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

Oasys LS-DYNA Environment: User Guide (Version 8.1)
For spring elements the calculation is:
t = 2(mk)
Where m = m 1m 2/(m 1 + m 2)
m 1,m 2 = mass of nodes
k = current spring stiffness
For dampers it is
ìt = 0.1m/λ
Where λ = damping constant
m = as above
For more details see Section 19 of the Theory Manual.
On commencing the solution phase and also on restart LS-DYNA reports the 100 elements with
the smallest timesteps to the .OTF file. There is also an option (IPNINT on
*CONTROL_OUTPUT) to output the initial timestep of all the elements in the model.
In most problems the stable timestep selected by LS-DYNA is small enough not to affect
accuracy (i.e. use of a smaller timestep would produce the same answers).
Exceptions occur when the highest modes in the model (i.e. single element modes) have a
significant effect on the results, e.g. mass-spring models and shock wave problems. In these
cases it is usually beneficial to reduce the timestep.
Three options are available for preventing the timestep falling below a given value (all of these
are on *CONTROL_TIMESTEP):
TSLIMT (applies only to shell elements of material types 3, 18, 19 and 24). This reduces
the Young's Modulus of elements which would otherwise demand a timestep less
than TSLIMT. The timestep for the whole model is then prevented from falling
below TSLIMT. This can be dangerous: if the critical element is not a shell
element of the material types given above, the model may go unstable and the run
may then crash.
DT2MS When set to a negative value, this tells LS-DYNA to add mass to any element
which would otherwise demand a timestep less than SCFT*DT2MS. Because
this option applies to all element classes (except spring elements) and material
types, it should be used in preference to TSLIMT. Use with ENDMAS on
*CONTROL_TERMINATION.
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