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Abstract
The main factors influencing high temperature fatigue crack growth in superalloy blade materials are discussed. The significance of temperature, frequency, environment, and minimum to maximum load ratio R are considered in turn. Transgranular cycle dependent and intergranular time dependent controlled cracking processes are identified. Conditions favouring each mechanism are clarified and it is shown that cumulative damage concepts can be applied to predict interaction effects. It is found that cycle controlled processes are most likely to dominate at high frequency and low values of R. Creep and oxidation mechanisms, which are favoured by low frequencies, high temperatures, and high values of R, have been identified as contributing to the time-dependent component of cracking. It is shown that creep and oxidation can enhance crack growth/cycle significantly and reduced blade lifetimes.
MST/517