Which is the middle stage of blade creep?

• A. Primary
• B. Secondary
• C. Tertiary
• D. Quaternary

Answer: (B)

The middle stage is steady-state creep, where the rate of deformation stays nearly constant over time. After the initial transient where the material work-hardens and the creep rate drops, recovery processes such as dislocation climb and substructure changes balance hardening. This balance yields a nearly constant, sustained creep rate, which is why it’s the "middle" phase in blade creep behavior. In turbine blade alloys, especially at high temperatures, this steady-state portion often lasts a long time and is central to life prediction, because the damage accumulation is at a steady pace during this period. The final stage, tertiary, features accelerating damage with cavity formation and microcracking leading to failure, while the initial stage, primary, is the transient high-rate period before steady-state creep sets in. Quaternary isn’t a standard stage in this framework.