THE EFFECT OF MATRIX YIELD STRAIN ON THE DATA REDUCTION TECHNIQUE OF THE SINGLE-FILAMENT FRAGMENTATION TEST

The effect of the elastic modulus and shear yield strength of the matr ix on the interfacial shear strength obtained from the fragmentation t est has been the subject of numerous studies. Little attention has bee n paid to the effect of the matrix yield strain. An axisymmetric finit e element model has been used to study the effect of matrix yield stra in on the shear stress at the fibre-matrix interface and the tensile s tress in the fibre and, consequently, the value of interfacial shear s trength obtained from the fragmentation test data using the constant s hear model of Kelly and Tyson. It was observed that saturation in fibr e fragmentation during the test depends on the matrix yield strain rat her than the fibre failure strain. Therefore, the matrix should be car efully selected to have the yield strain necessary for saturation. The maximum tensile stress that develops in the fragment depends on the t ensile yield strain of the matrix. For a matrix with high tensile yiel d strain, a higher tensile stress in the fibre will exist than in a ma trix of low tensile yield strain. Therefore, at saturation, the fragme nt lengths will be shorter so that an erroneously high value of the in terfacial shear strength will be estimated from the constant shear mod el. This particular observation can lead to the hypothesis of a rigid interphase in the case of very ductile or elastomeric matrices even th ough such an interphase is not present.