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The crack-tip opening displacement (CTOD) approach to the assessment of the risk of fracture, which has been incorporated into the British Standard's Published Document PD 6493, has served the United Kingdom welding fabrication industry well for the past 15 years. However, application of the approach and further research have drawn attention to several aspects which need improvement. For example, the PD 6493 document gives no guidance on the derivation of the critical CTOD value to be used in the analysis. Also, the fracture assessment method in PD 6493 suffers from a number of limitations.
This paper describes proposed improvements to the CTOD test and CTOD-based design approaches. A recently drafted appendix for weldment testing, which is proposed to be incorporated into the British CTOD test standard, is discussed briefly. Also, proposals for dealing with stable tearing, pop-in, and scatter in CTOD data are described. Finally, a three-tier fracture assessment procedure is proposed which allows flaws to be assessed at various levels of complexity, depending on the application. The three levels of assessment, in increasing order of complexity and accuracy, are 1. Modified version of the present PD 6493 method. 2. Strip yield model. 3. Reference stress approach.
All three levels utilize CTOD as the fracture toughness input. Some guidelines for choosing the appropriate assessment level are given.
fracture mechanics, crack-tip opening displacement, R-curve, plastic collapse, weld imperfections, nondestructive testing
Group manager, The Welding Institute, Abington Hall, Abington, Cambridge
Assistant Professor, Texas A & M University, College Station, TX