Three-dimensional effects of microstructures on short fatigue crack growth in an Al–Li 8090 alloy

Abstract

Al–Li 8090 alloy specimens were fatigued using a self-aligning four-point bend rig at R = 0.1 and room temperature, in air, under constant maximum stress control. The crystallographic characteristics of fatigue crack initiation and early growth were studied using EBSD. It was found that the growth behaviour of a short crack were controlled by the twist (α) and tilt (β) components of crack plane deflection across each of the first 20 grain boundaries along the crack path, and that the α angle at the first grain boundary encountered by a micro-crack was critical in determining whether the crack could become propagating or non-propagating. In addition to the orientations of the two neighbouring grains, the tilt of their boundary could also affect α across the boundary. A minimum α-map for a vertical micro-crack was calculated to evaluate the resistance to crack growth into a neighbouring grain with a random orientation. Such an α-map is of value in alloy design against fatigue damage by optimising texture components in the alloys.

Keywords:

• short fatigue crack
• grain boundary
• growth rate
• retardation
• deflection
• Al–Li 8090 alloy
• Euler space
• crystal orientation

Acknowledgements

This research was sponsored by the National Science Foundation through a CAREER Award (DMR-0645246). The authors acknowledge that some of the work in this paper was conducted with Dr Angus Wilkinson at University of Oxford, to whom T. Zhai is grateful.