The enhancement of adhesively-bonded aerospace-grade composite joints using steel fibres

Ductile steel fibres with high stiffness of 193 GPa, high strain-to-failure of 20% and diameter of 22 μm were used to enhance fracture toughness of composite adhesive joints. Two nano-toughened structural adhesives with significantly different mechanical and fracture properties were used to bond aerospace-grade composite substrates. Steel fibres were placed in the adhesive layer either longitudinally or transversely to the crack growth direction. Mode-I and mode-II fracture behaviour of the composite adhesive joints were studied using double cantilever beam test and end-loaded split test, respectively. The incorporation of steel fibres significantly increased both mode-I and mode-II fracture toughness, irrespective of the adhesive used. The improvement of mode-I and mode-II fracture energies was more pronounced when the steel fibres were placed transversely to the crack growth direction, due to the increased level of steel fibre bridging.