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DTD Handbook

Handbook for Damage Tolerant Design

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    • Sections
      • 1. Introduction
      • 2. Fundamentals of Damage Tolerance
        • 0. Fundamentals of Damage Tolerance
        • 1. Introduction to Damage Concepts and Behavior
        • 2. Fracture Mechanics Fundamentals
        • 3. Residual Strength Methodology
        • 4. Life Prediction Methodology
          • 0. Life Prediction Methodology
          • 1. Initial Flaw Distribution
          • 2. Usage
          • 3. Material Properties
          • 4. Crack Tip Stress Intensity Factor Analysis
          • 5. Damage Integration Models
          • 6. Failure Criteria
        • 5. Deterministic Versus Proabilistic Approaches
        • 6. Computer Codes
        • 7. Achieving Confidence in Life Prediction Methodology
        • 8. References
      • 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 2.4.1. Initial Flaw Distribution

A measure of initial quality in a component of service hardware is given by the distribution of initial crack sizes as illustrated in Figure 2.4.1.  For predictions of safety limits, the initial cracks larger than the nondestructive inspection (NDI) detectability limit are of principal concern.  Current specifications detail NDI limits and require verification/certification of contractor capability to detect cracks smaller than the specified NDI limits.  Normally, such certification is demonstrated with curves of the type shown in Figure 2.4.2.  The program of certification for a contractor’s quality control inspector/inspection techniques allows the USAF to assess the probability and confidence limits associated with detecting a given crack.  Section 3 will present a state-of-the-art summary of the technology and equipment that supports the establishment of initial flaws via nondestructive tools.


Figure 2.4.1.  Distribution of Initial Crack Size for a Given Type of Crack (e.g., Radial Cracks Growing from Fastener Holes)


Figure 2.4.2.  Certification of NDI Capability

Results generated by the F-4 Independent Review Team (IRT) provided a method of characterizing the initial flaw population (apparent initial quality) based on full-scale fatigue test-induced cracking behavior [Lozano, et al., 1974].  Given the measurable flaw distribution in a structure at some time subsequent to test startup, the initial flaw population can be backtracked by analysis.  The “back” extrapolation of the flaw population is conducted using the damage integration package.  The process is schematically illustrated in Figure 2.4.3.  Subsequently, the initial flaw distribution established as illustrated in Figure 2.4.3 can be used to estimate influence of load factors, mission profiles, and usage changes on the life of service hardware.  The F-4 IRT study also provided an evaluation of statistical methods for describing the large crack length extremes for initial flaw distributions established in this manner.  The resulting distribution of F-4 initial cracks is shown in Figure 2.4.4 [Lozano, et al., 1974; Pinchert, 1976].

Figure 2.4.3.  Determining Initial Quality by Back Calculation


Figure 2.4.4.  Initial Flaw Distribution for F-4 Based on Back Calculation