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AFGROW | DTD Handbook

Handbook for Damage Tolerant Design

  • DTDHandbook
    • About
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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
            • 0. Other Applications of Equivalent Flaw Size Distributions
            • 1. Durability Analysis
            • 2. Risk Analysis
        • 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 Durability Analysis

A probabilistic approach to characterizing structural durability has been extensively explored by Manning and Yang [1987, 1989].  For the durability analysis, the growth of a distribution of equivalent initial flaw sizes for a population of structural elements is calculated as a function of flight hours in the expected usage environment.  Durability is then characterized in terms of either the expected number of cracks that will exceed a fixed size as a function of flight hours or in terms of the distribution of flights to reach a crack of given size.  These concepts are illustrated in Figure 3.2.10, from Manning & Yang [1989], in which:

     EIFSD represents the equivalent initial flaw size distribution of initial quality;

     p(i,t) represents the distribution of number of cracks of a size larger than x;

     FT(x) (t) represents the distribution of service time to reach a crack of size x.

The EIFSD must be projected forward based on a crack growth methodology that is compatible with that used to produce the EIFSD. Manning and Yang recommend a combined deterministic crack growth analysis (DCGA) and stochastic crack growth analysis (SCGA) for projecting the EIFSD.

Figure 3.2.10.  Schematic Using the Equivalent Initial Crack Size Distribution (EIFSD) for Durability Analysis