How is damage tolerance analysis performed for aircraft structures?

Damage tolerance ensures safety with assumed damage present. Process: Identify principal structural elements (PSEs) - Failure would reduce structural capability below limit load; Assume damage - Initial manufacturing flaws or in-service damage (e.g., 0.05 inch surface crack); Predict crack growth - Paris law (da/dN = C*deltaK^m), stress intensity factors from geometry and loads; Determine inspection interval - Time for crack to grow from detectable to critical, with safety factor of 2. Analysis: Finite element for stress distributions, Fracture mechanics for stress intensity, Spectrum loading (flight-by-flight or block loading), Residual strength at any time. Certification: Show inspectable before critical size, Supplemental Inspection Documents define program, and Widespread Fatigue Damage assessment. Multiple site damage considered for aging aircraft. Inspection methods: Visual, eddy current, ultrasonic, radiography. Damage tolerance philosophy replaced safe-life for most structures after 1978 (FAR 25.571 amendment).