Micro-mechanical deformation mechanisms in the fracture of hybrid-particulate composites based on glass beads, rubber and epoxies

Two tougheners, glass beads and carboxyl terminated butadiene acrylonitrile copolymer (CTBN), are used to toughen and stiffen an epoxy thermoset. Rubber-encapsulated glass beads are used and the hybrid particulate composites containing them are compared with those containing non-encapsulated glass beads. Within a certain range of composition, the rubber encapsulation is found to change the interactions between glass beads and CTBN particles, resulting in an increase in fracture toughness. The toughening effect is explained by the fact that the cavities of CTBN particles are larger in encapsulation systems than in non-encapsulation systems. As more CTBN particles are incorporated into glass bead filled epoxies, the cavitation/shear yielding mechanism of CTBN particles replaces the micro-shear banding mechanism of glass beads as the major micro-mechanical deformation. Rubber encapsulation seems to enable this transition of major micro-mechanical deformation to occur at a lower volume fraction of CTBN.