Abstract
Three kinds of Ti-6Al-4V alloy, annealed at 750°C, and solution-treated and then aged at 520°C and 660°C, which are denoted as the materials 7N, 5T and 6T, respectively, were low-cycle fatigued under an axial strain-controlled condition.
Cyclic stress-strain response was most excellent in the material 5T, and the materials 6T and 7N followed in its superiority in this order. The logΔεp vs. logNf plot crooked downwards in the low plastic strain range for all the three materials. The material 5T exhibited slightly longer life than the other two materials of which life curves almost coincided with each other.
The crack growth rate da/dN plotted against cyclic J integral range ΔJ was approximately expressed by a linear line on the double logarithmic scales. The da/dN vs. ΔJ plot for the surface crack on the smooth specimen, was situated on the lower crack growth rate side than that for the long through-thickness crack. This might be due to the difference in the growth processes of the two cracks; the surface crack grows accompanying coalescence and branching, while the through-thickness crack extends relatively monotonously without any coalescence.
The resistance against the extension of surface crack was more superior in the materials 5T and 6T than in the material 7N. The materials 5T and 6T possess almost the same crack growth property.
