The tensile strength distribution of Fortafil-3 carbon fibres of circular cross-section has been investigated at different gauge lengths. The unimodal Weibull, unimodal Iognormal and bimodal Iognormal models were tested. Estimating the model parameters by the method of maximum likelihood, and testing each model by the Kolmogorov-Smirnov goodness-of-fit statistic at prescribed levels of significance, it is found that the data for untreated unsized fibres fit a bimodal Iognormal model best. The proportions of the low and high strength populations, p and q, respectively, did not show any well-defined trend with gauge length and had average values close to 0.5. But the Iognormal mean for each population showed an increasing trend with decreasing gauge length. It is inferred that p and q are, respectively, related to the presence of surface flaws and internal defects, both of which probably have the same structural origin. After electrodeposition of titanium di (dioctyl pyrophosphate) oxyacetate (TDPO), the fibre strength was still best approximated by a bimodal Iognormal distribution. But the weighting factor p for the weak population was reduced markedly, with a corresponding increase of q, indicating the healing of surface flaws during electrodeposition of a protective layer of TDPO. Furthermore, in contrast to the observations with untreated fibres, the Iognormal means for both the low and high strength populations of the electrocoated fibres were essentially unchanged with gauge length. Changes were also indicated in the number and severity of surface flaws, caused by concurrent electrochemical processes.
The electrodeposition of saturated copolymers onto carbon fibers is investigated, focusing particular attention on improvement of shear and impact properties of the corresponding composites. Carbon fibers are electrocoated with poly(ethy1ene-co-acrylic acid) and poly(methy1 vinyl etherco-maleic anhydride) from aqueous media, and fabricated into epoxy composites. The results of interlaminar shear strength (ILSS) tests, initially employed to assess fibermatrix adhesion, are vitiated by the occurrence of mixedmode failure. Interfacial shear strength (IFSS) is hence evaluated by stressing single-fiber composite specimens to obtain ultimate aspect ratios of the fiber fragments. The data are combined with fiber strengths by a recently developed statistical theory (1) to yield a distribution for IFSS. Both copolymer interphases improve fiber-matrix bonding to a n extent greater even than that obtained with commercial fiber surface treatment. Good fiber-matrix adhesion is further apparent from SEM studies of fractured ILSS test specimens. A key to this improved adhesion is the interpenetration of matrix resin and interphase polymer, revealed by electron microprobe analysis (2). Notched Izod impact strength is also increased over uncoated-fiber composites. These copolymer interphases behave as deformable interlayers, absorbing impact energy and blunting the growing crack tip. Further energy is absorbed in deflecting the crack through a more tortuous path. Simultaneous improvements in impact and shear strengths are thus obtained, which may be further enhanced by optimizing the electrodeposition parameters and the coating thickness. The influence of the interphase on composite properties is bettel understood from this study, paving the way for refinement in interphase design.
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