1993 Volume 42 Issue 481 Pages 1218-1223
In order to investigate the fatigue strength of Ti-6Al-4V alloy at a large number of cycles, ultrasonic fatigue tests were performed. Three heat treatment, i.e. mill anneal (MA), duplex anneal (DA), and solution treatment and aging (STA), were applied to the specimens. Although the fatigue strength obtained from this method was approximately 15 percent higher than that obtained from the conventional fatigue test, the general trend of these two S-N diagrams was the same. Usefulness of the ultrasonic fatigue test as an acceleration method to determine the S-N diagram was confirmed on this alloy and the fatigue strength of the above mentioned heat treated specimens at a large number of cycles up to 2×109 cycles was determined. The noteworthy result was that there seemed to exist a fatigue limit in this alloy from the data up to 108 cycles, but the fatigue strength became decreased at further number of cycles. This result was obtainable only by performing the fatigue test in a longer period like up to 109 cycles. Similarly to the tensile strength, the STA specimen showed the highest fatigue strength and the MA and the DA specimens followed. Judging from their FCP properties and fractographies, the difference in fatigue strength among these specimens with various microstructures was caused in the stages of crack nucleation and/or microcrack growth.
