Creep

Creep is the gradual, time-dependent deformation of a material under sustained stress. It becomes especially significant at elevated temperatures, where deformation can continue even when the applied load remains constant.

Primary creep: The initial creep stage, in which the deformation rate decreases as the material hardens. This stage begins the characteristic progression of creep strain under sustained load.
Stress: Internal force per unit area within a material, arising from applied loads or constrained deformation. Creep is driven by sustained stress rather than by a changing load alone.
Creep rupture: Failure of a material after prolonged deformation under sustained stress, often at elevated temperature. Rupture testing measures how long a component can withstand creep conditions before breaking.
Fatigue: Progressive material damage caused by repeated or fluctuating loading, often leading to cracking and failure. Fatigue is driven by load cycles, whereas creep is driven by sustained stress over time.
Secondary creep: The creep stage in which strain increases at a nearly constant rate. Its steady deformation rate is often used to compare materials and predict service life.
Temperature: A physical quantity that measures thermal state and governs the direction of heat transfer. Higher temperature generally accelerates the atomic processes that enable creep.
Creep strength: A material's ability to resist deformation under sustained stress at a specified temperature and duration. Designers use creep strength to set allowable stresses for long-lived hot components.
Stress relaxation: The decline in stress over time when a material is held at fixed deformation. It is the time-dependent counterpart to creep under fixed stress rather than fixed strain.
Tertiary creep: The accelerating final stage of creep, associated with damage and eventual rupture. Its rapid strain growth signals impending failure under sustained stress.
Elasticity: The reversible deformation of a material when applied forces are removed. Unlike elastic strain, creep accumulates with time and may not recover after unloading.

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