Sonic modulus in fiber-axis direction of carbon fibers

Abstract

The sonic modulus in the fiber-axis direction was measured on tows and on carbon fiber-epoxy resin composite strands for a series of polyacrylonitrile (PAN)-based carbon fibers. It was found out that the sonic modulus of composite strands Es,c can be represented by the rule of mixture Es,c = Es,fυf + Es,m(1 – υf), where Es,f and Es,m are the sonic moduli of the fiber and the matrix, respectively, and υf is the fiber volume fraction. It follows from this result that the measurement of sonic modulus of composite strands can be used as a convenient method for determining the volume fractions of fibers in composite strands if the sonic moduli of the fiber and the matrix are known in advance. The variation of sonic moduli of carbon fibers with applied stress σ at small stress ranges can be represented as Ef = [1 – (2S111/S11)σ]/S11 where S11 is the initial compliance. The factor, 2S111/S11, was found to be almost constant for a variety of PAN-based carbon fibers. The relations between the parameters representing the dependency of the sonic modulus on the applied stress and the fiber-structural parameters are also presented. © 1994, VSP. All rights reserved.