MICROMECHANICAL BEHAVIOR OF KEVLAR-149 S-GLASS HYBRID 7-FIBER MICROCOMPOSITES .1. TENSILE-STRENGTH OF THE HYBRID COMPOSITE/

To understand the mechanism of the 'hybrid effect' on the tensile prop erties of hybrid composites, single fiber type and hybrid microcomposi tes were fabricated by using Kevlar-149 as the low elongation fiber an d glass fibers as the high elongation fiber, in a DER 331/DER 732 epox y 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 hy brid effect for the failure strain but a negative hybrid effect for th e strength of the hybrid were observed. Tensile strength of the microc omposites predicted by Monte Carlo simulation agreed with the experime ntal results reasonably well. The tensile modulus of the hybrid follow ed the rule of mixtures. The fiber/matrix interface properties were in vestigated 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 sho rtened the ineffective length and contributed to the failure strain in crease of Kevlar-149 fibers in the hybrid.