Long-term flexural performance and microstructural evolution of 3D-printed carbon, glass, and aramid fibre-reinforced polymer under hygrothermal exposure

This study quantifies the hygrothermal durability of continuous-fibre 3D-printed thermoplastic composites, carbon fibre-reinforced polymer (CFRP), glass fibre-reinforced polymer (GFRP), and aramid fibre-reinforced polymer (AFRP), under fresh water and natural seawater immersion and develops Arrhenius-based service-life models for flexural capacity. A total of 177 Markforged-printed Onyx/PA-based beams with identical layups were conditioned for 30, 90 and 180 days at 25, 40 and 60 °C. Flexural strength was measured by three-point bending; moisture uptake was tracked gravimetrically, and degradation mechanisms were probed via SEM. Long-term predictions used two interfacial-debonding–consistent forms: a pure exponential and a plateau model. All composites exhibited an initial strength increase at 30 days followed by a progressive decline. The short-term over 100 % strength retention is attributed to limited moisture-induced plasticisation of the nylon matrix, which relaxes printing-induced residual stresses and promotes interlayer fusion and void closure, thereby enhancing flexural load transfer. With continued immersion, progressive matrix plasticisation and hydrolysis, hydrogen-bond disruption in the aramid system, and fibre–matrix interfacial debonding dominate; seawater ions further accelerate interfacial attack, particularly in GFRP and AFRP composites. CFRP showed the highest peak retention of about 141 % after 30 days in water at 60 °C, while its lowest retention fell to around 105 % after 180 days in seawater at 25 °C. GFRP reached a maximum retention of approximately 123 % after 30 days in water at 60 °C, decreasing to a minimum of about 84 % after 180 days in seawater at 25 °C. AFRP exhibited a peak retention of roughly 121 % after 30 days in seawater at 60 °C, with the lowest retention reducing to around 98 % after 180 days in seawater at 60 °C. Seawater exposure consistently accelerated post-peak degradation across all composites, with the most pronounced effects observed in GFRP and AFRP. Arrhenius-based time–temperature superposition models were employed to project long-term performance based on strength values normalised to each composite’s 30-day retention. Plateau-type fits predicted asymptotic retentions of about 83.0 % in water and 85.56 % in seawater for CFRP, 65.37 % and 83.0 % for GFRP, and 92.68 % and 84.79 % for AFRP, respectively. In contrast, pure exponential models provided more conservative forecasts, indicating a continuous decline without a defined plateau. These results reveal clear differences in moisture durability governed by fibre type and solution chemistry, confirming the superior long-term stability of CFRP, the higher moisture sensitivity of AFRP, and the pronounced vulnerability of GFRP. The predictive models developed here offer valuable service-life estimation tools for structural and marine design applications. • Fibre type strongly influenced durability of 3D-printed CFRP, GFRP, and AFRP in water and seawater. • CFRP showed highest strength retention, while GFRP degraded fastest in seawater. • Arrhenius models accurately predicted long-term flexural performance. • Findings support reliable design of 3D-printed composites for marine and coastal structures.