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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
          • 0. Material Tests
          • 1. Fracture Toughness Testing Methods
          • 2. Sub-Critical Crack Growth Testing Methods
            • 0. Sub-Critical Crack Growth Testing Methods
            • 1. Crack Growth Rate Testing
            • 2. Stress Corrosion Cracking
        • 3. Quality Control Testing
        • 4. Analysis Verification Testing
        • 5. Structural Hardware Tests
        • 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 Sub-Critical Crack Growth Testing Methods

Since the early sixties, sub-critical crack growth data have provided the basis for estimating the crack growth behavior of structural components under service conditions.  In the initial stages of damage tolerant design methodology and test development, the effects of stress ratio, environment and load sequencing were poorly understood.  Thus, the initial damage integration packages did not account for these effects; furthermore, testing capability was for the most part limited to constant amplitude or to block loading.  By the early seventies, understanding and capability had progressed to the point where evaluation of each major damage producing element in the service history could then be modeled by damage integration packages. 

The ASTM Committee E08 on Fatigue and Fracture Testing also played an important part in developing standards for collecting data which could be used to support damage integration packages.  Throughout the seventies, inter-laboratory testing programs were conducted which further refined the testing conditions that could be standardized by consensus.  The AF Materials Laboratory funded development of a standard test method to ensure a stable methodology for information used in aircraft damage tolerance assessments [Hudak, et al., 1978].  In 1978, ASTM issued the first standard based on these developments, ASTM E647, on fatigue crack growth rate (da/dN) testing.  Additional standards or additions to existing standards such as ASTM E1681 on environmentally assisted cracking testing (KIEAC), on corrosion fatigue, on automated methods and on threshold testing have and continue to evolve.  Methods for non-visual crack size monitoring such as compliance and electric-potential have been developed over the last 15 years and incorporated into nearly all of the fracture related standards.