Audi is writing a new chapter in its lightweight design success story. For the next generation of its A8 flagship model, an intelligent mix of 4 materials is being used for the first time in the weight-bearing body structure – more materials than in any of the brand’s previous production models. The luxury sedan is thus once again rightfully claiming its role as an innovation driver in automotive lightweight design: Its low weight and impressive rigidity offer greater performance, efficiency and safety.
In 1994, it was the first generation of this luxury sedan, with its aluminium unitary body, that made the Audi Space Frame an established presence in the automotive world. Since then, the company has built more than one million production cars in accordance with this design principle, and it has been continually building upon its know-how in the use of materials and joining techniques.
The lightweight design experts at Audi long ago abandoned the fixation on using a single material in lightweight design. With a mix of aluminium, steel, magnesium and carbon fibre-reinforced polymer (CFRP), they are establishing a new stage in multi-material construction in the Audi Space Frame (ASF) for the next generation of the A8 – in keeping with the principle ‘the right material in the right place and in the right amount’.
Audi consistently applies new material technologies and designs that directly benefit the customer – and not only in terms of weight.
In terms of its overall dimensions, an ultra-high-strength, torsionally rigid rear panel made of CFRP is the largest component in the occupant cell of the new A8, and it contributes 33% to the torsional rigidity of the total vehicle. To optimally absorb longitudinal and transverse loads as well as shearing force, between 6 and 19 fibre layers are placed one on top of the other, ensuring a load-optimized layout. These individual fibre layers consist of tapes which are 50 mm wide and can be placed individually in a finished layered package, with any desired fibre angle and minimal trimming of the fibres.
The innovative direct-fibre layering process specially developed for this purpose makes it possible to entirely dispense with the normally needed intermediary step of manufacturing entire sheets. Using another newly developed process, the layered package is wetted with epoxide resin and sets within minutes.
A high-strength combination of hot-formed steel components make up the occupant cell, which comprises the lower section of the front bulkhead, the side sills, the B-pillars and the front section of the roof line. Some of these sheet metal blanks are produced in varying thicknesses using tailoring technologies – meaning they are customized – and others also undergo partial heat treatment. That reduces weight and increases the strength, especially in areas of the vehicle that are particularly critical for safety.
The aluminium components make up 58% of the new bodyshell, the largest share in the mix of materials. Cast nodes, extruded profiles and sheets are the elements characteristic of the ASF design. And here too the competition of materials has been driving progress.
Rounding out the intelligent mix of materials is the magnesium strut brace. A comparison with the predecessor model shows that it contributes weight-savings of 28%. Aluminium bolts secure the connection to the strut tower domes, making them a guarantor of the body’s high torsional rigidity. In the event of a frontal collision, the forces generated are distributed to three impact buffers in the front end.