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AFGROW | DTD Handbook

Handbook for Damage Tolerant Design

  • DTDHandbook
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    • Sections
      • 1. Introduction
      • 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
        • 0. Damage Tolerance Testing
        • 1. Introduction
        • 2. Material Tests
        • 3. Quality Control Testing
        • 4. Analysis Verification Testing
        • 5. Structural Hardware Tests
          • 0. Structural Hardware Tests
          • 1. Test Conditions
          • 2. Initial and Continuing Damage
          • 3. Residual Strength Testing
          • 4. Damage Tolerance Test Articles
          • 5. Evaluation and Interpretation of Test Results
        • 6. References
      • 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 7.5.2. Initial and Continuing Damage

A 0.05 inch initial crack is assumed in slow-crack growth structure and in fail safe structures.  If the specimens for design development testing are not provided with artificial defects, the cracks, once initiated, will grow through the sizes mentioned above.  Crack-growth records would automatically cover the span of the requirements, provided the cracks can be detected.  Otherwise the recorded crack-growth curve would have to be extrapolated backwards.  If initial flaws are provided, it is recommended to make them the size assumed in the requirements or close to the size for analysis substantiation.

Continuing damage, from a testing standpoint, is more difficult to make as a result of the small initial sizes and the different growth requirements for different cases.  Consider the example configuration shown in Figure 7.5.1 where A is the primary damage site, and B, C, D, & E are continuing damage sites.  The four parts of the figure show (a) the initial damage assumed in the panel per JSSG-2006 paragraph 3.12.1 and Tables XXX and XXXI, (b) the initial damage and growth until the primary damage terminates at the edge, (c) the continuing damage that starts at B, the opposite side of the primary damage site which terminates in hole 2, and (d) the growth of continuing damage at C until termination at hole three. While the analysis can follow the assumptions required by JSSG-2006, it would be difficult (if not impossible) to manufacture the necessary continuing sizes either prior to test or after the primary damage (segment A) terminated at the edge.  Therefore common practice is to put in the primary damage and continuing damage starter flaws as shown in Figure 7.5.1.a and let the specimen crack growth proceed without additional perturbations.  Post test analysis of the crack growth data and fracture surface striation morphology will document this logic. 

Figure 7.5.1.  Primary and Secondary Damage Sites and Continuing Damage

In a AFRL/VA contracted study, Brussat et al. [1977] were able to show that the experimental fatigue lives of built-up structure with and without continuing damage flaws were about the same and that the primary crack damage chose the most effective path through the structure. Secondary cracks developed in a natural way during the test. Dormant periods when a crack ran into  a hole could be estimated and subtracted if the results were used for a check of the analysis.