Fatigue fracture tests on Al-Li 2198-T851 specimens under mixed-mode conditions

Aluminum-lithium alloys deliver numerous benefits for applications in which weight represents a key concern, thanks to their reduced mass density and enhanced mechanical properties compared to more traditional aluminum alloys. From the first generation of these alloys, significant anisotropy was observed in mechanical properties that was responsible of unpredictable failures during manufacturing. This paper reports an experimental campaign on the fracture anisotropy for the third generation aluminum-lithium alloy Al-Li 2198-T851. Particularly, fatigue crack-growth tests were carried out on the Middle Tension M(T) specimens with an initial central notch having different angles (30°, 45° and 60°) with the normal to the loading axis, in such a way to highlight the potential anisotropic characteristics of the material. Two types of specimen were manufactured: longitudinally (L) and transversally (T) with respect to the rolling direction of the sheets. Crack gauges and strain gauges were glued on specimens to measure the crack-growth rates and central alignment of load during tests. A significant difference of the crack-growth rates was observed for the two types of specimen, while cracks propagated perpendicularly to the loading axis irrespective of the angle of the central notch. Additionally, larger scatter of paths was observed for (L) specimens, while (T) specimens presented crack paths almost perpendicular to the loading axis, suggesting that cracks tended to propagate between the grains of the material.