PSI - Issue 77

Jiongyi Yan et al. / Procedia Structural Integrity 77 (2026) 135–142 J. Yan/ Structural Integrity Procedia 00 (2026) 000–000

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microstructural features imprinted by the toolpaths (Qamar Tanveer et al., 2022). Angular turns and corners, although ubiquitous as they can be created intentionally or accidentally: at perimeters, part corners, infill junctions, and internal lattices, are rarely studied. During printing, the nozzle decelerates when approaching a corner turn, leading to reduction in flow rate, whereas it accelerates when departing a corner and causes overshoot (Akhoundi et al., 2023). At the corner the polymer melt undergoes sharp transient spikes in shear rate and pressure, the extrudate is stretched on the outer radii and compressed on the inner radii (Mollah et al., 2022). More flow resistance is accumulated with higher shear rate for the inner edges than the outer edges, which may cause over-extrusion at the outer radii, leading to inhomogeneity and stress concentration (Z. Liu et al., 2020). For short fibre reinforced materials, corner toolpaths could perturb the orientation and distribution of fibres. Previous studies showed that fibres are highly aligned in the printing direction in straight paths (Yan et al., 2022), but they may be subject to misalignment, splaying, or buckling at corner. There are several research on fibre kinematics and orientation at corner toolpaths. Corner effects on short fibre orientation (Yan et al., 2023c) and long fibre jamming (Ding et al., 2024) have been identified. Corner effects on precision (Friedrich & Begley, 2020) and speed (Akhoundi et al., 2023) have been studied, and toolpath algorithm to compensate corner inaccuracy has been proposed (J. Liu et al., 2022). However, mechanical properties of angular corners are rarely characterised. In this study, we 3D printed short-fibre composites and varied the turn angles (30°, 60°, 90°, 120°, 150°). The 3D fibre orientation was quantified and used to explain the mechanical properties. Consistent trends were found where main-axis fibre alignment decreased at the corners. Static and cyclic tensile tests showed lower force and mechanical degradation for greater turn angles, which also correlated with corner‑induced misalignment and interlayer bonding for structure-dependent fatigue properties. We modelled the sharp 150° corner with a common composite model (Hashin damage criteria) to simulate structural failure under large deformation, and the simulation results effectively agreed with experiment by showing crack initiation and propagation. This study highlights the structure-dependent properties and mechanical weakness of angular corners in MEAM. It provides an insight into corner-aware toolpath and microstructure designs.

Nomenclature θ

Euler angle of a fibre in the out-of-plane direction ф in-plane deviation angle of a fibre to the longitudinal direction 11 fibre orientation tensor component: main-axis alignment along the printing direction 22 fibre orientation tensor component: lateral alignment transverse to the printing direction 33 fibre orientation tensor component: out-of-plane Z-direction alignment

2. Experimental Methodology 2.1. Materials and 3D printing

Polylactic acid reinforced with short carbon fibre (PLA-CF) filaments were obtained from Bambu Lab © , with filament diameter of 1.75 mm. The corner structure was designed by using Fullcontrol Gcode Designer (Python)(Gleadall, 2021), which allows direct coding of accurate toolpaths and generation of Gcode. The structure consisted of single-wall single-filament-extrusion with three sections (before corner, corner zone, after corner), and all have the same length 20 mm in the longitudinal direction and 10 mm height. In the corner section, different turning angles (defined as the angles at which the nozzle turned deviated from the original direction, not the apparent corner angles) of 30º, 60º, 90º, 120º, 150º were designed. A Bambu X1-Carbon printer was used to 3D print the designed structures. The nozzle temperature and bed temperature were 220 o C and 60 o C respectively, and the printing speed was 1000 mm.min -1 . The extrusion width, namely nominal wall thickness, is 0.6 mm, and the layer height is 0.2 mm.