The basic concept in damage tolerance design is to ensure the safety of the structure throughout the expected service life.  To provide the required safety, a structure must be designed to withstand service loads even when cracks are present or when part of the structure has already failed; i.e., the structure has to be damage tolerant.  The overriding philosophy is to maintain a minimum required residual strength so that catastrophic failure of the structure is prevented.

Figure 4.1.1 identifies the major sequence of events that ultimately define the residual strength requirements.  As can be noted from the figure, once a safety-of-flight-critical element is identified, either its structural configuration or its degree of inspectability will establish the allowable structural design concept and the inspection level categories.  Every safety-of-flight-critical element must be qualified in at least one design concept category and in one inspection category.  Each allowable combination of design concept and inspection category is coupled in JSSG-2006 to a residual strength requirement, a service life requirement, and a requirement to assume a level of initial damage.

Figure 4.1.1.  The Structural Configuration or Degree of Inspectability Controls the Subsequent Choices of Design Concept and Inspection Level

Figure 4.1.2 illustrates the residual strength and the service life interval requirements as well as a residual strength capability curve.  The residual strength capability curve defines the level of load that the structure can withstand without failing in the presence of a growing crack.  To account for the change in residual strength capacity as a function of time, it is necessary to determine the crack size as a function of time.  The crack-growth-life curve and its various properties are shown schematically in Figure 4.1.3. Shown are the various technology and specification requirements needed to define the crack growth curve which, in turn, establishes the life limit.

Figure 4.1.2.  Relationship Between the Life Expended and Residual Strength Capability

Showing a Monotonic Decrease in Load Carrying Capacity Due to Damage

Figure 4.1.3.  Relationship Between Crack Length and Life Expended Showing

a Monotonic Increase in Crack Length Up Until Failure

As can be seen from Figure 4.1.2, when the residual strength of the structure falls below the maximum stress in the service load history, failure can be expected.  To avoid such a failure, a thorough understanding of the problem is essential.  Significant advances have been made in recent years in procedures for analyzing damaged structures.  Assessments now consider residual strength, damage growth, interactive multiple damage sites and quantitative structural maintenance and in-service evaluations.

The application of existing fracture mechanics solution techniques has yielded effective methods for analyzing the residual strength of the cracked structure.  The necessary theories and methods for determining the residual strength capability of cracked structures are presented in this section.  Prior to presenting this information in the following sections, a few remarks are made about the residual strength requirements for the two damage tolerant design categories: slow crack growth structure and fail-safe structure.