This paper reviews some of the recent developments in the use of fracture mechanics to define the acceptable sizes and shapes of flaws in single-wall steel pressure vessels. The results of such appraisals are being incorporated in standard specifications and codes, for example in ASME and UK proposals. These steps make towards a worthwhile improvement in assessed reliability, and put in perspective particularly the considerable tolerance for porosity and slag inclusion flaws. Even so, for cracklike defects there are uncertainties still left after making such an analysis: these can be reduced by devoting sufficient attention to quality control of material fracture toughness, fabrication routes and inspection methods. The choice of materials of high intrinsic fracture toughness and low sensitivity to fabrication and process is of great importance. From the nondestructive examination side, reliability and certainty of reporting are more important than sensitivity, putting the emphasis on automated recording techniques and on overall inspection tools such as acoustic emission. Attention to these aspects must be given when considering permissible design stresses and permissible stress concentration levels.

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