Failure phenomena in two-dimensional multi-fibre microcomposites. Part 4: A Raman spectroscopic study on the influence of the matrix yield stress on stress concentrations

Raman spectroscopy was used to study the influence of the shear yield stress of the matrix on the stress situation in carbon/epoxy model composites containing a planar fibre array. The fibre used was a surface-treated high-modulus Tenax® HMS-40 carbon fibre showing good fibre/matrix adhesion. Three matrices were used, all consisting of a common epoxy resin and a mixture of a di-functional and a tri-functional aliphatic amine-based curing agent. By varying the ratio of the di-functional to the tri-functional curing agent, the shear yield stress of the matrix was varied. For all three matrices, it was found that in the area immediately neighbouring a fibre fracture, stress transfer takes place through a locally yielding matrix. More importantly, it was shown that the maximum interfacial shear stress approximately equals the shear yield stress of the bulk matrix. In addition, it was found that an increase in the shear yield stress of the matrix results in a decrease of both the ineffective length and the positively affected length. Further, the experimental results show that the shear yield stress of the matrix does not significantly influence the stress concentration in the fibres adjacent to a broken fibre.