Tensile failure of adhesively bonded CFRP composite scarf joints

Adhesively bonded scarf joints, comprising unidirectional carbon fibre-reinforced epoxy adherends and AF-163-2 film adhesive of 0.15 mm thick, are tested under uniaxial tensile loading. Test results revealed that failure occurred into two modes, namely fibre fracture and pull-out (Mode A) and cohesive shear failure of the adhesive film (Mode B). Mode A failure was observed for scarf angles less than about 2°, while Mode B failure was observed for scarf angles more than 2°. The knockdown in tensile strength was most prominent for scarf angles less than about 1°. For the largest scarf angle of 5°, the tensile strength dropped by 89% in comparison to the strength values of neat composites with no joints. The knockdown in tensile strength with increasing scarf angle was well described by our FEM predictions, which modeled the unidirectional composite as a homogeneous orthotropic continuum with Hashin–Lee failure criterion. Finally, the effect of adhesive bond line thickness on the joint strength is investigated by finite element simulations.