Oasys LS-DYNA Environment 8.1 VOLUME 3 ... - Oasys Software

Oasys LS-DYNA Environment: User Guide (Version 8.1)
ERODE (with DTMIN on *CONTROL_TERMINATION) Elements whose timesteps fall
below SCFT*DTMIN*DTSTART (where DTSTART is the initial timestep) will
be deleted provided that ERODE is set to 1. The disadvantage of this is that the
eroding contact types must be used if solid elements are present, with consequent
increase in memory and CPU requirements. Also the holes left by the deleted
elements can cause contact problems. If DTMIN is non-zero but ERODE is
zero, the run will simply terminate if the timestep falls below
DTMIN*DTSTART.
Note that none of the above options apply to Spring elements. For any Spring elements which
may control the timestep, it is recommended that lumped masses be added to their nodes. In
previous versions of LS-DYNA it was possible to force LS-DYNA to ignore spring elements
when calculating the timestep. This option was considered dangerous and has been discontinued.
Optimising the Timestep for Impact Models
Simply enter the required timestep (with a negative sign) as DT2MS on
*CONTROL_TERMINATION. LS-DYNA will add mass as required to bring the timestep up
to SCFT*DT2MS (N.B. SCFT is usually 0.9; it is advisable to allow for this when selecting a
value of DT2MS). Mass is added only to those elements whose timestep would otherwise have
been less than the specified value. The amount of mass added can be checked in the LOG and
OTF files. Clearly the more mass added, the less accurate the calculation; it is up to the analyst
to decide how much added mass is allowable. To reduce the added mass, reduce the specified
timestep.
Subcycling
Most models contain elements of a range of sizes, and generally the timestep for the whole model
is controlled by the smallest element. Thus CPU time is wasted in recalculating strains and
stresses in the larger elements more often than is required for stability. Subcycling overcomes
this by dividing the elements into groups by timestep, and by using a different timestep for each
group. If the smallest timestep in the model is DT, the timesteps for the groups are DT, 2DT,
4DT, 8DT, 16DT and 32DT. (Further explanation is given in the Theory Manual, Section 21.2).
Subcycling offers a most significant reduction in CPU requirement when there are relatively few
small elements, and the bulk of the elements have a stable timestep at least twice that of the
smallest element: for example, if one small region of a model is meshed much more finely than
the remainder. In a typical crash model, the mesh size is well planned and a large proportion of
the elements have a timestep within a factor of two of the minimum timestep. In these cases,
subcycling is unlikely to reduce CPU cost significantly and may even increase it, especially on
vector machines. To activate subcycling in a model, include *CONTROL_SUBCYCLE in the
input file. No further input is required.
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