About Creep and Creep Models

You can define classical deviatoric metal creep behavior for a material either by using user subroutine CREEP or by selecting a creep model and providing parameters for some simple creep laws. Creep behavior is also subject to the following requirements and standards:

• You can model either isotropic creep (using von Mises stress potential) or anisotropic creep (using Hill's anisotropic stress potential).
• Creep is active only during steps using the coupled temperature-displacement procedure, the transient soils consolidation procedure, and the quasi-static procedure.
• Creep requires that you define the material's elasticity as linear elastic behavior.
• You can use creep behavior in combination with creep strain rate control in analyses in which the creep strain rate must be kept within a certain range.
• Creep can potentially result in errors in calculated creep strains if anisotropic creep and plasticity occur simultaneously (as discussed in Modeling Simultaneous Creep and Plasticity).

Specify creep behavior by the equivalent uniaxial behavior—the creep "law." In practical cases, creep laws are typically of very complex form to fit experimental data; therefore, the laws are defined with user subroutine CREEP. Alternatively, you can use seven common creep laws: the strain hardening power law, the time hardening power law, the hyperbolic-sine law, the double power law, the Anand law, the Darveaux law, the Power law, and the Time Power law. You can use these standard creep laws to model secondary or steady-state creep. Include creep behavior in the material model definition to define creep. Alternatively, you can define creep in gasket behavior to define the rate-dependent behavior of a gasket.

The power law creep model (in both strain hardening and time hardening forms) is attractive for its simplicity. However, the time-hardening version of the power law creep model is typically used only in cases when the stress state remains essentially constant. Use the strain-hardening version of power law creep when the stress state varies during an analysis. Where the stress is constant and there are no temperature or field dependencies, the time-hardening version and the strain-hardening version of the power-creep law are equivalent. For either version of the power law, the stresses must be relatively low.

In regions of high stress, such as around a crack tip, the creep strain rates frequently show an exponential dependence of stress. The hyperbolic-sine creep law shows exponential dependence on the stress, $\sigma$ , at high stress levels ( $\sigma /{\sigma }^0\gg 1,$ , where ${\sigma }^0$ is the yield stress) and reduces to the power law at low stress levels (with no explicit time dependence).

The double power, Anand, and Darveaux models are particularly well suited for modeling the behavior of solder alloys used in electronic packaging and produce accurate results for a wide range of temperatures and strain rates.

None of the above models is suitable for modeling creep under cyclic loading. Generally, creep models for cyclic loading are complicated and must be added to a model with user subroutine CREEP or with user subroutine UMAT.

If creep and plasticity occur simultaneously and implicit creep integration is in effect, both behaviors might interact and a coupled system of constitutive equations needs to be solved. If creep and plasticity are isotropic, the solver properly takes into account such coupled behavior, even if the elasticity is anisotropic. However, if creep and plasticity are anisotropic, the solver integrates the creep equations without taking plasticity into account, which might lead to substantial errors in the creep strains. This situation develops only if plasticity and creep are active at the same time, such as would occur during a long-term load increase. You would not expect to have a problem if there is a short-term preloading phase in which plasticity dominates, followed by a creeping phase in which no further yielding occurs.