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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 Depot Level Inspectable Category

The capability of inspection in the field is generally less than at the depot.  The sizes of damage assumed to exist following inspection are specified in Table 1.3.2.  For this example, assume that penetrant or ultrasonics will be used at the depot both exterior and interior to the lower surface.  If this type of inspection is conducted, the damage likely to be found will be much smaller than the failed skin panel.  From Table 1.3.3, the minimum damage size to be assumed at the hole is a through crack of 0.25 inch uncovered length.  The locations of the 0.25 inch flaw in both the skin and the splicing stringer should be selected on the basis of inspectability but should be the location most critical to subsequent growth.  Assume for purposes of illustration, that the damage is as indicated in Figure 1.3.14.  The 0.005 inch flaw away from the primary damage site represents the initial manufacturing type damage as explained in Section 1.3.4.

Figure 1.3.14.  Illustration of Primary Damage Following a Depot-Level Penetrant or Ultrasonic Inspection

Residual Strength Load, Pxx

The required level of residual strength Pxx for the depot or base level inspection category is PDM, as shown in Table 1.3.4.  This is the maximum load that would occur in the planned ¼ lifetime (10,000 hour) inspection interval.  The method for establishing this particular load level follows the method outlined in Example 1.3.3 where the one life time exceedance curve is multiplied by a factor of 5 rather than 20.

Analysis Guidelines

Figure 1.3.15 illustrates the slow crack growth and residual strength guidelines for this category, as established by JSSG-2006 paragraph A3.12.2.  This figure specifically shows that the post-inspection damage is restricted from growing a crack to critical size and thereby causing failure of the structure due to the application of PDM in two times the inspection interval (½ lifetime, 20,000 flight hours).

Figure 1.3.15.  Illustration of Damage-Growth and Residual Strength Guidelines for Example Problem Qualified as Depot-Level-Inspectable