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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 Fastener Policy

In practice, the growth of flaws from fastener holes can be retarded by the use of interference fit fasteners, special hole preparation such as cold work, and to some degree, by joint assembly procedures like friction due to joint clamp-up.  Because these procedures delay flaw growth, the slow crack growth lives (or intervals) can be significantly longer than those obtained from structure containing conventional low torque clearance fasteners

Experience has shown that to achieve the beneficial effects of these techniques consistently, exceptionally high quality process control is required during manufacture.  However, this is not always obtained.  As a result, it is thought unwise to consider all interference or hole preparation systems effective in retarding crack growth.

As stated in JSSG-2006 paragraph A3.12.1.g, to maximize safety of flight and to minimize the impact of manufacturing errors, the damage tolerance guidelines should be met without considering the beneficial effects of specific joint design and assembly procedures such as interference fit fasteners, cold expanded holes, and joint clamp-up.

Exceptions to this policy can be considered.  The limits of the beneficial effects used in design should be no more than derived from assuming a 0.005 inch corner flaw as initial damage in an as-manufactured, non-expanded hole containing a neat fit fastener in a non-clamp-up joint.