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AFGROW | DTD Handbook

Handbook for Damage Tolerant Design

  • DTDHandbook
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    • Sections
      • 1. Introduction
      • 2. Fundamentals of Damage Tolerance
      • 3. Damage Size Characterizations
      • 4. Residual Strength
      • 5. Analysis Of Damage Growth
      • 6. Examples of Damage Tolerant Analyses
        • 0. Examples of Damage Tolerant Analyses
        • 1. Damage Tolerance Analysis Procedure
        • 2. Damage Development And Progression
          • 0. Damage Development And Progression
          • 1. Slow Crack Growth Structure
          • 2. Multiple Load Path, Fail Safe Structure
          • 3. Crack Arrest, Fail Safe Structure
        • 3. Slow Crack Growth Structure
        • 4. Multiple Load Path Structure
        • 5. Fail Safe Multiple Load Path Structure
      • 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 6.2.2. Multiple Load Path, Fail Safe Structure

The second example is academic, but illustrative.  It is a multiple load path dependent beam consisting of members A, B, C (Figure 6.2.2a).  Assume that crack 1 is the critical crack.  (If the critical location was at the other side of the hole, damage development would be similar as in Figure 6.2.2).  Due to assembly drilling the two members, A and B should both be assumed flawed.  The damage development is shown in Figure 6.2.2b, c.

Figure 6.2.2.  Damage Development in Multiple Load Path, Fail Safe Structure

Cracks 1 and 2 both start as 0.05 inch flaws.  Crack 1 is assumed to grow faster, and when K = KIc rapid crack propagation (instability) will occur at which point member A is assumed failed.  Remaining structure damage has to be assumed in the adjacent member (crack in member C).  It is a 0.005 inch crack plus its prior growth, Da.

Due to member failure the stress-intensity factors of all cracks will show a jump.  Therefore, cracks 2 and 3 will grow much faster than before:  Final failure will occur when the stress-intensity factor (K) of crack 3 reaches KIc, or when K of crack 2 reaches Kc, whichever occurs first.  (It is assumed that plane stress prevails in the thin member B).  The period between failure of member A and final failure (indicated by t in Figure 6.2.2c) has to be adequate for one of the options of remaining structure damage inspection.  Otherwise, the structure would not qualify as Multiple Load Path, Fail Safe structure.