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INNOVATIVE LIGHTWEIGHT DESIGN CONCEPTS FOR HOODS Modern steel technologies and lightweight design concepts are making hoods over 20 % lighter. This lightweight design potential can be accessed by using the flexurally stiff and light steel-polymer composite Litecor without having to compromise on performance or safety. Low lightweight design costs additionally ensure that this solution is economically very attractive. A multi-material concept with magnesium sheet metal even enables a weight advantage of up to 40 %.
As part of InCar plus, ThyssenKrupp has studied various concepts for light and cost-optimized hoods, ➊. The Litecor steel-polymer composite, which offers optimized stiffness and consists of two
very thin steel cover sheets and a plastic core, is particularly suitable for meeting the partly contradictory requirements of stiffness and pedestrian protection. The thickness of the steel cover sheets and
Hood benchmark Segment
Mid-size
Material Weight Weight per unit area [kg / m²]
InCar®plus
Upper mid-size
Steel
Aluminum
20.4 kg
20.1 kg
20.9 kg
11.0
10.9
11.6
InCar ®plus reference
11.0 kg
12.2 kg
13.7 kg
5.7
6.2
6.9
InCar ®plus solution LITECOR®
Design
InCar ®plus solution steel-magnesium hybrid
Steel LITECOR®
the plastic core can be varied, thus enabling tailored solutions for special applications and requirements. The project kicked off with extensive research work, in which the state of the art for both aluminum and steel hoods in the mid-size and upper mid-size class was determined. The steel reference hood developed for InCar plus is oriented towards the best hoods in terms of global stiffness and oil canning. Pedestrian protection is an important criterion in the design of a hood. All hood variants are therefore subjected to impact simulations according to the Euro NCAP process. At 168 measuring points on the front structure of a vehicle, an example dummy head impact is used to determine how compatible the structure is with the objectives of pedestrian protection. The InCar plus reference hood and all of the studied concepts offer the potential for five stars even without active measures such as raising the hood using accordingly designed hinges.
Magnesium
Weight Weight per unit area [kg / m²]
19.3 kg 10.2
14.9 kg 7.9
➊ Benchmark of hoods (top) and InCar plus hood concepts (bottom)
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11.3 kg
6.0
LITECOR OUTER PANEL WITH WEIGHT- OPTIMIZED INNER STRUCTURE
The outer hood panel manufactured using the Litecor steel-polymer composite displays outstanding stiffness properties and low weight per unit area. Thus, it is 2.9 kg lighter than the outer panel of
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the reference hood. The inner structure also offers further potential for reducing weight. In comparison with the reference outer panel’s sheet metal thickness of 0.70 mm, the Litecor layer structure is slightly larger, with an overall thickness of 0.85 mm, thus offering higher resistance to oil canning. As a result, the inner structure can be adapted by reducing the sheet metal thickness and optimizing the strut design. In comparison with the reference structure, this reduces the weight by a further 1.5 kg. In total, the hood with Litecor outer panel and optimized inner part is therefore 4.4 kg lighter than the reference.
200
Point A48
Force [N]
150 100 50 0 0
2
4
6
8
10
8
10
Displacement [mm] 250
InCar®plus LITECOR®
Force [N]
200
STRUCTURE-MECHANICAL DESIGN
150 100 50 0
2000
2
4
6
Acceleration curve for point A 4 8
Simulation
Displacement [mm]
Acceleration [m/s²]
The Litecor hood with weight-optimized inner part was tested as regards the load cases of torsional stiffness, front lateral stiffness, longitudinal stiffness and rear lateral stiffness. It meets the global stiffness target values. The highrating level of oil Pedestrian protection canning resistance which is achieved is 1,700 < ∞ 0 < 650 1,000 < 1,350 similar to that of the reference hood, ➋. 650 < 1,000 1,350 < 1,700 The Euro NCAP pedestrian protection assessment is undertaken virtually on the overall vehicle. Potential for a five-star rating is achieved in consideration of the HIC values (Head Injury Criterion) of all 168 impact points. Rig tests were performed on prototype hoods to validate the virtual development. The test results reveal a good match with the simulation, ➌.
