Effect of infill ratio on fracture toughness of Fused Filament Fabrication (FFF) polymeric adhesive joints

In order to optimize the stiffness/weight ratio and to reduce the manufacturing time, Additive Manufacturing (AM) components are built up by a solid skin and a partially filled core. The filled/total volume ratio is called infill ratio and it is one of the printing parameters to be selected. Since the infill ratio affects the global as well as the local adherend stiffness, i.e. the stiffness close to the joint interface, this paper aims at exploring the effect of this parameter on the Mode I fracture toughness of adhesive joints of AM materials, for a given infill pattern. Therefore, PLA (PolyLactic Acid) AM adherends have been manufactured with an FFF (Fused Filament Fabrication) printer at infill ratios ranging between 20% and 100% and bonded with a structural epoxy adhesive to form a DCB (Double Cantilever Beam) joint for Mode I fracture toughness testing. Finite Element (FE) simulation of a stationary crack in the adhesive layer has been done to evaluate the strain energy release rate along the crack front at different infill ratios in order to highlight local changes in its distribution that can correlate the variation of fracture toughness with infill ratio recorded in the experiments.