A MODIFIED RULE-OF-MIXTURES FOR PREDICTION OF TENSILE STRENGTHS OF UNIDIRECTIONAL FIBER-REINFORCED COMPOSITE-MATERIALS

Measured ultimate tensile strengths in unidirectional fibre-reinforced composite materials have been observed to deviate from the linear pre dictions of the classical rule-of-mixtures relationship. The physical factors responsible are fibre-fibre interaction, inhomogeneous fibre d istribution in the matrix and fibre misorientation to the loading dire ction. A recent modification to the classic rule-of-mixtures to accoun t for fibre-fibre interaction has already resulted in good agreement b etween measured and predicted values of ultimate tensile strengths at high fibre volume fractions for Kevlar/epoxy composites. Additional mo difications to the rule of mixtures to incorporate both fibre misorien tation and inhomogeneous spread have been presented here. These modifi cations result in greater agreement between measured and predicted ult imate tensile strengths at low fibre volume fractions while retaining the accuracy of prediction at higher fibre volume fractions. Good agre ement between measured and predicted values of inhomogeneous fibre spr ead were obtained at high fibre volume fractions. Furthermore, these a dditions to the classic rule-of-mixtures can be used to gauge the exte nt of each of the physical factors responsible for ultimate tensile st rength reduction in unidirectional composite materials.