Experimental and Numerical Investigation of the Interfacial Bonding in Multi-Material Fused-Filament Fabrication Using Mode-I Fracture Toughness

Multi-material fused-filament fabrication (FFF) enables the integration of two or more materials within a single printed component, offering significant potential for producing functionally graded and hybrid structures with tailored mechanical properties. However, the effects of processing conditions on interfacial quality and fracture behavior in multi-material FFF remain insufficiently understood. This study investigates the Mode-I interfacial fracture behavior of multi-material FFF using compact-tension (CT) specimens through combined experimental testing and finite-element simulation. CT specimens were fabricated from polylactic acid (PLA), thermoplastic polyurethane (TPU), and polyethylene terephthalate glycol (PETG) using five raster configurations, including rectilinear 0°/90°, ±45° and aligned-rectilinear 0°, 45°, 90° configurations. The load-extension response, peak load (Pmax), critical stress-intensity factor (KIC), and critical strain-energy release rate (GIC) were evaluated. Two-way ANOVA showed that material pairing was the dominant statistically significant factor affecting peak load, whereas raster orientation and its interaction with material pairing were not statistically significant. The PETG–TPU interface exhibited the highest fracture resistance, with the AR-45 orientation producing an average Pmax of 130.90 N, KIC of 0.761 MPam, and GIC of 375.5 N/m. Fracture-surface observations supported these findings, revealing greater deformation, material transfer, surface roughness, and mechanical interlocking in the PETG–TPU specimens. The calibrated cohesive-zone model demonstrated the stress contours and the localized interfacial damage patterns. Overall, material pairing primarily governed interfacial fracture resistance, whereas raster orientation showed no statistically significant effect under the tested conditions.