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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
        • 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 2.4.0. Life Prediction Methodology

Currently, within the Air Force, airframe life predictions are based on a crack growth damage integration package that uses a data base and analysis to interrelate the following six elements:

a)                  The initial flaw distribution which accounts for size variations and location of cracks in a given structure;

b)                  aircraft usage describing the load spectra data base;

c)                  constant amplitude crack growth rate material properties accounting for stress ratio and environmental effects;

d)                  crack tip stress intensity factor analyses which account for crack size, shape, and structural interactions;

e)                  damage integrator model which assigns a level of crack growth for each applied stress application and accounts for load history interactions; and

f)                    the fracture or life limiting criterion which establishes the end point of the life calculation.

Prior to describing each of the above itemized elements in separate subsections, the damage integrating equation will be introduced to show how the various elements interact.  As expressed in a numerical form, the damage integrating equation is


where Daj is the growth increment associated with the jth time increment.  The purpose of Equation 2.4.1 is to determine the life tf.  The various elements affect the quantities in Equation 2.4.1 in the following manner:

1.                  acr is determined interrelating elements b, d, and f.

2.                  ao is determined using element a.

3.                  Daj is determined by interrelating elements a, b, c, d, and e.