Creep is material deforming under load over time in such a way as to tend to relieve the stress. Creep can also be a function of temperature and neutron flux level. The term relaxation is sometimes used interchangeably with creep.
The von Mises or Hill stress potentials can be used for creep analysis. For the von Mises potential, the material is assumed to be isotropic and the basic solution technique used is the initial-stiffness Newton-Raphson method.
The options available for creep are described in Rate-Dependent Plasticity (Viscoplasticity) in the Material Reference.
Four different types of creep are available and the effects of the first three may be added together except as noted:
Primary creep is accessed via C6. (Ci values refer to the
ith value given in the TBDATA command with TB,CREEP.) The creep calculations are bypassed if C1 = 0.Secondary creep is accessed with C12. These creep calculations are bypassed if C7 = 0. They are also bypassed if a primary creep strain was calculated via C6 = 9, 10, 11, 13, 14, or 15, as they include secondary creep in their formulations.
Irradiation-induced creep is accessed with C66.
User-specified creep may be accessed with C6 = 100. For more information, see User Routines and Non-Standard Uses.
The creep calculations are also bypassed if:
(change of time)
10-6(input temperature + Toff)
0 where Toff = offset temperature
(input on TOFFST command). For C6 = 0 case: A special effective strain based on εe and εcr is calculated. A bypass occurs if it is equal to zero.