In most cases, Fcr1 and Fcr3 depend on stress, temperature and time, while secondary creep, Fcr2, depends only on stress level and temperature. Normally, secondary creep is the dominant process. Tertiary creep is seldom important because it only accounts for a small fraction of the total lifetime of a structure or mechanical component.

The mathematical description of a general creep model can be summarized by a set of equations given in a similar format as the flow theory of plasticity. The creep strain is given by a flow rule that defines the relationship between the rate of the creep strain tensor εcr and the current state of stress. This flow rule can be written as

where λ is the creep multiplier, Qcr is the creep potential, and σ is the stress tensor. The creep multiplier is an explicit function of stress σ, temperature T, time t, and the equivalent creep strain εce, and can be given on a general format as

where the generic functions f, g, and h that define the creep rate and can be combined to construct different types of creep models. The hardening rule, h(εce, t), defines the relationship between the creep strain tensor εcr and the equivalent creep strain εce. The equivalent stress σe is a scalar representation of the Cauchy stress tensor, and given associative creep flow, it also defines the creep potential so that

The creep rate function f is typically a function of the equivalent stress σe or any other scalar measure of the current stress state. In COMSOL Multiphysics, there are several built-in creep rate functions:

It is also possible to define an arbitrary expression for f so that, in principle, any creep model can be implemented. The expressions used for the built-in creep rate functions are described in the following sections.

The Norton model is the most common secondary creep model, where the creep rate is assumed proportional to a power law of the equivalent stress σe such that

where A is the creep rate coefficient, n is the stress exponent, σref is a reference stress level.

At very high stress levels the creep rate is proportional to the exponential of σe. Garofalo showed (Ref. 8, Ref. 9) that the power-law and exponential creep are limiting cases for the general empirical expression

This equation reduces to a power-law (the Norton model) when ασe < 0.8 and approaches exponential creep for ασe > 1.2, where 1/α is a reference equivalent stress level. The Garofalo (Hyperbolic sine law) creep model implemented in COMSOL Multiphysics is given as

For metals at low stress levels and high temperatures, Nabarro and Herring (Ref. 6, Ref. 7) independently derived an expression for the creep rate as a function of atomic diffusion, so called diffusional creep. The Nabarro-Herring model implemented in COMSOL Multiphysics is given as

where d is the grain diameter, Dv is the volume diffusivity through the grain interior, b is the Burgers vector, kB is the Boltzmann’s constant, T is the absolute temperature.

Another model for diffusional creep is the Coble creep model (Ref. 6, Ref. 7), which is closely related to Nabarro–Herring creep, but takes into account the grain boundary diffusivity through the parameter Dgb. The Coble creep model implemented in COMSOL Multiphysics is given as

For metals at intermediate to high stress levels and temperatures close to the melting temperature of the solid, the creep mechanism can be assumed to be diffusion-controlled movements of dislocations in the crystal lattices (Ref. 7). This type of creep deformation can be described by the Weertman model, which is given as

Generally, the stress exponent n in the Weertman model takes a value between 3 and 5 when modeling dislocational creep.

For metals and alloys, creep is in most cases deviatoric (volume preserving), and can often be described by a von Mises equivalent stress

where J2 is the second invariant of the stress deviator. If creep is orthotropic, the Hill orthotropic equivalent stress can be used, which is given by

where F, G, H, L, M, and N are Hill’s coefficients that relate the anisotropy to the local coordinates. In the above equation it is assumed that an average equivalent stress can be computed from F, G, and H using a von Mises like expression; this assumption leads to the first term on the right-hand-side of Equation 3-71.

For soils and other geological materials, creep is often volumetric and can be described by a Pressure equivalent stress

where p is the pressure.

The dimensionless tensor nD, which gives the direction of the creep flow, is computed from the creep potential Qcr as

When von Mises equivalent stress is used, the potential equals Qcr = σmises, and the deviatoric tensor nD is defined as done for J2 plasticity (see Isotropic Plasticity)

The same hardening rule is also used when the Hill orthotropic equivalent stress is selected.

For the Pressure equivalent stress, the flow direction is volumetric

When a user defined equivalent stress is used, a von Mises like hardening rule is used according to Equation 3-72.

