extra
October 2014
Extract Body:
APPLICATION POTENTIAL OF
LITECOR IN THE BODY
The Project
ThyssenKrupp InCar plus
Solutions for Automotive Efficiency
BODY 
APPLICATION POTENTIAL OF
LITECOR IN THE BODY
In addition to outer panel components, ThyssenKrupp has also manufactured structurally-relevant inner parts
using the Litecor steel-polymer composite in a potential analysis. Technical component forming feasibility is
checked in simulations; the joining technology boundary conditions for the individual components are also
­scrutinized. The body’s technical performance is analyzed in stiffness, NVH and crash simulations. In this study,
the body reveals potential for the application of 14 Litecor parts. With the same performance, these are a total of
19.1 kg or around 20 % lighter than conventional components.
The declared objective of developing
the Litecor composite product is costattractive lightweight design for large
shell components – both for inner parts
and in outer panel quality. Litecor is a
three-layer composite which combines
the high strength of steel with the low
density of plastic and is also suitable
for cataphoretic painting, 1 . It consists
of an upper and lower steel cover
sheet, each with a thickness of 0.20 to
0.25 mm, which is attached to a plastic
core layer by an adhesive of between
0.30 and approximately 1.0 mm to form
a sandwich material. The thermoplastic compound layer with variable thickness acts as a firm spacer, with the
result that even a slight increase in the
core thickness results in a disproportionately high increase in bending and
LITECOR® sandwich structure
Steel cover sheet
Plastic core layer
Steel cover sheet
1Litecor composite
10 8
buckling stiffness. Virtually no added
weight occurs due to the polymer’s low
density of 1.03 g/cm³. The weight
reduction in comparison with steel
blanks with the same bending stiffness
is up to 40 %.
Besides its weight and stiffness advantage, Litecor is also suitable for implementing typical steel design features
such as striking styling edges in forming. At the same time, the lightweight
design costs are lower than those of
alloy components.
To meet the technical forming requirements, certain of which are more
demanding, an IF steel grade which can
be easily formed is used. Its strength is
higher than that of soft deep-drawing
steels. The dent resistance required e.g.
in the event of hail impact or minor car
park bumps is therefore ensured. The
cover sheets are electrogalvanized on
both sides to meet automotive corrosion
requirements.
For use as structural components,
higher strength steel grades are recommended as material for the cover sheets.
In a crash, these offer energy-transforming properties with structural stability of
 BODY
the composite structures at the same
time. Possible Litecor structural components include the firewall and floor panels, for example, 2.
LITECOR® body parts
6
5
4
COMPREHENSIVE VIRTUAL
ANALYSIS
This potential analysis encompasses a
total of 14 Litecor parts with a sandwich
structure which meets the requirements.
On selection of the Litecor parts, the
outer panels are taken into consideration
due to their high weight potential and
stiffness requirements. These components have an external steel cover sheet
with a thickness of 0.25 mm and therefore meet the dent resistance requirements, e.g. in the event of hail impact.
On the inner side, a 0.20-mm thick steel
sheet is used for maximum weight saving. Inner parts with reduced crash relevance are also designed in ­Litecor to
achieve further weight savings. The
­Litecor parts can be integrated into existing scenarios with minimum effort.
­Litecor’s thermal expansion is similar to
that of sheet steel, as is its recycling
process.
The individual sandwich components
are initially dimensioned through forming simulations. A forming simulation
model which enables realistic predictions for the classic evaluation criterion
of crack and crease formation is available
for Litecor, 3. On the basis of shell and
volumetric elements, the physical behavior of the sandwich material is plausibly
simulated even for complex local forming processes such as hemming. Adaptations for trimming and grid refinement
functions with volumetric elements have
been created in cooperation with software developers to provide method planners with a practical tool for forming
simulations.
1
3
10
8
9
2
5
7
4
8
3
No.
Part
Composite structure
(thickness of individual layers in mm)
1
Roof outer panel
0.25 / 0.40 / 0.20
2
Hood outer panel
0.25 / 0.40 / 0.20
3
Fender front right/left
0.25 / 0.40 / 0.20
4
Front door outer panel right/left
0.25 / 0.40 / 0.20
5
Rear door outer panel right/left
0.25 / 0.40 / 0.20
0.25 / 0.40 / 0.20
6
Tailgate outer panel
7
Firewall
0.25 / 0.40 / 0.25
8
Main floor middle right/left
0.20 / 0.40 / 0.20
9
Floor panel rear seat
0.25 / 0.40 / 0.25
Floor rear
0.20 / 0.40 / 0.20
10
2Overview of Litecor body components and their layer structure
FEM analysis
LITECOR® real part
LITECOR WELL SUITED FOR
JOINING
To be able to use Litecor effectively in
body-in-white design, it is particularly
necessary to employ resistance spot
welding in combination with adhesive
bonding. Due to its special material
3Comparison of the simulation (left) and component (right) in a critical forming location
October 2014 ThyssenKrupp InCar p lus
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BODY 
structure, Litecor is only conditionally
suitable for the thermal joining processes
(resistance spot, laser beam and arc
welding) used in vehicle design.
