In 1972, Section III of the ASME Boiler and Pressure Vessel Code adopted new fracture toughness requirements for Class 1 components based on fracture mechanics concepts. In the ensuing several years, new toughness requirements were developed and adopted for the other Section III classes of components covered by Subsections NC, ND, NE, and NF. This paper describes the chronology of these developments and the bases of the rules and criteria. The formulation of specific rules is discussed and examples of data used to support the criteria are described. The rules and requirements as initially adopted, and subsequent revisions and modifications are discussed to show their evolution into the current version.

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