Creep is the slow, permanent deformation of a material under a constant load or stress. It is most important when a component operates for a long time at high temperature, such as a turbine blade, boiler tube, pressure pipe, or furnace support. Even if the stress is below the material's yield strength, creep can gradually change the shape of the part until it no longer functions safely.
Engineers must predict creep to set temperature limits, stress limits, inspection intervals, and service lifetimes.
Key Facts
- Creep strain is the time-dependent permanent strain that occurs under sustained stress, especially at high temperature.
- A common rule of thumb is that creep becomes important above about 0.4 Tm in kelvin, where Tm is the melting temperature.
- Creep rate is the slope of the creep curve: creep rate = dε/dt.
- Primary creep has a decreasing creep rate, secondary creep has an approximately constant creep rate, and tertiary creep has an increasing creep rate before rupture.
- Minimum creep rate often follows Norton law: creep rate = Aσ^n, where σ is stress and A and n depend on material and temperature.
- Stress rupture life is the time until fracture under a constant stress and temperature: rupture time decreases as stress or temperature increases.
Vocabulary
- Creep
- Creep is the slow permanent deformation of a material under sustained stress over time.
- Creep strain
- Creep strain is the amount of permanent strain that accumulates because of creep during loading.
- Creep rate
- Creep rate is the rate at which creep strain increases with time, usually written as dε/dt.
- Stress rupture
- Stress rupture is fracture that occurs after a material has been held under constant stress and temperature for a period of time.
- Secondary creep
- Secondary creep is the stage of creep where strain increases at an approximately steady rate.
Common Mistakes to Avoid
- Using yield strength alone to judge high-temperature safety is wrong because creep can occur at stresses below yield strength when the load lasts long enough.
- Ignoring temperature in creep problems is wrong because creep rate can increase dramatically as temperature rises, especially near a large fraction of the melting temperature.
- Confusing elastic strain with creep strain is wrong because elastic strain disappears when the load is removed, while creep strain is permanent.
- Assuming the creep curve is linear for the entire lifetime is wrong because primary and tertiary creep have changing slopes, and tertiary creep accelerates toward failure.
Practice Questions
- 1 A steel pipe operating at high temperature accumulates creep strain from 0.002 to 0.006 over 2000 hours during secondary creep. Calculate the average creep rate in strain per hour.
- 2 A turbine blade has a measured secondary creep rate of 1.5 × 10^-7 per hour. If the allowable creep strain is 0.003, estimate the time to reach this strain assuming the secondary creep rate stays constant.
- 3 Two identical pressure tubes carry the same stress, but one operates at 450°C and the other at 650°C. Explain which tube is more likely to be creep-limited and why.