Flexural properties of additively manufactured continuous fibre reinforced thermoplastic polymers after exposure to elevated temperatures

Additive manufacturing composite components are gaining popularity for their design flexibility and fibre reinforcement potential, making it essential to evaluate their mechanical properties under diverse loading and environmental conditions. This study experimentally examines the flexural properties of 3D-printed continuous fibre reinforced polymer (FRP) composites with an Onyx matrix using a 3-point bending test, following exposure to elevated temperatures up to 200 °C. Three types of fibres, including carbon, glass, and Kevlar were investigated. The findings indicated that irrespective of the fibre type, all composites exhibited an initial increase in flexural strength for all conditioned samples compared to the reference samples up to 140 °C. This was followed by a drop in strength at around 145 °C, corresponding to the heat deflection temperature of Onyx. Above 160 °C, the flexural strength retention increased again. The observed increase in flexural strength in conditioned samples can be attributed to annealing process and increased flowability of the thermoplastic matrix as well as the geometrical changes in the cross-sectional area of the specimens, which increased the second moment of area of the composite. Additionally, elevated temperatures may have caused further polymer chain alignment or reduced internal stresses within the composite matrix, contributing to the observed improvements in mechanical performance. The findings of this study highlights the critical role of understanding the thermal properties and behaviour of 3D-printed thermoplastic composites, emphasising the need to optimise their mechanical performance in high-temperature engineering applications.