Plastic with string in it(aka CFRP):
“Brittle behaviour is an unavoidable feature of the CFRP. It is necessary that the polymer matrix be glassy; otherwise, the flexing of the tubing during use would dissipate energy. That is, the bicycle would have a “spongy” feel to a rider. Because the polymer matrix is not viscoelastic, it cannot act as a microcrack arrestor when the fibres start to break. The result is that the CFRP used for bicycle tubing is inherently brittle (as opposed to fishing rods or vaulting poles, which are intended to be flexible). Because of this behaviour, it can be risky to make a component like the front fork of a bicycle out of CFRP. Front forks can receive large, sudden loads, and a brittle failure here would obviously threaten rider safety.”
Introduction to Engineering Materials: The Bicycle and the Walkman.
C.J.McMahon and C.D.Graham