CompositesWorld

SEP 2018

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NEWS 43 CompositesWorld.com Bicycle Design Integrity But Stier and others point out that this combination, by itself, makes a fairly brittle composite. And that, left unmitigated, can be a safety risk. at brittleness, however, can be offset by adding thermoplastics in some form to the composite. "eir higher strain energy release rate (or toughness) might reduce the conser- vatism demanded from the rather brittle carbon epoxies and, thus, yield more safety due to increased fatigue- and damage- tolerance," he suggests. Epoxies can be "toughened" via the addition of rubber compounds. Epoxy suppliers can provide them toughened in the formulation process. But bicycle designers and manufacturers are discovering ways to incorporate the mix of thermosets and thermoplastics into their design processes. For example, Jen Hanna, director of business develop- ment for Innegra Technologies (Green- ville, SC, US), says Innegra high-modulus polypropylene (HMPP) UD and woven fiber has become a thermoplastic constit- uent of choice for some bicycle manu- facturers. When mixing thermoplastics with thermoset matrices, however, cure cycles need to be carefully considered. "We recommend processing temps below 150°C [302°F]. Innegra works very well in a 250°F [121°C] prepreg." She adds that that would present a challenge to the bike manufacturer on a quest for quicker cure cycles, which often means higher temperatures, noting that in that scenario, "Innegra would struggle to be a fit." But a small sacrifice in production speed can be a good trade-off for the benefits that HMPP brings to bicycle performance. "e ductile nature and excellent flex- fatigue properties allow the material to withstand impacts, reduce vibrations and limit or stop fracture propagation in composite parts," she points out, "creating a safer cycling product." ermoplastics may enable bike parts to better handle temperature swings in very hot or cold climates: "Innegra is stable on cryogenic levels down to -90°C [-130°F] and probably further," Hanna notes. "We have tested to that degree and saw absolutely no change in mechan- ical properties. On the higher end, 165°F [74°C] is the melt temperature." She quips, "I hope no one is cycling in that climate!" TeXtreme spread-tow carbon fiber reinforcements from Oxeon AB (Borås, Sweden) also are being used to reduce weight and improve performance under load — especially to protect laminates from micro-cracking. Fredrik Ohlsson, product development director for Oxeon, explains: "By mixing and varying the layup sequence, by combining thicker materials (standard tow) and thinner materials (spread tow), it is possible to load a composite in its transverse direction to a much higher degree before cracks occur in the matrix between the filaments." is is reportedly a great advantage in bicycles, where each push of the pedal induces a cyclic load path in multiple directions through the bike frame. Commingling TeXtreme spread tow with Innegra's HMPP fiber endows the part with higher elongation and ductility. "If you have a catastrophic failure of the composite part, it stays together OMAX abrasive waterjets deliver industry leading part accuracy. Get the whole story at omax.com/precision VISIT US AT IMTS 2018 IN BOOTH #236222

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