Temperature‐Dependent Failure Mechanisms of Hydrothermally Aged Additively Manufactured Continuous Basalt Fiber‐Reinforced Polylactic Acid/Polyethylene Terephthalate Glycol Composites

ABSTRACT Growing industrial demand for sustainable materials has drawn attention to naturally derived reinforcements for thermoplastic composites. Continuous basalt fiber (CBF), made from volcanic rock, is attracting interest in composite additive manufacturing because of its high strength, corrosion resistance, and high‐temperature performance, which suit it to aerospace, construction, defense, and automotive applications. However, limited studies have focused on how CBF composites fail under combined moisture and high‐temperature exposure. This study examines the hydrothermal degradation and failure mechanisms of additively manufactured continuous basalt fiber‐reinforced composites with a polylactic acid/polyethylene terephthalate glycol blend matrix at different exposure temperatures. Specimens were immersed in water at room temperature, 60°C, and 90°C and tested at several aging stages. Moisture uptake was evaluated with Fickian diffusion analysis, while tensile and flexural tests were used to follow changes in mechanical performance and the progression of failure. The diffusion coefficient increased from about 0.003 mm 2 /h at room temperature to 0.019 mm 2 /h at 90°C. Tensile and flexural strengths dropped considerably with aging, especially at higher temperatures. Failure analysis identified the damage mechanisms linked to aging. Prolonged exposure at 60°C caused progressive damage in the composite, whereas exposure at 90°C led to severe loss of mechanical properties and premature failure, even after a short exposure. Fourier‐transform infrared analysis showed shifts in relative band ratios but no distinct new absorption bands, and scanning electron microscopy revealed voids and interfacial damage in the aged specimens. These results show that both exposure temperature and duration control the failure response.