Abstract
Ti alloys, with the advantageous tensile strength/density ratio and the chemical stability, have been used widely in the aerospace and chemical engineering industries and their usages are still expanding in various industrial areas. In the automotive industry, because of their superior merits of weight reduction and fuel saving, Ti alloys are expected to be used as various part materials including connecting rods, engine valves, springs and retainers, which are all subjected to the fatigue loads. In this study, using two Ti alloys of Ti-6Al-4V and Ti-3Al-2.5V, the effects of temperature, frequency and microstructure change on fatigue crack propagation have been investigated.