Anisotropic Fatigue Crack Initiation and Small Crack Growth in Ti-6Al-4V Alloy.

Abstract

This paper deals with the effects of specimen orientation and microstructure on small fatigue crack growth in Ti-6Al-4V alloy. Fatigue tests have been carried out under axial loading at a stress ratio of-1 using smooth specimens of annealed (AN) and solution-treated and aged (STA) materials. Specimens were cut from the materials so that the loading axis was parallel to (L-orientation) and perpendicular to (T-orientation) the rolling direction. T-orientation showed higher fatigue strength than L-orientation, which was more remarkable in STA material. In both materials, T-orientation also indicated higher resistance to crack initiation than L-orientation. In AN material, there was a distinct difference in small fatigue crack growth between both orientations : L-orientation exhibited faster crack growth rates than T-orientation at the early stage of crack growth, but STA material showed no significant difference. The orientation dependence of small fatigue crack growth was strongly related to stage I facets developed at the crack initiation site. Although stage I facets were not seen clearly in STA material, the sizes of stage I facets for L-orientation in AN material were significantly large compared to those for T-orientation. Since AN material had a texture with the basal plane (0002) parallel to the rolling direction. the preferred orientation led to faster and extended stage I crack growth for L-orientation.