Anisotropy and Defect Tolerance of FFF-Manufactured PETG: A Combined Tensile, Compressive and Mode I Fracture Study

Fused filament fabrication (FFF) enables the rapid, low-cost manufacture of thermoplastic components, but its layer-by-layer deposition imparts a pronounced, orientation-dependent anisotropy that remains only partially characterised for polyethylene terephthalate glycol (PETG), a polymer whose fracture behaviour in particular has scarcely been reported. This work presents an integrated characterisation of unreinforced FFF PETG in which the tensile, compressive and mode I fracture response are measured within a single experimental campaign and under a common set of standards, so that the influence of build orientation can be resolved consistently across all three loading modes. Specimens were manufactured at two orientations, with the deposited filaments aligned with and transverse to the loading direction, and tested under quasi-static conditions. To move beyond the qualitative descriptions that dominate the literature, a dimensionless anisotropy index was defined for each property and used to compare the loading modes with one another and to benchmark PETG against reported values for polylactic acid (PLA) and acrylonitrile butadiene styrene (ABS). A distinctive anisotropy signature emerges: the tensile response is by far the most orientation-sensitive, the fracture response is markedly but more moderately anisotropic, and the compressive response is almost insensitive to build orientation. Although the transverse orientation loads the inter-layer bonds and fails in a brittle, low-energy manner, PETG retains a far larger fraction of its fracture resistance in this weak orientation than literature values reported for ABS, on this benchmark making it the apparently more forgiving material precisely in the property that governs the integrity of cracked or notched parts. Combining the measured strength and toughness within a linear-elastic fracture-mechanics framework further shows that build orientation governs the size of the intrinsic defect the material can sustain before unstable fracture, a defect-tolerance reading that neither property yields on its own, obtained under an idealised small-scale-yielding assumption whose applicability is examined explicitly for each orientation. These findings establish build orientation as a first-order design parameter for FFF PETG and provide a consistent, standard-based and cross-material reference dataset linking print orientation to the strength, stiffness, toughness and defect tolerance of the printed material.