Crack growth transitions in Udimet 720

Loo Morrey, Marianne (1997) Crack growth transitions in Udimet 720. University of Southampton, Doctoral Thesis.

Abstract

Nickel base superalloys have been in widespread use as components in jet engines for the past 30 years. Due to the economic pressures to produce ever more efficient engines nickel superalloys are constantly under development to improve their high temperature performance. Fine grained Udimet 720 (U720) is a nickel based superalloy under consideration for use in the turbine discs in gas turbine engines. In order to determine safe service lifetimes for such critical components it is necessary to have a thorough understanding of their fatigue mechanisms.

The intrinsic fatigue resistance of U720 has been studied at a range of temperatures in both air and vacuum. The aim of these investigations was to identify the Stage II to Stage I crack growth transition and the parameters controlling it. This crack growth transition was found to be controlled by the size of the cyclic crack tip plasticity in both air and vacuum, with Stage I faceted crack growth occurring when the cyclic plastic zone was smaller than the average grain size of the material. Testing in a vacuum environment led to a dramatic increase in the observed crack growth threshold. This high crack growth threshold is believed to be attributed to the tortuous crack paths observed in vacuum.

Atypical fatigue failures have been observed in corner notched specimens of fine grained U720 tested at 300^oC, this phenomenon has become known as tear drop cracking. Corner notched specimens were tested at 20, 300 and 600^oC in both air and vacuum to investigate the effect of temperature and environment on this failure mechanism. The R-ratio and initial loading conditions were also varied in order to study the effect on macroscopic crack shape development. The onset of shear lip formation has been found to be linked to a critical ΔK level.

Fractographic studies of the fatigue fracture surfaces have shown that the formation of secondary cracks plays an important role in crack growth transition in U720. A number of simplistic two dimensional elastic analyses have been used to attempt to quantify the extent of shielding experienced by the crack tip, and to estimate the crack tip mixity associated with these transitions. A limited amount of mixed mode I/II testing has been performed using these estimations in an attempt to reproduce the shear crack growth mechanism observed in the shear lip regions of the tear drop specimens.

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