Strain rate-dependent failure modes of material extrusion-based additively manufactured PETG: A study on crack deflection and penetration

The layer-by-layer nature of fused filament fabrication (FFF) introduces interfaces along the build direction (z-axis). A crack approaching an interface may deflect or penetrate subsequent layers, based on the relative strengths of the interface and the matrix. This study evaluates the applicability of the Cook & Gordan (C&G) model for predicting crack deflection or penetration in glycol-modified poly(ethylene terephthalate) (PETG) printed structures, considering different print orientations and layer heights. The loading rate was varied between 0.1 and 1000 mm/min to identify potential rate-dependent effects. Interface and matrix strengths were determined through tensile testing, and their ratio was used to assess the validity of the C&G criterion. The results, supported by fracture mechanical validation experiments, indicate that the C&G model can effectively predict crack paths in FFF-printed PETG structures, provided that the assumptions of linear elastic fracture mechanics are not significantly violated. Accurate predictions were unattainable at the lowest loading rate (0.1 mm/min). For loading rates ≥ 10 mm/min the criterion appears plausible, aligning with previous studies.