The damage tolerance design guidelines are illustrated in Figure 1.3.1 in a diagrammatic form.  Since residual static strength generally decreases with increased damage size, the residual strength and growth guidelines are coupled through the maximum allowable damage size, i.e. the damage size growth limit established by the minimum-required residual strength load.  The safe growth period (period of unrepaired service usage) is coupled to either the design life requirement for the air vehicle or to the scheduled in-service inspection intervals.  While the specific guidelines of JSSG-2006 may seem more complex than described in Figure 1.3.1, all essential elements are as illustrated.  The remainder of Section 1.3 will describe these individual elements.

A structure can be qualified under one of two categories of defined damage, either Slow Crack Growth or Fail Safe.  In the Slow Crack Growth category, structures are designed such that initial damage will grow at a stable, slow rate under service environment and not achieve a size large enough to cause rapid unstable propagation. In the Fail Safe category, structures are designed such that propagating damage is safely contained after failing a major load path by load shift to adjacent intact elements or by other damage arrestment features.

Figure 1.3.1.  Residual Strength and Damage Growth Guidelines

In Slow Crack Growth qualified structure, damage tolerance (and thus safety) is assured only by the maintenance of a slow rate of growth of damage, a residual strength capacity and the assurance that sub-critical damage will either be detected at the depot or will not reach unstable dimensions within several design life times. 

In Fail Safe qualified structure, damage tolerance (and thus safety) is assured by the allowance of partial structural failure, the ability to detect this failure prior to total loss of the structure, the ability to operate safely with the partial failure prior to inspection, and the maintenance of specified static residual strength through this period.  Section 1.3.2 discusses the design categories.

Each structure must qualify within one of the designated categories of in-service inspectability (referred to as “The Degree of Inspectability” in JSSG-2006), including the option to designate Slow Crack Growth qualified structure as “in-service non-inspectable.”  The various degrees of inspectability refer to methods, equipment, and other techniques for conducting in-service inspections as well as accessibility and the location of the inspection (i.e., field or depot).  These degrees of inspectability are discussed in Section 1.3.3.

The selection of the most appropriate damage tolerance category under which to qualify the structure is the choice of the designer/analyst.  The choice of degree of in-service inspectability is somewhat limited, however, to those described in JSSG-2006.  The inspection guidelines have been developed based upon past and present experiences and are felt to be reasonable estimates of future practice.

The intent of the guideline is to provide for at-least design limit load residual strength capability for all intact structure, i.e., for sub-critical damage sizes in slow crack growth structure and damage sizes less than a failed load path in fail safe qualified designs.  This requirement allows for full limit load design capability and thus unrestricted aircraft usage.  The imposition of the requirement constrains structure qualified as Slow Crack Growth to either depot level inspectable or in-service non-inspectable.

As described in Section 1.3.2, fail safe structure must meet both the intact structure and remaining structure guidelines.  Slow crack growth structure will meet either the depot level inspectable or the non-inspectable structure guidelines.  For each structure, evaluation of the following parameters is required:

·        Design Category

·        Degree of In-Service Inspectability

·        Inspection Intervals

·        Initial Damage, In-Service Damage and Continuing Damage Assumptions

·        Minimum Required Residual Strength

·        Damage Size Growth Limits

·        Period of Unrepaired Service Usage

·        Remaining Structure Damage Sizes

Each of these are described in the following sections, and Section 1.3.7 shows several examples.