Microstructure-failure interactions in continuous carbon- and glass-fibre reinforced nylon 3D-printed composites under compression

Continuous fibre-reinforced nylon produced by fused filament fabrication (FFF) offers great potential for lightweight structural applications; however, process-induced defects such as porosity, fibre misalignment and inter-bead discontinuities can significantly reduce compressive performance. This study investigates the role of these microstructural features in governing damage evolution and collapse in continuous carbon fibre-reinforced (CFRP) and glass fibre-reinforced (GFRP) nylon under uniaxial compression using in-situ synchrotron X-ray computed tomography (XCT). XCT scans recorded before and after compression are analysed using a deep learning-based segmentation framework to quantify phase fractions, fibre orientation and void morphology. CFRP exhibits higher void content (11%–12%) and lower fibre volume fractions (30%–33%) than GFRP with approx. 70% and 50% of fibres oriented within 2.5° of the extrusion direction. For both CFRP samples and one GFRP sample, kinking was observed out-of-plane of the printed layers, accompanied by fibre fracture and delamination. In the other case, the GFRP exhibited more distributed deformation characterised by fibre reorientation, bending instability and interlaminar damage. This was ascribed to a reduction in lateral constraint arising from interlaminar voids, but the lower degree of fibre alignment for that sample may also contribute. These results show how compressive failure in continuous-fibre FFF composites is controlled by the coupled interactions between fibre architecture, void morphology and nonuniform local stress field, providing a basis for microstructure-informed process optimisation and improved structural performance.