Improvement in interfacial fracture toughness of multi-material additively manufactured composites through thermal annealing

An experimental investigation is performed to investigate the effect of post-processing heat treatment on the interfacial fracture toughness of bi-material additively manufactured semi-crystalline polymer composite. An asymmetric double cantilever beam (ADCB) and single-leg bending (SLB) specimens made of polylactic acid (PLA) and Nylon are considered for the mode-I and mixed-mode fracture characterization, respectively. Specimens are isothermally heated in a forced convection oven for a wide range of temperatures and durations. Fracture toughness decreases significantly for both mode-I and mixed-mode conditions when specimens are annealed below the melting temperature of PLA (150 °C). An increase of the crystallinity at the high-temperature annealing prevents the polymer chain mobility, hinders the neck growth, and provides poor intermolecular diffusion resulting in decrease of fracture toughness by 88% as compared to the unannealed specimen. Annealing at 160 °C improves the bi-material interfacial fracture toughness by a maximum of 1225% via sufficient interfacial wetting, higher molecular diffusion, and a longer polymer chain entanglement process. Material transfer, void collapse, and filament impression on the fracture surface of high temperature annealed specimen indicate a better molecular diffusion and strong interlaminar bond at the interface.