Two different modes of fracture propagation can occur when a crack encounters a weak interface in a fused filament fabricated (FFF) part: the crack either deflects into the interface, or penetrates the subsequent layers. The objective of this work is to verify the suitability of an energy- and a strength-based criterion for predicting which failure mode will occur in FFF printed parts. Four different materials, glycol-modified poly(ethylene terephthalate), polylactide acid and two different poly(methyl methacrylate) grades were examined. Fracture mechanical tests were performed on single edge-notched bending specimens for the energy-based approach and tensile tests performed on dumbbell specimens for the strength-based approach. Additionally, porosity measurements and thermal analysis were carried out to provide structural information. The energy-based approach proved unreliable for failure mode prediction. Potential problems include failure to meet the requirements of linear elastic fracture mechanics and issues with notch design. The strength-based approach, in contrast, correctly predicted the crack path for all tested materials and seems a promising candidate for failure mode prediction in FFF materials.