Creep

Remarks

Example: a spring of modulus $E$ in series with a dashpot of viscosity $\eta$ (the Maxwell material) has $\varepsilon(t) = \sigma_0/E + \sigma_0 t/\eta$, whose creep strain $\sigma_0 t / \eta$ grows linearly. Non-example: a linear elastic solid has $\varepsilon(t) = \sigma_0/E$ for all $t > 0$, so its creep strain is zero. Metals at high temperature typically show three stages. They are primary creep with decreasing strain rate, secondary creep with a nearly constant rate, and tertiary creep with an increasing rate that ends in rupture. The steady secondary rate is often fitted by Norton's power law $\dot\varepsilon = A \sigma_0^n$, with constants $A > 0$ and $n \ge 1$ that depend on the material and the temperature.