Briefly describe creep and identify its three stages.

• A. Time-dependent plastic deformation at elevated temperature; primary (decreasing rate), secondary (steady-state), tertiary (accelerating toward failure).
• B. Time-independent deformation with constant rate.
• C. Instantaneous deformation.
• D. Diffusionless transformation.

Answer: (B)

Creep is time-dependent plastic deformation that occurs when a material is under a sustained load at elevated temperature (or high stress). The deformation continues even if the applied stress remains constant, and its rate changes over time in three characteristic stages.

In the first stage, the creep rate is high initially but slows down as time progresses because dislocations become more difficult to move due to work hardening. The material adapts to the load, so deformation proceeds more slowly.

The second stage is characterized by a nearly constant, steady-state creep rate. Here the processes that harden the material are balanced by recovery mechanisms, so the strain increases at a roughly constant pace.

The third stage shows accelerating creep, where the deformation rate increases rapidly as the material nears failure. This is driven by mechanisms such as cavity or void formation, grain boundary sliding, microcracking, and accelerated damage accumulation.

The statement describing time-independent deformation with a constant rate does not capture creep, since creep fundamentally involves deformation that develops over time under sustained conditions.