Madymo-Applications Manual

1.1 Airbag-related features
The user will often have to deal with the same important airbag-related features and
aspects when building an airbag model. This section gives explanations on those
aspects.
1.1.1 Time step and computation time
To be predictive, finite element airbag models require the use of a relatively small time
step. In general, a time step of 1·10-6 s is advised in order to describe contact
interactions correctly. This is particularly true in applications where the airbag
deployment must be modelled accurately, and also in applications with FE dummy
models. Therefore, particular attention has to be paid to the definition of the contact
groups, since a more selective choice of the elements involved in contact can
significantly reduce the computation time. Another way to improve the efficiency of
simulations with respect to CPU time is to activate the detection of the contacts only
when this becomes relevant. For that, the STATE.CONTACT and SWITCH.* elements
can be combined.
Note that no sub-cycling is allowed between the thermo-dynamical Gas Flow (GF)
calculation and the Finite Element (FE) calculation of the FE model containing the GF
module. When one of the required GF or FE time-step is smaller than the other one, the
smallest is used. Due to small GF cells and high gas velocities the GF time step might
drop, causing the related FE time step to drop as well. When contact is defined
between this FE model and another FE model, the time step of this other FE model will
be set to the same value as the first one, in order to obtain a synchronisation of the
contact.

1.1.2 MADYMO Folder program
The MADYMO Folder program is able to fold any flat mesh. Meshes can be folded
either with the standard Folding module or with the newly-developed Mesh
Independent Folding module of the MADYMO Folder. Before using the standard
module the mesh has to be prepared in such a way that the nodes are distributed along
the folding lines, so that the elements are not distorted during folding. With the Mesh
Independent Folding module there is no need to prepare the mesh in advance, since
the module makes use of a special algorithm that enables re-meshing before folding. It
is strongly recommended to check the general aspect of the mesh at the end of the remeshing
process, to ensure that no extremely small elements have been created that
could influence the time step of the simulation.
When using the MADYMO Folder program, the user should avoid intersected
elements and minimise the number of distorted elements in the folded airbag. It is
possible to detect both within the MADYMO Folder program, and even the strains in
the mesh can be checked. In general, thin folds will affect fewer elements than thick
folds, but will cause slightly larger local deformations. To minimise distorted elements
the user can also make use of two folds of 90° instead of one of 180°, if this is possible.
The user should always try to find a compromise between relatively low strains in the
mesh and relatively large gaps between the fabric layers.

http://www.cadfamily.com/HTML/Tutorial/Madymo-Applications%20Manual_305903.htm