Evolution of damages and mechanical properties in 3D printed C-CFRPs laminates under compressive load

Variations in porosity resulting from the layer-by-layer deposition process can lead to distinct kink band damage modes. In this study, mechanical testing and in situ X-ray computed tomography (X-ray CT) compression experiments were conducted to investigate the influence of lay-up sequence and print-induced porosity on the compressive response of 3D-printed continuous carbon fibre-reinforced polymer (C-CFRPs) laminates. A U-Net deep learning neural network semantic segmentation approach was employed to quantitatively characterize the internal porosity. The nucleation and evolution mechanisms of kink bands were further elucidated by correlating the segmented defect morphology with damage progression. The results revealed that the fibres were placed in the inner layers, where the surrounding non-0° layers provided lateral constraint. This stabilization reduced the buckling risk and improved the longitudinal compressive strength. Type 2 kink bands were observed in specimens exhibiting a large interlayer porosity mismatch (≈5%), whereas Type 1 kink bands were associated with nearly uniform porosity distributions (≈0.3% or lower) across adjacent layers. For Type 1 kink bands, observations suggest a trend that damage evolution is initiated in zones with relatively higher porosity and subsequently extends toward neighbouring lower-porosity regions. Porosity asymmetry in adjacent layers triggers Type 2 fibre micro-buckling (leading to kink bands) first, while uniform porosity promotes Type 1. Type 2 failure initiates earlier than Type 1.