A4 Creep

Topic

When subjected simultaneously to a sustained load and a sustained elevated temperature, this metal deforms continuously over time—even when the sustained load alone, at room temperature, would be insufficient to produce any additional permanent deformation; this time-dependent deformation is precisely the phenomenon known as creep. The fact that the sustained load acts in conjunction with the sustained elevated temperature—rather than each acting in isolation—causes the metal's deformation to continue accumulating over time, rather than ceasing once the instantaneous elastic or plastic deformation associated with that load has been reached.

At moderately elevated temperatures, this creep occurs primarily because dislocations within the metal—aided by the heat—overcome the obstacles that would normally impede them, continuing to move steadily through the crystal lattice; this dislocation-controlled creep is described by Norton's Law. According to Norton's Law, a higher sustained stress causes the rate of this dislocation-controlled creep to increase at a rate that is more than proportional to the stress itself.

At even higher temperatures, however, creep occurs primarily because the metal's atoms undergo net diffusion from grain boundaries subjected to compressive stress toward those subjected to tensile stress; this diffusion-controlled creep is described by the Nabarro-Herring model, which directly utilizes the diffusion coefficient established by another model in this set. According to the Nabarro-Herring model, a higher diffusion coefficient at a given temperature results in a higher rate of this diffusion-controlled creep. For this reason, the model—framed by the concepts of creep and time-dependent deformation—encapsulates the combined effect of two creep mechanisms (dislocation-controlled at moderate temperatures and diffusion-controlled at higher temperatures) into two practical parameters: the Larson-Miller parameter, which allows for the extrapolation of the metal's creep life across various temperature-time combinations without the need to test each combination individually; and the Monkman-Grant relationship, which directly links the creep rate—once the steady-state (secondary) stage is reached—to the time to final rupture. A higher secondary creep rate implies, according to the Monkman-Grant relationship, a shorter time to final rupture for the metal.

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