Cold mechanical joining processes such
as punch riveting and bolting were therefore analyzed in an initial step. During
generally suitable semi-tubular punch riveting, Litecor, as the underlying material,
should not be the last element in the joint
structure to be penetrated by the rivet.
Using semi-tubular punch rivets in combination with adhesive bonding is generally recommended. In this case, adhesive
bonding is subject to the same boundary
conditions as those applicable to galvanized steel sheets.
Under the influence of thermal stress,
creeping effects may occur in the core
layer in the case of mechanical joining
elements with preload force, such as
bolts. This effect can be countered by
locally pre-conditioning the joining point,
e.g. in the forming tool, prior to joining.
MIG, MAG and laser beam welding
are not possible due to the material
structure. Laser brazing at low temperatures can be used after adapting the
process parameters and the auxiliary
materials. Production process suitability
must be verified in the individual case.
In the second step, a new resistance
spot welding process was developed to
qualify Litecor for this process and for
combined spot welding and adhesive
bonding. In a practical test program,
­Litecor revealed process consistency in
resistance spot welding with different
steel grades in both two- and three-sheet
versions with only minor modifications
to a standard welding system. The
attained joint qualities and strength
­values meet the requirements, and the
robustness of the process was confirmed.
With a view to use in production, a prototype welding system is being developed
at ThyssenKrupp to enable near-production testing and qualification of the pro-
cess. The firewall serves as a demonstrator; resistance spot welding and adhesive
bonding will be employed to join it to the
surrounding steel body components such
as the tunnel, A-pillar, firewall crossmember and reinforcement elements, 4.
For all outer panel components
­ThyssenKrupp recommends IF steel
grades which meet requirements for oil
canning and dent resistance. As for conventional solutions, the dent repair methods (“dent doctor”) are available for permanent dents caused e.g. by hail impact
or minor car park bumps. Steel cover
sheets with a thickness of 0.25 mm or
more are usually selected for structural
components to meet the higher strength
requirements. In two cases, higher
strength steel grades are also used to
increase the load level that can be withstood in a crash. According to current
information, ­Litecor offers forming capability similar to that of monolithic sheets
of the same basic grade.
Resistance spot welding
LITECOR®
2
BETAMATE 1620
DP-K® 330Y590T
2-sheet joint:
LITECOR® 0.25 / 0.40 / 0.25 mm
DP-K® 330Y590T, t = 1.0 mm
Joining method:
Resistance spot welding
1
DP-K® 590Y980T
LITECOR®
2
1
MBW® 1500
4
3
3-sheet joint:
DP-K® 590Y980T, t = 1.0 mm
LITECOR® 0.25 / 0.40 / 0.25 mm
MBW® 1500, t = 1.0 mm
Joining method:
Resistance spot welding
4Resistance spot welding of Litecor with typical material pairings in body design
110
LITECOR ®
DP-K® 590Y980T
2-sheet joint:
LITECOR® 0.25 / 0.40 / 0.25 mm
DP-K® 590Y980T, t = 1.0 mm
Joining method:
Resistance spot welding
+ Adhesive bonding BETAMATE 1620
LITECOR®
MBW® 1500
CR300LA
3-sheet joint:
LITECOR® 0.25 / 0.40 / 0.25 mm
MBW® 1500, t = 1.0 mm
CR300LA, t = 1.0 mm
Joining method:
Resistance spot welding
 BODY
Crash simulation
200
190
180
150
130
110
90
70
50
30
10
-10
Intrusion [mm]
5Litecor firewall intrusion after Euro NCAP frontal crash
SAFE AND LIGHT
Noise, vibration and harshness (NVH)
simulations are also performed on the
basis of the numerical stiffness model to
determine natural frequencies and avoid
critical vibration modes. As part of these
simulations, moderate thickening is
required for certain steel reinforcement
parts in the body. As a result, support
effects and local yields are optimized in
the design. The resulting, additional
weight of 2.6 kg is taken into consider­
ation in the Litecor body’s weight balance. At the same time, increased damping can be exploited to undertake specific minimization at the secondary
acoustic materials, possibly resulting in
a further weight and cost advantage with
Litecor.
In parallel with the NVH and stiffness
simulations, the body is tested and eval-
uated as regards five representative crash
load cases (Euro NCAP Front, IIHS
SORB, FMVSS 301, Euro NCAP Pole) in
crash analyses. In this case, individual
steel components are replaced with
­Litecor parts in iterative loops in the
crash model and their technical performance is analyzed. Following adaptation
of the layer thicknesses and, if necessary, the choice of the steel cover sheets’
strength class, the final Litecor body
­variant meets all crash requirements in a
manner similar to the reference body, 5.
However, it is 19.1 kg lighter in com­
parison. Litecor is therefore outstandingly suitable for meeting further weight
reduction requirements.
system suitable for volume production of
Litecor. Volume production of Litecor
material for inner parts is initially
planned in the medium term, with outer
panel material to follow.
OUTLOOK
At present, ThyssenKrupp is working
intensively to construct a manufacturing
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