Influence of interface notch geometry on the tensile behaviour of FGF-printed parts

Purpose: The paper investigates the influence of interface notches-formed due to extrudate geometry-on the tensile performance of parts manufactured by fused granular fabrication (FGF). Although FGF enables fast, cost-effective, large-scale printing with thermoplastic pellets, its adoption in industry remains limited, mainly due to poor mechanical performance. Interface notches have been identified as a key contributor to this limitation. The study investigates the effect of layer height on notch severity and its impact on the tensile behaviour of PETG samples, to improve understanding and guide process optimisation. Design/methodology/approach: Single-extrudate-thick PETG walls were printed at two different layer heights (3 mm and 4.5 mm) using a large-scale pellet-fed 3D printer. Tensile specimens were extracted from these walls and tested using digital image correlation (DIC) to map full-field strain. Interface notch geometry was characterised through optical microscopy and image analysis, focusing on key parameters including notch depth, angle, and root radius. The geometric features were then correlated with the tensile properties and localised strain distribution observed during loading. Findings: Increasing the layer height resulted in deeper, sharper interface notches that raised the strain concentration factor and caused more premature failure. The 4.5 mm layer height sample showed a 31% reduction in ultimate stress and a 29% decrease in strain at break compared to the 3 mm sample. DIC analysis confirmed that the strain localised at the notch roots, highlighting the impact of severe notches on tensile performance. Research limitations/implications: Future studies could expand on this work by exploring a wider range of layer heights, nozzle sizes, materials, and interfacial healing conditions. Quantifying the evolution of notch geometry across multiple process variables could help establish predictive models for failure. Practical implications: Understanding the role of notch geometry in FGF-printed parts helps inform optimal printing strategies for improving part strength and reliability. Findings can guide design choices in the structural applications of large-format printing, where the extrudate shape has a significant influence on performance. Originality/value: The paper highlights the often overlooked role of interface notch geometry as a key driver of failure in FGF-printed structures. By combining DIC and quantitative notch characterisation, it offers new insights into the geometric mechanisms behind anisotropy and strength reduction in material extrusion additive manufacturing.