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DTD Handbook

Handbook for Damage Tolerant Design

  • DTDHandbook
    • About
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    • PDF Versions
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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 1.3.4.1. Intact Structure Primary Damage Assumption

The basic premise in arriving at the initial damage sizes is the assumption that the as-fabricated structure contains flaws of a size just smaller than the maximum undetectable flaw size found with the NDI procedures used on the production line.  These flaw size shapes which are intended to be covered by the initial flaw size assumptions include radial tears, drilling burrs, and rifle marks at fastener holes as well as forging defects, welding defects, heat treatment cracks, forming cracks, and machining damage at locations other than fastener holes.

Table 1.3.2 and Figure 1.3.2 summarize the initial damage assumptions as specified in JSSG-2006 paragraph A3.12.1 and Table XXX.  For slow crack growth and fail safe primary element structure, the assumed initial flaw at holes and cutouts is a 0.05 inch through the thickness flaw at one side of the hole if the material thickness is equal to or less than 0.05 inch.  For thicker materials (> 0.05 inch), the assumed initial flaw is a 0.05 inch radius corner flaw.

At locations other than holes, the assumed initial flaw is a semi-circular surface flaw with a length of 0.25 inch and depth of 0.125 inch, or, for material thickness less than 0.125 inch, a through thickness flaw of 0.25 inch length.

These assumptions - relative to the size, shape and location - were based on a review of existing NDI data.  The crack length values given in Figure 1.3.2 and Table 1.3.2 were selected as most appropriate for the types of cracks considered and for the two design categories.

Table 1.3.2.  Initial Flaw Assumptions for Metallic Structure, JSSG-2006 Appendix Table XXX

Category

Critical Detail

Initial Flaw Assumption*

Slow crack growth and Fail Safe primary element

Hole, Cutouts, etc.

For t £ 0.05”, 0.05” through thickness flaw
For t > 0.05”, 0.05” radial corner flaw

Other

For t £ 0.125”, 0.25” through thickness flaw
For t > 0.125”, 0.125” deep x 0.25” long surface flaw

Welds, embedded defects

TBD

* - Flaw is oritented in the most critical direction

 

Figure 1.3.2.  Summary of Initial-Flaw Assumption for Intact Structure

 

The Slow Crack Growth initial damage sizes are based on NDI probability of detection (POD) data (90 percent probability of detection with 95 percent confidence).  The 0.050 inch crack size for holes and cutouts is based on POD data obtained in the lab using eddy current inspection with fastener removed.  The surface flaw size, 0.250 inch long by 0.125 inch deep, was obtained from Air Force sponsored inspection reliability programs where several techniques were used including ultrasonic, dye penetrant and magnetic particle.  In these programs, most techniques were found to be sensitive to both surface length and flaw depth and thus the NDI capability must be judged in terms of the flaw shape rather than simply surface length or crack depth.