# /DT1/BRICK/Keyword3/Iflag

Engine Keyword Alternative time step calculation method for /TETRA4 using /PROP/SOLID Itetra4=1000 solid elements.

It results in a lower but more accurate time step than the default calculation method. The result will be more robust and stable. Otherwise, this option is identical to /DT/BRICK/Keyword3/Iflag.

## Format

/DT1/BRICK/Keyword3/Iflag

$\text{Δ}{T}_{sca}$ $\text{Δ}{T}_{\mathrm{min}}$

If using /DT1/BRICK/CST/1, insert the next line

COL_min Vdef_min

If using /DT1/BRICK/DEL/1, insert the next line:

COL_min Vdef_min ASP_max Vdef_max

## Definition

Field Contents SI Unit Example
Keyword3 Time step control type. 1
CST
Elements are switched to small strain formulation to maintain the constant time step, $\text{Δ}{T}_{\mathrm{min}}$ . Only valid for certain values of Ismstr as defined in element properties. 4
DEL
The element which is controlling the time step is removed so that the constant time step, $\text{Δ}{T}_{\mathrm{min}}$ , can be maintained.
STOP
The run will stop if the time step reaches $\text{Δ}{T}_{\mathrm{min}}$ and a restart file will be written.

If Keyword3 is not specified, that is, /DT1/BRICK, then the default STOP option is used.

$\text{Δ}{T}_{sca}$ Scale factor on critical time step.

Default = 0.9

$\text{Δ}{T}_{\mathrm{min}}$ Minimum time step.
Iflag Number of additional (optional) cards. Only required for /DT1/BRICK/CST/Iflag or /DT/BRICK/DEL/1.
= 0 or blank
Only one card for $\text{Δ}{T}_{sca}$ and $\text{Δ}{T}_{\mathrm{min}}$ is read.
= 1
COL_min Minimum collapse ratio to switch solid element to small strain formulation (to keep constant time step) or to deleted solid element. If Ndir1, the element is deleted and SPH elements are activated. 5 6

Default = 0

Vdef_min Minimum volume ratio (V/Vo) to switch solid element to small strain formulation (to keep constant time step) or to deleted solid element. If Ndir1, the element is deleted and SPH elements are activated. 5 6

Default = 0

ASP_max Maximum aspect ratio to deleted solid element. If Ndir1, the element is deleted and SPH elements are activated. 6

Used only if different from 0.

Default = 0

Vdef_max Maximum volume ratio (V/Vo) to deleted solid element. If Ndir1, the element is deleted and SPH elements are activated. 6

Used only if different from 0.

Default = 0

1. The time step control will be activated when the time step of the element controlling the time step of the simulation becomes less than:

$\frac{\text{Δ}{T}_{\mathrm{min}}}{\text{Δ}{T}_{sca}}$

2. /DT1/BRICK/Keyword3 can be used with /DT/NODA/Keyword3. It will also result in a more robust calculation of the time step, and a more stable computation.
3. This time step calculation is actually the same as default one, except for /TETRA4 using Itetra4=1000.
4. /DT1/BRICK/CST is only active when Ismstr=2 or 12 is defined in /PROP/SOLID. It is compatible with /TETRA4 with Itetra4=1000, /TETRA10, and /BRICK solid elements with Isolid=1, 2, 14, 17, or 24.
5. With /DT/BRICK/CST/1, solid elements will be switched to small strain formulation when criteria COL_min, Vdef_min is reached.
• COL_min uses the same collapse ratio calculation as in HyperMesh element quality check.
• Elements failing the COL_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 can arise.
6. With /DT/BRICK/DEL/1, solid elements will be deleted, or switch to SPH elements if Ndir1 in the solid property, when criteria COL_min, Vdef_min, ASP_max or Vdef_max is reached.
• COL_min and ASP_max use the same collapse ratio and very similar aspect ratio calculation as in HyperMesh element quality check. For thickshell, thickness direction is not taken into account, then aspect ratio 2D and collapse ratio 2D (L_min/L_max) are used to compute COL_min and ASP_max.
• Elements failing the COL_min or ASP_max criteria will be used to delete or switch to SPH elements (if Ndir1 in the solid property) at the beginning or during a simulation.
• Vdef_min or Vdef_max can be used to delete or switch to SPH elements (if Ndir1 in the solid property) when the solid element is highly deformed.