Syntactic foams comprising hollow glass microspheres (GMS) in an epoxy matrix are critical materials for lightweight structures, being extensively used in marine and aerospace as cores for composite sandwich panels. They exhibit low density and high specific strength, but their use is limited by their brittleness. To increase the fracture toughness of the syntactic foam, the use of two toughening modifiers were investigated: carboxyl-terminated butadiene-acrylonitrile (CTBN) rubber, and milled carbon fibres (MCF). The morphology, thermal, mechanical, and fracture properties, and toughening mechanisms of the modified foams were determined.

The fracture energy of the unmodified syntactic foam was measured to be 192 J/m2, which increased to 296 J/m2 with the addition of 12 wt% CTBN to the epoxy matrix. The formation of co-continuous rubber structures was responsible for this increase in fracture energy. However, this increase is much smaller than for the CTBN modification of bulk epoxy polymers, where an increase from 101 J/m2 to 1112 J/m2 was observed for the same CTBN concentration. There is therefore little transfer of toughness from the bulk epoxy polymers to the syntactic foams.

When the syntactic foams were modified with MCF, the fracture energy increased to 505 J/m2 with the addition of 40% MCF:GMS weight ratio, showing good promise for MCF as a toughener in syntactic foams. Scanning electron microscopy identified the toughening mechanisms as crack deflection, debonding and subsequent plastic void growth, and fibre pull-out. The fracture energies predicted using analytical modelling of these toughening mechanisms showed excellent agreement to the experimental data. Improvements in the tensile modulus (of over 60%), tensile strength (of over 50%), and fatigue performance (of 550%) were also recorded. This successful toughening of syntactic foams can greatly increase their usefulness, enabling lighter and more damage-resistant structures to be produced.