Advancing the Capability of Additively Manufactured Continuous Fibre-Reinforced Polymers for Structural Applications: The Effect of Nitrogen-Purging and Post-Annealing on the Tensile Performance

Additively manufactured continuous fibre-reinforced polymers (CFRPs) offer promising mechanical properties for engineering applications, including aerospace and automotive load-bearing structures. However, challenges such as weak interlayer bonding and low strength compared to traditional composites remain. This paper presents an experimental investigation into the effects of nitrogen (N2) purging during printing and thermal anneal- ing after printing on the tensile performance of additively manufactured CFRPs. Tensile tests were conducted on Onyx specimens produced by material extrusion and reinforced with continuous carbon fibre filaments (CFF), glass fibre filaments (GFF), or Kevlar fibre filaments (KFF). Results showed that N2-purging and post-annealing had different effects on the tensile properties of various CFRPs. Particularly, N2-purging, post-annealing, and their combination enhanced both the Young’s modulus and ultimate tensile strength (UTS) of KFF/Onyx specimens. For GFF/Onyx specimens, both treatments had a minor effect on the Young’s modulus but enhanced UTS. CFF/Onyx specimens exhibited improved Young’s modulus with N2-purging, while both treatments reduced UTS. The different response of the CFRPs was associated with diverse governing failure mechanisms, as proved by microstructural and fracture surface inspection. Additionally, differential scan- ning calorimetry (DSC) and X-ray diffraction (XRD) analyses also revealed the thermal behaviour and crystal structures that influence the mechanical properties of CFRPs.