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AFGROW | DTD Handbook

Handbook for Damage Tolerant Design

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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 Incremental Damage Growth Da

The remaining structure analysis of damage growth and residual strength considers damage in the adjacent structure at the time of load path failure which has grown an amount Da from the time of manufacture (Figure 1.3.19).  Since the structure must met the single design lifetime requirement, it becomes necessary to establish at what point during the lifetime the failure of the load path is assumed to take place so that the proper amount of growth Da can be computed to represent growth during this time segment. 

Figure 1.3.22 illustrates the growth of the 0.005 inch manufacturing type damage from time zero for one design lifetime.  In this example, the walk-around-visual inspection is used to detect the failure of the major load path and the inspection interval is 10 flights.  JSSG-2006 Table XXXIII requires a factor of 5 on this interval and thus the damage growth life requirement is 50 flights.  Therefore, the maximum amount of Da and the condition to be met would be growth for one design lifetime minus 50 flights. 

For any other in-service inspection interval the amount Da would be computed in a similar manner.  For example, if the walk-around-visual inspection was not conducted and fail safety was dependent upon discovery of damage at the scheduled 10,000 hour depot level inspection, then the increment of growth Da would be one design lifetime minus 2x 10,000 hours, as in Figure 1.3.23.


Figure 1.3.22.  Development of Increment of Growth Da2 for Walk-Around-Visual Inspectable Damage


Figure 1.3.23.  Development of Increment of Growth Da2 for Depot-Level-Inspectable Damage