4.1.1         Slow Crack Growth Structure

In a slow crack growth structure, the damage tolerance must be assured by the maintenance of a slow rate of crack growth, a residual strength capacity, and the assurance that subcritical damage will either be detected at the depot or will not reach unstable dimensions within the design lifetime of the structure.  The residual strength curve for a structure which is inspected periodically is schematically shown in Figure 4.1.4.  As a result of the inspections, the initially assumed cracks do not grow to a critical size and the structure is restored to its original load carrying capability when an inspection capability equal to that of the manufacturer’s is employed.

Figure 4.1.4.  Strength Criteria for Periodically Inspected Damage Tolerant Structure

Single-load-path “monolithic” structure must be qualified in this category; the residual strength estimation procedure for this type of structure is fairly straightforward.  Built-up (multiple-load-path) structure can be qualified either in this category or in the fail-safe category.