# Creep

Started by Lauren Hartle, Fall 2011.

## Definition

Creep is the time dependent change in Strain of a material subject to a constant Stress. Creep is distinct from Plastic flow, which is often defined as time-independent permanent deformation. A Creep test and Stress relaxation test attempt to quantify the same material behavior: the timescale and functional form of molecular and/or atomic rearrangement that occurs when a material is irreversibly deformed. The mechanism of creep differs depending on the material. In a crystal, mechanisms for creep include the movement of dislocations and the diffusion of atoms along grain boundaries or through grains.

The general equation for describing creep is:

$\frac{\mathrm{d}\varepsilon}{\mathrm{d}t} = \frac{C\sigma^m}{d^b} e^\frac{-Q}{kT}$

where the left hand side is the strain rate due to creep, Q is the activation energy of creep, d is the grain size, $\sigma$ is the stress in the material, T is the temperature, and m and b are constants that depend on the mechanism of creep.

For a general visco-elastic material,