The effect of interfacial conditions on the elastic-longitudinal modulus of fibre reinforced composites

A model study is conducted on the prediction of the elastic longitudinal modulusE _CL of a unidirectional fibre reinforced composite. It is assumed in our model that the fibres are aligned in the uniaxial loading direction and that the representative volume element (RVE) consists of three coaxial cylinders, namely the fibre, the interphase and the bulk matrix. The interphase represents the third phase developed between the constituent phases of the composite and it is characterized by mechanical imperfections, physicochemical interactions and limited mobility of macromolecules due to their absorption on the filler surface. Thus the interphase properties are varied within this third phase in an unknown way. In the present study it is supposed that the elastic modulus of the interphase materialE _i is varied along the thickness following an exponential law of variation. Moreover, the interphase thickness is determined according to existing theory which is based on thermal capacity measurements. PredictedE _CL values agree well with respective experimental results. In addition the effect of an abrupt variation of the elastic modulus at the fibre surface onE _CL is considered. Results showed that this type of variation is of minor importance in predictingE _CL.