Dependence of the coefficient of friction on fibre orientation in the sliding of a HSS punch on a unidirectional CFRP surface

The dynamic coefficient of friction, f, for a high speed steel/carbon fibre reinforced plastic (HSS/CFRP) combination was measured, varying both the fibre orientation with respect to sliding direction, ??, and the normal stress level, ??. It was found that f is dependent on fibre orientation, as well as on normal stress. In particular. the lowest f value concerns the orientation ?? = 0°, whereas f attains its maximum value for ?? = 120°. Only for very low stress levels the Amonton's rule, stating the independence of f on ??, is followed. When sufficiently high normal stresses are applied, the coefficient of friction is an increasing function of ??. This behaviour is opposite to metals, where f undergoes a decrease as ?? increases. In all, f mean values in the range 0.2 to 0.65 were measured. The experimental data were statistically treated, to estimate the expected scatter in the coefficient of friction. It is shown that the scatter is normally distributed, and quite large variations in f values are predicted, especially when ?? = 150°. Finally, some limitations of the results obtained in this work are highlighted: using cutting force data available in the literature, typical values of the normal stress developing at the tool face-work material interface during actual machining are estimated. It is shown that the ?? levels adopted in this work are at least an order of magnitude lower than the real ones. Therefore, additional tests are recommended in order to extend the present findings to cutting operations.