The d.c, electrical properties of 80×80×3 mm 3 polypropylene plaques filled with 6.5 µm diameter stainless steel fibres have been studied for volume fractions in the vicinity of a critical threshold at which the volume resistivity changes very rapidly with filler concentration. By the use of very low power inputs to eliminate any possibility of local temperature changes, the samples have been established to be ohmic conductors with resistivities ranging from 12 to 0.61 O cm for fibre volume fractions of 1 to 3%. It is suggested that percolation conditions i.e. continuous chains of metal fibres are produced at low volume fraction of filler because of a special fibre geometry i.e. a substantial proportion of the fibres are three dimensionally folded into shapes of roughly helical form, thus enhancing the probabilities of contact between adjacent fibres. For simplicity a model structure of perfect helices with identical diameters and pitch has been examined. The model leads to a critical volume fraction at the percolation threshold, which is in good agreement with experiment and proportional to the square of the ratio of fibre diameter to helix diameter. The threshold resistivity range predicted by the model is also a function of fibre and helix diameter and this resistivity also decreases with mean fibre length. It is argued further that there exists an optimum value of fibre aspect ratio for which the critical volume fraction is a minimum. The predicted threshold resistivity is in good agree ment with experiment providing that a small amount of the size coating is assumed to have been removed during manufacture of the plaques, thus allowing a small fraction of the fibre fibre contacts to be conducting
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