Throughout the structural hardware test program, there is substantial attention given to cracking problems.  Such problems, when they surface, identify areas where the design should be modified to ensure the soundness of the final product (see Figure 7.5.2).  Each structural problem is analyzed to determine the specific cause of the problem so that appropriate candidate solutions can be incorporated into production aircraft.

Figure 7.5.2.  Summary of Interactions Resulting from Structural Failure Per JSSG-2006 Requirements

One final check on the adequacy of the structural design is the teardown inspection that follows the full-scale durability test (two lifetimes or economic life) and the damage tolerance test (one lifetime).  The teardown inspection is required by JSSG-2006 paragraph 4.11.1.2.2.e to provide assurance that no critical area has been overlooked in the course of normal inspections, and to characterize the state of crack development in selected structural areas.  In relation to the characterization of the state of crack development, the teardown inspection will typically include the sectioning of the structure for additional fatigue testing, residual strength testing, and/or microscopically tracking cracks back to the start of the durability test.  The crack population at the end of the durability test and damage tolerance test becomes the basis for assessing the quality of the production structure through the use of the equivalent initial quality concept illustrated in Figure 7.5.3 (see Section 3 for more details).

Figure 7.5.3.  Equivalent Initial Quality Distribution Obtained by Backtracking Cracks Found in Durability Test Articles.  Backtracking Procedures Involve Fractography and Fracture Mechanics Crack Growth Analyses