The maximum size of keyway crack sectioned was 4.5 mm, although many smaller cracks
were observed. The application of fracture mechanics to the geometry of the discs for the
fracture toughness of the steel involved showed that in the presence of such stress corrosion
cracks spontaneous initiation of a catastrophic brittle fracture would occur, when the speed of
the turbine-generator reached 3200 rev/min, the speed achieved during a proving test of the
overspeed protection when the failure occurred. Similar proving tests, at approximately
monthly intervals over the 4½ years of service prior to the failure, had involved maximum
speeds of 3450 rev/min, indicative of the absence or slow growth of the stress corrosion