Temperature Dependence of the Intrinsic Small Fatigue Crack Growth Behaviour in Ni-Base Superalloys Based on Measurement of Crack Closure

Abstract

Ni-base superalloys have been used with high reliability for blades and vane applications in gas turbines for the last decades [1–13]. Recently the application of directionally-solidified and single crystal alloys has become common in advanced gas turbines. On these materials, several kinds of studies, such as on the effects of temperature [1–5], loading frequency [1,4,6], stress multiaxiality [9], environment [3], microstructures [13], and crystallographic orientation [8] on the crack growth during fatigue, creep and creep-fatigue, have been performed. On the other hand, experimental evidence has also shown that small crack growth behaviour may not conform to that measured conventionally with physically long cracks [10–19]. However, most of the foregoing studies have dealt with the propagation of physically long cracks by using the compact tension specimen as an example, and there is little quantitative information on the process of small crack initiation and the subsequent propagation in Ni-base superalloys, which generally dominates the major part of the fatigue life [10–13]. Furthermore, the crack closure phenomenon [20], which plays an important role in fatigue crack propagation, has not been taken into account even in the foregoing few studies at all, due to the difficulty of the measurement at high temperatures.