The use of fracture mechanics in assessing 'fitness for purpose' of engineering structures is generally considerably more involved than the the simple illustrations given in this tutorial. In the UK, Engineering Critical Assessment (ECA) of potential or actual defects in engineering structures is codified in two prime documents - PD6493 : 1991 [1] and the CEGB R6 procedure [2], both of which have been developed over some 20 years. PD 6493 has now been replaced by the more extensive procedures in BS7910 : 1997 [3].
The original PD6493 : 1980 document paid scant attention to the possibility of plastic collapse, as it was primarily concerned with assessment of defects at welds. The approach of the Central Electricity Generating Board (CEGB) was different, as much of their equipment operated at high temperatures where plastic collapse might assume equal, or greater, importance with fracture. They therefore proposed a two parameter technique of assessing the possibility of plastic collapse and fracture separately, and then plotting these possibilities as the axes on a Failure Assessment Diagram (FAD). This was the basis of the R6 : 1976 document. There were other differences between the approaches taken by PD6493 and R6, and more details can be found in reference 4. This gives papers which trace the development of the R6 document, provide background on US approaches to Failure Assessment Diagrams (based on the J-integral) and outline the PD6493 philosophy.
In revisions to both R6 and PD6493, account was taken of useful developments in the other code and, currently, although BS7910 and R6 Revision 3 still are very different in format, they perform many of the same tasks. BS7910 is perhaps more accessible in formulation to the informed 'layperson', but either will produce reliable results.
The concept of Failure Assessment Diagrams immediately leads to a need for fracture parameters able to cope with fairly extensive plasticity. R6 essentially provides a special form of J-integral analysis with additional safeguards imposed at the plastic collapse limit, while PD6493 was primarily intended for use with CTOD, although in many cases a K based analysis is acceptable. BS7910 allows the use of CTOD, K or 'equivalent K' derived from the J-integral.
A typical FAD is shown below (Level 2 in BS7910):
Lr is a load ratio defined as a reference stress over the lower yield strength of 0.2% proof stress. The reference stress characterises the possibility of plastic collapse and is calculated in specific ways. Kr is the fracture ratio and is defined as the applied stress intensity factor over the material toughness. Essentially, a flaw is stable if the assessment point lies inside the FAD curve.
This is a very cursory introduction to a complex topic and much more information is available in the references given. It does, however, serve to indicate that fracture is only one of a number of failure mechanisms and that, generally, the possibility of plastic collapse also needs consideration.
References
1. British Standards Institution, Guidance on methods for assessing the acceptability of flaws in fusion welded structures, PD6493 : 1991, BSI, London.
2. Nuclear Electric, Assessment of the integrity of structures containing defects, R/H/R6 Revision 3, Barnwood, Gloucestershire.
3. British Standards Institution, Guide on methods for assessing the acceptability of flaws in structures, BS7910 : 1997, BSI, London.
4. Institution of Mechanical Engineers, 20 years of R6, proceedings of a seminar held in November 1996, IMechE, London.