To establish the residual strength capability of a given structure under certain loading conditions, prediction techniques must be developed with a thorough understanding of the complexities involved in evaluating the residual strength.  For monolithic or single load path structures which must be classified as slow crack growth structures, the estimation of residual strength capability is straightforward.  In multiple load path, built-up structures, whether classified as slow crack growth or fail-safe structures, the strength analysis can become complicated due to the complex geometric construction of the built-up components.  In general, the prediction techniques are based on the critical value of the stress-intensity factor for a given geometry and loading.  Using fracture toughness failure criteria as explained earlier, the decay in critical stress can be obtained in terms of crack size.

As described by Figure 4.1.2, the residual strength capability is a function of service time for a given structure.  This is because the residual strength capability depends on the size of the crack in the structure and the crack grows as a function of time.  Thus, to obtain a residual strength capability curve (Figure 4.1.2), one needs two types of data: (a) the relationship between crack length and time, and (b) the relationship between fracture strength (s_f) and crack length.  Section 5 is devoted to obtaining the crack length-time relationship and the remainder of this section is devoted to presenting methods and procedures for obtaining the fracture strength-crack length (s_f vs. a) relationship.  It is to be noted that the s_f vs. a relationship is independent of time and has been referred to in the general literature as the residual strength diagram.  This section presents useful information about residual strength diagrams for single load path and for multiple load path structures.