MORE THAN CAR­BON

The his­tory and use of the ma­te­rial that changed our world

Cycle World - - Elements - By KEVIN CAMERON / Pho­tog­ra­phy by DREW RUIZ and JEFF ALLEN

The fin­ished parts that we ca­su­ally call “car­bon fiber” are more than that. They are com­pos­ites made of su­per-strong crys­talline car­bon fibers, held to­gether by an epoxy resin. The proper name is Car­bon Fiber Re­in­forced Plas­tic, or CFRP. Luck­ily, no one in­sists on it.

The idea of em­bed­ding fine, high-strength fibers in a plas­tic ma­trix goes back to 1920, when A.A. Grif­fith, em­ployed in Eng­land’s Royal Air­craft Fac­tory, showed that the low strengths of prac­ti­cal ma­te­ri­als re­sulted from sur­face or in­ter­nal de­fects. With­out these de­fects, con­sid­er­a­tions of atomic bond­ing would pre­dict tremen­dously greater strengths.

A good ex­am­ple is glass, which in han­dling ac­cu­mu­lates mi­cro­scratches on its sur­faces. Ap­pli­ca­tion of even mod­er­ate stress causes one or more of these to open and prop­a­gate as a crack. Also re­vealed by Grif­fith’s anal­y­sis was that be­low some crit­i­cal size (the Grif­fith crack length), such mi­cro­c­racks did not prop­a­gate. Herein lay a path to prac­ti­cal su­per-strong ma­te­ri­als—to pro­duce fibers too small to con­tain de­fects of that size.

Fiber­glass-re­in­forced plas­tic (FRP) found ap­pli­ca­tion as radomes dur­ing World War II, and fiber­glass-bod­ied cars such as the Kaiser Dar­rin and Chevro­let Corvette de­buted soon af­ter. The beau­ti­ful, hand-beaten alu­minum fair­ings of rac­ing Guzzis and NSUS of the 1950s dis­ap­peared, re­placed by eas­ily molded fiber­glass. You coated the in­side of a mold with mold re­lease, then gel­coat, then resin, af­ter which you could lay in and “wet out” one or more lay­ers of wo­ven fiber­glass cloth. Thou­sands of boats were built in sim­i­lar fash­ion, but the method of fiber­glass ap­pli­ca­tion was dif­fer­ent: the dreaded “chop­per gun,” which spewed forth a mix­ture of prickly chopped-glass fibers and resin.

A fix­ture of the 1960s and ’70s Cal­i­for­nia bike-rac­ing scene was the one-man fiber­glass shop. Its pro­pri­etor wore jeans so cov­ered in half-cured resin and fiber that they stood up by them­selves, and in a dis­tress­ing num­ber of cases, his mind had been al­tered by the fog of MEK per­ox­ide (cat­a­lyst for the polyester resin used in fiber­glass work) in which he spent his days.

Even stronger fibers were on the way. When in doubt, look at atomic bond strengths and pick a likely el­e­ment. Car­bon got the nod. Let’s ex­trude tiny fibers of poly­acry­loni­trile (PAN) and then heat the day­lights out of it in a non­re­ac­tive at­mos­phere. Ev­ery­thing but the car­bon is driven off, and the car­bon as­sumes a strong crys­talline form. Or, do the same with pitch—a heavy hy­dro­car­bon.

As with so many things, the idea is just the be­gin­ning. When in 1976 an “in­for­ma­tion of­fi­cer” at AVCO handed me a demo piece of uni­di­rec­tional car­bon prepreg (fibers al­ready em­bed­ded in a layer of un­cured resin), I was nat­u­rally think­ing about car­bon reed valves for two-strokes. Twenty years later, Erv Kanemoto was try­ing such valves in his Honda NSR500. And 41 years would pass be­fore Boe­ing rolled out its 787 Dream­liner, most of whose struc­ture is CFRP.

Car­bon fiber is avail­able in many forms. So-called tow is bun­dled fibers wound onto a spool. John Brit­ten used tow to make the “bones” of his “skin-and-bones” chas­sis struc­ture of the early ’90s—wet­ting it out with resin af­ter wind­ing it in place.

Fibers can also be wo­ven into fab­ric, and it is this black fab­ric, seen through a per­fectly smooth resin sur­face, that tells us if a non­struc­tural part such as a fen­der or rider’s heel guard is made of CFRP. Un­for­tu­nately, the stiff­ness of car­bon pre­vents it from ly­ing limply, con­form­ing to a mold as fiber­glass does. To force it into place, it is “bagged”—a vac­uum pump pulls the air out of the bag, and at­mo­spheric pres­sure holds bag and cloth in place against the mold sur­face. Best cures take place in a heated au­to­clave.

For high­est strength, CFRP lam­i­nates are made (like tires) from uni­di­rec­tional fiber prepreg sheets whose fiber di­rec­tion and num­ber are planned to de­liver the de­sired strengths in cho­sen di­rec­tions. Prepreg has the ad­van­tage that the cor­rect vol­ume ra­tio of fiber-to-resin is pro­vided in ad­vance—no “wetout” re­quired. The sheets are coated with tack­i­fier to en­able easy as­sem­bly of the lam­i­nate. Prepreg is held un­der re­frig­er­a­tion un­til use to pre­vent cure from oc­cur­ring in stor­age.

The tremen­dous strength of CFRP has de­liv­ered a large im­prove­ment in race­car-driver safety. In the days of alu­minum struc­tural “tubs,” crashes burst or crushed fuel blad­ders, lead­ing to fire and loss of life. Cured high-strength CFRP can have the ten­sile strength of al­loy steel with one-fifth the weight.

Why doesn’t ev­ery Mo­togp bike have CFRP struc­ture? There are two rea­sons, one of which was known in ad­vance. First, the need for elab­o­rate mold­ing and cur­ing ar­range­ments meant that with a CFRP bike, Kenny Roberts and Kel Car­ruthers could not have sawn off their YZR Yamaha’s steer­ing-head and welded it back on at a bet­ter an­gle. With CFRP, the teams would not be able to quickly

BE­LOW LEFT: Un­cured car­bon weave up close feels soft, al­most del­i­cate. BE­LOW RIGHT: Car­bon-ce­ramic brake discs (these from NCR) are more heat-tol­er­ant and much lighter than metal coun­ter­parts.

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