Micromechanical behavior of Kevlar-149/S-glass hybrid seven-fiber microcomposites: I. Tensile strength of the hybrid composite

To understand the mechanism of the ‘hybrid effect’ on the tensile properties of hybrid composites, single fiber type and hybrid microcomposites were fabricated by using Kevlar-149 as the low elongation fiber and S glass fibers as the high elongation fiber, in a DER 331/DER 732 epoxy mixture (70/30, w/w). Kevlar-149 fiber showed a significantly higher tensile strength in the microcomposite than as a single filament. For the hybrid, Kevlar-149 fibers usually broke one by one. A positive hybrid effect for the failure strain but a negative hybrid effect for the strength of the hybrid were observed. Tensile strength of the microcomposites predicted by Monte Carlo simulation agreed with the experimental results reasonably well. The tensile modulus of the hybrid followed the rule of mixture. The fiber/matrix interface properties were investigated by using single fibre pull-out from a microcomposite (SFPOM) test, which showed a significant difference between the interfacial shear strength (IFSS) of Kevlar fiber/epoxy in single-fiber type (SFT) and that in the hybrid at a constant fiber volume fraction, which shortened the ineffective length and contributed to the failure strain increase of Kevlar-149 fibers in the hybrid.