Towards predicting failure modes in multi-material extrusion-based additive manufactured PETG/TPC structures

The presence of defects in Fused Filament Fabrication (FFF) multi-material components can lead to various failure mechanisms, mainly depending on the interface quality between adjacent materials. This study investigates the predictability of crack deflection or penetration using two established criteria from Cook & Gordon (C&G) and He & Hutchinson (H&H). Samples are printed from glycol-modified poly(ethylene terephthalate) (PETG) and a compliant thermoplastic elastomer on copolyester basis (TPC), where TPC serves as a compliant interlayer (IL) within a PETG matrix. To evaluate the stress-based C&G model, tensile tests are conducted on mono- and multi-material specimens. The fracture toughness of the TPC IL is determined using the Essential Work of Fracture approach. Interface fracture toughness between PETG and TPC is assessed using a stiffness drop technique combined with a finite element model, which applies the J-integral method. Two IL thicknesses of 0.3 and 0.8 mm are tested. The results from mechanical testing show that IL thickness does not significantly affect interface strength, but show that the choice of specimen geometry plays a key role regarding fracture behavior. Fracture tests reveal that increasing IL thickness enhances the macroscopical interface fracture toughness, although the TPC IL fracture toughness itself remains unaffected. The C&G criteria prove unreliable due to high nonlinearity, mainly due to the TPC layer. In contrast, the H&H criteria correctly identify the failure mode. Nevertheless, further investigations are necessary to validate the given transition value, as the toughness difference between the TPC IL and the interface is too large for conclusive interpretation.