Examining the role of water vapor on small fatigue crack growth behavior in Ti-6242S using ultrasonic fatigue and scanning electron microscopy

Abstract

The influence of water vapor on small fatigue crack growth rates in the near alpha titanium alloy Ti-6242S was investigated using a combined ultrasonic fatigue/environmental scanning electron microscope system (UF-SEM). Small fatigue cracks were grown at a cyclic frequency of 20 kHz in laboratory air, partial pressures of water vapor ranging from 65 to 1330 Pa, and vacuum (1.7 × 10-4 Pa). Fatigue cracks grew at higher rates in all water vapor pressures investigated versus in vacuum, with a nearly identical growth rate observed in laboratory air and 1330 Pa water vapor. A linear dependence of small fatigue crack growth rate on water vapor pressure was observed that is consistent with a pressure dependent transport rate of water vapor molecules to the crack-tip. This work indicates that the amount of water vapor present in the environment (humidity) plays an important role in determining the fatigue life of Ti-6242S and must be accounted for even at very high cyclic frequencies, such as the 20 kHz cyclic frequency commonly employed in ultrasonic fatigue testing.