What is fracture toughness and how is it measured?

Fracture toughness measures resistance to crack propagation. Parameters: KIc - Plane strain fracture toughness (critical stress intensity for Mode I crack); KIe or Kc - Plane stress (thin sections, higher values); and JIc - J-integral for elastic-plastic fracture. Testing: Compact tension (CT) or bend specimens per ASTM E399; Pre-crack specimen by fatigue; Load to failure, measure critical load; Calculate KIc = f(P, a, W) based on geometry. Requirements: Valid test requires plane strain conditions (thickness requirement); Specimen size must meet criteria for valid KIc. Material trends: Higher strength often means lower toughness; 2024-T3 higher toughness than 7075-T6; Temper affects toughness (T7 better than T6 for 7xxx). Application: Damage tolerance analysis uses KIc; Material comparison for flaw-critical applications; Residual strength prediction. Units: MPa-sqrt(m) or ksi-sqrt(in). Typical aerospace aluminum: 25-40 MPa-sqrt(m); titanium: 40-100; steel varies widely.