Twenty-five years of carbon fiber

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Few sailors think of carbon fiber masts and spars as cutting edge gear. Parts made from this stippled black cloth can be found in nearly every anchorage these days.

One of the reasons for this is the work of companies like GMT Composites in Bristol, R.I. This year, GMT celebrates its 25th year working with carbon fiber — making masts and spars and rudders as well as parts for robotic, medical, aerospace and defense applications.

Back in 1984, when GMT started to use carbon fiber, the impressive characteristics of the material were well known in the aerospace field. Stronger, stiffer and lighter than aluminum, it was a perfect material to use for aircraft where weight is always an issue. A problem for marine companies using carbon fiber was simply getting an adequate supply, noted David Schwartz, president of GMT, in an email and subsequent conversation. “Twenty-five years ago the carbon fiber fabric was available in only limited varieties,” Schwartz said. “We were only able to get what was left over from various government aerospace projects.”

Early on, GMT realized that a great application for carbon fiber was in the manufacturing of rudder posts for racing sailboats. Up to that point, rudder posts were made from stainless steel or aluminum. Steel is heavy and susceptible to crevice corrosion, while aluminum, though lighter, is also prone to attack from salt water. “We realized that carbon fiber held together by epoxy resin would solve both of these problems,” Schwartz said. “This composite is three times as strong as steel yet only 15 percent of the weight. As a side benefit, the material is not affected by salt water, so all corrosion issues disappear. We could take a 120-pound metal rudder post and replace it with one of carbon that weighed 20 pounds.”

Though carbon fiber offers big benefits, there was a learning curve involved. According to Schwartz, unidirectional carbon cloth, with all the fibers running one way, could be challenging. In the early years, epoxy resin was applied to the cloth by hand. “It was always a struggle to keep the yarns from unraveling.”

Another wrinkle with this wet lay-up approach was the need to race the clock. Workers had roughly two hours to complete a part and get it vacuum bagged before the resin hardened. “If you don’t get it under vacuum in time, you have to throw the part away,” Schwartz said. That could get expensive fast. A hand lay up of a mast could have 150 pounds of fiber and resin, materials costing many thousand dollars.

If hand lay up had remained the only way to build with carbon fiber, the material might not have taken off and achieved its current success. In the late 1980s, however, a solution appeared called pre-preg. Carbon fiber pre-impregnated with resin was delivered in refrigerated rolls. These pre-preg rolls were coated with a viscous layer of resin and could be worked for much longer than the short window of the two-hour hand lay up. “Our technicians could then unroll what they needed, fit the ply to the part and then continue to add additional plies,” Schwartz said. “They would have days or weeks to work on the part so that all fibers could be accurately placed.”

The next step in carbon fiber’s climb to world domination was improved epoxy resins in the early 90s. Previously, parts often needed to be cooked in a device called an autoclave — a combined pressure cooker and oven. The pressure inside the autoclave was used to force the resin into the carbon fibers. But in the 90s, pre-preg makers concocted a resin that, when heated, would flow and wet out the fiber without the need for increased pressure. Removing the autoclave from the loop meant eliminating the restriction of making parts that could fit inside the closed autoclave. Just having to heat the part meant a more flexible open-ended oven could be used. “Our suppliers developed a resin system with flow and cure characteristics that would produce excellent parts using a vacuum bag,” Schwartz said, “When a part is being cooked, the pre-preg resin must first get less viscous (more runny) and flow. This happens in a vacuum bag which presses on the laminate, driving the resin into the fibers and expelling any air.” After less than an hour of heating at the proper curing temperature, the resin in the bagged pre-preg starts to cross-link and transforms into a hard composite.

Another resin improvement was the addition of small amounts of rubber-based materials into the epoxy. These tougheners help make the finished part less likely to crack when subjected to impact loads, like a winch handle dropped on a deck-stored carbon spinnaker pole.

The resin continues to improve, increasing the options for builders like GMT. “In the last five years, we have seen the introduction of resin systems that can be cured at lower temperatures. Our standard system cures at 250° F. Some of the newer systems can be cured at 150° F.”

The arc of carbon fiber’s development as a composite building material follows that of its older cousin, fiberglass. Just as the builders of early fiberglass boats often erred on the side of caution and overbuilt their fiberglass hulls, early work in carbon fiber involved the same unknowns. GMT decided that rather than use lab test results supplied by carbon fiber suppliers, the best approach was to build parts on the shop floor, then get the parts tested. “We sent these out to testing labs where they were pulled apart and checked for voids,” Schwartz said. “The results allowed us to design parts using allowable strengths that reflected what we would actually achieve. We will not use a new material without first making sample parts and testing them to destruction.”

Given carbon fiber’s place as the go-to material for strength and light weight, are there any materials coming up to supplant it? One possibility for the future might be an exotic material like carbon nanotubes. But this form of carbon has a long way to go. “Nanotubes are quite expensive,” Schwartz said. “More importantly, we will not use them because the health risks have not been fully addressed.”

Twenty-five years of carbon fiber experience has given builders like GMT Composites a thorough understanding of this composite material. And this expertise has made carbon spars, rudder shafts and other parts a reliable choice for voyagers.

By Ocean Navigator