Element Time Step Control

The element time step can be controlled using the Engine options:

/DT/Eltyp/Keyword3/Iflag
$Δ T_{s c a}$ $Δ T_{\min}$

Where,

Eltype: Element type
Keyword3: Time step control type used

If Iflag=1, then a second line with additional input is entered for some element types and times step control methods.

For all the Keyword3 options, the action taken will occur when with the calculation of the element time step defined in Finite Elements. For element time step control, only one Keyword3 option is available per Eltyp, which means it is not possible to use /DT/BRICK/CST and /DT/BRICK/DEL in the same model, but /DT/SHELL/STOP and /DT/BRICK/CST can be used together.

/DT/Eltyp/CST

Eltyp=BRICK or SHELL.

This option controls the times step by switching elements to the small strain formulation if they cause the simulations time step to be less than $Δ T_{\min}$ . This only occurs when using certain I_smstr values defined in the property definition /PROP for the element. Although, switching to small strain formulation is not as accurate as large strain, the reduction in accuracy is often necessary to obtain a solution in a reasonable time.

If using /DT/BRICK/CST/1, then a second extra line is added.

$Δ T_{s c a}$ $Δ T_{\min}$
ASP_min VDEF_min

Where,

ASP_min: Minimum aspect
VDEF_min: Minimum volume ratio (V/V₀)

Elements will be switched to small strain formulation when one of the two minimum values for ASP_min or VDEF_min has been reached. ASP_min uses the same tet collapse calculation as HyperMesh element quality check. Elements failing the ASP_min criteria will be switched to the small strain formulation at the beginning or during a simulation. VDEF_min can be used to switch highly deformed or compressed elements to the small strain formulation during the simulation thus avoiding negative volume issues that sometimes arise during a simulation.

When the switch to small strain formulation occurs, a constant Jacobian matrix is assumed and all spatial variables (characteristic lengths, volumes, …) are considered in their initial configuration. At this point, the element deformation is not taken in account anymore when calculating the time step. If the sound speed is constant, the time step thus becomes and stays constant. For some solid material laws, the sound velocity is pressure dependent. It is, therefore, possible that the time step becomes a little smaller than $Δ T_{\min}$ . If an elements time step is less than $Δ T_{\min}$ at the beginning of a simulation, then the element will switch to the small strain formulation and assuming that element has the smallest time step the simulation will run at a time step less than $Δ T_{\min}$ . When an element switches to the small strain formulation, a message is printed to the Engine output file.

Advantages and Disadvantages

The switch to small strain formulation stops a decrease in time step which if allowed to continue would cause a simulation to run much longer. Most times the slight loss in accuracy caused by a few elements using the small strain formulation is worth it to make sure results are obtained in a reasonable time. Switching to small strain is better than adding mass and thus kinetic energy in dynamic simulation or removing element and thus removing kinetic energy. Switching to small strain to maintain a time step can be a better solution in dynamic simulations rather than adding or losing kinetic energy associated with adding mass or deleting elements.

There are less limitations for shell elements than for solid elements. In crash applications, the main shell deformation is bending and the small strain formulation has no effect on the bending description if membrane deformation is small.

A large number of elements switching to the small strain formulation typically indicates a model issue that needs investigated.

/DT/Eltyp/DEL

Deletes the element that causes the simulation time step to become less than $Δ T_{\min}$ . When elements are deleted, the physics of the model is changed because removing an element changes the mass, inertia and possibly the stiffness of the model. When an element is deleted, a message is printed to the Engine output file.

/DT/Eltyp/STOP

Stops the simulation when the simulation’s time step drops below the entered $Δ T_{\min}$ value. Many times, a reduction in time step is caused by a model's instability, so stopping the simulation can be useful to diagnose the issue.