Hybrid Body Panels with Structural Foam Reinforcements Equal Big Weight Savings

By on September 28, 2019 in IN THE NEWS

Replacing all-metal components doesn’t decrease crash safety.

Ajoint study investigating the possibilities of further automotive lightweighting in body and closure parts has revealed that about 40 kg of weight per car can be saved if conventional allmetal components are replaced by hybrid designs using high-performance structural foam.

The approach leverages the synergy of a specially developed lightweight structural-foam technology from Henkel, a leader in adhesive technologies, in combination with advanced engineering design by RLE International, which provides development, technology and consultation services to the mobility and engineering industries. Extensive crash simulations have validated the concept.

Lightweighting remains a continuing challenge for automotive manufacturers even as more and more vehicles are turning electric and losing their traditional combustion components. Although the energy capacity of advanced lithium-ion batteries has improved considerably over recent years, their weight may still account for 600 to 800 kg depending on car size and targeted reach.

Body components and especially closures have always been a focus of lightweighting in automotive engineering since they form some of the biggest parts of vehicles. Even if you reduce their thickness by just one-tenth of a millimeter, you can save a lot of weight. However, on most modern cars, these parts have already become so thin that further thickness reductions will create problems in terms of buckling strength and rigidity, resulting in insufficient crash protection.

In the light of these constraints, Henkel and RLE International have undertaken a collaborative study targeted at reducing the weight of closures, fenders, pillars, bumpers, etc., without compromising stiffness and crash performance in line with standard automotive crash scenarios.

The ambitious approach is based on the extensive replacement of common all-metal (steel or aluminum) components by hybrid designs with rigid local reinforcement patches using high-performance structural foam. The image at the beginning of the article illustrates the investigated parts and potential of this concept.

ABOUT HYBRID STRUCTURAL PARTS
The hybrid parts consist of a fiber-reinforced frame or carrier—e.g., injection molded in 30 percent glass-filled polyamide resin—and structural foam injected at predefined sections (Image 1). The foam expands in the e-coat oven and creates a stiff connection between the hybrid structure and other parts of the Body-in-White unit.

Compared to all-metal platforms, the design freedom and efficient processability offered by fiber reinforced plastics (FRP) and foam materials facilitate the adaptation of each structure to the required geometry, with locally stiffening foam ribs added precisely where needed. The structural foam underlying the study is Teroson® EP 1450, a commercially available epoxy-based material from Henkel.

Body components and especially closures have always been a focus of lightweighting in automotive engineering since they form some of the biggest parts of vehicles.

Part fixation can be done using either clips or spot welding, making a simplified assembly process. Each component design is fully engineered and optimized by Henkel and RLE for all crash-load scenarios at one of the partners’ six development centers around the world. The final parts will then be manufactured for on-time delivery at sites located close to customers.

Automakers who want to use the same vehicle platform for as many different models as possible could simply modify an existing internal combustion engine car platform and replace common steel reinforcements by strategically adding structural foam, as required, thus also saving development time and manufacturing costs.

The starting point of the actual study was a radical review of standard component designs by focusing on their required rigidity and performance in crash situations.

COMPONENT TESTING
The starting point of the actual study was a radical review of standard component designs by focusing on their required rigidity and performance in crash situations. The final designs were arrived at in several consecutive optimization cycles, including extensive crash simulations and other pertinent testing.

Bumper beam
Image 2 exemplifies the concept by means of a structural hybrid reinforcement for use in the right and left sections of the front bumper beam. In particular, the solution also addresses small overlap front-crash performance requirements, reducing beam thickness, while saving weight.

Adding panel
reinforcements to the side outer body panel, fender and doors also provides valuable weight savings. At a wall thickness reduced by 0.2 mm in aluminum panels, overall weight savings are achieved. Special finite element analysis (FEA) was also performed to establish and confirm the bendability strength of the door panels.

Liftgate
Another major area for applying the hybrid structural design concept is the liftgate, where a material switch of the panels from steel to aluminum and an optimized design of the hinge and the inner panel reinforcement (Image 3) resulted in weight savings versus all-steel liftgates—notably without compromising the torsional and bending strength of the liftgate.

In the case of existing aluminum inner and outer panels with steel reinforcements, replacing the latter with hybrid structural reinforcements can still save weight while increasing the torsional and bending performance.

Rocker panel, pillars, rear
Similar optimized solutions were provided for the rocker panel, the A-, B- and C/D-pillars, the rear roof section, as well as the rear side rail and impact plate, by eliminating steel reinforcements, reducing the wall thickness of the outer panels and adding structural hybrid reinforcements.

The changes recommended for the rocker panel, which plays an essential role in the effective crash protection of battery packs in electric vehicles, have been evaluated in a separate battery protection white paper. Altogether, the lightweighting measures in the Henkel and RLE study result in total weight savings of more than 40 kg per car, with subsequent lower fuel or electrical power consumption and carbon emissions.

Reducing the thickness of the outer side panels by 0.2 mm was rather difficult to achieve with aluminum. One particular requirement to meet was the bendability strength of the door panels under load. However, in dedicated FEA, it was found that the hybrid structural reinforcements were even capable of exceeding the expected performance and result in a significant reduction of displacement when compared with the initial all-aluminum design.

VEHICLE CRASHES
Front vehicle collisions, mostly occurring at an offset, account for more deaths and severe injuries than any other type of accident. The crash performance of the Henkel/ RLE-optimized bumper design under Euro NCAP ODB 64 km/h test conditions results in controlled crumpling of the collapsible front zone, leaving the passenger compartment more or less undeformed.

Front vehicle collisions, mostly occurring at an offset, account for more deaths and severe injuries than any other type of accident.

In contrast to the NCAP ODB scenario, small overlap frontal crashes mostly involve the outer areas of the collapsible zone, subsequently directing forces straight into the suspension, wheel and firewall areas. Serious foot and leg injuries can occur as a consequence of intrusion into the passenger compartment. For this reason, the crash performance according to the IIHS small overlap test was examined by simulating the worst-case conditions of the wheel.

Although the results showed sufficient protection against intrusion, it was found that the rocker panel might kink, which is unacceptable for OEMs and could compromise the safety of the battery tray in electric vehicles. Therefore, the hybrid rocker panel was further optimized with local structural foam ribbing.

SIDE-IMPACT PROTECTION
In the U.S. NCAP pole test simulation, the final hybrid structural designs of the rocker panel, doors and B-pillar showed excellent side-impact protection, although the rocker panel required an extra round of optimization to keep the maximum intrusion into the battery pack below the limit of 10 mm specified by most OEMs.

Likewise, the FMVSS 301 rear-impact test simulation produced good results with regard to fuel-tank and EV-battery protection. The main purpose of this test is to limit the spillage of fuel in the event and after a rear crash, thus minimizing the risk of petrol-fume ingestion and fires.

Figure 4 illustrates the strength-to-weight ratio established in line with IISH roof-crush specifications. Even when maximizing the lightweighting of pillars, struts and upper rear/roof sections using hybrid structural patches and redesigns, the SWR rating is well above 4, meeting the expectations of premium OEMs.

In addition, the entire vehicle body was subjected to dynamic torsion and bending simulations to confirm the adequate stiffness of new designs such as the hybrid structural C/D-pillars.

INNOVATING AUTOMOTIVE DESIGN
Henkel and RLE International have opened a fresh, new perspective on innovative automotive design. In addition to helping drive weight reduction and cost control in next generation vehicles, the dedicated use of structural foam in optimized hybrid components can provide significant improvements in stiffness and strength to meet demanding crash performance levels.

 

Authored by David Caro

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