InCar® plus reference
150
HIC value: 519 ➋ Oil canning resistance of the reference hood (top) and the Litecor hood (bottom) Test HIC value: 616
100 MULTI-MATERIAL DESIGN AS A CONCEPT 50 One innovative option for saving weight is the combination of magne0 metal for the inner strucsium sheet 0 0.005 0.01
ture and very thin but higher strength sheet steel for the outer panel. Thanks to the low density of magnesium, a hood with this multi-material design weighs just 11.3 kg, around 8 kg less than the reference. 0.015
0.02
0.025
0.03
Time [s]
Head impact simulation
Head impact test
➌ Comparison of simulation (left) and test (right) October 2014
ThyssenKrupp InCar plus
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Cost and weight comparison € 77.10
[200,000 vehicles/year]
€ 53.70 € 44.20 19.3 kg 14.9 kg 11.3 kg
➍ The InCar plus solutions offer a weight advantage of up to 8 kg per hood
Global warming potential [kg CO2 equivalent]
500
InCar ®plus reference
InCar ®plus solution LITECOR®
InCar ®plus solution steel-magnesium hybrid
Global warming potential
InCar ®plus reference
400 InCar ®plus solution LITECOR®
300
InCar ®plus solution steel-magnesium hybrid
200
100
0 0
50,000
100,000
150,000
200,000
Mileage [km] ➎ Both InCar plus solutions are better than the reference in terms of global warming potential
The extensive lightweight design potential is concealed beneath the outer panel in the form of an optimized inner hood structure. A honeycomb geometry supports the outer panel over its entire area, thus enabling especially thin steel sheets to be used for the outer panel. This newly developed concept offers good oil canning performance even with an outer panel which is only 0.50 mm thick. The global stiffnesses correspond to the target values. Potential for a 5-star rating is achieved in Euro NCAP pedestrian protection. ThyssenKrupp has extensively tested the technical manufacturing feasibility of the magnesium sheet component. Magnesium is formed at a temperature of
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approximately 240 °C in a heated die. A forming simulation confirms that the magnesium sheet components can be manufactured. The magnesium sheet hinge and latch reinforcements can be joined to the inner magnesium panel using spot welding. The cycle times are around 0.4 to 0.65 s per spot weld and are comparable with those of steel components. The outer panel, consisting of extremely thin steel sheet, is joined to the inner magnesium sheet structure by means of table-top hemming. Additional corrosion protection is also required for the magnesium sheet components. In this case, multilayer wet coating for high corrosion requirements is used.
As the mechanical and thermal properties of steel and magnesium are different, a material-friendly design is vital. Larger cross-sections in the surrounding profiles enable individual adaptation of the global stiffness to customer requirements. To ensure that the hood continues to meet the pedestrian protection specifications, specific weakening of the profiles by means of slitting is possible. ATTRACTIVE COSTS, POSITIVE OUTLOOKS
Both concepts have been extensively evaluated in terms of manufacturing and costs, and are suitable for integration into the conventional body-in-white. Detailed body-in-white planning together with a cost breakdown is available for each concept, ➍. Litecor hemming only requires minor adaptations to the table-top hemming system. Roller hemming may also prove sensible for small quantities. In total, lightweight design costs of less than € 2.20/kg are incurred for the Litecor hood. In the multi-material concept, the material costs for the magnesium sheet and its further processing lead to lightweight design costs of € 4.11/kg. Both concepts have been analyzed as regards their environmental compatibility, ➎. If raw material acquisition, material manufacturing and recycling are included in the balance, both solutions are ecologically very attractive. Litecor’s suitability for use in production has already been successfully demonstrated in the hood fitted on the VW Polo World Rally Car. In the mediumterm, Litecor is initially to be manufactured on a large scale for inner parts. This will be followed by visible components with outer panel quality.
