The Griffith study usefully identifies two distinct stages in crack evolution, initiation and propagation. Of these, initiation is by far the less amenable to systematic analysis, governed as it invariably is by complex (and often illdefined) local nucleation forces that describe the flaw state. Accordingly, we defer investigation of crack initiation to chapter 9. A crack is deemed to have entered the propagation stage when it has outgrown the zone of influence of its nucleating forces. The term ‘propagation’ is not necessarily to imply departure from an equilibrium state: indeed, for the present we shall concern ourselves exclusively with equilibrium crack propagation. Usually (although not always), a single ‘well-developed’ crack, by relieving the stress field on neighbouring nucleation centres, propagates from a ‘dominant flaw’ at the expense of its potential competitors. In the construction of experimental test specimens for studying propagation mechanics such a well-developed crack may be artificially induced, e.g. by machining a surface notch. This pervasive notion of a well-developed crack, taken in conjunction with the fundamental Griffith energy-balance concept, provides us with the starting point for a powerful analytical tool called fracture mechanics, the many facets of which will become manifest in the remaining chapters.

The formulation of fracture mechanics began with Irwin and his associates round about 1950. The impetus for the development of this discipline originally came from the increasing demand for more reliable safety criteria in engineering design.