User-Defined Thermal Material Behavior

Any material constants that are needed in user subroutine UMATHT or VUMATHT must be specified as part of a user-defined thermal material behavior definition. Any other thermal material behaviors included in the same material definition will be ignored: the user-defined thermal material behavior requires that all thermal behavior calculations are programmed in user subroutine UMATHT or VUMATHT.

When the conductivity is defined in user subroutine UMATHT as a strong function of temperature, the heat transfer equilibrium equations become nonsymmetric and you may choose to invoke the unsymmetric equation solution capability; otherwise, convergence may be poor.

Many thermal constitutive models require the storage of solution-dependent state variables. These state variables might include microstructure or phase content information when the material undergoes phase changes. You should allocate storage for these variables in the associated material definition (see Allocating Space for Solution-Dependent State Variables). There is no restriction on the number of state variables associated with a user-defined material.

User subroutine UMATHT is called for each material point at each iteration of every increment. It is provided with the thermal state of the material at the start of the increment (solution-dependent state variables, temperature, and any predefined field variables) and with the increments in temperature, predefined state variables, and time.

User subroutine VUMATHT is called for blocks of material points at each increment. When the subroutine is called, it is provided with the state at the start of the increment (temperature, internal energy per unit mass, solution-dependent variables, and any predefined field variables) and the state at the end of the increment (temperature, spatial gradients of temperature). The VUMATHT user material interface passes a block of material points to the subroutine on each call, which allows vectorization of the material subroutine.

User subroutine UMATHT must perform the following functions: it must define the internal energy per unit mass and its variation with respect to temperature and spatial gradients of temperature; it must define the heat flux vector and its variation with respect to temperature and spatial gradients of temperature; and it must update the solution-dependent state variables to their values at the end of the increment. The components of the heat flux and spatial gradients in user subroutine UMATHT are in directions that depend on the use of local orientations (see Orientations).

User subroutine VUMATHT must return the following values at the end of the increment: the internal energy per unit mass and its variation with respect to temperature, the heat flux vector, solution-dependent state variables and an effective conductivity. The effective conductivity is required to estimate the element stable time increment size in Abaqus/Explicit. Since the stable time increment size is inversely proportional to conductivity, you can provide the largest value in conductivity tensor to obtain a conservative estimation of the stable time increment size.

Any other user subroutine that does not deal with constitutive thermal behavior for a material can be used in conjunction with user subroutines UMATHT and VUMATHT.

In Abaqus/Standard user subroutine UMAT can be used in conjunction with UMATHT to define the constitutive mechanical behavior of the material. The solution-dependent variables allocated in the material definition are accessible in both UMATHT and UMAT. In addition, user subroutines FRIC, GAPCON, and GAPELECTR are available for separately defining mechanical, thermal, and electrical interactions between surfaces; and user subroutine UINTER is available for defining general interfacial constitutive behavior. User subroutine HETVAL for user-defined internal heat generation can be used in conjunction with user subroutine UMATHT.

In Abaqus/Explicit user subroutine VUMAT can be used in conjunction with user subroutine VUMATHT to define the constitutive mechanical behavior of the material. The solution-dependent variables allocated in the material definition are accessible in both VUMATHT and VUMAT. In addition, user subroutines VFRIC, VFRICTION, VUINTER, and VUINTERACTION are available for defining interfacial constitutive behavior. User subroutine VHETVAL for user-defined internal heat generation can be used in conjunction with user subroutine VUMATHT.