Innovations for Lightweighting: Audi brings CFRP to 2018 A8

May 1, 2017

Reported by Stuart Birch

2018 Audi A8L spaceframe showing mixed-materials architecture. Note CFRP rear bulkhead.

Audi helped pioneer aluminum-intensive bodies-in-white in volume production but for its recently unveiled 2018 A8 the company adopts a broader mixed-materials approach. Besides retaining aluminum alloys for most of the car’s BIW, Audi incorporates steel, magnesium and carbon-fiber reinforced polymer (CFRP).

Claimed torsional rigidity is up 24% compared with the incumbent A8. But the car still fights the dreaded “mass creep” due to the addition of new safety/autonomous driving technologies and the provision for plug-in hybrid propulsion. Despite the body engineers’ best efforts, the new A8’s BIW weighs 621 lb (282 kg) versus 509 lb (231 kg) for the incumbent model.

CFRP is used for the rear panel and associated parcel shelf, the largest component in the occupant cell of the A8. It provides some 33% of the torsional rigidity for the whole car, according to Audi engineers. They explained that to optimally absorb longitudinal and transverse loads and associated load-shearing forces, between six and 19 fiber layers are placed one above each other to deliver load optimization. The individual fiber layers comprise 2-in-wide (50-mm) tapes placed in a finished layered panel “with any desired fiber angle and minimal trimming of the fibers.”

The process obviates the need for an intermediary step of manufacturing entire sheets of carbon fiber. A further new process sees the layered panel wetted with epoxy resin and cured “within minutes.” Although hot-formed high-strength steel is used for the A8 occupant cell, cast-aluminum nodes and extruded profiles and sheets account for 58% of the car’s body.

The new heat-treated cast alloys provide a tensile strength of more than 230 MPa (33,359 psi); the corresponding yield strength in the tensile test is over 180 MPa (26,107 psi), engineers said. For the profile alloys, “significantly higher” values are delivered than those previously achieved.

Magnesium is used for a front brace linking the A8’s suspension towers and provides a 28% weight saving, the engineers claim. Aluminum bolts are used to secure it to the strut tower domes to boost torsional rigidity. In terms of safety performance, a frontal collision would distribute resultant generated forces to three impact buffers in the front end.

A further significant aspect of the A8 build is the use of 14 different joining processes in body assembly. These include roller hemming, grip punch riveting and remote laser welding of aluminum, claimed to be a world-first. Grip punch riveting fixes the side wall frame in position and is supported by structural bonding. Audi is particularly proud of its adaptation of these joining technologies, combining the aluminum side wall frame with the hot-formed HSS sheets at the B-pillar, the roof line and the thin-flange sills.