The creep hardening function h is typically a function of time t and the equivalent creep strain εce. In COMSOL Multiphysics, there are two built-in hardening functions, Strain hardening and Time hardening, and it is also possible to specify a user defined expression. Also, it is possible to add hardening to any of the built-in creep models.

by taking the time derivative of this generic expression at constant stress, it is found that the creep strain rate and stress are related by the time hardening function

where the time hardening function is given by the power law

here, tshift is a time shift, tref is a reference time, and m is the hardening exponent.

Similarly, the strain hardening function is given by the power law

where εshift is the equivalent creep strain shift, tref is a reference time, and m is the hardening exponent.

The strain shift εshift and the time shift tshift in the hardening functions serve two purposes:

The temperature shift function g typically depends on the absolute temperature T. In COMSOL Multiphysics, an Arrhenius temperature function is built-in, but it is also possible to specify a user defined expression.

The Arrhenius temperature function is given as

where Q is the activation energy, R is the gas constant, T is the absolute temperature, and Tref is a reference temperature.

Using a combination of f, g, and h together with the definition of the equivalent stress σe makes it possible to construct sophisticate creep models. For example, the often used temperature-dependent isotropic Norton creep model is obtained by the following settings:

Several creep rate contributions can be added to the total creep rate to construct more elaborate models. This is done by adding one or more Addition Creep subnodes to an existing Creep node. There are no restrictions on how to combine models; for example, a von Mises based creep flow contribution can be combined with a Pressure based creep flow contribution.

The rate equations given by the creep flow and hardening rules have to be integrated in time to compute the value of the creep strain tensor εcr and the equivalent creep strain εce at each time step. This can be done using any of the following methods:

where Gcr and Gce are tensors that describe the direction of εcr and εce, respectively.

The Backward Euler method is used to discretize these equations as

where n+1 indicates that the variable is evaluated at the current time step, and Δt is the time step. The creep strain tensor and equivalent creep strain at the previous time step are stored as internal state variables. Equation 3-75 and Equation 3-76 define a system of nonlinear equations that is solved locally at each Gauss point for and using Newton’s method.

The Forward Euler method is used to discretize Equation 3-73 and Equation 3-74 as

such that all quantities on the right-hand-side are evaluated at the previous time step. This means that Equation 3-77 and Equation 3-78 define an explicit update of and given only known quantities from the previous time step. The forward Euler method is only conditionally stable, and the time step of the global time marching scheme has to be constrained to preserve stability and accuracy of the method. In COMSOL Multiphysics the limit for a stable time step is estimated as

where εel is the elastic strain tensor. This estimate in principle restricts the creep increment to be smaller than the current state of stress, and 1/4 is a constant to make the estimate conservative. The final value is computed as the minimum value over all Gauss points in the domain where the creep feature is active.

For Domain ODEs, Equation 3-73 and Equation 3-74 are converted to weak-form and solved as part of the general initial-boundary value problem. The components of the creep strain tensor εcr and the equivalent creep strain εce are then treated as degrees-of-freedom.

The equations described in the previous sections about the different creep models differ from the forms most commonly found in the literature. The main difference lies in the introduction of normalizing reference values such as the reference stress σref and reference time tref. These values are in a sense superfluous and can in principle be chosen arbitrarily. The choice of reference values, however, affects the numerical values to be entered for the material data. This system has two advantages:

Material data for a Norton law is often available as the parameters AN and n in the equation

The coefficient AN has a physical dimension that depends on the value of n, and the unit has an implicit dependence on the stress and time units. Converting the data to the format used in COMSOL Multiphysics (e.g. Equation 3-70) requires the introduction of the reference stress σref. It is here convenient to use the implicit stress unit for which AN is given as reference stress. The creep rate coefficient A will then have the same numerical value as AN, and you do not need to do any conversions.

The physical dimension of A is, however, (time)−1, whereas the physical dimension of AN is (stress)−n(time)−1.

