Superstrong nuts and bolts

Using composite flow molding (CFM) and a polyaryletherketone resin reinforced with long-carbon fibers, Icotec's lightweight nuts and bolts sport ultrahigh specific strengths of up to 2.4 million psi/lb/in.3 This contrasts to aluminum and titanium with specific strengths of about 0.2 million and 1.2 million psi/lb/ in.3, respectively. The composite's semicrystalline polymer matrix called PEEK, from Victrex USA Inc., Greenville, S.C. (victrex.com), also has a melting point of 649°F, a glass-transition temperature of 289°F, and a continuous-use temperature of 500°F.

According to Chris Karas, spokesperson for the resin supplier, "CFM feeds the pultruded composite rods into a closed preheating cavity. From there, the rods move to the mold and are formed to shape under pressure with helically reinforced threads which are protected against abrasion by the fastener tapping the bore." The high-volume CFM process produces fasteners in a single step without the need for any posttreatment.

Even with high fiber loadings, fastener threads can sustain high mechanical loads without shearing, thanks to a transitional fiber-orientation zone that transfers forces away from the threads to the fastener core. In contrast, conventional thermoplastic processing techniques don't permit extremely high fiber content nor can they process long fibers without damaging them. Consequently, fasteners subjected to high mechanical loads, as seen in Formula 1 race cars, are typically machined from a blank.

In addition to the motor-sport application, CFM nuts used in marine applications reportedly resist pounding vibrations without loosening. Various aerospace agencies are also evaluating CFM fasteners to further reduce weight in assemblies and to better match fastener thermal expansion coefficients to those of the composite they assemble.

Fasteners made from carbon-fiber filled Victrex PEEK also sport flexural strengths ranging from 116 to 145 kpsi (800 to 1,000 MPa) and tensile strengths up to 65 kpsi (450 MPa) depending on fiber type and loadings.