Fatigue Endurance of Al-Cu-Li Alloys

Al-Cu-Li alloys show a considerably improved fatigue resistance compared to conventional aerospace alloys, particularly those showing the same strength level. This has been demonstrated in open hole fatigue tests, in high load transfer fatigue specimens, and in notch fatigue specimens that have been subjected to surface treatment. The effect is maintained when specimens are subjected to variable amplitude loading.  The origin of this improvement is not yet clear and is the subject of this paper.

The present work examines the early stages of crack initiation and growth using optical microscopy, scanning electron microscopy and eddy current local defect detection. A reference 7050 T74 alloy is compared to an Al-Cu-Li, AIRWARE^TM 2050 alloy, using open hole specimens. Initiation sites are identified as a function of stress level.

The results show that constituent particles are the main initiation site. Less expected is the fact that slip bands are more common as initiation sites at low stress level.

A micro-crack growth model is fitted to the observations mentioned above, using in particular the inter-striation distance. This model is then included in a statistical model for fatigue life prediction based on the distribution of constituent particles.  This allows merging the data from both alloys in a consistent data set. The conclusion obtained from the modeling is that both the constituent distribution and the short crack growth rate contribute to the better behavior of AIRWARE 2050 versus 7050. However the latter effect is larger.

The model can be used as a tool to predict the effect of alloy modifications.