Another popular way of representing creep data is to supply the stress giving a certain creep rate. As an example, σc7 is the stress at which the creep strain rate is 10−7/h. Data on this form is also easy to enter: You set the reference stress σref to the value of σc7 and enter the creep rate coefficient A as 1e-7[1/h].

Since the stress inside in the Garofalo law appears as an argument to a sinh() function, it must necessarily be dimensionless. Most commonly this is however written as

In this case, there is no arbitrariness in the choice of σref, since α is an actual material parameter.

Anand viscoplasticity (Ref. 9) is a deviatoric viscoplastic model suitable for isotropic viscoplastic deformations. As for the model described in Creep, the Anand model has no yield surface.

The viscoplastic strain εvp is here given by a flow rule that defines the relationship between the rate of εvp and the current state of stress.

Given that an associative flow and von Mises equivalent stress σe are used, so Qvp = σe, the rate of εvp is coaxial to the stress deviator and the flow rule is written as

where Qvp is the viscoplastic potential, σ is the stress tensor, and nD = ∂Qvp / ∂σ is the viscoplastic flow direction.

The rate of εvp is determined by the viscoplastic multiplier λ which is a function of stress σ and temperature T, and is given by

where A is the viscoplastic rate coefficient, Q is the activation energy, ξ is the stress multiplier, m is the stress sensitivity, and R is the gas constant.

The deformation resistance, sa (SI unit: Pa), controls the hardening behavior of the Anand model. In COMSOL Multiphysics it is derived for the normalized resistance factor, sf, and the deformation resistance saturation coefficient, ssat, such that sa = ssat sf. The resistance factor sf is an dimensionless internal variable that is computed by the evolution equation

with the initial condition sf(0) = sinit / ssat. The evolution of the resistance factor sf is controlled by the dimensionless hardening function

were h0 (SI unit: Pa) represents a constant rate of athermal hardening in the curve stress versus strain (Ref. 9). The use of the sign function and the absolute value in the hardening function permits the modeling of either strain hardening or strain softening, depending on whether sf is greater or smaller than the saturation value sf*.

The saturation value sf* is calculated from the expression

where n is the deformation resistance sensitivity exponent.

The rate equations given by the viscoplastic flow rule and evolution equations for the two internal variables are integrated in time to compute the value of the viscoplastic strain tensor εvp, the equivalent creep strain εvpe, and the resistance factor sf at each time step. This can be done using any of the following methods:

The Backward Euler method is used to discretize these equations as

where n+1 indicates that the variable is evaluated at the current time step, and Δt is the time step. The viscoplastic strain tensor , equivalent viscoplastic strain and the resistance factor n at the previous time step are stored as internal state variables. Equation 3-79 to Equation 3-81 define a system of nonlinear equations that is solved locally at each Gauss point for , and using Newton’s method.

For Domain ODEs, the evolution equations of the Anand model are converted to weak-form and solved as part of the general initial-boundary value problem. The components of εvp, εvpe and sf are then treated as degrees-of-freedom.

The viscoplastic strain rate tensor is in the Chaboche viscoplastic model given by

Here, A is the viscoplastic rate coefficient (SI unit: 1/s), n is the stress exponent (dimensionless), σref a reference stress level (SI unit: Pa), and nD is a deviatoric tensor coaxial to the stress tensor. The Macaulay brackets are applied on the yield function, which is defined as done for plasticity

The equivalent stress is either the von Mises, Tresca, or Hill stress, or a user defined expression; and σys is the yield stress (which may include a linear or nonlinear Isotropic Hardening model). The stress tensor used in the equivalent stress is shifted by what is usually called the back stress, σback when Kinematic Hardening is included.

The deviatoric tensor nD is computed from the viscoplastic potential Qvp

When von Mises equivalent stress is used, the associated flow rule reads Qvp = Fy, and the deviatoric tensor nD is defined as done for deviatoric creep

Some authors denote the viscosity as the quotient η = σref/A.

Since creep and viscoplasticity are inelastic processes, the dissipated energy density can be calculated by integrating the creep dissipation rate density or viscoplastic dissipation rate density given by

in time. The total energy dissipated in a given volume can be calculated by a volume integration of the dissipated creep energy density Wcr or dissipated viscoplastic energy density Wvp.