1990 Volume 76 Issue 6 Pages 924-931
In titanium alloys subsurface fatigue crack initiation occurs apparently without the existence of any defects such as inclusion or pore. We investigated subsurface crack initiation and sub-crack for Ti- 6Al-4V alloys at cryogenic temperatures, and discussed the growth mechanism of microcrack.
As the maximum stress was decreased, the morphology of subsurface crack initiation sites was changed from consisting of one facet to of more facets. The facet was identified as a cracked α phase by comparing their chemical composition and morphology. Sub-cracks were also produced in α phase. The subsurface crack initiation sites and the ub-cracks were not perpendicular to the applied stress. Hence it is concluded that a microcrack initiates in a α grain, grows into neighbor β plates and α grains, and finally forms an initiation site for a main fatigue crack. The lower the maximum stress, the greater the size of subsurface crack initiation sites. We adopted the size of the projection of subsurface crack initiation site on the main crack propagating plane, as a shape parameter of three-dimensional crack. Using this parameter, the dependence of initiation site size on the maximum stress range can be accounted for by an assumption that the microcrack growth is controlled by a threshold condition.
