TABLE OF CONTENTS In three dimensions there are numerous possible crack configurations (Pook 1986). In general, cracks three dimensional cracks observed in service cannot be approximated by two dimensional stress intensity factor solutions.
Naturally occurring cracks, and crack like flaws, are often irregular in shape, for example the casting defects in Figures 3.17 and 7.2. The three main types of cracks encountered in practice are shown schematically in Figure A.29. These are surface cracks, internal cracks,and through the thickness cracks. Methods of estimating stress intensity factors for irregular cracks have been of interest for many years (Paris and Sih 1965, Anon. 1980b, Chang 1982, Pook 1982a, Murakami 2002). Normally, stress intensity factors vary along a crack front, and it is the largest value of the Mode I stress intensity factor, K I, that is usually of interest. Adjacent cracks, as in Figure 7.1, can increase stress intensity factors through interaction effects. These are difficult to assess when cracks are not coplanar.
In the analysis of laboratory and service failures, the size and shape of cracks can usually be ascertained in detail by the examination of fracture surfaces. However, the results of non destructive testing will usually supply only a limited amount of information. For example, only the surface length and maximum depth of a surface crack might be available (see Appendix C).
Initially, an intuitive approach was used by various authors to estimate stress intensity...
