What is creep and when must it be considered in design?

Creep is time-dependent plastic deformation under constant stress, significant at temperatures above 0.3-0.4 of melting point (Kelvin). The creep curve shows primary (decreasing rate), secondary (constant rate), and tertiary (accelerating to rupture) stages. It must be considered in gas turbine blades, boiler tubes, jet engine components, and nuclear reactors. Design uses creep rupture data and appropriate safety factors.