Chapter 13: Creep & Stress Rupture

Creep & Stress Rupture on creep and stress rupture explores the time-dependent deformation of engineering materials exposed to elevated temperatures, where atomic diffusion and dislocation mobility significantly alter mechanical stability. The text begins by defining the high-temperature materials problem, distinguishing between time-independent elastic strain and time-dependent phenomena such as anelasticity, the thermoelastic effect, and internal friction, which is quantified using the logarithmic decrement and damping capacity. A major portion of the chapter is dedicated to the engineering creep curve, dissecting its three stages: primary (transient) creep where strain hardening dominates, secondary (steady-state) creep where recovery and hardening are balanced, and tertiary creep which accelerates toward fracture. The discussion categorizes the fundamental mechanisms governing these deformations, including dislocation glide, dislocation creep (power-law creep) mediated by vacancy diffusion, and diffusional flow mechanisms known as Nabarro-Herring creep (lattice diffusion) and Coble creep (grain-boundary diffusion). These mechanisms are contextualized using deformation mechanism maps, or Ashby maps, which graphically predict the dominant behavior based on stress and homologous temperature. The chapter further examines superplasticity, a state of extreme ductility driven primarily by grain-boundary sliding in ultrafine-grained materials. Significant attention is given to high-temperature fracture modes, explaining the transition from transgranular failure to intergranular cracking at the equicohesive temperature, often caused by the nucleation and growth of voids or cavities on grain boundaries. The text also reviews the metallurgical principles behind developing creep-resistant materials, such as nickel-based superalloys that rely on precipitation hardening and dispersion strengthening. Finally, it outlines essential engineering methodologies for extrapolating short-term laboratory data to long-term service life, utilizing time-temperature parameters like the Larson-Miller, Sherby-Dorn, and Manson-Haferd relations, as well as analyzing complex loading scenarios involving combined stresses and creep-fatigue interactions.