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Handbook for Damage Tolerant Design

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Section 5.1.0. Basic Information

Crack growth is a result of cyclic loading due to gusts and maneuvers (fatigue cracking), or of the combined action of sustained loading and environment (stress-corrosion cracking), or both.  The most common crack growth mechanisms are fatigue crack growth and environment-assisted (corrosion) fatigue crack growth.  Certain aircraft parts, especially high-strength forgings, may be liable to stress-corrosion cracking.  Since there is a design threshold for stress corrosion, proper detail design and proper material selection can minimize or prevent stress corrosion.  Fatigue cracking is difficult to prevent, but it can be controlled.

To predict crack growth behavior such as illustrated in Figure 5.1.1, the following information must be available:

·        The stress-intensity factor, described as a function of crack size, for the relevant structural and crack geometry;

·        The stress (load) – time history, described for the structural location component or structure under consideration;

·        The baseline crack growth properties (constant amplitude crack growth rate data), described as a function of the stress intensity factor, for the material and for the relevant environment;

·        A damage integration routine that integrates the crack growth rate to produce a crack growth curve, and uses the proper stress-time history, the proper stress intensity formulation, and an appropriate integration rule.

This section provides guidelines to arrive at crack growth estimates, and points out where deficiencies in knowledge and analysis methods lead to inaccuracies.


Figure 5.1.1.  Typical Crack Growth-life Curve