Development of ultrasonic fatigue for rapid, high temperature fatigue studies in turbine engine materials

A system for ultrasonic fatigue testing at temperatures as high as 700°C and at positive mean stresses has been developed. Its use is demonstrated by examining the fatigue behavior in the lifetime range of 10 ⁵ to 10⁹ cycles for the nickel-base superalloy Rene' 88 DT at 20 and 593°C for a load-ratio of 0.05. Crack initiation occurred from large grains and from inclusions, consistent with crack initiation behavior at conventional test frequencies. Surface condition influenced fatigue behavior at ambient temperature, where electropolished specimens had considerably shorter lives than as-machined specimens. At 593°C, however, no effect of surface condition on fatigue lifetime was observed since subsurface initiation occurred for both electropolished and as-machined specimens. Fatigue life, at a given stress appears to be shorter when testing at ultrasonic frequencies compared to the behavior observed at conventional frequencies and the exact causes for this remain to be determined. It is also demonstrated that fatigue cracks could be initiated and grown from micronotches with dimensions on the order of grain size.