NEAR THRESHOLD FATIGUE CRACK GROWTH IN A 8090 LITHIUM CONTAINING Al ALLOY

Abstract
Near threshold fatigue crack growth behaviour of a 2.2 % Li, 1.1 % Cu and 0.7 % Mg 8090 Al alloy has been studied in air and vacuum at load ratios ranging from 0.01 to 0.8, using compact tension specimens machined in TL and LT orientations in three aging conditions : 2 h 30 at 160 °C, 12 h at 190 °C (T651) and 18 h at 210 °C. Comparison of the behaviour of the 8090 alloys with classical alloys 2024 T351 and 7075 T651 shows that lithium addition consists in a significant improvement of the near threshold crack growth resistance which has been analysed as a consequence of the planar slip mechanism observed in δ' hardened matrix which results in substantial roughness-induced closure effects. The influence of load ratio and texture are shown to be mainly related to crack closure and the propagation behaviour to be rationalized in terms of the effective stress intensity factor range ƊK_eff for given environmental and aging conditions. The influence of environment is discussed in terms of water vapour embrittlement from metallurgical studies.