What Is Creep?
Creep is time-dependent plastic deformation under constant stress. Unlike the instantaneous deformation that occurs when a load is applied, creep continues to accumulate over hours, months, or years. At low temperatures (below about 0.3T_melting in Kelvin — the homologous temperature), creep rates are negligibly slow for most engineering applications. As temperature rises toward 0.5T_melt, creep becomes the dominant design consideration. This temperature dependence means that "high temperature" is relative to the material. For tungsten (melting point 3422°C), creep is negligible at 1000°C. For lead (melting point 327°C), creep is significant at room temperature — which is why lead pipes and roofing sheets noticeably deform over decades. For structural steel (melting point ~1500°C), creep becomes significant above about 400°C, which is why fire resistance of structures is a major design consideration. For concrete, creep occurs even at room temperature under sustained load — the Pantheon in Rome shows measurable creep deformation accumulated over 2000 years.
The Three Stages of Creep
A constant-stress creep test at elevated temperature produces a characteristic strain-time curve with three distinct regimes:
- Primary creep: The creep rate decreases with time as the material work-hardens internally. Dislocation density increases, making further motion more difficult. The rate falls from its initial high value toward a minimum.
- Secondary (steady-state) creep: Work-hardening and recovery (thermal softening) reach a balance. The creep rate is approximately constant — the minimum rate. This is the most important regime for engineering design, because most of a component's service life is spent here.
- Tertiary creep: The creep rate accelerates as the cross-section necks down (reducing the load-bearing area while the load stays constant, increasing the true stress), voids nucleate and grow along grain boundaries, and microcracks develop. This stage ends in rupture.
Stress Relaxation: Creep's Twin
Stress relaxation is what happens when a material is held at constant strain (rather than constant stress) at elevated temperature. The material creeps internally, rearranging dislocations, and the elastic strain (which generates the stress) gradually converts to inelastic strain. The total strain stays constant, but the stress falls. This is exactly what happens to bolted joints at elevated temperature. The bolt is stretched to a target clamping force. Over time, the bolt and flanges creep; the elastic strain in the bolt decreases; the clamping force falls — sometimes to levels where the joint no longer seals or transmits load as designed. This is why high-temperature bolted joints require periodic re-torquing, or why gasket materials with low creep rates are specified for steam pipework.
Engineering Design Against Creep
The primary design tools against creep are: limiting stress below the creep-limiting stress for the required life (using Larson-Miller data from materials standards); choosing materials with higher creep resistance (nickel superalloys instead of steel, single-crystal turbine blades instead of polycrystalline); reducing temperature (every 10°C reduction in turbine inlet temperature approximately doubles creep life); and using internal cooling to maintain metal temperatures below the alloy's creep threshold despite hot gas temperatures that would cause rapid failure in uncooled blades. EngForge's fatigue life tool computes S-N curves, Goodman correction, and safety factors. For high-temperature applications, combine with your material's Larson-Miller data to check both fatigue and creep limits simultaneously.