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AFGROW | DTD Handbook

Handbook for Damage Tolerant Design

  • DTDHandbook
    • 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 4.1.1. Slow Crack Growth Structure

4.1.1         Slow Crack Growth Structure

In a slow crack growth structure, the damage tolerance must be assured by the maintenance of a slow rate of crack growth, a residual strength capacity, and the assurance that subcritical damage will either be detected at the depot or will not reach unstable dimensions within the design lifetime of the structure.  The residual strength curve for a structure which is inspected periodically is schematically shown in Figure 4.1.4.  As a result of the inspections, the initially assumed cracks do not grow to a critical size and the structure is restored to its original load carrying capability when an inspection capability equal to that of the manufacturer’s is employed.

Figure 4.1.4.  Strength Criteria for Periodically Inspected Damage Tolerant Structure

Single-load-path “monolithic” structure must be qualified in this category; the residual strength estimation procedure for this type of structure is fairly straightforward.  Built-up (multiple-load-path) structure can be qualified either in this category or in the fail-safe category.