The existing crack tip opening displacement (CTOD) design curve approach is summarised, and new proposals are described for the assessment of finite width plates, plain materials and welded joints that have been post-welded heat treated. Recommendations are also made concerning the mechanical properties of welds.
All engineering materials contain cracks or crack-like flaws at some level of examination. It follows that all engineering structures have an inherent tolerance to specific combinations of cracks and applied stresses or strains. Recognising this fact, the primary objective of the CTOD design curve approach (1–3) is to determine tolerable or maximum allowable crack sizes, i.e. the crack sizes that are significantly smaller than the critical values for brittle fracture or plastic collapse.
It is important to note that the CTOD design curve approach is based on the results of full section thickness BS 5762 CTOD tests (4), a design curve, and a number of simplifying assumptions. These provide an easy and rapid basis for determining the tolerable sizes of cracks in welded structures.
There have been many examples over the last 15 years where the CTOD design curve approach has saved large amounts of time and money by avoiding such alternatives as more sophisticated analyses, unnecessary weld repairs, and even some unnecessary post weld heat treatments (5). For these situations the CTOD design curve approach continues to have a valuable role (6). Nevertheless, it is recommended that the CTOD design curve approach should be regarded as a first coarse filter in fitness-for-purpose assessments, whilst retaining the option of using alternative, theoretically more accurate, but less conservative fracture mechanics methods. Two such alternatives have been proposed in some recent complimentary studies at The Welding Institute (7,8), and these have led to a further proposal for a three tier fitness-for-purpose assessment procedure (9).
While the existing CTOD design curve approach (3) has been widely publicised by its inclusion in PD 6493 (6), this document does not draw attention to some of the limitations of the approach. For example, the present CTOD design curve approach should be limited to through thickness crack length to plate width ratios less than approximately 0.1 and uniform gross-section stresses less than yield. Fortunately, most industrial applications of PD 6493 fall within these limitations. On the other hand, guided only by PD 6493, some experimentalists and analysts have worked outside the limitations and have concluded that the CTOD design curve approach may be unsafe in some instances.
This paper summarises the existing CTOD design curve approach and some proposed finite width modifications (10). In the spirit of the CTOD design curve approach, the modifications are deliberately simple in order to avoid the iterative calculations associated with alternative more accurate analyses. This paper also gives some new recommendations concerning material properties and the various assumptions that may be necessary when applying the CTOD design curve approach to plain materials and welded joints.
