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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
            • 0. Residual Strength Guidelines
            • 1. Fail-Safe Structure at Time of Load Path Failure
            • 2. Determining the Residual Strength Load for Remaining Structure
          • 6. Required Periods Of Safe Damage Growth
          • 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 Residual Strength Guidelines

The residual strength capability is defined as the amount of static strength available at any time during the service exposure period considering that damage is initially present and grows as a function of service exposure time.  The strength degrades with increased damage size, as shown in Figure 1.3.8.  The intent of JSSG-2006 paragraph 3.12.2 is to provide residual strength capability for intact structure of at least design limit load at all times throughout the service life of the structure.  The requirement to maintain limit load capability is considered necessary to allow unrestricted operational usage.


Figure 1.3.8.  Residual Strength Diagram

The residual strength guidelines are specified in terms of the minimum internal member load Pxx that must be sustained.  The magnitude of Pxx depends upon the service exposure time of the structure between inspections and the overall degree of inspectability.  The load Pxx is intended to represent the maximum load that the aircraft might encounter during the time interval between inspections. 

The required Pxx is at least design limit load for all intact structure whether the structure is being qualified as Slow Crack Growth or Fail Safe.  The required Pxx is also at least design limit load when the only planned safety inspections are at the depot (i.e., the depot or base-level inspection category).

In addition, all Fail Safe Structure must be designed to be at least depot level inspectable, and Pxx over this inspection interval must be at least limit load.  This restriction is obvious since the only means to protect the safety is not to allow damage growth to degrade the strength of the structure to less than design limit load.  Where partial failure is allowed and subsequent detection of failed load path is required, the limit load requirement on intact structure has two benefits.  First, it is the only way that the operational force can be maintained with unrestricted capability; and second, when coupled with the intact structure damage growth guidelines, it provides assurance that, under normal situations, early nuisance cracking will not occur as a result of lower stress.