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AFGROW | DTD Handbook

Handbook for Damage Tolerant Design

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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
      • 8. Force Management and Sustainment Engineering
      • 9. Structural Repairs
      • 10. Guidelines for Damage Tolerance Design and Fracture Control Planning
        • 0. Guidelines for Damage Tolerance Design and Fracture Control Planning
        • 1. Design Loads Spectrum
        • 2. Material Selection
          • 0. Material Selection
          • 1. Crack Growth Resistance and Fracture Toughness
          • 2. Material Property Control
        • 3. Structural Configuration Analysis
        • 4. Manufacturing Process
        • 5. References
      • 11. Summary of Stress Intensity Factor Information
    • Examples

Section 10.2.1. Crack Growth Resistance and Fracture Toughness

The material properties used for the selection criteria must be obtained for conditions that correspond to those expected in the structural usage environment.  Crack growth resistance as expressed in the da/dN data should be obtained from tests conducted using thickness similar to the anticipated structure applications and in similar environments.  Some alloys are quite susceptible to corrosive media such as may be experienced in aircraft fuel bays or during operation near salt water.  Effects of these variables are shown in Figure 10.2.2 [Circle & Conley, 1980].



Figure 10.2.2.  Illustration of Effects of Environment on Crack Growth Rates [Circle & Conley, 1980]

For ease of application in the design process, the crack propagation data is usually described by an empirical relationship, such as the Forman equation, given as:



                        Kc        - fracture toughness

                        DK       - stress-intensity factor range

                        C, n     - constants dependent on material, obtained from curve fitting techniques

It may be necessary to model the data in several parts over the DK range of interest in order to achieve adequate representation.

Ekvall, et al. [1982] presents a method for evaluation of weight savings due to the usage of advanced materials.  The utilization of materials having improved damage tolerance characteristics as evidenced by a higher allowable stress value was shown to effect a weight savings from 1-3 percent for an improvement in allowable stress of 10-25 percent.

Simenz and Guess [1980] discusses material properties and characteristics of some new materials based on obtaining high strength with good durability and damage tolerance properties.  This is mentioned to make the reader aware of current efforts to improve structural materials.  Goals stated in this report are to increase the static strength, decrease the crack growth, and increase the temperature capability of aluminum alloy.