. The influence of microstructure associated to the T8 temper and of the specimen texture on fatigue crack growth at constant amplitude loading is discussed on the basis of EBSD identification of slip planes orientation along the crack path. The discussion is particularly focussed on the deviation in the crack path in ambient air from the reference propagation plane for stage II crack, which is normal to the loading axis. Reference tests run in high vacuum (P<3e^-4 Pa) clearly show the existence of a substantial influence of ambient air on the propagation crack path, and hence, on the propagation mechanisms. These results are discussed on the basis of an existing framework modelling. Variable amplitude loading tests in ambient air, consisting in blocks of 1000 constant amplitude loadings in similar condition as above plus a 70% overload, were also performed to evaluate the applicability of the PREFFAS model, for industrial application.