Glossary

In industrial engineering, creep is slow, time-dependent, mostly permanent deformation that occurs when a material is held under constant (or near-constant) stress—most importantly at elevated temperature. The key “gotcha” is that the stress can be below the yield strength, yet the part can still stretch or distort gradually over thousands of hours until it no longer fits, seals, or carries load safely.

In high-temperature equipment (think boilers, superheaters/reheaters, steam lines, pressure vessels, turbine components, furnaces, heat exchangers), creep is a life-limiting design concern because it can lead to creep rupture (stress-rupture failure) after long exposure. The National Board sums it up plainly: time-dependent deformation at elevated temperature under constant stress; when it fails, that’s a creep (stress-rupture) failure.

Engineers often describe creep with the classic three-stage creep curve. Primary creep starts fast then slows as the material strain-hardens; secondary (steady-state) creep runs at a roughly constant rate and is usually the main regime used for design life predictions; tertiary creep accelerates as damage accumulates (voids, grain boundary separation, cracking) until rupture.

It’s also helpful to separate creep from its close cousin, stress relaxation. Creep is “stress held roughly constant, strain increases with time.” Stress relaxation is the flip: “strain held roughly constant, stress drops with time.” In bolted joints, gaskets, and high-temp bolting, you’ll see both—creep in the material and relaxation showing up as lost preload.

How engineers manage creep is a whole discipline: pick materials with better creep strength, limit metal temperature, design to code allowable stresses for time-at-temperature, and validate life using creep data (creep rate, rupture curves, etc.). In many industries this is baked into design rules and allowable stress tables for high-temperature service.

Creep resistance is the ability of a material to withstand time-dependent deformation under constant stress at elevated temperature. In high-temperature environments—such as in turbine blades, power plant piping, jet engines, or chemical reactors—materials experience a gradual, permanent strain called creep, even when the applied stress is well below their normal yield strength. The higher the temperature (usually above about 0.4 times the material’s melting temperature in Kelvin), the more pronounced the creep behavior becomes.

The rate at which creep occurs during long-term service is described by the Norton–Bailey power law, expressed as ε˙ = Aσⁿe^(−Q/RT). In this equation, ε˙ represents the steady-state creep strain rate, A is a material constant, σ is the applied stress, n is the stress exponent (indicating how strongly stress influences creep rate), Q is the activation energy for creep, R is the universal gas constant, and T is the absolute temperature. The exponential term, e^(−Q/RT), shows how sensitively creep rate increases with temperature—small temperature rises can dramatically accelerate deformation.

Creep typically progresses through three stages: primary creep, where deformation slows as the material strain-hardens; secondary creep, the steady-state phase where the rate is nearly constant and most design calculations focus; and tertiary creep, where damage accelerates, leading to rupture. Materials that possess high creep resistance, such as nickel- and cobalt-based superalloys, stainless steels, titanium alloys, molybdenum, and tungsten, are engineered to limit dislocation motion and grain boundary sliding—the two main mechanisms that drive creep. Techniques like solid solution strengthening, precipitation hardening, and grain boundary stabilization enhance their ability to resist this slow, temperature-driven deformation.