Size effect on the fatigue behaviour of IMI 829 titanium alloy under dwell conditions

The fatigue behaviour of titanium alloy IMI 829 was examined at room temperature to study the effects of dwell-time on the fatigue lives for different sizes of specimen. The base line tests were conducted on smooth cylindrical specimens under load control with a triangular wave of 0.5 Hz frequency. When a two minute dwell-time was imposed at the peak of each cycle a fatigue life reduction ranging from 5 to 45 times was observed compared to the fatigue lives obtained under the triangular wave. The dwell effect on the fatigue life was more severe for smaller specimens than for larger specimens, contradicting the observations made by other investigators. There was a significant amount of irreversible strain recorded during the dwell-time at a stress level of about 90% of the yield strength. The total strain accumulated during the tests was plotted against the number of cycles for all the specimens. Smaller specimens showed much higher strain accumulation rates and shorter lives. It is believed that sustained load effects played an important role in the dwell effect at room temperature, and the greater fatigue life reduction associated with the smaller specimens is believed to be related to the microstructures of the alloy.