Composites in cars and buses

Closures and semi-structural parts

In passenger cars, glass fibre composites are used for closures and semi-structural parts — rear hatches, roofs, doors, brackets — where they let several parts be combined into one, resist corrosion and damp vibration. Carbon fibre composites had, by 2015, mostly appeared in semi-structural parts such as hoods and roofs, and in low-volume production.^[1]

Many non-structural car parts are compression-moulded from short-glass-fibre sheet and bulk moulding compounds; fibre-filled thermoplastics are injection-moulded into interior parts. Both are high-volume routes, and in the 2015 account both were used mainly for non-structural parts.^[1,2]

Structural carbon fibre

Structural use of carbon fibre is the harder step. The BMW i3 of 2013 has a body frame made by resin transfer moulding from robotically laid-up carbon fibre preforms, and resin transfer moulding has been named the method with the greatest potential for large integrated car structure.^[1,2]

What held wider structural use back, as of 2015, was a set of needs: crash failure-mode data for body structures, cost parity with steel, design tools and dependable joining, all at production speed.^[1]

Buses: the sandwich monocoque

Electric buses have prompted designs in which the whole body is a sandwich-composite monocoque rather than a metal frame with panels. One reported mass-transit bus-body design used glass/polypropylene face sheets on a polypropylene honeycomb core and saved 55 per cent of the weight of a metal-framed bus; a 24-seat electric microbus was designed from glass fibre/epoxy face sheets on high-density polyurethane foam cores, with carbon fibre added in the battery tray and subfloor where more bending stiffness was needed.^[1,2]

These are the same ingredients as the aircraft sandwich panel — laminate skins on a cellular core — chosen for the same reason: stiffness at low weight.^[1,2]

Crash safety

A bus body must protect its occupants in a rollover, under UN-ECE Regulation No. 66, and in a frontal impact, through pendulum tests under UN-ECE Regulation No. 29 and barrier impact under FMVSS 208. Most crash analyses had been done for metal structures, and as of 2022 knowledge of how composite buses behave in a crash was described as very limited.^[1]

A finite-element simulation of the sandwich microbus in a 50 km/h frontal crash found the front panel, A-pillars and front sidewalls failing in fibre compression and the occupant space intruded by up to 221 mm; stiffening the front panel and thickening the foam cores improved protection markedly for 35.6 kg of added structure. These are modelled results for one design, not test results.^[1]

Commonly misunderstood