Under the leadership of Germany's "Multi-material lightweight design for system integration for electric vehicle development" (hereinafter referred to as "SMiLE") alliance, an ambitious multi-year project has developed a prototype of a car bearing floor module.

As part of the larger mixed-body body-in-white structure, it highlights the huge application prospects of composite materials and non-ferrous metals in medium-volume production.

This rear load floor for battery electric vehicles is composed of two types of thermoplastic composite materials plus metal profiles and inserts, and it functions as a luggage compartment and rear passenger compartment floor.

It is connected to the second mixed/thermoset composite load-bearing floor by means of adhesive and mechanical connection.

The second load-bearing floor is a structure used in the front half of the car to support the battery. It is produced using carbon fiber reinforced epoxy resin and molded by resin transfer molding. It has metal inserts and a part containing polyurethane foam core material. Sandwich structure.

The entire load-bearing floor module is connected and bolted to the aluminum sill/side wall. The sill and the side wall itself are bolted to the aluminum single body beam.

The design of the entire load-bearing floor module demonstration component has achieved weight reduction, providing significant impact energy absorption performance for 300 cars of this mass production per day.

                       Design decision

The alliance members who participated in the research of bearing floors after participation include:

? Automaker Audi Motors (also the leader of the entire SMiLE project) and its parent company Volkswagen

? Institute of Vehicle System Technology, Karlsruhe Institute of Technology, Germany

? Fraunhofer Institute of Chemical Technology (leader of front and rear bearing floor projects)

? Fraunhofer Institute of Material Mechanics

? BASF, a supplier of thermoplastic composites

? Machine manufacturer Dieffenbach

? Frimo, the tooling manufacturer

For the front and rear load-bearing floors and the larger body-in-white structures to which they belong, the development team's goal is to absorb higher impact energy while reducing weight and cost.

Therefore, they decided to use thermoplastic composite materials and metal inserts to produce the rear load-bearing floor.

The team wanted to add a trunk function and a second-row seat belt connection structure, but they also wanted to use the load-bearing floor to absorb a lot of collision energy.

Normally, car manufacturers mainly rely on the metal profiles on the side of the metal bearing floor to manage the rear collision energy of passenger cars. Therefore, considering the impact strength of thermoplastic composite materials, researchers want to know: Whether the entire width and length can be used to manage the collision load, and whether it can absorb higher collision energy.

The researchers examined the common automotive thermoplastic composite materials. Polypropylene (PP) and polyamide 6 (PA6) and other matrix materials have entered their field of vision, but due to temperature reasons, they gave up PP, because the back bearing floor will follow The body in white is rust-proofed by high-temperature electrophoretic coating.

Due to the need for continuous fiber reinforced materials to achieve the highest rigidity and strength, they mainly focused on fabric-reinforced organic sheets (a form of glass fiber felt thermoplastic (GMT) composites) and Continuous Fiber Reinforced Thermoplastic Prepregs Unidirectional Tapes（UD-Tapes ）

For various reasons, they chose to use prepreg tape for further prototype development.

The researchers knew that the geometry of the back-loaded floor would be very complicated, so they used an automated tape laying machine (ATL) so that the UD tape could be laid in any direction, making the material for the window/hole It is less than using organic boards to reduce waste, reduce weight and cost, and allow the most effective application of fibers to local and entire parts.

Moreover, since the fibers laid by ATL are flat or parallel in each layer of the laminate, and are not woven like fabrics, there is no fluctuation and subsequent loss in rigidity and strength.

By selectively using discontinuous/chopped direct long-fiber thermoplastic (D-LFT) composites, these problems are solved because they are flowable, allowing a high degree of functional integration/component integration, and without fiber In the case of bridging, it is easier to form complex ribs and absorb a lot of impact energy.

With D-LFT, it is also easy to embed metal accessories, especially when it is necessary to pre-drill the insert so that the composite material flows over and wraps around the metal, so that a strong combination can be achieved by mechanical interlocking.

In addition, D-LFT is cheaper than strip or organic board, and it is easier to form in thick parts.

Because compounding is done next to the press, D-LFT simplifies material inventory management and provides a high degree of flexibility for development projects, which can quickly change material properties, including fiber length and type, fiber volume percentage (FVF) and matrix materials.

During the production process, the settings of materials/processes are controllable, so that a high degree of reproducibility and reproducibility (R&R) can be achieved-that is why the automotive industry has used this process for medium and large scales in the past 20 years In mass production.

Because the researchers hope that the rear load-bearing floor is thin and light, and can absorb high impact loads while resisting buckling, they used glass fiber-reinforced and carbon fiber-reinforced strips and D-LFT, according to different fiber weight percentages ( FWFs), through the simulation and preliminary development of small parts to evaluate the mechanical properties and filling performance.

Although carbon fiber composites produce thinner, lighter, and more rigid structures compared to glass fiber composites, due to cost issues, plus the use of carbon fiber reinforced materials in the front load floor, the researchers In the process of expanding to full-size parts, glass fiber is used to reinforce the rear bearing floor.

The Ultramid B3K PA6 D-LFT with 40% glass fiber weight percentage provided by BASF and the 8-layer Ultratape B3WG12 PA6 with 60% glass fiber weight percentage were applied here.