643 Damage Tolerant Design

Accurate knowledge of the size of a crack in a structure and its propagation rate as a function of the applied stress-strain field are the major ingredients for a fatigue damage-tolerant approach (FDTA). FDTA has been originally developed by the

FIGURE 6.22 Schematic representation of radial cracks emanating from fiber breaks due to residual stresses.

airframe industry to ensure local failure within fail-safe boundaries previously established through failure analysis techniques. In this sense, linear elastic fracture mechanics (LEFM) is used to relate crack size with nondestructive and fracture surface observations in order to estimate load history and typical service life. Thus, the application of fracture mechanics methods as part of damage-tolerant assessment involves the consideration of four parameters: (a) initial cracks (a^); (b) threshold stress intensity factor for crack propagation (Kth); (c) the final crack length (af); and (d) the crack growth rate (da/dN). Typical FDTA techniques can be found elsewhere [102, 103].

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