FIBER INTERACTIONS IN 2-DIMENSIONAL COMPOSITES BY MICRO-RAMAN SPECTROSCOPY

In the study of fracture processes in composite materials, the interac tions between broken and intact fibres are of critical importance. Ind eed, the redistribution of stress from a failed fibre to its unfailed adjacent neighbours, and the stress concentration induced in these, de termine the extent to which a break in one fibre will cause more break s in neighbouring fibres. The overall failure pattern is a direct func tion of the stress concentration factors. In this paper we use laser m icro-Raman spectroscopy to study the extent of stress transfer and red istribution caused by fibre fracture in two-dimensional Kevlar 149 bas ed microcomposites. The strain along the fibres was mapped at differen t levels of load, and specimens with different inter-fibre distances w ere used to study the fibre content effect. The experimental stress co ncentration factors were compared with values predicted from various t heoretical models. The stress concentration factors generally agreed w ith those literature models that include interfibre distance and matri x effects. The overall failure pattern was found not to be a direct fu nction of the stress concentration factors in this system, as fracture propagates from fibre to fibre even at large interfibre distances, an d is apparently accompanied by relatively low values of the stress con centration factors. The 'critical cluster size', beyond which final fr acture of the composite occurs in a catastrophic manner, was found to be larger than five adjacent fibre breaks in the present system, for a ll interfibre distances studied.