Fracture-toughness tests on 3Cr–Mo and 3Ni–Cr–Mo– V turbine–disc steels are reported. The tests were invalid by ASTM criteria, although failure always occurred by unstable crack propagation. Where the fracture surfaces showed no evidence of stable crack extension before the instability, plane–strain fracture–toughness values werepredicted using (a) a post–yield theory of fracture based upon the Bilby–Cottrell–Swinden model of yielding ahead of a crack tip and (b) the equivalent–energy technique due to Witt. Good agreement was obtained between the two techniques. Some of the practical and conceptual difficulties of interpreting the results of invalid tests are discussed.
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