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

Oasys LS-DYNA Environment: User Guide (Version 8.1)
vertical fold on either side of the module, with the outer portions of the bag initially pointing
towards the occupant. Another way is to use an initially shrunken airbag and the “reference
geometry” facility (see later).
However, in many cases the interactions of interest occur as the airbag is unfolding, for example
where the occupant is out of position or when the behaviour of the airbag module is being
studied. Such simulations must use a fully folded airbag mesh.
OASYS PRIMER contains special features to create a folded airbag mesh from an unfolded one.
The PRIMER manual contains a tutorial. Here are some tips:
! As well as straight mesh lines along the fold positions, also make the next-door mesh lines
straight and parallel to the folds
! When creating the unfolded mesh in the normal pre-processor, space the two layers slightly
more than one fabric thickness apart.
! The fold thickness should typically by slightly more than one fabric thickness
! Adjust the fold “scale factor” to achieve parallel lines at the folds
! Use thorough visual checking of the folded mesh in PRIMER to check that there are no initial
penetrations.
RUNNING FOLDED MODELS
Folded airbags must have a single-surface contact defined to prevent one part of the airbag
penetrating another. The recommended contact type to use is
*CONTACT_AIRBAG_SINGLE_SURFACE, which has been specially formulated for airbag
unfolding.
Even when an airbag has been folded and checked carefully in PRIMER, a number of initial
penetrations are often detected: check for warnings in the otf file. There should be no initial
penetrations if the airbag contact is to work correctly. Initial penetrations can be avoided by
controlling the contact segment thickness as a function of time via a load curve. The load curve
is defined using the LCIDAB in the *CONTACT cards. Typically the curve will begin at 0.3 to
1.0 times the real fabric thickness, rising to 1 to 2 times the fabric thickness at 1-2 millisec, and
remain constant thereafter.
Use trial and error to find the maximum value of the initial thickness that prevents warnings. If
the value is less than about 0.1mm the folded mesh is probably not good enough.
Usually, the timestep has to be reduced to prevent contact problems as the bag unfolds. This is
most important in the first millisecond; there is less need for a small timestep as the unfolding
process continues. The timestep can be controlled by a load curve on *CONTROL_TIMESTEP.
The ideal curve is problem-dependent and must be found by trial and error. With very tightly
Page 16.3