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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
          • 5. Residual Strength Guidelines
          • 6. Required Periods Of Safe Damage Growth
          • 7. Illustrative Example Of Guidelines
            • 0. Illustrative Example Of Guidelines
            • 1. Slow Crack Growth Structure
            • 2. Fail Safe Structure
              • 0. Fail Safe Structure
              • 1. In-Service Inspection Consideration
              • 2. Initial Flaw Considerations
              • 3. In-Service Flaw Assumptions Following Inspection
              • 4. Adjacent Structure Damage Following the Failure of the Major Load Path
              • 5. Analysis of Intact Structure–Residual Strength Guidelines and Damage Growth Limits
              • 6. Analysis of Intact Structure (Alternate Requirement)
              • 7. Discussion of Intact Structure Analysis
              • 8. Analysis of Remaining Structure Subsequent to Load Path Failure
              • 9. Derivation of Residual Strength Load
              • 10. Incremental Damage Growth Da
              • 11. Alternative-Analysis of Remaining Structure Subsequent to Load Path Failure
              • 12. Summary and Comments
        • 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 Derivation of Residual Strength Load

In the analysis of the intact structure, the critical damage limit is failure of the skin panel ­.  The mode of failure was slow growth of either depot level inspection type damage or initial manufacturing damage (Figure 1.3.18 and 1.3.19, respectively).  In each case, the damage is assumed to grow in a stable manner until the critical damage size in the skin panel is reached.  The critical damage size for this case would be that size at Pxx = PDM or Pxx = PLT where Pxx is bounded by

            Plimit £ Pxx £ 1.2Pone lifetime

For a balance fail safe design, the remaining structure must be capable of withstanding the effects of the major load path failing, including the redistribution of load to adjacent members at the time of load path failure.  This is the basis for the requirement that the remaining structure must support the Pyy residual strength load.  The load Pyy is dependent upon the design allowable for the first panel (Panel ­ in this case). 

Assume for example that the Pxx allowable for first panel failure is exactly Plimit.  The remaining structure must be capable of supporting Plimit with adjacent panels carrying the increment or that portion originally carried by panel ­ at Plimit.  This is illustrated in Figure 1.3.21 where the amount of load in panel ­ at the limit design condition, i.e. P2 is redistributed after it is multiplied by 1.15 to account for dynamic effects (DP1+DP2+ DP3+ DP4+ DP5).  The total redistribution increment then is


The residual strength capability of the remaining structure is then checked against this condition; the Pyy requirement for panel ® is .


Figure 1.3.21.  Illustration of Redistributed Panel Load P2 to Adjacent Structure