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AFGROW | DTD Handbook

Handbook for Damage Tolerant Design

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    • Sections
      • 1. Introduction
      • 2. Fundamentals of Damage Tolerance
      • 3. Damage Size Characterizations
        • 0. Damage Size Characterizations
        • 1. NDI Demonstration of Crack Detection Capability
        • 2. Equivalent Initial Quality
          • 0. Equivalent Initial Quality
          • 1. Description of Equivalent Initial Quality Method
          • 2. Example Application of Equivalent Initial Quality Method
          • 3. Other Applications of Equivalent Flaw Size Distributions
        • 3. Proof Test Determinations
        • 4. References
      • 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 3.2.0. Equivalent Initial Quality

The requirements of JSSG-2006 specify that initial flaws shall be assumed to exist as a result of manufacturing and processing operations.  Small imperfections, equivalent to a 0.005 in. radius corner crack, resulting from these operations shall be assumed to exist in each hole of each element in the structure.  These assumed cracks provide the basis for the fastener policy requirements as well as the continuing damage and remaining damage assumptions.  However, if the contractor has developed initial quality data on fastener holes, these data may be submitted to the procuring activity for review and serve as a basis for negotiating a different size than the specified 0.005 in. radius corner flaw.

One method of accounting for the initial quality is to represent the quality in terms of an equivalent fatigue crack of a particular size and shape.  Such a method of quantifying the initial quality is the Equivalent Initial Quality Method [Rudd & Gray, 1976; Rudd & Gray, 1978; Pinckert, 1976; Dumesnil, et al., 1977; and Potter, 1978].  The Equivalent Initial Quality method for characterizing manufacturing quality is described in the Subsection 3.2.1 and demonstrated by example in Subsection 3.2.2.

The concept of a distribution of flaw sizes for a population of structural details that will experience equivalent stresses in operational usage has been applied in more general contexts than characterizing initial quality.  In particular, this concept plays a central role in a probabilistic approach to characterizing structural durability and in structural risk analyses.  These uses of flaw size distributions will be briefly discussed in Subsection 3.2.3