Enhancing transverse mechanical properties of continuous carbon fibre reinforced composites via staggered-layer printing method

Abstract

The transverse mechanical performance is known to be usually weak in Continuous Carbon Fibre Reinforced Composites (CCFRCs) fabricated through Fused Filament Fabrication (FFF), impeding their engineering applications. To address this issue, a Staggered-Layer Printing (SLP) method is proposed based on FFF to produce the CCFRCs with enhanced transverse mechanical properties. First, the optimal melt deposition width was determined based on microscopic characterizations. And then, multi-scale models were constructed based on the mesoscopic features of CCFRCs manufactured using both the conventional fabrication method, i.e., Aligned-Layer Printing (ALP) and the proposed SLP method. Finally, the tensile tests and short beam shear tests were performed to obtain the mechanical properties of printed specimens. Experimental validations on 90° specimens were performed, showing that the transverse Young's modulus and tensile strength of the ALP specimens are 3.6 GPa and 22.4 MPa respectively, while those of the SLP specimens are 4.8 GPa and 44.1 MPa respectively. Furthermore, the change of the meso-structure due to the SLP method has enhanced the printed specimens' critical fracture toughness, resulting in a substantial improvement of 97% in the transverse strength of additively manufactured CCFRCs. The main reason is attributed to the stress redistribution that impedes the crack propagation along the weak intra-layer interfaces in SLP specimens. Accordingly, the numerical model was developed to evaluate the critical fracture strength and fracture toughness inside printed filaments in the transverse direction as 73 MPa and 0.6 mJ/mm², respectively. Hence, the specific meso-structure generated from SLP is effective to significantly improve the transverse load-bearing capacity of FFF-printed CCFRCs without compromising their longitudinal properties.