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

Oasys LS-DYNA Environment: User Guide (Version 8.1)
folded bags, an initial timestep as low as 0.1 microsec may be necessary, rising to the LS-DYNA
default by 5 millisec.
REFERENCE GEOMETRY
By default, the undeformed state of the fabric elements is defined by the initial geometry.
However, this is not always wanted.
By using the airbag reference geometry feature, it is possible to define the flat, unfolded mesh
geometry separatety from the initial geometry. LS-DYNA then compares the current geometry
of an element with the reference geometry, instead of the undeformed geometry, to calculate the
strains and stresses in an element. Stresses are zero until an element size reaches that of its
equivalent in the reference geometry.
There are two main reasons for using this feature:
! Folding the mesh distorts the elements around the fold lines so that the initial geometry is
inaccurate. This can lead to an incorrect shape when the bag is inflated unless the reference
geometry is used.
! A “shrunken” unfolded bag may be used to avoid initial contact with the occupant or to
achieve a reasonable initial volume. The airbag will inflate to the correct size if it is defined
using the reference geometry.
The coordinates of the airbag nodes in the folded or reduced state is included under *NODES as
usual. The coordinates of the same nodes in the flat, full-size state are given under
*AIRBAG_REFERENCE_GEOMETRY. For reduced size airbags, use
*AIRBAG_REFERENCE_GEOMETRY_RDT. This sets the timestep based on the reference,
not the reduced state.
If the fabric is defined as orthotropic, the angles are relative to the reference geometry: this
simplifies the definition for folded airbags.
JETTING
In a real airbag, the pressure on the portions of fabric directly in front of the inflator will be
higher than the pressure on other parts of the bag, because the fabric in front of the inflator feels
momentum forces from the gas entering the bag. However, the internal pressure in an LS-DYNA
control volume is constant throughout the volume at any given time.
LS-DYNA can simulate the effect of the momentum forces from the gas that rushes out of the
inflator. It does this by applying extra forces to nodes that are in the inflator’s line-of-sight. The
user defines the inflator position and direction, and a “cone angle” over which the forces are to
be applied. The jetting forces are calculated automatically from the mass inflow and gas
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