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AFGROW | DTD Handbook

Handbook for Damage Tolerant Design

  • DTDHandbook
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    • Sections
      • 1. Introduction
        • 0. Introduction
        • 1. Historical Perspective on Structural Integrity in the USAF
        • 2. Overview of MIL-HDBK-1530 ASIP Guidance
        • 3. Summary of Damage Tolerance Design Guidelines
          • 0. Summary of Damage Tolerance Design Guidelines
          • 1. Summary of Guidelines
          • 2. Design Category
          • 3. Inspection Categories and Inspection Intervals
          • 4. Initial Damage Assumptions
          • 5. Residual Strength Guidelines
          • 6. Required Periods Of Safe Damage Growth
          • 7. Illustrative Example Of Guidelines
            • 0. Illustrative Example Of Guidelines
            • 1. Slow Crack Growth Structure
              • 0. Slow Crack Growth Structure
              • 1. Initial Flaw Sizes Assumed to Result from Manufacturing
              • 2. Choice of Inspection Category
              • 3. In-Service Non-Inspectable Category
              • 4. Depot Level Inspectable Category
            • 2. Fail Safe Structure
        • 4. Sustainment/Aging Aircraft
        • 5. References
      • 2. Fundamentals of Damage Tolerance
      • 3. Damage Size Characterizations
      • 4. Residual Strength
      • 5. Analysis Of Damage Growth
      • 6. Examples of Damage Tolerant Analyses
      • 7. Damage Tolerance Testing
      • 8. Force Management and Sustainment Engineering
      • 9. Structural Repairs
      • 10. Guidelines for Damage Tolerance Design and Fracture Control Planning
      • 11. Summary of Stress Intensity Factor Information
    • Examples

Section In-Service Non-Inspectable Category

For this example case, no special in-service and no depot level inspections will be required to protect the integrity of the lower wing structure shown in Figure 1.3.11.  The implication is that no inspections are desired; however, there are cases in which the flaw size at failure is so small that such a flaw might easily be overlooked during an inspection.  Thus, the in-service non-inspectable category covers those cases where inspections are neither desirable nor practical.

Residual Strength Load, Pxx

From Table 1.3.4, the required level of residual strength Pxx for non-inspectable structure is PLT.  This is the maximum load that could occur in one lifetime.  Example 1.3.3 describes the method for establishing this load level.

Analysis Guidelines

The slow crack growth and residual strength guidelines for this category are illustrated in Figure 1.3.13.  This figure specifically shows that the initial manufacturing damage is restricted from growing to critical size and causing failure of the structure due to the application of PLT in two (2) design service lifetimes.  Note that the damage limit is the ultimate failure of the wing.  Engineering judgement may dictate that a more reasonable limit and, perhaps, an easier situation to adhere to, would be to establish the limit at some intermediate point, such as the failure of the primary load path panel ­.  This might be accomplished in design at very little expense to overall weight.

Figure 1.3.13.  Illustration of Damage-Growth and Residual-Strength Guidelines for Example Problem Qualified as Slow Crack Growth Non-Inspectable