Test conditions effect on the fracture toughness of hollow glass micro-sphere filled composites

Low-density sheet-moulding compounds based on hollow glass micro-spheres are usually classified as syntactic foams if the filler content is relatively high. Syntactic foams are potentially suitable materials for applications where impact loads occur as they are able to reduce impact force. The addition of hollow micro-spheres tends to increase the specific values in terms of impact force and, marginally, in flexural modulus for high-volume fractions of micro-spheres. In this study, the effects of load rate and of immersion of the specimens in water up to 67 days were studied on the flexural mechanical properties and particularly on the fracture toughness, K-IC. Hollow micro-spheres (Verre ScotchitTM-K20) with epoxy and polyester polymer binder were used. Fracture toughness, K-IC, flexural stiffness modulus and ultimate strength were obtained as functions of load rate and immersion time. The increase of load rate tends to increase stiffness modulus, but effects on K-IC were found to be only marginal. Ultimate strength increases significantly with the increase of load rate for epoxy-based composites, but for the case of the polyester-based foams, only a negligible effect was observed. The increase of the immersion time in water tends to reduce stiffness modulus. K-IC decreases slightly after 15 days for the polyester-based composites and after 67 days for epoxy-based foams, and only negligible effects on ultimate strength were observed.