12-12-2012, 12:07 PM
Manufacturing Simulation of an Automotive Hood Assembly
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ABSTRACT
This paper presents the results of applying the finite element method to calculating
the spring back of an automotive hood assembly, and its application to the functional
build method. The assembly was comprised of six individual panels: an inner panel,
an outer panel, a major reinforcement, a latch reinforcement, and two hinge reinforcements.
Finite element simulations were conducted for forming each of the six components.
Each component was formed, trimmed, and positioned in car position. The outer
panel required several secondary forming operations including a re-meshing, remapping,
trim, and flanging operation.
Once in car position, the components were moved so that they just contacted each
other, and were “spot welded” together through the application of nodal constraints.
Mastic between components was simulated with tied contact. Contact between
components was simulated with contact interfaces. Finally, a spring back analysis
was conducted.
The models clearly illustrate that it is possible to predict spring back of large automotive
assemblies, and that the assembly process yields different final shapes than
those obtained from spring back of individual components. With this newly developed
tool it is possible to predict whether or not the assembly process can correct
out-of-spec components, a key factor in utilizing the functional build method.
INTRODUCTION
A typical automotive body is a complex structure comprised of many sub-assemblies
each made of many parts. Often times these parts are sheet metal stampings that
must be joined together using spot weld or hemming processes to make the subassemblies.
It has been shown by Hammett et al.1 that the final shape of automotive structures is
not only affected by residual stresses in the individual stamped parts, but often times
by the assembly process itself. Automotive companies typically use a sequential
validation process whereby individual stampings are compared to their printed specifications
during the die buy-off stage. If the parts do not conform, then the die is reworked
until the stamped parts do conform. Often times this effort is wasted as the
part would take its desired shape when joined to a stiffer part during the subsequent
assembly process. Alternatively, the assembly process itself can distort the shape of
individual panels that were produced within specification. The functional build approach
recognizes this fact and attempts to take advantage of it: relatively cheap
assembly fixtures are used to ensure that during assembly the out of spec components
are brought within spec.
While the functional build approach is gaining acceptance in the automotive industry,
it is not without its detractors. One of the serious drawbacks is that the functional
build approach is a downstream activity. All components to be assembled must first
be manufactured before the functional build activity can begin. The investment in
tooling for forming each component may be lost if the functional build approach fails
to produce an in-spec assembly and the components must be modified so as to bring
the assembly into spec. This represents a significant risk to the automotive manufacturers
whose risk tolerance decreases dramatically closer to production.
Forming Simulations
The components to be assembled are shown in Figure 1. The components are from
a production vehicle and are typical of many hood assemblies constructed with a
cruciform inner panel. Although the production vehicle has steel components, they
were modeled as if produced from aluminum. For confidentiality reasons, pictures of
the inner panel and outer panel are cropped so as to hide some of their identifying
features.
Spring Back Calculations
Spring back was calculated for each of the individual panels, and for the assembly.
Model run times, memory requirements, and number of steps and iterations are listed
in Table 2.
The SMP LS-DYNA implicit solver was used because the MPP version does not
currently support contact in implicit analyses. Because plasticity rarely occurs during
spring back, the material was modeled as being a linear elastic material. A nonx
y
linear solution is required nonetheless, because of the geometric non-linearities related
to large deformation during spring back.
Spring Back of the Assembly
The six components were positioned in car position as a starting point for the assembly
process. Each of the dynain files from the forming processes were then read
into DYNAFORM Version 4.0, as this version is capable of reading in multiple dynain
files and automatically reordering the nodes and element numbers to ensure unique
numbering when combining input files. A new dynain file was created which contained
all six components in roughly the correct position for assembly
Summary and Conclusions
With recent advances in LS-DYNA, it is now possible to simulate an assembly process
for a component comprised of multiple sub-components. The implicit solver has
become more robust and is now capable of handling significantly larger models than
in the past. Extension of the MPP code to include implicit contact is required before
that code can be used for doing implicit spring back calculations, but once this ongoing
work is completed, it should allow even larger assemblies to be studied.