Influence of acetylene plasma treatment on the torsional fatigue of carbon-fiber-reinforced composite strands

Plasma treatments of high-performance fibers have been found to improve the adhesion at the fiber/matrix interface without deteriorating the strength of the fibers. However it is not always clear how much this improvement affects the performance of the composite material. To study the possible effects, a 3 k tow of carbon fibers was exposed for one minute to an acetylene/oxygen (2:1) plasma treatment. The tow was then impregnated in epoxy to form a 500 μm diameter strand (Vf=0.52). The strands were subjected to fully reversed torsion at a frequency of 3 Hz for up to 106 cycles. The fatigue, measured as the decay in torsional modulus, was recorded and compared with the decay observed in untreated fiber-reinforced strands; both showed logarithmic decay. The mean normalized torsional modulus after 106 cycles for the treated strands was 0.79 compared with 0.71 for the untreated. This difference was significant at α=0.04.