Understanding UV-driven degradation in 3D-printed continuous fibre composites: Impact, hardness, and surface evolution

This study examined the effects of ultraviolet (UV) ageing on the impact performance, surface hardness, and roughness evolution of 3D-printed continuous fibre-reinforced Onyx® composites fabricated with carbon (CFRP), glass (GFRP), and aramid (AFRP) fibres. Specimens were exposed to UVA-340 radiation for 720, 1440, and 2160 h, simulating up to approximately three years of outdoor exposure. Charpy impact testing, Shore D hardness measurements, and surface roughness (R_a) profiling were performed to evaluate mechanical and morphological changes. Results showed different fibre-specific behaviours. Impact retention was strongly fibre-dependent, with AFRP achieving up to 124 % retention after 2160 h and GFRP maintaining about 116–118 %. In contrast, CFRP exhibited initial losses below 90 % at 1440 h before partial recovery to 94 % after 2160 h. Hardness increased for almost all composites due to UV-induced cross-linking of the Onyx matrix, while surface roughness also increased significantly, especially in CFRP. The findings indicate that UV degradation mechanisms involve competing processes of surface cross-linking and photodegradation, mediated by fibre-specific interfacial chemistry. Generally, GFRP and AFRP exhibited enhanced dynamic durability under UV ageing, whereas CFRP's performance was limited by surface embrittlement and localised photothermal oxidation. These findings provide practical guidance for designing and selecting 3D-printed fibre composites with tailored UV resistance, enabling longer service life and safer use of polymer-based structural components in outdoor engineering applications.