FRAGMENTATION OF ARAMID FIBERS IN SINGLE-FIBER MODEL COMPOSITES

Raman spectroscopy has been used to monitor the state of axial stress along fragmented, high-modulus Kevlar 149 aramid fibres in an epoxy re sin matrix by monitoring the peak position of the strain-sensitive 161 0 cm(-1) aramid Raman band along individual fragments. It is shown tha t the interfacial shear stress along each fragment, derived from the s train-distribution profiles, is not constant as assumed by conventiona l fragmentation analysis. The fragmentation process of as-received Kev lar 149 fibres is compared to that of irradiated Kevlar 149 fibres exp osed to ultraviolet light where the tensile strength and modulus of th e fibres have been reduced. It is found that the derived interfacial s hear stress and interfacial shear strength values are higher for those fibres exposed to ultraviolet light compared with the as-received fib res. It is also clearly demonstrated that the values of interfacial sh ear strength calculated at high matrix strains from conventional fragm entation analysis are considerably lower than the maximum value of int erfacial shear stress prior to fibre fracture that was found to be clo se to the shear yield stress of the resin matrix. Hence the determinat ion of the interfacial shear strength following the saturation of the fragmentation process may give rise to misleading results.