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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
            • 0. Initial Damage Assumptions
            • 1. Intact Structure Primary Damage Assumption
            • 2. Continuing Damage
            • 3. Fastener Policy
            • 4. In-Service Inspection Damage Assumptions
            • 5. Demonstration of Initial Flaw Sizes Smaller Than Those Specified
          • 5. Residual Strength Guidelines
          • 6. Required Periods Of Safe Damage Growth
          • 7. Illustrative Example Of Guidelines
        • 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 Demonstration of Initial Flaw Sizes Smaller Than Those Specified

The choice of smaller initial damage must be justified either through an NDI demonstration or a proof test.  The NDI demonstration program is described in JSSG-2006 paragraph 4.12.1.a.  The program must be formulated by the contractor and approved by the Air Force and must verify that, for the particular set of production and inspection conditions, flaws will be detected to the 90% probability level with 95% confidence.

Where no other means of NDI is available or where it is cost effective, the proof test can be an effective means of screening structure for flaws.  Proof testing generally has been successful for the more brittle materials which exhibit plane strain fracture behavior (e.g. high strength steels) and for small structural components.  The application of proof testing to complete airframe structure in the USAF has been somewhat limited.  The notable exception has been the cold proof tests (-40° F) of the F-111 aircraft to clear the D6AC steel wing carry-through and appendage components for flight usage. 

In general, proof testing has only been used on major airframe components as a last resort to allow operation (usually restricted) until extensive modifications are made to the structure (e.g. wing reskin modification of the B-52D).  In deriving estimates of the initial flaw size associated with the proof test conditions, approved upper-bound fracture toughness values shall be used for the materials under proof test conditions.  Section 3 also presents more information on the proof test concept.