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

Oasys LS-DYNA Environment: User Guide (Version 8.1)
12.0 TIMESTEP CONTROL
The timestep in LS-DYNA is generally limited by stability. Usually, the timestep falls during
an analysis as elements become deformed. It is also possible for the timestep to rise. LS-DYNA
automatically calculates the largest timestep which can be used without triggering numerical
instability; it is not possible to force the code to use a timestep larger than this. It is, however,
possible to force the code to use a timestep smaller than the calculated value, either by defining
a multiplying factor less than the default value of 0.9 (SCFT on *CONTROL_TIMESTEP), or
by specifying a timestep-vs-time loadcurve (LCTM on *CONTROL_TIMESTEP).
Instability (shown by rapidly rising energy and a "floating overflow" error) will occur if the
period of any mode of deformation in the model is less than π times the timestep. Thus the
timestep must divide the period of the highest mode by at least π. Generally the highest mode
is a single element mode, i.e. oscillation of a single brick, shell, beam or spring element.
LS-DYNA checks all elements when calculating the required timestep. The timestep can be
estimated roughly using the formula
ìt = 0.9 L/c (solid, shell, beam)
where l is the smallest element dimension and c the speed of sound in the material. Shell
thickness and beam section dimensions are ignored when finding l. Rigid elements are not
included.
More accurate estimates can be obtained using l and c instead of l and c as follows:
L = Volume/Area of largest surface (solid elements)
L = as per ISDO on *CONTROL_TIMESTEP.
c = (E/ρ)
E = K + 4G/3 = E(1-ν)/[(1+ν)(1-2ν)]
It is possible for flexure modes of Belytschko-Schwer beams to control the timestep especially
if they are short in comparison to their section dimensions. In this case,
t =014l 2 (A(12r + 1)/I(3r + 1))
c'
Where A = cross-sectional area
I = maximum of I ss, I tt, J
r = I/(Al 2 )
Page 12.1