The cleavage fracture of quenched and tempered steels at a sharp crack is seen to involve a statistical competition between different sized crack nuclei in the rapidly changing stress gradient ahead of the crack tip. A procedure is proposed, based on this statistical fracture model, whereby it is possible to estimate the cleavage fracture toughness of a steel containing spheroidal carbide particles from a knowledge of the carbide particle radius distribution. Predictions so made are seen to be in good agreement with experimentally determined fracture toughness values over a range of different temperatures and microstructures.
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