Fast fracture of rubber-toughened thermoplastics used for the shells of motorcycle helmets

The effects of outdoor ageing and of accelerated UV exposure on the grade of acrylonitrile butadiene styrene copolymer (ABS) used by UK manufacturers of motorcycle helmets were compared. Although the surface layer becomes embrittled the helmets still survive an impact test after conditioning at -20° C because the cracks are arrested before they penetrate the full thickness of the shell. A 4 mm thick rubber-toughened polycarbonate, used in helmets from continental suppliers, shows a transition from “tough” to “brittle” crack propagation behaviour at -15° C. For 4 mm thick ABS this transition had not occurred at the lowest test temperature of -20° C, but if 6 mm sheet is tested the transition is at -15° C. Temperature increases in the 40 sec delay between conditioning a helmet at -20° C and impact testing are shown to be significant.