CompositesWorld
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JUNE 2015 28 CompositesWorld WORK IN PROGRESS is governed by the modulus and weight of the chosen material." As a bonus for manufacturing economics, the improved properties were shown to be achievable using currently available equipment, with minor modifcations. Another advantage of carbon fber for this material system is its high modulus, and the relationship of modulus translation to fber aspect ratio in the fnal composite, Purcell says. Modulus translation It is known that the strength of a composite part is primarily a function of the fber reinforcement. Te greatest strength is provided by continuous fber, and the use of chopped fbers, rather than continuous fbers, considerably decreases the strength of the fnal part. However, there is evidence that longer segments of chopped fber in parallel orientation, or distribution, will cause the part to have strength approaching the strength provided by the continuous carbon tow. Tis phenomenon is explicated in the Halpin-Tsai isotropic approximation for longitudinal and transverse moduli, developed by J.C. Halpin at Wright Patterson Air Force Base (WPAFB, Dayton, OH, US) and Stephen W. Tsai, professor emeritus, Structures and Composites Laboratory, Stanford University (Stanford, CA, US). Te Halpin-Tsai equations show that if an aspect ratio (L/D) of 100 — 0.72 mm length/0.0072 mm diameter in the case of carbon fber — can be reached, then the mechanical strength of the fber will be about 21.5 GPa in tensile modulus, or 83% of the 26 GPa theoretical maximum modulus for a molded laminate that is 35% by weight of carbon fber with PA6 polymer. Purcell notes that in the trials, some fbers retained lengths of as much as 2 mm, but the average length was much shorter. "Because we had some fbers that made it through the process at 2 mm, it shows that it's tech- nically possible to make it happen," he notes, but empha- sizes that "it's the average fber length that matters more and drives the mechanical performance. A few fbers at 2 mm length did not seem to contribute to the mechan- ical performance." In any case, the theoretical curve fattens out above the L/D ratio of 100, delivering diminishing benefts as fber length increases. Tus, the theoretical estimated fber translation (modulus translation) for the same part at a fber aspect ratio of 278 — that for an average 2 mm fber length — is ~24.5 GPa, or 94% of the 26 GPa theoretical maximum. While the higher L/D is better, attaining 2 mm length would be a considerable challenge, for only a 10-11% improvement in properties (see Fig. 1, this page). In Zoltek's study, the actual properties measured were consis- tent with the theoretical curves. Te majority of the study samples held L/D ratios between 14 and 42; that is 0.1 to 0.3 mm length in the fnal molded laminate. "More work is needed to drive up the fber lengths to at least 0.72 mm average and, ideally, 1 to 2 mm average in the fnal end product," Purcell says. Run Condition Fiber- Wt (%) RPM Charge- Position Circ Tested Tensile Panels Tested Flexural Panels 1 30% 50 Edge 0.16 1 1 2 30% 70 Edge 0.16 2 2 3 30% 100 Edge 0.16 2 2 4 30% 50 Center 0.16 2 2 5 35% 50 Edge 0.16 2 2 6 35% 50 Edge 0.16 1 1 7 35% 50 Edge 0.16 2 2 8 35% 50 Center 0.16 2 2 9 35% 70 Edge 0.16 2 2 10 40% 50 Edge 0.16 2 2 11 40% 50 Center 0.16 2 2 12 40% 70 Edge 0.16 2 2 13 45% 50 Edge 0.16 1 1 14 30% 50 Edge 0.16 1 1 15 30% 50 Edge 0.108 2 2 Processing parameters Set Value Melt temperature [°C] 270 Mold temperature [°C] 125 Pressing pressure [MPa] 33 Cooling time [seconds] 30 Fig. 1 Isotropic modulus vs. fber aspect ratios and various fber weight percentage. Note that the fgure shows fber weight percent in the compound to be an impor- tant factor in achieving higher modulus. Table 3 Overview of variable processing parameters from 15 experimental trials. Table 4 Constant processing parameters. 35 30 25 20 15 10 5 0 Isotropic Modulus (GPa) 1 10 100 1000 L/D ratio 45% 40% 35% 30% 25% 20% 15% 10% 5% Read this article online: short.compositesworld.com/CFinD-LFT • RPM = speed of the extruder screw in revolutions per minute. • Circ. = circumference, in this case, describing the length of one fber roving drawn into the extruder per screw revolution.