The COD Approach and Its Application to Welded Structures

Abstract

The crack opening displacement (COD) approach has, since its inception as a fracture initiation parameter in yielding fracture mechanics, gained increasing acceptance both as a viable research tool and an engineering design concept. In the United Kingdom, The Welding Institute has pioneered the application of COD in the structural fabrication industry largely through the development of the COD design curve. This paper is a representation of the philosophy underlying design curve applications and illustrates the practical significance of COD by drawing on case studies from various welded structures. Following a brief appraisal of the origins of the design curve, the paper outlines procedures for the use of COD in design, that is, either as a basis for material selection or in setting up acceptance levels for weld defects. The reliability of a small-scale test prediction from the design curve has been investigated on a statistical basis from a survey of more than 70 wide-plate tension test results in which the material had also been categorized by COD. Specific practical examples are then discussed covering the various types of application, material selection defect assessment, and failure investigation. Structures included in these examples are offshore rigs, oil and gas pipelines, pressure vessels, etc., with special emphasis on the manner in which small-scale COD test results are translated to the structural situation. Finally, the paper includes information on the considerable range of structures to which the concept has been applied during the past five years.