How is fatigue crack growth characterized and used in damage tolerance analysis?

Fatigue crack growth (FCG) characterizes crack extension under cyclic loading. Paris Law: da/dN = C(deltaK)^m, where da/dN is crack growth rate, deltaK is stress intensity range, C and m are material constants. Regions: Region I (threshold) - Below threshold deltaK_th, no crack growth; Region II (Paris regime) - Log-linear relationship, stable growth; Region III - Accelerating growth approaching fracture toughness. Testing: Compact tension (CT) specimens per ASTM E647, measure crack length vs cycles, calculate deltaK and da/dN. Application: Input to damage tolerance analysis, predict inspection intervals, compare materials for crack-critical applications. Factors: R-ratio (min/max stress) affects growth rate, Environment (corrosive accelerates), and Temperature. Material selection: Lower da/dN at given deltaK means slower crack growth, longer inspection intervals. 2024-T3 has better FCG resistance than 7075-T6. Databases (NASGRO, AFGROW) contain material FCG data.