Correlating Scale-Specific Void Architecture with Tensile and Flexural Performance in Fused Filament Fabricated Continuous Carbon Fibre-Reinforced PA6-Based Composites

Continuous carbon fibre–reinforced thermoplastic composites produced by fused filament fabrication (FFF) exhibit mechanical properties influenced by manufacturing-induced voids near bead-fusion zones and fibre–matrix interfaces. In this work, a replicated 3 × 3 full-factorial design of experiments (DOE) quantified tensile strength (TS) and flexural strength (FS) across fibre volume content (20–40 vol%) and stacking sequence (Equidistant, Borders, and Borders & Centre) in continuous carbon fibre–reinforced Onyx composites (CFROC). A mechanically informed sub-DOE was then analysed using synchrotron micro-computed tomography (SMCT; 3.6 μm voxel edge length) and scanning electron microscopy (SEM) of as-printed surfaces. Connected void objects were segmented from SMCT volumes and partitioned by equivalent spherical diameter (ESD) into micro (ESD < 0.100 mm), meso (0.100 ≤ ESD < 0.500 mm), and macro (ESD ≥ 0.500 mm), with a flagged macro-tail subset (ESD ≥ 1.0 mm) used to capture deposition-scale discontinuities. Across the DOE, responses spanned TS = 230–435 MPa and FS = 399–636 MPa; the highest fitted responses occurred at 40 vol% with Borders, whereas 20 vol% with Equidistant defined the lower bound. In the SMCT subset, global porosity (Pglobal) ranged from 5.58 to 10.21%, but strength ranking was not explained by porosity magnitude alone. Ranking was more consistent with scale-resolved porosity partitioning, meso/macro discontinuity morphology, and macro-tail continuity. PO references showed connected macro discontinuity networks, whereas high-performing CFROC specimens exhibited curtailed macro-span continuity and a greater share of void volume in micro/meso populations; SEM observations supported corresponding differences in fusion quality and interface-adjacent discontinuities. The study presents a DOE-linked, object-wise, scale-resolved void-architecture framework for FFF CFROC that enables identification of mechanically consequential discontinuity families beyond Pglobal within a controlled reinforcement-architecture window.