Fracture, Fatigue and Creep
The principal mechanisms of sudden, cyclic and time-dependent failure.
Failure has a mechanism
Fracture, fatigue and creep may interact, but they are not interchangeable labels. Correct diagnosis begins with load history, temperature, environment and fracture morphology.
Fracture
Ductile fracture involves substantial plasticity and void growth. Brittle fracture can propagate with little visible deformation. Fracture mechanics relates crack size, stress and toughness, recognising that flaws are unavoidable.
Fatigue
Repeated loading can initiate and grow cracks at stresses below monotonic yield strength. Surface condition, mean stress, notches, residual stress and environment strongly affect life.
Creep
At elevated homologous temperature, materials deform with time under sustained load. Diffusion, dislocation motion and grain-boundary processes control the rate. Creep rupture may occur after long apparently stable service.
Takeaway
Ask whether failure was controlled by a critical crack, repeated cycles, sustained high-temperature load—or a combination of all three.