Anomalous crack shape development (tear drop cracking) in turbine disc material Udimet 720

Abstract

An unusual crack growth mechanism has been observed in fine grained Udimet 720 at moderate temperatures. Fracture surfaces consisting of a tear drop shaped region of stage II crack growth bounded by regions of shear crack growth have been observed above room temperature. The current study investigates the effect of temperature, environment, and specimen geometry on the tear drop cracking phenomenon. Vacuum conditions are found to retain tear drop cracking behaviour at higher temperatures than observed by other workers. Tear drop cracking has been established in a number of specimen geometries and it is demonstrated that further analysis of the effects of crack tip shielding during this macroscopically tortuous crack growth is required to fully evaluate the effects of such crack growth behaviour on component lifing.

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