Ductile vs. brittle fracture mechanisms, how stress concentration around a flaw or crack sets fracture toughness (K_IC) as a genuine material property, and the ductile-to-brittle transition temperature that decides whether structural steel fails safely or catastrophically.
Ductile fracture warns you first — necking, gross deformation, visible distress before final separation. Brittle fracture does not: it propagates near the speed of sound through a part that looked fine a moment earlier, which is exactly why this module treats fracture mechanics as its own discipline rather than an extension of static strength. This module builds from stress concentration and Griffith's energy-balance insight up to fracture toughness (K_IC) as an intrinsic material property, why K_IC is only valid under plane-strain conditions, and why section thickness changes what toughness value actually applies.
By the end of this module you should be able to explain why structural steel needs to stay above its ductile-to-brittle transition temperature in service, using the Liberty ship failures as the historical case that ties stress concentration, low-temperature brittleness, and inadequate fracture toughness into one real, catastrophic consequence. Module 11's fatigue crack-growth content and this module's K_IC together form the basis of the damage-tolerant design approach covered here, and Module 13's failure analysis returns to this exact combination of factors directly.