The effect of fiber surface treatment on its strength and adhesion to the matrix

A procedure is suggested of evaluating the efficiency of fiber treatments by the extent to which the fiber reinforcement strength is realized in the composite. For this purpose, the effects of surface treatments on the scale effect of carbon-fiber strength and adhesion to epoxy resin and polyamide matrices was investigated by the single-fiber composite test. The effect of electrochemical, ozone and cryogenic treatments on high-tenacity carbon fiber UKN-01 has been studied. All surface treatments result in a decrease of the scale dependence of fiber strength. It is shown that there are essential differences in the critical-length distributions for treated and untreated carbon fibers. This may be accounted for in terms of adhesion heterogeneity of the fiber surface. It is suggested that the average length of fragments formed at the beginning of adhesive failure should be used as the critical length for practical calculations because of the large scatter of fragment lengths at the end of a single-fiber composite rest. The cryogenic treatment leads to the strengthening of carbon fibers by the removal of pyrolyzate deposits from the surface. The alternation of different kinds of carbon-fiber surface treatment (which change the structure of the surface layer to a certain depth) with cryogenic treatments (which remove these layers from the fiber) permits a study of the radial structure of the carbon fiber. (C) 1997 Elsevier Science Limited.