During the development cycle, the manufacturer will subject major assemblies and structural components to flight-by-flight fatigue loadings that approximate the operational environment.  Some tests are specifically identified as damage tolerant tests or as durability tests, but other tests serve a dual function - first as a durability test (two lifetimes) and then as a damage tolerant test. Component durability tests or component dual function tests are normally scheduled to precede the full-scale durability test by a sufficient amount of time that would allow incorporating suggested structural modifications into the full-scale durability test article.  The scheduling of the full scale damage tolerant test follows (and uses) the full scale durability test article.

The major assemblies and components selected for damage tolerant testing are chosen to provide further assurance that major elements will not fail during service and thus impact the operational readiness of the force due to safety-of-flight failures.  Several examples of major assemblies and components selected during recent weapon system acquisition programs are listed in Table 7.5.1.

The damage tolerant articles will include artificially induced damage such as scratches, elox notches, sawcuts, and other types of non-crack damage, and are then subjected to an interval (about one-quarter lifetime) of flight-by-flight loading that is designed to initiate the desired starter cracks.  The test interval subsequent to the precracking is up to one design lifetime with a P_xx loading applied at the end of the lifetime to verify residual strength capability.  Crack growth should be monitored throughout the test.  In-service inspection procedures should be employed whenever possible to evaluate the ability of these procedures to locate and measure the cracks.

Table 7.5.1.  Major Assemblies and Components Tested to Support

Damage Tolerant Design Verification

* Damaged subsequent to durability test (2 lifetimes).