About Coupled Thermal-Stress Explicit Steps

A coupled thermal-stress explicit step performs a thermal analysis and an explicit dynamic stress analysis (using Abaqus/Explicit solvers) simultaneously, which captures the effects of temperature and stress distributions on each other.

A coupled thermal-stress explicit step is a procedure that you create within a general analysis case. A general analysis case supports thermal, structural, and electrical potential degrees of freedom, such that you can run multiphysics simulations simultaneously.

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Defining Coupled Thermal-Stress Explicit Steps

A coupled thermal-stress explicit step includes inertia effects, and it models transient thermal response. The solvers integrate the heat transfer equations using the explicit forward-difference integration rule, while they integrate the mechanical equations using the explicit central-difference integration rule. Since both the forward-difference and central-difference integrations are explicit, the solvers can obtain the heat transfer and mechanical solutions simultaneously by an explicit coupling. Therefore, the solvers do not require iterations or tangent stiffness matrices. Instead, the coupled thermal-stress explicit step performs a large number of small time increments efficiently.

Time Incrementation

A coupled thermal-stress explicit step must use a time increment that is smaller than the stability limits of the central- and forward-difference operators. Failure to use a time increment that satisfies this requirement results in an unstable solution. When a solution becomes unstable, the time history response of solution variables (such as displacements) usually oscillates with increasing amplitudes. In addition, the total energy balance changes significantly.

There are two strategies for controlling time incrementation: automatic time incrementation (where the solvers account for changes in the stability limit) and fixed time incrementation. For more information, see Automatic Time Incrementation and Fixed Time Incrementation.

Automatic Time Incrementation

The solvers use two types of time estimates to determine the stability limit during automatic time incrementation: element-by-element estimates (for the thermal and mechanical solution responses) and global estimates (for the mechanical solution response). An analysis that uses automatic time incrementation always uses the element-by-element estimates initially, but it can switch to using the global estimates under certain circumstances. For more information, see Element-by-Element Estimation and Global Estimation.

To reduce the chance of a solution becoming unstable during automatic time incrementation, you can specify a scale factor to adjust the initial stable time increment that the solver computes. The scale factor scales the time increment from the default global estimate, the time increment from the element-by-element estimate, or the fixed time increment that is based on the initial element-by-element estimate. You cannot use a scale factor to scale a fixed time increment that you specified directly.

Element-by-Element Estimation

In a coupled thermal-stress explicit step with automatic time incrementation, the solvers initially use element-by-element estimates to determine the stability limit. The element-by-element estimates use the smallest time increment size arising from the thermal and mechanical solution responses in each element across the whole model. An element-by-element estimate is conservative; it is a smaller stable time increment than the true stability limit, which is based on the maximum frequency of the entire model.

In general, constraints (such as boundary conditions and kinematic contact) have the effect of compressing the eigenvalue spectrum, and the element-by-element estimates do not take these into account.

Global Estimation

As a coupled thermal-stress explicit step proceeds, the solvers might switch from element-by-element estimates to global estimates to determine the stability limit for the mechanical solution response. The switch to using global estimates occurs when the algorithm determines that the accuracy of the global estimates is within an acceptable margin. Cases in which the solver does not switch to using the global estimates include:

  • You specify that the solver uses only the element-by-element estimates to determine the stability limit.
  • You specify a fixed time incrementation.
  • A condition prevents the use of the global estimates.

The solvers do not use global estimates to determine the stability limit for the thermal solution response. The solvers always determine the stability limit for the thermal solution response using element-by-element estimates.

Fixed Time Incrementation

Defining a coupled thermal-stress explicit step with fixed time incrementation is useful when you require a more accurate representation of the higher-mode solution response. In this case, you specify a time increment size smaller than the stability limit from the element-by-element estimate. You can obtain the stability limit from the element-by-element estimate by running a simulation check. If you choose to use fixed time increments that are the size of the stability limit based on the initial element-by-element estimate, the solver (at the beginning of the step) uses the dilatational wave speed and the thermal diffusivity in each element to compute the fixed time increment size.

To reduce the chance of a solution becoming unstable during fixed time incrementation, you can specify a scale factor to adjust the initial stable time increment that the solver computes. Alternatively, you can specify a time increment size directly.