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DTD Handbook

Handbook for Damage Tolerant Design

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    • About
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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
            • 0. Required Periods Of Safe Damage Growth
            • 1. Slow Crack Growth Structure
            • 2. Fail Safe Multiple Load Path Structure
            • 3. Fail Safe Crack Arrest Structures
          • 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 6.2.1. Slow Crack Growth Structure

The first example is a heavy-section spar cap (Figure 6.2.1a).  The spar cap will be treated as Slow Crack Growth structure.  The initial flaw has to be assumed at the most critical location.  Assume that this is location A (Figure 6.2.1a).  Due to assembly drilling the skin is assumed to be flawed also.  If there is load transfer from the cracked spar cap to the skin, it should be taken into consideration.  The damage development for the spar cap is shown schematically in Figure 6.2.1b, the change of the stress-intensity factor is shown schematically in Figure 6.2.1c.


Figure 6.2.1.  Damage Development in Slow Crack Growth Structure

Crack 1 starts as a 0.05 inch crack.  It grows until the remaining ligament fails at K = KIc.  The continuing damage is a 0.005 inch flaw at the other side of the hole (point B).  Its prior growth need not be considered, since the primary damage terminated by ligament failure (JSSG-2006 paragraph A3.12.1e).  Hence, it may be assumed to have been stationary thus far.

At ligament failure, crack 2 is suddenly introduced and the stress-intensity factor is determined by the total damage size, consisting of the failed ligament, the hole, and a 0.005 inch crack.  This damage grows to failure by the growth of crack 2.

Now consider the case that B is the critical location (Figure 6.2.1a).  In that case, crack 1 would be absent (ligament intact), but crack 2 would start at 0.05 inch (dashed lines in Figure 6.2.1b,c).  Due to the absence of crack 1 it will grow slower.

Now assume that C is the most critical location.  This case is depicted in Figure 6.2.1d, e.  Crack 3 will start as a 0.05 inch crack, and terminates in the next hole.  Continuing damage is a 0.005-inch crack at the other side of the hole, plus its prior growth, Da, must be assumed (JSSG-2006 paragraph A3.12.1e).  Contrary to the previous case, the 0.005 inch crack was growing previously.  Its independent growth, Da, has to be calculated.  Due to this previous growth there is an increase of K.  When crack 3 terminated in the next hole the stress-intensity factor of crack 4 jumps, because crack 3 and 4 together now constitute the total damage.  Therefore, the growth of crack 4 will be much faster than before.