In the strength design phase of high speed rotating machinery such as turbines and turbocompressors, structural integrity assessment of impellers or disks is of prime importance. Since the rotating disks are, in many cases, made of high toughness material, stable crack growth occurs from material defects, preexisting cracks, or notches in case of over speed, followed by unstable fracture, i.e., a final burst. It has been reported that the onset of stable crack growth can be predicted by the JIc fracture criterion and that the unstable ductile fracture of a rotating disk with inner symmetrical notches can be predicted by the average stress criterion or the modified average stress criterion.
In the present study, rotating tests were performed on disk specimens with fatigue cracks or notches existing at various locations in order to determine the effects of notch sharpness and its location upon unstable ductile fracture strength. The validity of the J-R curve as an unstable fracture criterion was also investigated.
The results obtained are as follows:
(1) Notch sharpness exerts no influence on burst strength provided that the location and length of the notch remain constant.
(2) The unstable fracture strength of rotating machinery having cracks or notches can be estimated with a sufficient accuracy by the modified average stress criterion. Strictly speaking, this criterion is prone to overestimate the unstable fracture strength of disk with neighboring cracks, notches or unsymmetrical cracks.
(3) The R curve methodology using the J integral calculation with body force and thickness effects taken into account can adequately predict the stable crack growth and the unstable fracture strength of rotating disks.
