Compression failure of continuous carbon fiber FFF composites: CT-verified effects of isotropic and concentric deposition patterns

Continuous-fiber fused filament fabrication (FFF) enables local reinforcement of polymer composites, but the effect of fiber deposition strategy on transverse compressive response remains insufficiently quantified. This work investigates cylindrical Onyx specimens reinforced with continuous carbon fiber layers printed with two deposition patterns: isotropic (0°/45°/90°/135°) and concentric. The fraction of reinforced layers was varied from 10% to 100%, and Onyx-only specimens were tested as a reference. Unlike previous studies on compression properties, this work integrates compression tests with transverse-strain measurements and X-ray CT inspection of both untested and failed specimens to directly relate nominal print settings to the effective as-built fiber architecture, porosity, and damage mechanisms. Increasing the reinforced-layer fraction increased stiffness and maximum compressive stress, while reducing lateral expansion relative to Onyx-only specimens. Although concentric paths deposited a higher effective fiber volume, isotropic paths provided higher stiffness and strength efficiency. Both layouts showed a change in transverse-strain response at approximately 2–3%, consistent with the onset of carbon-fiber damage at the structural scale. CT and SEM observations revealed pattern-dependent fracture mechanisms: concentric specimens failed through circumferential fiber breakage and tortuous crack propagation, whereas isotropic specimens showed inclined d shear-like fracture and interlaminar delamination. Linear regressions described the dependence of maximum compressive stress on effective fiber volume fraction for the two patterns. The results support deposition-pattern selection for compressively loaded continuous-fiber FFF components, especially where stiffness, strength and lateral expansion constraints must be balanced.