Vehicle Crashworthiness and Occupant Protection - Chapter 3

Finite Element Analytical Techniques
and Applications to Structural Design
Normally, every sheet metal component in the car body is meshed separately
using CAD surface data. In order to become a precise model of the sheet in the
midplane, offsets of the surface data are carefully performed. The sheet is then
meshed in a regular way using meshlines that are as much as possible parallel and
orthogonal to the incoming pressure wave and using triangles only where
necessary. Triangles are thus found in areas of mesh transition or areas of high
double curvature (warpage) only. At the assembly of the individual sheets, further
offsets may be necessary in order to guarantee a minimum gap between all parts
so that no initial penetrations are generated. These gaps are also necessary to
ensure a good performance of the contact algorithms and avoid deep penetrations
through the midplane of the opposing part.
Once this task of generating the geometrical model has been performed,
connections (spot welds) must be generated without deforming the geometry of
the flanges on the individual sheet metal parts. This task can be completed in a
number of ways and currently no clearly superior method can be distinguished.
Several automatic procedures to generate spot weld elements of different kinds
are currently being developed by software vendors as well as by automotive
companies. Some fundamental problems remain, however. One is the real rotational
stiffness of the spot weld, which will be discussed in a subsequent paragraph.
Another problem lies in the desire to make the spot weld element location
independent of the finite element mesh on both flanges. Since flanges are currently
meshed with two elements over the width, automatic generation of a regular
pattern for the spot weld elements proves a rather elusive goal. Still, it should be
appreciated that in a modern vehicle model, between 3,000 and 5,000 spot welds
are modeled individually and roughly in their exact locations.
Although the car body model accounts for approximately 75 percent of the entire
model, more and more care is been given to modeling of the many other vehicle
components. First to be mentioned are structural parts such as the subframe,
doors, hood and wings, which use roughly the same modeling rules as the car
body.
Particular care must be given to the connections between car body and subframe,
which are often realized by bolts containing rubber bushings. Whereas the
modeling of the rubber parts would require a prohibitively fine mesh in the context
of a full vehicle simulation, it is still necessary to correctly account for the relative
rotations between the body-in-white and subframe which can severely influence
the acceleration response calculated in the passenger compartment. Therefore, a
rigid body connection as well as a spring element connection will both lead to
erroneous results. It is necessary to correctly simulate the motion of the bolt in
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