Fatigue crack initiation and fatigue life prediction of Ti-6Al-4V ELI

Fatigue crack initiation characteristics of Ti-6Al-4V ELI with equiaxed alpha and Widmanstatten alpha structures, which had been standardized as a biomaterial by ASTM, were investigated with relating microstructure. Relationship between fatigue damage and mechanical properties of the same alloy were also investigated in this study. The fatigue crack initiates mainly at the interface between primary alpha grains in equiaxed alpha structure while that initiates in the colony along about 45 degrees direction versus stress axis in Widmanstatten alpha structure. The fatigue crack initiation period occupies about 40 similar to 50 % portion of total fatigue life in equiaxed alpha structure while that occupies more than 90 % assumed that the crack initiation occurs when the surface crack length, 2a, is 30 mu m. The tensile strength of fatigued specimens with equiaxed alpha and Widmanstaten alpha structure at various fatigue conditions increases clearly according to the fatigue steps at the first period of low cycle fatigue (LCF) region, while the elongation and impact toughness show the reverse trend. The hardness far from the specimen surface is greater than that near the specimen surface at the early stage of LCF. Both hardness is however nearly equal each other at the latter stage of fatigue. The damage ratio of mechanical properties after fatigue is relatively greater in equiaxed alpha structure. Each mechanical property after fatigue and fatigue life relates each other systematically in LCF region.