Hygromechanical properties of 3D printed continuous carbon and glass fibre reinforced polyamide composite for outdoor structural applications

The additive manufacturing of structural composites is a disruptive technology currently limited by its moderate mechanical properties. Continuous fibre reinforcements have recently been developed to create high performance composites and open up encouraging prospects. However, to increase their use, deeper understanding of the relationship between process and induced properties remains necessary. In addition, to apply these materials to engineering applications, it is of high importance to evaluate the effect of environmental conditions on their mechanical performances, particularly when moisture-sensitive polymer is used (PolyAmide PA for instance) which is currently lacking in the literature. This present article aims to investigate in more detail the relationship between the process, the mechanical behaviour and the induced properties of continuous carbon and glass fibres reinforced with a polyamide matrix manufactured using a commercial 3D printer. In addition, their hygromechanical behaviour linked to moisture effect is investigated through sorption, hygroexpansion and mechanical properties characterization on a wide range of relative humidity (10-98% Relative Humidity RH). The printing process induces an original microstructure with multiscale singularities (intra/inter beads porosity and filament loop). Longitudinal tensile performance shows that the reinforcing mechanism is typical of composite laminates for glass and carbon. However, the rather poor transverse properties are not well fitted by the Rule Of Mixture (ROM), thus underlining the specificity of the printing-induced microstructure and an anisotropic behaviour in the material. Non-negligible (5-6%) moisture uptake is observed at 98% RH, as well as orthotropic hygroscopic expansion of PA/carbon and PA/glass composites. The consequences of various moisture contents on mechanical properties are studied, showing a reduction of PA/carbon stiffness and strength of 25 and 18% in the longitudinal direction and 45 and 70% in the transverse direction. For PA/glass composites, we obtain a reduction in strength of 25% in the longitudinal direction, along with a 80% reduction of stiffness and 45% in strength in the transverse direction. A wetting/drying cycle underlines reversible phenomena in the longitudinal direction and mainly non-reversible degradation in the transverse direction.