Fracture mechanics in design and service: ‘living with defects’ - Review of traditional design codes and their relevance in relation to fracture

Abstract

A review of the traditional design codes, in the context of fracture prevention, is made by considering the requirements of British Standards relating to low temperature storage tanks and quality pressure vessels. The use by the codes of the results of wide plate testing in controlling permissible sub-zero design temperatures is discussed together with the effects of heat treatment on material toughness and residual weld stresses. An estimate is made of stress levels implied by code requirements both below and above the threshold for the onset of creep effects. The activities of manufacture, inspection and testing are considered for their contributions to fracture prevention. The general techniques of fracture assessment (two-criteria approach and the c.o.d. approach) are simplified by restricting the scope to code-designed structures, so that flaw assessment charts can be produced. The charts are used to estimate stable flaw sizes under sub-zero and ambient temperature conditions, with comment made on the elevated temperature situation. Finally, a survey is made of techniques available to assess flawed components under creep conditions, which meet the requirements of the appropriate codes. While a comprehensive methodology is not yet available, an example is given of a flawed cylinder under creep conditions that would be applicable provided that the material remained ductile.